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MSTP FUNDAMENTALS
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Contents
1 Evolution of MSTP..................................................................................................................................... 1
1.1 Emergence of MSTP......................................................................................................................... 1
1.2 First Generation MSTP ..................................................................................................................... 2
1.2.1 Virtual Concatenation Technology ......................................................................................... 2
1.2.2 Link Capacity Adjustment Scheme ........................................................................................ 3
1.3 Second Generation MSTP................................................................................................................. 4
1.3.1 Resilient Packet Ring Technology ......................................................................................... 4
1.3.2 Multiple Protocol Label Switching Technology..................................................................... 5
2 Theory of EOS............................................................................................................................................ 7
2.1 Ethernet Fundamentals...................................................................................................................... 7
2.1.1 Ethernet Frame Format .......................................................................................................... 7
2.1.2 MAC Address......................................................................................................................... 8
2.2 Ethernet Switching Principle............................................................................................................. 9
2.2.1 Operation Principle of Transparent Bridge ............................................................................ 9
2.2.2 MAC Address Learning ......................................................................................................... 9
2.2.3 Transfer and Filtering Mechanism ....................................................................................... 10
2.2.4 Loop Avoidance: Spanning Tree Protocol............................................................................ 13
2.2.5 VLAN................................................................................................................................... 13
2.3 EOS Fundamentals.......................................................................................................................... 19
2.3.1 What is EOS......................................................................................................................... 19
2.3.2 Function Model of EOS ....................................................................................................... 19
2.3.3 Ethernet Frame Encapsulation ............................................................................................. 20
2.3.4 Contiguous Concatenation and Virtual Concatenation......................................................... 23
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3 Theory of ATM..........................................................................................................................................25
3.1 ATM Fundamentals..........................................................................................................................25
3.1.1 Generation Background of ATM Technology.......................................................................25
3.1.2 ATM Features........................................................................................................................26
3.1.3 ATM Cell Structure...............................................................................................................27
3.1.4 Fundamentals of ATM Switching .........................................................................................27
3.1.5 ATM Statistics Multiplexing.................................................................................................30
3.1.6 ATM Protocol Reference Model ...........................................................................................31
3.1.7 ATM Service Type ................................................................................................................34
3.1.8 ATM Communication QoS ...................................................................................................36
3.2 ATM Processing in MSTP Devices..................................................................................................36
3.2.1 Background of ATM Application on MSTP ........................................................................36
3.2.2 Key Technology of ATM Service Processing .......................................................................37
3.2.3 ATM Layer Processing Function of MSTP Devices .............................................................39
4 Theory of RPR ..........................................................................................................................................41
4.1 Overview of RPR Technology .........................................................................................................41
4.1.1 Emergence of RPR Technology............................................................................................41
4.1.2 Basic Concepts and Features of RPR Technology................................................................43
4.2 Fundamentals of RPR Technology ..................................................................................................45
4.2.1 RPR Ring Network Architecture ..........................................................................................45
4.2.2 RPT Technology ...................................................................................................................47
4.2.3 RPR Network Hierarchy Model ...........................................................................................50
4.2.4 RPR MAC Data Frame Processing.......................................................................................52
4.2.5 RPR Fairness Algorithm .......................................................................................................54
4.2.6 RPR Topology Discovery .....................................................................................................56
4.2.7 RPR Protection .....................................................................................................................58
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4.3 RPR Implementation Scheme ......................................................................................................... 60
4.3.1 Three Implementation Schemes of RPR .............................................................................. 60
4.3.2 System Architecture of RPR-Embedded MSTP................................................................... 62
5 Theory of MPLS....................................................................................................................................... 63
5.1 Introduction to MPLS ..................................................................................................................... 63
5.2 Architecture of MPLS ..................................................................................................................... 66
5.2.1 Basic Working Mode of MPLS ............................................................................................ 67
5.2.2 Advantages of MPLS ........................................................................................................... 72
Appendix A Abbreviations.......................................................................................................................... 77
Chapter 1 Evolution of MSTP
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1 Evolution of MSTP
Key points
Evolution of the MSTP technology
Difference between the MSTP and traditional SDH technology
Current state of the MSTP technology
1.1 Emergence of MSTP
In the telecommunication era with the principle part of voice services, Synchronous
Digital Hierarchy (SDH) systems guarantee the real-time transmission of voice
services as carrier networks through the mapping of tributaries, cross-connects and the
point-to-point quality assurance mechanism. Increasingly, with the great and
high-speed development of IP data services based on packet exchange, it is difficult for
SDH networks, which are based on the transmission mechanism of time-division
switching, to carry IP services efficiently while satisfying the transmission
requirements of voice services. It used to be the discussing focus of professionals in the
telecommunication industry whether reestablish carrier networks without using the
SDH technology, or approach some new technologies to reconstruct SDH networks and
solve the problem at network edges (access end), thus achieving good passing
characteristic of IP services over SDH networks. Undoubtedly, the latter one is more
operable. Because with this solution, not only current network resources can be used
more efficiently, but also some inherent characteristics of the SDH technology can
make up for certain shortcomings of Ethernet, such as Quality of Service (QoS).
Then the concept of Multi-Service Transport Platform (MSTP) over SDH appears
which is also called as a new generation of SDH. It is different from traditional SDH
equipment. From the point of position in networks, the MSTP should be located at the
access layer, which means it is connected to various service interfaces at the customer
side while to SDH transmission equipment at the network side. In other words, the
MSTP is just like a long-haul passenger/freight junction center station. The objective of
the station is to separate passengers from freights efficiently, and then transport them to
corresponding destinations safely and fast according to different requirements.
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1.2 First Generation MSTP
The initial purpose of MSTP is to implement the transparent point-to-point
transmission of IP data packets over SDH through mapping Ethernet frames into
containers (C) of SDH frames directly. However, the fixed sizes of payload in different
SDH containers make it difficult to load 10/100 M Base-T frames or Gigabit Ethernet
frames into SDH containers perfectly. As a kind of point-to-point transparent
transmission mechanism, the earlier MSTP can not implement many functions such as
flow control, QoS of Ethernet services, and the statistics and multiplexing of different
Ethernet traffics. At that time, no commercial values could be offered by the MSTP.
To improve the carrying efficiency of IP data services over SDH, the virtual
concatenation technology and Link Capacity Adjustment Scheme (LCAS) are
presented in the first generation MSTP.
1.2.1 Virtual Concatenation Technology
An effective solution for mapping Ethernet frames appears: Cascade Virtual Container
(VC) units to form an appropriate loading unit. For instance, bind five VC-12 units as a
unit, which can carry 10 M Ethernet service pretty well. But it also brings a new
problem. If adjacent containers are cascaded as VC-n-Xn, the cascaded loading units
have to keep the same route and continuous bandwidth during the whole transmission
process. On the other hand, the passing equipment must also support the concatenation
function to guarantee the point-to-point transmission of the integrated loading unit after
concatenation. All these require too much for long-haul transmission, and will block
the practical development of services consequently.
The virtual concatenation technology solves the problem above thoroughly. The
difference between it and the adjacent concatenation technology is that VC-n units can
belong to different STM-N with independent structure and corresponding Path
Overhead (POH) sequence. These VC units are bound as a big virtual container
(VC-n-Xv, or called as VC group), in which each VC unit is identified with the
Multi-frame Identifier (MFI) of the virtual concatenation, and the Sequence (SQ)
identifier (belonging to LCAS and VC control frames). In this way, each independent
VC-n can be transmitted through different routes as different member in the VC group,
before converging at the destination end. And the concatenation function is
unnecessary for passing equipment.
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1.2.2 Link Capacity Adjustment Scheme
The virtual concatenation technology just provides a possible scheme to combine
loading units more efficient. A real management scheme is needed to ensure the
high-efficiency point-to-point transmission of IP data services over SDH carrier
networks. Just like a highway extending in all direction, on which there are various
cars. It can not become a fine transportation system without a good dispatch station.
The key point is how to manage and dispatch VC units efficiently, especially for the
virtual concatenation. Unlike the adjacent concatenation technology, VC-n in virtual
concatenations can be located in different STM-N with various combination modes,
which may result in unexpected problems without a proper dispatching scheme.
Then Link Capacity Adjustment Scheme (LCAS) emergences. It is a bidirectional
control information system established between sources and destinations. The control
information can be used to adjust the number of members in VCs dynamically
according to actual demands, and thus implement the real-time management of
bandwidth. It improves the network utilization ratio greatly while ensure the quality of
services at the same time.
From the point of integrated technology, the virtual concatenation technology and
LCAS make it possible to carry IP services efficiently over SDH networks, and finally
form the first generation MSTP with practical values. The emergence of MSTP
equipment drives SDH networks into further development. As an efficient networking
technology for Metro Area Network (MAN), the MSTP plays an important role at the
access section in the network. Furthermore, it improves the reliability of IP services
greatly with the quality guarantee characteristic inherited from SDH.
Cooperating with the flow control at the Media Access Control (MAC) layer, the
LCAS increases or decreases members in VC groups to avoid packet loss of IP services
when the network runs normally. Even when the optical signal intensity at the optical
receiving end does not reach the receiving sensitivity (maybe caused by fiber cut), or
the bit error rate is higher than the threshold, the network can implement protection
switching in 50 ms with the inherent protection capability of SDH. The application of
LCAS and flow control may only cause losses of a few packets, which will not
influence normal services. Ethernet has no this characteristic.
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1.3 Second Generation MSTP
The MSTP technology itself develops further due to its high commercial values. With
the application of Resilient Packet Ring (RPR) technology and Multiple Protocol Label
Switching (MPLS) technology, the second generation MSTP appears.
1.3.1 Resilient Packet Ring Technology
Ring networks share the features of low cost and convenient management in
comparison with star networks, bus networks and tree networks. At the beginning, the
token ring technology is developed for the data flow transmission in ring networks.
However, data packets in token ring networks roam in the whole network. Thus the
shared bandwidth will decrease drastically with the increase of nodes in the network.
This drawback restricts the further development of token ring technology.
The RPR works at the MAC layer supports better transmission of data flows in ring
topologies. The RPR technology has the following features.
● Dual-ring structure:
Two physical paths between every two adjacent nodes guarantee the high
reliability of networks.
● Ring bandwidth control and Spatial Reuse Protocol (SRP)
Unicast data can be transported at different parts of the ring, thus the capacity of
the ring increases accordingly. In this way, the bandwidth decreasing caused by
the addition of nodes will be eased to a certain degree. Moreover, the RPR can
discover the new network topology and update it automatically when the ring
topology changes. With this function, the man-made errors caused by manual
configuration can be avoided. It facilitates the management and maintenance of
networks.
● Dynamic bandwidth allocation and statistical multiplexing principle
Each node maintains the data load passing through itself and transports
corresponding data to adjacent nodes in the ring. Other nodes can find how many
available bandwidths can be achieved from the source node according to the
information.
To sum up, with the features above, the RPR technology shortens the transmission
process of data flow in the ring network for the maximum route between any two nodes
Chapter 1 Evolution of MSTP
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is only half of the ring. The network topology discovery and update capability is
achieved through exchanging topology identification information with the algorithm
such as Open Shortest Path First (OSPF). It can not only avoid the infinite loop of
packets efficiently, but also improve the self-healing ability of ring networks.
1.3.2 Multiple Protocol Label Switching Technology
The MPLS is a kind of data transfer technology. It combines the Layer-3 (network
layer) routing and Layer-2 (data link layer) switching. Based on the mechanism of label
switching, the MPLS separates the route selection from data transfer, and specifies the
path of a packet through a network with the label. It implements the conversion from
the connectionless IP services to connection-oriented label switching. The technical
characteristics of MPLS include traffic engineering, load balancing, failure recovery
and path priority etc.
Its typical application in Ethernet is Virtual Private Network (VPN) services based on
MPLS, which offer the following benefits:
● Providing seamless connections for intranets
● Restricting the spreading of VPN route information, and guarantee the security
through adopting MPLS forwarding only for members in the VPN
● Allowing different customers using the same VLAN ID through embedding
Layer 2 MPLS technology, and thus extending the address space for VLAN
● Implementing multilevel services in VPN, and setting up different priority
between VPNs
The involvement of the MPLS technology in the MSTP provides the label switching
function in addition to those features of MPLS mentioned above. Then the process of
adding/removing labels at the edge of IP networks is unnecessary. The real
point-to-point label switching is implemented through connecting the MSTP to core
routers with the label switching function directly.
The evolution process of MSTP, from the traditional SDH which can not carrying IP
services efficiently to the first generation MSTP which is competent to carry IP
services, and then to the increasingly robust MSTP supporting RPR and MPLS, is
always driven by practical applications. We can imagine, in the future, more other
functions and technologies will be involved in MSTP.
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2 Theory of EOS
Key points
Ethernet frame structure
MAC address and address learning
Transfer and filtering mechanism of Layer 2 switching
Layer 2 loop
Spanning tree and fast spanning tree
VLAN
2.1 Ethernet Fundamentals
2.1.1 Ethernet Frame Format
Fig. 2.1-1 shows the Ethernet frame format.
1 octet
64-1518 octet
FCSPADDATALENSADASFDPRE
46-1500 octet7 octet 6 octet
PADDATALENSADASFDPRE
6 octet 2 octet 4 octet
PRE = Preamble
SFD = Start-of-Frame Delimiter
DA = Destination Address
SA = Source Address
LEN = Data Length
FCS = Frame Check Sequence
Fig. 2.1-1 Ethernet Frame Format
● The Preamble (PRE) is a 7-octet sequence of alternating bits (10101010) used to
reach synchronization.
● The Start-of-Frame Delimiter (SFD) is a special octet (10101011) to identify the
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beginning of the Ethernet frame.
● Destination Address (DA): The first bit indicates whether the address is an
individual address or a group address. “0” identifies an individual address, while
“1” identifies a group address.
The frame with a group address will be transferred to all stations specified in the
address. The interface of each station recognizes its own address and responds to
it when the interface detects the group address. If all bits in the destination
address are “1”, the frame will be broadcasted to all stations on the network.
● The Source Address (SA) indicates where the frame comes from.
● The Data Length (LEN) field indicates the number of octets in the data field and
pad field.
● The Data (DATA) field comprises all the data originated from the upper layer.
● Pad (PAD) field: The length of Data field should be no less than 46 octets. If it is
less than 46, the Data field must be extended by adding octets (pad) to make the
actual Data field length meet the minimum length.
● Frame Check Sequence (FCS): It provides error detection with 32-bit Cyclic
Redundancy Check (CRC) sequence.
2.1.2 MAC Address
MAC address is 48 bits long, which can be transferred to 12 hex bytes. These bytes are
divided into three groups, which are separated with a dot between groups. Each group
comprises four bytes. The MAC address is burned into the Network Interface
Controller (NIC).
The IEEE manages all MAC addresses to ensure their uniqueness. Each MAC address
consists of two parts: the vendor code and the serial number.
● Vendor code: As the first six hex bytes (24 binary bits) in the MAC address, it
identifies the NIC vendor.
● Serial number: The vendor manages serial numbers of MAC addresses. The
serial number is the last six hex bytes in the MAC address. If all serial numbers
after a vender code are used up, the vendor must apply for another vendor code.
Chapter 2 Theory of EOS
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2.2 Ethernet Switching Principle
2.2.1 Operation Principle of Transparent Bridge
Fig. 2.2-1 illustrates the operating principle of transparent bridge.
Station C
Station B
Port 1
Segment A
Segment B
Station A
Station D
Port 2
Station C
Station B
Port 1
Segment A
Segment B
Station A
Station D
Port 2
Fig. 2.2-1 Operating Principle of Transparent Bridge
In Ethernet, the process of determining transfer is called as transparent bridge
connection.
The meaning of “transparent”: Terminal equipment connected to the bridge do not
know whether they are connected to shared medium or switching equipment, that is,
the equipment is transparent to terminal users. On the other hand, the bridge does not
change or process the frames transferred through it (except trunk lines of VLAN).
The transparent bridge has the following three main functions:
● Address learning function
● Transfer and filtering function
● Loop avoidance function
All these three functions are performed in the transparent bridge, and they works on the
network at the same time. Moreover, Ethernet switches also perform three main
functions same as those of transparent bridge.
2.2.2 MAC Address Learning
The bridge determines whether to transfer frames based on the destination MAC
address. Therefore, it must “catch” the position of MAC address first. Only by this, can
the bridge make a decision about transfer correctly.
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When the bridge is connected to physical network sections, it checks all frames
detected. After reading the source address of a frame, the bridge associate the frame to
corresponding receiving port and record the relation in the MAC address table. Then
the process of MAC address learning completes.
2.2.3 Transfer and Filtering Mechanism
1. Transfer/filtering
Once all workstations have transmitted data frames, the switch learns all
one-to-one relationships between MAC addresses and ports and records them in
the MAC address table.
For example, as shown in Fig. 2.2-2, the workstation A sends a unicast data
frame to workstation C. The switch recognizes that the destination address of the
frame already exists in the MAC address table and is associated to the E2 port.
Then it transfers this frame to the E2 port directly.
The switch will not transfer the data frame to other ports in the network, and this
is considered as the filtering operation.
E0: 0260.8c01.1111E2: 0260.8c01.2222E1: 0260.8c01.3333E3: 0260.8c01.4444
0260.8c01.1111
0260.8c01.2222
0260.8c01.3333
0260.8c01.4444
E0 E1
E2 E3
XXXX DC
A B
MAC address tableE0: 0260.8c01.1111E2: 0260.8c01.2222E1: 0260.8c01.3333E3: 0260.8c01.4444
0260.8c01.1111
0260.8c01.2222
0260.8c01.3333
0260.8c01.4444
E0 E1
E2 E3
XXXX DC
A B
MAC address table
Fig. 2.2-2 Transfer and Filtering
2. Transfer of broadcast/multicast frames or frames with unknown MAC addresses
As shown in Fig. 2.2-3, when the workstation D sends a data frame, the switch
recognizes that the frame is a broadcast frame or a multicast frame, or a frame
whose MAC address is unknown (that is, the MAC address of this frame does
not exist in the MAC address table of the switch). Then the switch floods the
network with the frame, that is, it transfers the frame to all the other ports except
the entrance port.
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0260.8c01.1111
0260.8c01.2222
0260.8c01.3333
0260.8c01.4444
E0 E1
E2 E3 DC
A B
E0: 0260.8c01.1111E2: 0260.8c01.2222E1: 0260.8c01.3333E3: 0260.8c01.4444
MAC address table
0260.8c01.1111
0260.8c01.2222
0260.8c01.3333
0260.8c01.4444
E0 E1
E2 E3 DC
A B
E0: 0260.8c01.1111E2: 0260.8c01.2222E1: 0260.8c01.3333E3: 0260.8c01.4444
MAC address table
Fig. 2.2-3 Transfer of broadcast/multicast frame or frame with unknown MAC address
Note
If the switch supports multicast functions such as Internet Group Multicast Protocol
(IGMP) interception, it will not transfer multicast data frames with the flooding mode.
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3. Transfer/filtering procedure
Fig. 2.2-4 shows the transfer/filtering procedure diagram.
Fig. 2.2-4 Transfer/Filtering Procedure
The processing procedures after the switch receives a data frame at a port are
described as follows.
The switch judges whether the MAC address of the data frame is a broadcast
address or a multicast address. If the answer is yes, it will perform the flooding
operation.
If the MAC address is a unicast one identifying a network device, the switch
will look up the address in the MAC-Port table. Once the switch can not find it
in the table, it will transfer the frame with flooding mode too.
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If the switch finds the address in the MAC-Port table, it will transfer the data
frame to the corresponding port associated with the destination address.
2.2.4 Loop Avoidance: Spanning Tree Protocol
In Layer 2 networks, Layer 2 loop may be generated once the physical loop comes into
being. The Layer 2 loop will damage the network drastically, and the network can not
recover by itself sometimes. The problems caused by the loop may be broadcast storm,
repeated replication of frames, and the instability of switches' MAC address tables
(MAC address drift), etc.
However, complicated multi-loop connections always exit in actual networks. Layer 2
loops should be avoided when there are physical loops in networks. Then the Spanning
Tree Protocol (STP) appears to avoid the generation of Layer 2 loops.
A switch supported STP can automatically recognize a loop in the network with
redundant paths, and it will keep the best path for frame transfer while blocking other
redundant paths. When the network topology changes, STP automatically reconfigure
switch ports and ensure that only one path exists between any two stations and the
network does not run into a loop situation.
2.2.5 VLAN
1. Overview
Local Area Network (LAN) can be either a network consisting of a few
computers or an enterprise network composed of hundreds of computers. Virtual
LAN (VLAN) is a special LAN segmented by routers, that is, a kind of
broadcast domain. Members in a VLAN work like sharing the same physical
network section. Members in different VLANs can not access each other
directly.
In a VLAN, there is no physical or geographical limit for members divided into
the same broadcast domain. They can be connected to different switches in a
switching network. Broadcast packets, unknown packets and data packets
between members are all restricted in the VLAN.
Another explanation of VLAN is that it offers a method to divide one physical
network into multiple broadcast domains.
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Note
A broadcast domain is a restricted area in which broadcast frames (all bits of the
destination address are 1) can be transmitted to all other devices. Strictly speaking, not
only broadcast frames, but also multicast frames and unknown unicast frames can be
transmitted in a broadcast domain without blocks.
VLAN provides the following functions or advantages:
● VLAN can divide a switching network or a broadcast domain into multiple
broadcast domains, as if multiple separate physical networks. In this way, a
network is segmented, and in each segment the number of computers
decreases accordingly so as to improve the network performance.
● VLAN is very flexible for users to configure a VLAN, add/remove or modify
members in the VLAN just on the switch. Generally, it is unnecessary to
change the physical network or add new devices.
● When a network is divided into VLANs, computers in different VLANs can
communicate with each other only through Layer 3 devices. The security of
Layer 3 can be ensured by configuring the Access Control List (ACL) on
these devices. To sum up, the communication between VLANs is
implemented under control. The security of VLANs is better than those
networks without VLAN division, in which computers communicate with
each other directly. Furthermore, if a customer wants to join in a VLAN, only
after the network administrator configures it on the switch, can the customer
be added in the VLAN. All these improve the security of networks
accordingly.
For example, no VLAN has been configured on a Layer2 switch, as shown in
Fig. 2.2-5. Any broadcast frames will be transferred to all the other ports of the
switch except the receiving port. The switch floods the broadcast information
received from the computer A to port 2, 3 and 4.
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Fig. 2.2-5 Switch without VLAN Division
Two VLANs are configured on a switch: VLAN I and VLAN II, as shown in Fig.
2.2-6. The port 1 and 2 belongs to the VLAN I, while port 3 and 4 to the VLAN
II. If the computer A sends a broadcast frame, the switch will transfer it only to
the other port in the same VLAN, that is, port 2 in VLAN I. It will not transfer
the frame to the ports in VLAN II.
In the same way, the broadcast information output from the computer C will be
transferred only to the port in VLAN II instead of ports in VLAN I.
Fig. 2.2-6 Switch with VLAN Division
VLAN divides the broadcast area through limiting the transferring range of
broadcast frames. To illustrate different VLANs clearly, Fig. 2.2-6 identifies two
VLANs with different colors. In actual application, “VLAN ID” is used to
identify the VLAN.
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2. VLAN division modes VLAN
The popular VLAN division mode now is the static division mode based on
ports.
The network administrator sets ports as those in a specified VLAN. Then
computers connected to these ports belong to this VLAN.
The advantage of this division mode is that the configuration is easy and has no
influence on transfer performance of the switch. However, every port of the
switch should be configured into the VLAN it belongs to. Once the user moves,
the network administrator has to reconfigure corresponding ports for the switch.
Other VLAN division modes include: division based on MAC address, division
based on protocol, division based on IP address subnet, division based on
application, division based on user name and division based on password etc.
3. Operation process of VLAN
Each VLAN can be regarded as a physically isolated bridge. Members in
different VLANs can not access each other directly.
VLAN can pass over switches. Members belonging to the same VLAN on
different switches are in the same broadcast domain; therefore they can access
each other directly.
Because the VLAN division is based on physical ports of switches, when the
switch receives a data frame from one port connected to a computer, it can
recognize which VLAN the data frame belongs to.
But the link connecting two switches carries data frames from different VLANs.
Then the ports connecting to the link on the switches does not belong to a
specified VLAN. If not tag the data frame, the switch can not know which
VLAN the frame received from such link belongs to. So every data frame is
“tagged” with a prefix before being transferred to such links by the switch. The
tag is used to identify the VLAN which the data frame belongs to.
The tag of VLAN enables the switch combining traffics from different VLANs
and transmitting them through the same physical line.
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4. Link type
● Access Link
The access link connects a non-VLAN-aware workstation to a LAN section
of a VLAN switch port, that is, the access link is used to connect terminal
equipment and switches. If the VLAN division is based on ports, an access
link can only belongs to one VLAN.
The access link may be either an isolated network section, or multiple
network sections or workstations connected with non-VLAN-aware bridges
and switches. The access link can not carry tagged packets.
● Trunk Link
The trunk link is the one carrying tagged packets (with VLAN ID). Therefore
a trunk link can carry data from multiple VLANs. It supports devices which
can recognize VLAN frames and membership. The trunk link is always used
to connect two VLAN switches. It enables VLAN passing over multiple
switches.
The trunk link may also be a shared LAN section connected with multiple
VLAN switches and VLAN-aware workstations.
VLAN1
VLAN1
VLAN2VLAN3
Backbone
VLAN1 VLAN2 VLAN3
VLAN1
VLAN1
VLAN2VLAN3
Backbone
VLAN1 VLAN2 VLAN3
Fig. 2.2-7 Trunk Link
5. IEEE 802.1Q
IEEE developed a general VLAN standard IEEE 802.1Q. The standard
● Defines the architecture of VLAN for the purpose of providing VLAN
services for current IEEE 802 bridge LAN.
● Defines the tagged VLAN frame format for Ethernet IEEE 802.3 and token
ring IEEE 802.5
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● Defines the protocol and mechanism for VLAN-aware devices to configure
information and communicate membership information.
● Defines the principle and procedures for VLAN-aware devices to transfer
frames on networks.
● Specifies the requirements to ensure the interoperability and coexistence of
non-VLAN-aware devices. The non-VLAN-aware device is a workstation or
router which can not receive or transmit tagged VLAN packets, and neither
can recognize information about VLAN membership.
Fig. 2.2-8 shows the 802.1Q frame format.
Fig. 2.2-8 802.1Q Frame Format
The addition of 4-byte tag head to the original Ethernet frame makes the
maximum length of Ethernet frame up to 1518 bytes. This number exceeds 1514
bytes specified in IEEE 802.3, which is expected to be modified to support
1518-byte long tagged VLAN frames.
The 4-byte tag head carries the following information:
● Tag Protocol Identifier (TPID): Two-byte field consisting of the hexadecimal
value 81-00. It carries the 802.1Q/802.1p tag type.
● Tag Control Information (TCI): The fields contained in the TCI are described
as follows.
The three-bit user priority field is capable of representing the priority of the
frame while the IEEE 802.1p-supported switch transferring it.
The one-bit Canonical Format Indicator (CFI) field indicates that all MAC
address information carried by the frame is in Canonical format.
The twelve-bit VLAN Identifier (VID) field uniquely identifies the VLAN to
which the frame belongs.
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2.3 EOS Fundamentals
2.3.1 What is EOS
Ethernet Over SDH (EOS) is a part of the MSTP architecture to implement the
transmission of Ethernet services through SDH nodes in the network. It has the
functions of the transparent transmission and Ethernet Layer 2 switching etc.
2.3.2 Function Model of EOS
Fig. 2.3-1 shows the diagram of EOS function model.
MSOH = Multiplex Section Overhead
RSOH = Regenerator Section Overhead
Fig. 2.3-1 EOS Function Model
As shown in the diagram above, the data frame from Ethernet interface is transmitted
transparently to the Layer 2 for switching. And after being encapsulated, the frame is
mapped into the virtual container. The Multiplex Section Overhead (MSOH) and
Regenerator Section Overhead (RSOH) are inserted to form a STM-N frame which is
transmitted over the SDH network.
The EOS transfer node supporting Layer 2 switching must be provided with the
following basic functions:
● Transmission link bandwidth is configurable
● Ensuring the transparency of Ethernet services
● Having the transfer and filtering function for Layer 2 data frames
● Supporting IEEE 802.1d Spanning Tree Protocol
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2.3.3 Ethernet Frame Encapsulation
Fig. 2.3-2 shows the encapsulation architecture of EOS.
IEEE 802.3 MAC
PPP/LAPS/GFP
VC12/VC3/VC4
Multiplex Section
Regenerator Section
PHY
Fig. 2.3-2 Encapsulation Architecture of EOS
The encapsulation protocol stack specifies the functions of link control from
point-to-point Ethernet to SDH network, rate adaptation and delineation.
There are three encapsulation protocols: Point-to-Point Protocol (PPP), Link Access
Procedure for SDH (LAPS) and Generic Framing Procedure (GFP).
1. PPP encapsulation
The Point-to-Point Protocol adopts the RFC 1662 PPP in HDLC-like Framing of
the byte synchroneous link. And the encapsulation procedures include three
steps: MAC frame extraction, PPP framing and HDLC processing.
1) MAC extraction
Check the MAC frames, filtering CRC errors and other abnormal frames, and
then remove the preambles of Ethernet frames and gaps between frames.
2) PPP framing
The Address, Control and Protocol provides controls of multiple protocol
encapsulation, link initialization and authentication In addition, errors are
controlled with the Frame Check Sequence (FCS) through CRC16 and CRC32.
3) HDLC processing
Process the PPP frame transparently through changing 0x7e to 0x7d and then
change 0x7d to 0x7d, 5d. Delimit the PPP frame by adding 0x7e to the
header and trailer. And then adapt the rate of PPP frame to SDH VC channel by
filling 0x7e.
Chapter 2 Theory of EOS
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Fig. 2.3-3 PPP Encapsulation Procedure
The PPP protocol is the first encapsulation protocol. It has been used widely due
to its technology maturity. The PPP protocol is the link layer protocol used
commonly to support the communication between two devices connected
directly on a point-to-point link. For example, the connection between the
computer and the access server during dial-up adopts the PPP protocol; and the
connection between Digital Data Network (DDN) routers adopts the PPP
protocol too. However, devices provided by different vendors can not interwork
because there are no unified requirements to apply the PPP protocol.
2. LAPS encapsulation
The LAPS encapsulation is similar to the PPP. It simplifies the processing of
link control and implements the rate adaptation with the additional transmitting
sequence (0x7d, 0xdd). Comparing to the PPP encapsulation, the LAPS
completes the packing and adaptation at the same time. Fig. 2.3-4 illustrates the
LAPS encapsulation procedure.
Fig. 2.3-4 LAPS Encapsulation Procedure
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3. GFP encapsulation
The GFP is a kind of generic mapping technology. Variable-length or
fixed-length data packets can be adapted and processed together, thus
implementing the transmission of data services through various high-speed
physical transmission channels.
Fig. 2.3-5 GFP Frame Format
Fig. 2.3-5 shows the format of GFP frame. Generally, a GFP frame contains the
core header and payload.
The encapsulation efficiency of GFP, being independent of the payload contents,
is higher then those of PPP and LAPS.
In addition, the GFP is more robust. Even if there are odd-bit errors in the GFP
frame header, it will not cause synchronization loss (out of frame), while it will
do for the PPP/LAPS encapsulation.
The GFP can use the system bandwidth more efficiently. With the channel
identifier, the GFP can combine multiple physical ports to one channel, while a
physical port can only associated to one channel for the PPP/LAPS.
The GFP support ring networks besides point-to-point networks.
Chapter 2 Theory of EOS
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2.3.4 Contiguous Concatenation and Virtual Concatenation
Concatenation is a data encapsulation mapping technique on MSTP that allows the
transmission of large-granularity services by combining multiple virtual containers as a
single container which keeps the integrity of bit sequence.
Concatenation is divided into contiguous concatenation and virtual concatenation.
For contiguous concatenation, adjacent virtual containers in the same STM-N data
frame are combined asC-4/3/12-Xc which is transferred as a whole structure.
For virtual concatenation, virtual containers (with the same route or different route) in
different STM-N data frames are combined to form a virtual VC-4/3/12-Xv with big
structure which is transferred as a whole.
1. Contiguous concatenation
The contiguous concatenation technology is first adopted to support the
transmission of traffics occupying more than one virtual container over the
transmission network. The advantage of contiguous concatenation is that every
part of the data can be transferred without delay and the transmission quality is
high for the traffics are transmitted as a whole.
However, the application of contiguous concatenation technology has its
limitation. It requires that all passing networks and nodes support the contiguous
concatenation.
2. Virtual concatenation
With the virtual concatenation technology, virtual containers (VC-n) in different
STM-N can be cascaded to form a big virtual structure (VC-n-Xv) for
transmission, in which each VC-n has independent and integrated structure and
corresponding POH. The virtual concatenation of multiple C-n is quite like the
interleaving of multiple VC-n. Unlike the contiguous concatenation, the virtual
concatenation enables the independent transmission for each VC-n through
different paths. And there are no special requirements for middle devices except
terminal devices at both sides of the transmission path, which should comply
with corresponding protocols to support the virtual concatenation.
The virtual concatenation shares the following features:
● Passing-independent over networks and multi-path transmission
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Because devices supporting concatenation traffics and those not supporting
concatenation traffics have different interpretation for pointers, generally
existing SDH devices can not transfer adjacent-concatenation traffics.
However, the application of virtual concatenation can meet the bandwidth
demands of broadband services.
Generally, the virtual concatenation should implement functions in both the
transmitting and receiving direction. In the transmitting direction, it converts
C-4/3/12-Xc to C-4/3/12-Xv, and converts adjacent concatenation traffics to
virtual concatenation traffics which can be transmitted over SDH devices. In
the receiving direction, C-4/3/12-Xv is transformed to C-4/3/12-Xc, and the
virtual concatenation traffics are converted to adjacent concatenation traffics.
In this way, adjacent concatenation traffics can be transmitted through SDH
devices.
● Supporting LCAS
The LCAS is applicable to virtual concatenation. It can adjust the link
capacity without damage for virtual-cascaded signals passing through the
transmission network. Based on the existing bandwidth, the LCAS can
increase or decrease the bandwidth capacity dynamically, and thus adapt to
the change of virtual-cascaded traffics. Moreover, the LCAS improve the
robusticity of virtual-cascaded traffics and service quality as well.
There are still some problems about the application of virtual concatenation
should be considered.
From the point of technology, the main problem of virtual concatenation is delay
comparing to contiguous concatenation. Because the passing path for each
virtual container in the virtual concatenation may be different, transmission time
difference may appears between virtual containers. At worst, the virtual
container with the latter sequence reaches the sink terminal node before the one
with the sequence in front. This makes it difficult to recover original signals. At
present, the effective way to solve this problem is using a large delay alignment
memory to buffer data for the purpose of data re-alignment.
For multi-path transmission, ZTE’s MSTP products can compensate the path
delay difference of 32 ms. Calculated by 5 us/km, the maximum path difference
is 6400 km.
25
3 Theory of ATM
Key points
Features of ATM
ATM cell structure
Fundamentals of ATM switching
ATM protocol reference model ATM
ATM communication QoS
VP-Ring technology
ATM service type
Basic connection functions of ATM
3.1 ATM Fundamentals
3.1.1 Generation Background of ATM Technology
In modern times, people need to transmit and process more and more information. And
a variety of information appears. The demand for new broadband services, such as
video conference, high-speed data transmission, tele-education and Video On Demand
(VOD), is increasing rapidly.
The earlier networks can transfer only one type of service. For example, the telephone
network can only provide telephone services, while the data communication network
can only provide data communication services. Such networks are costly and
inconvenient for either users or network carriers. Then the concept of Integrated
Services Digital Network (ISDN) is presented which is expected to carry various
services with just one kind of network.
The narrowband ISDN (N-ISDN) was presented at first in 1972 due to the limitation of
technology and service demands of the time. Now, the N-ISDN technology is mature,
and there are many mature N-ISDN networks in the world already. However, the
N-ISDN still has limitations such as narrow bandwidth, limited service integrated
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capability, various relay networks, and weak adaptation to new services. Therefore a
more flexible new network with broader bandwidth and stronger service integrated
capability is needed. From the 80th in 20 century, the development of basic
technologies related to telecommunication, such as micro-electronics and
photoelectronics, provides bases for the realization of new networks. In this
background, the broadband ISDN (B-ISDN) appears.
The B-ISDN can
● Enable the high-speed transmission of services.
● The network devices is independent from the characteristics of services
● The information transfer mode is independent from service type.
People have sought many solutions to develop a transfer mode adapting to the B-ISDN,
such as multi-rate circuit switching, frame relay, and fast packet switching etc. Finally,
the most appropriate transfer mode for B-ISDN was found, and that is the
Asynchronous Transfer Mode (ATM).
As the core technology of B-ISDN, the ATM technology has been specified as the
unified information transfer mode by ITU-T in 1992. The ATM technology excludes
the limitations of circuit switching mode and packet switching mode. It adopts the
optical telecommunication technology, and improves the transmission quality. At the
same time, it simplifies the operation on the network node and thus decreases network
delays. A series of other technologies are also adopted to meet all the requirements of
B-ISDN.
3.1.2 ATM Features
1. Statistics time division multiplexing and channel utilization ratio improvement
2. The respond time is short with the transmission unit of fixed-53-byte cell.
3. Connection-oriented working mode with transmission resource reservation
4. In ATM networks, the error control and flow control hop by hop are cancelled,
which are moved to edges of the network.
5. The ATM supports integrated services.
Chapter 3 Theory of ATM
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3.1.3 ATM Cell Structure
Fig. 3.1-1 shows the structure of ATM cell. 8 7 6 5 4 3 2 1 8 7 6 5 4 3 2 1
GFC VPI VPI
VPI VCI VPI VCI
VCI VCI
VCI PTI CLP VCI PTI CLP
HEC HEC
PAYLOAD
PAYLOAD
(a) UNI header format (b) NNI header format
Fig. 3.1-1 ATM Cell Structure
The cell header contains the following parts:
● GFC: Generic Flow Control. It has four bits, and all of them are set to a default
value “0000”currently. The GFC is only applicable to User to Network
Interfaces (UNI) and may be used to control flows in the future.
● VPI: Virtual Path Identifier. It has 12 bits for Network to Network Interfaces
(NNI), while has 8 bits for UNI.
● VCI: Virtual Channel Identifier. It identifies the virtual path part of a virtual path.
The VCI and VPI can be combined to identify a virtual connection.
● PTI: Payload Type Identifier. It is a 3-bit field to identify the payload type.
● CLP: Cell Loss Priority. It is a bit used to distinguish the priority of cell loss. “1”
indicates the cell is of low priority, while “0” indicates it is of high priority. The
cell of low priority will be discarded when congestion occurs.
● HEC: Header Error Control. It is an 8-bit error control byte to detect the cell
with error. It can also correct the “1” bit error in the cell header. In addition, the
HEC is used for cell delineation.
3.1.4 Fundamentals of ATM Switching
In an ATM network, a physical transmission channel is divided into multiple Virtual
Paths (VPs). A VP may be multiplexed by thousands of Virtual Channel (VC). Both the
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VP and VC are used to describe the unidirectional transmission route of ATM cells.
The ATM cell can be switched either on the VP level or on the VC level.
Each VP can accommodate 65536 virtual channels at most with the multiplexing mode.
Cells in a cell group belonging to the same VC have the same VC Identifier (VCI).
Different VCs belonging to the same VP have the same VP Identifier (VPI). Both the
VCI and VPI are transmitted with the cell as parts of the cell header. The transmission
channel, VP and VC are three important concepts in the ATM technology. Fig. 3.1-2
shows the relationship among them.
Transmission Channel
VP
VP
VP
VP
VP
VP
VC
VC
VC
VC
VC
VC
Fig. 3.1-2 Relationship among Transmission Channel, VP and VC
The call proceeding in the ATM is based on the concept of virtual call in packet
switching instead of routing control for cells one by one. The cell proceeding route
related to a call is established in advance before transmission. All cells of the same call
must pass through this route until the call ends.
The proceeding procedure is as follows. The calling party sends a control signal of call
request via a UNI. The called party receives the control signal and accepts the request.
After that, switching nodes in the network forms a virtual circuit between the calling
and called party after exchanging signaling. The virtual circuit is represented with a
series of VPI and VCI. While setting up the virtual circuit, all switching nodes on the
circuit arranges a routing table for the purpose to convert the input cell VPI/VCI to
output cell VPI/VCI.
After establishing the virtual circuit, the transmitted information is segmented into cells,
which are transferred to the called party over the network. If the transmitting end wants
to forward more than one message to different receiving ends at the same time,
different virtual circuits can be built up respectively to corresponding receiving ends.
And the cells will be output alternately.
Chapter 3 Theory of ATM
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In the virtual circuit, the VCI/VPI value of cells in two adjacent switching nodes keeps
unchanged. Between these two nodes, a VC link is formed. A bunch of VC links forms
the VC Connection (VCC). Similarly, the VP link and VP Connection (VPC) are
formed.
1. VP switching
While the cell passing the ATM switching node, this node modifies the VPI
value in the input cell to a new value according to the destination connected to
the VP. Then the node assigns the new value to the cell and output it. This
process is called as VP switching.
As shown in Fig. 3.1-3, all VC links in a VP are transferred to another VP in the
VP switching, while the VCI values in these VC links keep unchanged. The
implementation of VP switching is very simple. Generally, it can be realized
through the cross-connection of digital multiplex cables at some level in the
transmission channel.
VCI=7VCI=8
VCI=1VCI=2
VCI=1VCI=2
VCI=7VCI=8
VPI=1
VPI=2
VPI=4
VPI=5
VP Switching
Fig. 3.1-3 VP Switching
2. VC switching
The VC switching should be performed with the VP switching simultaneously.
Because when a VC link ends, corresponding VP connection ends too. Then all
VC links on this VPC will carry out switching respectively, being added to
VPCs in different directions, as shown in Fig. 3.1-4.
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VCI=4
VP Switching
VCI=3
VPI=2
VPI=3
VPI=1VCI=1VCI=2
VC Switcing
VCI=1VCI=2
VCI=3
VCI=4
Fig. 3.1-4 VC Switching
3.1.5 ATM Statistics Multiplexing
It is the greatest feature of ATM to achieve the best resource utilization ratio in
networks under any traffic distribution mode. For this purpose, the statistics
multiplexing of network resources is necessary. The statistics multiplexing refers to the
dynamic distribution of network resources between traffics while guaranteeing the
services quality according to the statistics characteristics of various services, and thus
getting the best resource utilization ratio.
As shown in Fig. 3.1-5, the data from user A, C and D are arranged in turn on the line
carrier. The user B will not occupy bandwidth resource of output line because he/she
has no data for transmission at the time. From this point, the ATM link is a virtual link.
Fig. 3.1-5 ATM Multiplexing
Chapter 3 Theory of ATM
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3.1.6 ATM Protocol Reference Model
The ATM reference model contains three flats, the user flat, control flat and
management flat. Here we mainly introduce the user flat, including data flow directions
and functions of each layer, specially the ATM Adaptation Layer (AAL), AAL1 ~
AAL5 and services type such as CBR/VBR/UBR/ABR and A/B/C/D.
The user plat contains the physical layer, ATM layer, AAL layer and highest layer same
as from the OSI model. Fig. 3.1-6 illustrates the data transmission between these
layers.
AAL Layer
ATM Layer
AAL- SDU AAL-PCI
48-byte transmitting information
Highest layer inforamtion
48-byte payload5-byte header
53-byte cell
53-byte cellPhysical Layer
Bit Flow
Fig. 3.1-6 Data Transmission between Layers
The functions of each layer are as follows.
1. Physical layer
The physical layer is the layer to carry information flow. It contains two
sublayers, the Transmission Convergence (TC) sublayer and Physical Medium
(PM) sublayer.
1) TC sublayer
This layer embeds ATM cells to the transmission frame of the current medium,
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or extracts valid ATM cells from the transmission frame on the contrary.
The procedure of embedding ATM cells in the transmission frame is: ATM cell
demodulation (buffering) → generating Header Error Control (HEC) → cell
delineation → adapting the transmission frame → generating the transmission
frame
The procedure of ATM cells extraction from the transmission frame is: receiving
the transmission frame → adapting the transmission frame → cell delineation →
checking header error control → ATM cell queuing.
The main functions of TC layer is cell delineation and header error control.
2) PM sublayer
The PM sublayer is based on the ITU-T and ATMF recommendations. It
includes the following connections:
● Connections based on direct transmission of cells
● Connections over PDH networks
● Connections over SDH networks
● Direct optical transmission of cells
● Connections between Universal Test & Operation PHY Interfaces for ATM
(UTOPIA)
● Connections between Operation And Maintenance (OAM) interfaces for
management and monitoring information flow
2. ATM layer
This layer mainly implements the multiplexing/demultiplexing of cells,
operations related to headers and flow control.
The multiplexing and demultiplexing of cells completes at the interface between
the ATM layer and the TC sublayer of physical layer. The sending ATM layer
combines cells with different VPI/VCI and transfers them to the physical layer
as a whole. The receiving ATM layer recognize the VPI/VCI in cells received
from the physical layer and then sends each cell to different modules for
processing. If the cell is a signaling cell, it will be sent to the control plat; while
the cell will be sent to the management plat, if it is a managing one.
Chapter 3 Theory of ATM
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The header operations is the translation of VPI/VCI based on the value of
VPI/VCI allocated while establishing the link.
3. AAL layer
The AAL layer works on the top of the ATM layer. It cares about services,
adopting different adaptation methods for different services. For each adaptation
mode, the information flow (with different length and rate) from the highest
layer is split into ATM service data cell in 48 bytes long. At the same time, it
reassembles cells from the ATM layer, recovers the flow and sends it to the
highest layer. For there are various kinds of information at the highest layer, the
ALL layer is divided into two sublayers for the complicated processing
procedure. They are the Convergence Sublayer (CS) and Segmentation and
Reassembly (SAR) sublayer.
To improve the rate of switching networks, the ATM layer has been simplified as
possible. However the ATM layer does not provide functions concerning about
the quality of service, such as cell loss, transmission error, delay and jitter. These
functions are performed by the ALL layer.
It is necessary to provide different adaptation for different services. Four classes
of service are defined according to requirements of timing, bit rate and
connection modes between the source and destination. These classes of service
correspond to ALL protocol ALL1, ALL2, ALL3/4 and ALL5 respectively.
● ALL1 supports constant bit rate and connection-oriented traffics. And the
timing information needs to be transferred between sources and sinks.
Common services of this class include 64 kbit/s voice services,
uncompressed video traffics with constant bit rate and leased lines in private
data networks.
● ALL2 is provided for point-to-point variable bit rate traffics with timing
relations. Common services of this class are compressed packet voice
communication and compressed video transmission. One characteristic of
these services is the transmission delay, which is caused by the reassembly of
uncompressed voice and video information in the receiver.
● ALL3/4 is provided to adapt two kinds of data services in the ATM network,
the data service corresponding to remote LAN interworking and the
connection-oriented data service.
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● AAL5 supports variable bit rate traffics without synchronization
requirements between transmitting and receiving ends. It provides similar
services as ALL3/4, which are mainly used to transmit computer data, UNI
signaling information and frame relay in the ATM network. The purpose of
ALL5 is to reduce overheads and provide a simple and efficient ALL.
3.1.7 ATM Service Type
The ATM layer provides five types of services: Constant Bit Rate (CBR), Real Time
Variable Bit Rate (rt-VBR), non-realtime Variable Bit Rate (nrt-VBR), Available Bit
Rate and Unspecified Bit Rate. These services concern of the flow characteristic and
QoS of the network.
1. CBR services
CBR services are provided for links with static bandwidth in the life cycle. The
bandwidth depends on the value of Peak Cell Rate (PCR).
The network provides the basic guarantee for CBR users that the negotiated
ATM layer QoS should be ensured for all the cells which have passed the
consistency check once the link is established. That is, the QoS must be
guaranteed for CBR services whenever the source terminal sends cells at the
peak cell rate in any duration.
Generally, CBR services include realtime services having high requirements for
delay variation (such as voice, video and circuit emulation) but not limited to
these services. For CBR services, the source end can transmit cells at the
negotiated PCR or at the rate below PCR (or just stop transmitting). The
performance is considered being drastically decreased if the cell delay is greater
than the specified maximum cell transfer delay (maxCTD).
2. rt-VBR services
As a real time application, the rt-VBR service limits the delay and delay
variation strictly. Rt-VBR services mainly include voice and video services. The
characteristics of rt-VBR links are represented by Peak Cell Rate (PCR),
Sustainable Cell Rate (SCR), Maximum Burst Size (MBS) and Cell Delay
Variation Tolerance (CDTV). The cell rate at the source end is variable; it can
also be considered that the source “bursts out”. Rt-VBR services support the
statistics multiplexing of real time resources.
Chapter 3 Theory of ATM
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3. nrt-VBR services
Nrt-VBR services support the burst non-realtime application. The link
characteristics are represented by PCR, SCR and MBS. The nrt-VBR service can
ensure a very low cell loss ratio for those cells meeting the flow contract, but it
will not restrict the delay. Nrt-VBR services support continuous statistics
multiplexing.
4. UBR services
The UBR service is a kind of non-realtime application. It does not limit the
delay and delay variation strictly. UBR services include traditional computer
communication applications, such as file transfer and E-mail etc.
UBR services will neither ensure the quality of services, nor the amount of cell
loss ratio and cell transfer delay. Networks can determine whether use the PCR
in Connection Admission Control (CAC) and Usage Parameter Control (UPC).
The PCR value has no sense when the network has no forced limitation on PCR.
The congestion control for UBR links is carried out on the highest layer based
on point-to-point.
5. ABR services
The transfer characteristics of ABR services on the establishment of links can be
changed later. There is a flow control mechanism supporting the feedback of
source to control the transfer rate of cells at the source end. Such feedback is
realized by a special control cell – Resource Management (RM) cell. Low cell
loss ratio is expected when the interruption system is controlling flows
according to the feedback. Then a fair and available bandwidth can be accessed.
To a specified link, the ABR service has no boundary limitation for delay and
delay variation, that is, the ARB service does not support real time applications.
While the ABR establishing a link, the interruption system will assign a
maximum bandwidth and a minimum available bandwidth needed. They are
represented by the PCR and Minimum Cell Rate (MCR). The MCR can be “0”.
The bandwidth provided by the network can be variable but can not less than the
MCR.
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3.1.8 ATM Communication QoS
The Quality of Service (QoS) is an important part of the ATM networking, for ATM
networks are always used for real time transmission, such as the transfer of voice and
video.
The contract of QoS has two major parts. The first one is about traffic descriptor. It
characterizes the load to be offered. The second part of the contract specifies the QoS
desired by the customer and accepted by the carrier.
To get the detailed traffic contract, the ATM standard defines a few QoS parameters
whose values the customers can negotiate with the carriers. These parameters are called
PCR, SCR, MCR, MBS and CDVT. Each parameter is defined by the worst case
performance and the carrier to meet or exceed it. In some cases, the parameter is the
minimum value, in others it is the maximum value. Here, the QoS is defined separately
in each direction.
The correlativity between five classes of ATM service and QoS parameters are shown
in Table 3.1-1, where the “√”indicates “Specified”, while the “-” indicates
“Unconcerned”.
Table 3.1-1 Correlativity between ATM services and QoS parameters
ATM Service
QoS Parameter CBR rt-VBR nrt-VBR ABR UBR
PCR,CDVT √ √ √ √ √
SCR,MBS,CDVT - √ √ - -
MCR - - - √ -
3.2 ATM Processing in MSTP Devices
3.2.1 Background of ATM Application on MSTP
The transfer and processing of Ethernet services are well done with the application of
various new technologies of new generation MSTP, such as GFP encapsulation, virtual
concatenation, LCAS, embedded Resilient Packet Ring (RPR) and enhanced Layer 2
etc. However, the Ethernet can not ensure the QoS because it is
non-connection-oriented. The ATM is connection-oriented and has the features of
statistics multiplexing and QoS. Therefore, the ATM technology is used widely in
bandwidth access and 3G networks. The new generation MSTP with new ATM
Chapter 3 Theory of ATM
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functions, such as inverse multiplexing and statistics multiplexing, enable carriers
meeting the demands of ATM service application and leading ahead in the metro area
network construction.
3.2.2 Key Technology of ATM Service Processing
1. Inverse Multiplexing for ATM (IMA) technology
Taking various devices in current use into account, they can provide functions of
T1/E1 line interfaces, frame generation and ATM transfer convergence (TC)
layer etc. And the ATM cell transfer bus (UTOPIA) specified in ATM forum is
accepted and used widely. Accordingly, a solution defined between the TC layer
and ATM layer appears, that is the inverse multiplexing technology.
The IMA technology is contrary to the traditional multiplexing technology. It
disassembles a single ATM cell flow at the transmitting end and them allocates
them to multiple low-rate links for transmission. At the receiving end, the
services carried in cells are integrated and then recovered to the high-rate ATM
cell flow.
This technology enables broadband ATM cell flows being transferred through
multiple T1 or E1 lines. As shown in Fig. 3.2-1, three T1 links are assembled as
a 4.6 M bandwidth. The IMA technology is applicable to both public networks
and private networks. By adopting enough low-cost narrowband line terminals,
users can enjoy many advantages of ATM, such as QoS, service cutting-in,
scalability and capability of transmitting mixed data voice and video flow
conveniently. And it is unnecessary to apply expensive broadband transfer
devices, such as T3, E3 and SONET/SDH devices to get all these advantages.
Fig. 3.2-1 Principle of IMA Technology
The IMA technology provides functions similar to virtual concatenation and
LCAS in Ethernet. When there are much traffic on the ATM Digital Subscriber
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Line Access Multiplexer (DSLAM) which can not be transferred through a
single path, the IMA can detach the ATM service into multiple low-rate E1 links
with IMA timing, and transmit them transparently through VC12 of different
spare paths in the current transmission network. With the function similar to
LCAS, the IMA can adjust the bandwidth of ATM services dynamically. In this
way, the IMA can still ensure the QoS of other E1 links when one E1 link
failures. Furthermore, it can dynamically adjust the bandwidth combination
mode of links for the purpose to access bursting services at any moment.
2. Virtual Path Ring (VP-Ring) technology
For ATM services, the standard SDH can also implement the transmission
function of ATM 155/622 M interfaces. Considering burst data services, the
statistics multiplexing of ATM services on the ring should be carried out on the
MSTP through taking advantage of characteristic great dynamic variation of
actual data service flow. The ATM VP-ring technology transfers data services
through the VC4 of SDH, and implements the statistics multiplexing and
protection of ATM service access nodes. It also specifies the service
convergence processing principle.
As shown in Fig. 3.2-3, the ATM DSLAM, node B, and Radio Network
Controller (RNC) are accessed to the MSTP via 155 M interfaces. Actually, the
bandwidth for transmission is dynamically variable. With the application of
ATM VP-Ring, all ATM nodes on the ring can share a VC4 of the SDH path,
thus improving the bandwidth utilization ratio greatly.
Fig. 3.2-2 Principle of ATM VP-Ring
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3.2.3 ATM Layer Processing Function of MSTP Devices
For the ATM layer processing function of the MSTP nodes over SDH, their protocol
reference model and hierarchical function should comply with the recommend I.321,
and the functional characteristics should comply with the recommend I.731 and I.732.
The main function of the ATM access board in MSTP equipment is implementing the
convergence from ATM services to the SDH transmission network. Fig. 3.2-3 shows
the functional block model.
Fig. 3.2-3 ATM Function Model of MSTP Devices
1. ATM service types provided
The SDH-based multi-service transport node provides the following ATM
services for ATM service source with different characteristics.
● CBR service
● rt-VBR service
● nrt-VBR service
● UBR service
2. Basic connection functions
The VP-Ring supports the building up and removal of Permanent Virtual Circuit
(PVC) by commands, as well as the ordered establishment and removal of user
data path between ATM interfaces.
● Point-to-point connection function
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It supports the establishment of PVC by commands.
● Point-to-multipoint connection function
Multipoint network connection: supporting network interconnection between
two or more physical interfaces.
ATM multicast: it supports replicating the VP/VC of the input cell flow to
multiple output ATM links during ATM switching.
Space multicast: the output ATM link can be located at two or more physical
interfaces while each interface has only one ATM link.
Logic multicast: two or more output ATM links share one physical interface.
● Connection management function
This function includes the following two parts.
Network resource control: including the management of VPI/VCI and
network bandwidth, as well as the routing of services.
Flow control: providing contracted QoS for ATM data flows, which includes
traffic shaping, Usage Parameter Control and Network Parameter Control
(UPC/NPC), Connection Admission Control (CAC), Selective Cell Discard
(SCD), frame discard, user data buffer and QoS type management etc.
3. ATM layer service protection switching
The ATM layer service protection mode is the ATM virtual path (VP) protection.
Generally, the ATM service protection switching is a kind of layered protection.
The physical layer adopts SDH protection, such as multiplex section protection;
while the ATM layer adopts ATM VP protection. When the switching of the
ATM layer and physical layer has been enabled, the switching between layers is
implemented by delaying the ATM layer switching so as to avoid the
overlapping of these two switching.
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4 Theory of RPR
Key points
RPR overview and features
RPR networking architecture
Concepts and functions of RPR technology
RPR network hierarchy model
RPR fairness algorithm
RPR topology discovery
RPR protection
Implementation scheme of RPR
System architecture of MSTP with embedded RPR
4.1 Overview of RPR Technology
4.1.1 Emergence of RPR Technology
The Resilient Packet Ring (RPR) technology was presented in 2000 as the solution of
some limitations of the SDH, ATM and Ethernet technology used widely in metro area
networks. As the Time Division Multiplexing (TDM) path, the SDH is not good at
supporting packet services, and thus the resource utilization ratio is low. The metro
area network structure constructed with SDH technology is complicated, and it is
difficult to share bandwidth. Therefore, the SDH is generally used in existing TDM
networks to supplement data services. Although the ATM has the advantage of QoS,
the complexity of this technology leads to high cost and more cell overheads. And it
can not keep pace with the development of IP networks. As a low-cost and simple
technology, the Ethernet is widely used in local area networks. However, it can not
satisfy the carriers’ requirements due to the lack of effective QoS, network recovery
and protection, and network management mechanism.
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Currently, the EOS product combining the Ethernet and SDH technology is employed
on a large scale too. It is the main and mature supporting product with the application
of earlier MSTP technology. It meets the demands of data transmission in early times
of TDM networks by transferring Ethernet frames with SDH virtual containers. It
encapsulates Ethernet frames into virtual containers directly with the GFP, HDLC/PPP
or LAPS protocol. In this way, the EOS technology resolves the problem that
expensive Packet Over SONET (POS) interfaces must be used if there is no packet
service interface in the SDH optical network. Generally, services are converged
through an 802.3 switching module before encapsulation in order to improve the
bandwidth utilization ratio. The technology is just the second generation MSTP
technology mentioned in the first chapter of this book. The main disadvantages of the
second generation MSTP are as follows.
● Complicate configuration: Services between sites should be configured one by
one, and the passing sites should be configured as straight-through. For a
complex network, a lot of works should be done for configuration and
maintenance.
● Lack of sharing characteristic: Traffics are connected through VC. As the carrier
of traffic, the link can not share its bandwidth with other links.
● Bandwidth with low utilization ratio: In order to avoid broadcast storm, the STP
protocol should be performed, and thus some bandwidth can not be used at best.
On the other hand, the MSTP needs SDH protection because it has no fast
protection mechanism (STP protection at second level is too slow). However, the
SDH protection will waste 50 percent bandwidth regardless of its high speed.
● Special requirements on convergence ratio: In convergence networks, boards
with multiple system directions are needed to respond linking requests from
various sites to the convergence site.
● Difficult to ensure the QoS in ring networks: Although EOS equipment can
avoid the problems in part mentioned above by constructing Ethernet ring, they
can not be avoided completely while constructing ring networks because
network complying with 802.3 is not designed originally for ring networks.
Besides, it will result in interaction between traffics from upstream and
downstream sites, and thus the QoS can not be guaranteed. Therefore, so far
there is seldom such application.
Chapter 4 Theory of RPR
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All these problems are those the RPR technology should solve.
4.1.2 Basic Concepts and Features of RPR Technology
The IP industry recognized the value of ring networking architecture for a long time. It
has done a lot of works on this field and developed solutions such as token ring and
Fiber Distributed Data Interface (FDDI). However, these solutions can not meet either
the demands of IP traffic flow and optical fiber bandwidth, or the requirements of the
development of IP transmission and service transfer, such as keeping high bandwidth
utilization ratio and forwarding traffic in the congestion case, ensuring the balance
between nodes, recovering quickly from node or transmission medium failures, and
being capable of plug-and-play etc. Therefore, ring networks such as token ring or
FDDI are not applicable to metro area network. Service providers and enterprises need
a technology with good scalability, which can be applied in Metro Area Network
(MAN) and Wide Area Network (WAN) with the capability of transferring IP
information packets at Gigabit rate
In Nov. 2000, IEEE set up the 802.17 Resilient Packet Ring Workgroup (RPRWG)
formally, which was expected to develop a RPR MAC standard and thus optimize the
data packet transfer on rings in the LAN, MAN and WAN topology.
The Resilient Packet Ring is a new MAC layer protocol to optimize the data service
transmission in ring architectures. It makes the advantage of ring architecture, and
solves problems in packet service transfer through the supporting protocol on the MAC
layer, such as bandwidth sharing, protection and QoS guarantee etc. Because the
bandwidth of low-priority traffic is controlled by the inner algorithm, having the
characteristic of automatic adjustment, this technology is called as resilient packet ring.
The RPR is adaptive to various physical layers such as SDH and Ethernet, and can
transmit various types of service such as data, voice and video service. It not only
involves the Ethernet features of economy, flexibility and scalability, but also absorbs
the advantage of quick protection (50 ms) in SDH ring networks. Besides, the RPR
technology has the advantages such as the automatic discovery of network topology,
bandwidth sharing of ring, fair allocation, and strict Classification Of Service (COS)
etc. The purpose of RPR is to provide more economical and efficient metro area
network solution without the cost of network performance and reliability.
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The RPR technology has the following features.
● High bandwidth efficiency
Traditional SDH networks need 50% of the ring bandwidth as redundancy, while
the RPR does not need. The RPR technology remains the protection mechanism
similar to that of SDH networks. It protects services by using two rings in
reverse directions and allows the data traffics being transferring at the full speed
on the ring between the source node and the destination node. With the
application of Spatial Reuse Protocol (SRP), the destination node does not
occupy bandwidth of the ring after extracting data frames dropped in this node,
which is released to the downstream section. Spatially, there are no repeated
traffic flows can use their own bandwidths without influencing others. To sum
up, normally data are transmitted on the shortest arc between the source node
and the destination node, and multiple nodes can intercommunicated at the same
time. In this way, many nodes can receive and transmit packets at the same time,
and thus improving the utilization of ring bandwidth; especially for the ring with
many nodes, the improvement is more evident.
● Fair bandwidth allocation protocol (QOS guarantee)
The RPR enables the bandwidth sharing for data services by using the effective
fairness algorithm. In the network, the traffic of the user access end is
paroxysmal in nature, while the traffic of the core part of the network is
comparatively smooth, which thus can be predicted. By classifying services, the
RPR technology enables carriers providing low-priority networks to access
services (such as some data services) only when there are spare bandwidths. It
not only makes full use of the inherent characteristics of these traffics, but also
avoids the bandwidth un-fairness between upstream and downstream sites.
● Quick protection mechanism
The RPR can provide 50 ms service protection which is similar to the Automatic
Protection Switching (APS) in SDH networks. At present, two methods can be
used to avoid failures: Wrapping and Steering. When “Wrapping” is used, the
adjacent node of the failure will loop the traffic on a ring to another ring, such as
looping the traffic on the internal ring to the external ring. This way can keep the
continuity (sequence) of the data even the traffic flow arrives at the destination
node through a long path. The “Steering” method is used to reverse the traffic
Chapter 4 Theory of RPR
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flow in direction, which will reach the destination node through another path.
● Seamless connection with SDH networks
The RPR system with embedded SDH can be connected to SDH networks
seamlessly, for most SDH networks are ring networks while the RPR is ring in
structure. It makes full use of bandwidth in ring networks. Unlike the existing
MSTP, it should avoid looping traffics with STP protocol or manual
configuration, which sometimes may cause the bandwidth waste.
● Simple service configuration
One objective of RPR technology is distributed access. The distributed access
together with the quick protection, automatic traffic re-establishment provides
the plug-and play mechanism for inserting or removing nodes quickly. The RPR
is a packet switching technology using shared bandwidth in the ring. In the ring,
each node knows the available capacity of the ring. Under the traditional circuit
switching mode, each connection in the whole network should be configured
point to point. But the RPR only needs to configure the connection relationship
between the access end and the ring. It is unnecessary to configure the
connections between nodes, the flow direction of traffics. All these simplified
the configuration greatly. Furthermore, such service configuration mode avoids
the convergence ratio problem existing in traditional EOS devices. Neglecting
the bandwidth limitation, the RPR can almost get the unlimited convergence
ratio.
4.2 Fundamentals of RPR Technology
4.2.1 RPR Ring Network Architecture
The RPR is of the dual-ring architecture, as shown in Fig. 4.2-1. Being similar to the
bidirectional multiplex section ring topology of SDH, it consists of two rings with
reverse directions: the clockwise one is calls as Ringlet 0 while the counter-clockwise
one is Ringlet 1. Each node on the ring is called as site, which is identified with a
48-bit address. The connection between sites is SPAN, and each SPAN based on Time
to Live (TTL) is a hop.
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Fig. 4.2-1 RPR Ring Network Architecture
The data transmission based on RPR supports the unicast, multicast and broadcast
mechanisms. For the multicast or broadcast, each node can detect the data after it is
sent by the source node. The node analyzes the address information in the frame header;
if this node meets the address, it copies the data and then forwards it to the next node.
After the data passing through the ring and returning to the source node, it is stripped
from the ring by the source node. For the unicast, the data packet is transferred only on
the shortest arc between the source node and the destination node. The source node
sends the data, and the destination node receives the data and strips the data packet
from the ring. The RPR is more efficient than token ring or FDDI in metro area
networks.
The topology of RPR is a dual symmetrical reverse ring: one is the inner ring; the other
is the outer ring. Such architecture provides the following benefits.
● Two paths between each pair of node ensure the high reliability.
● Two protection mechanisms can be used.
One is the wrapping mechanism. When the adjacent node detects the failure,
the ring stops working and the traffic on this ring is looped to another ring.
The other one is the steering protection. When a failure occurs, the protection
messages are quickly dispatched to all nodes on the ring. The source node
has the right to select the ring for data transfer and finally enable the data
bypassing the failure point. IEEE 802.17 specifies the steering protection as
the default mechanism.
● The longest path is half of the ring, for data can be transmitted in two
directions and nodes in the ring can select the shortest transfer path.
Chapter 4 Theory of RPR
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● It is possible for spatial reuse. In the unicast mode, data can be transferred on
different parts of the ring at the same time, and thus the capacity of the whole
ring will be a few multiple of that of single fiber.
The “Resilient” in the RPR represents the RPR protection protocol.
The Operation, Administration and Management (OAM) protocol of RPR involves four
functions defined by ISO: fault management, configuration management, performance
management and accounting management.
The fault management is the core part of OAM, responsible of detecting, isolating and
correcting exception conditions in the network and reporting them to the network
management system. IEEE 802.17 specifies that network elements must detect two
types of failure: Loss Of Continuity (LOC) and Remote Defect Indication (RDI).
4.2.2 RPT Technology
As a superset of RPR, the Resilient Packet Transport (RPT) technology goes beyond
RPR, attempting to extend resiliency protection further.
The RPT classifies services into different levels which are identified by the MPLS
COS field. It adopts the special synchronization mechanism to provide reliable clock
and delay, jitter assurance, and support TMD services such as voice service. RPT
devices provide a lot of Ethernet interfaces. And the Layer 2 switching based on logic
MAC address simplifies the IP packets transmission in metro area networks greatly,
and thus decreases the network construction cost.
The RPT technology provides data services and voice services with a platform for
Layer 2 statistics multiplexing. For the private line services or data services, they can
be transmitted on the same bandwidth once being accessed to the network. Because all
services share the bandwidth together, the RPT improves the bandwidth utilization
ratio greatly.
1. Quick layer 2 packet switching
The RPT layer 2 adopts the encapsulation mode of resilient packet transport
frame. The header overhead of frame contains the logic MAC address (1-byte)
and standard MPLS field. All nodes on the ring are assigned with a unique logic
MAC address, and 255 nodes can be identified at most (while 16 nodes can be
identified at most on an SDH ring). All nodes can perform quick Layer 2
switching based on logic MAC address. The passing node can judge the
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destination according to the logic MAC address of the traffic flow, and forwards
the traffic quickly without any processing.
2. Flexible interfaces
Currently, the RPT technology supports various tributary interfaces such as E1,
10 M/100 M, GE, STM-1 and POS. And the rate of line interface rate can reach
GE, 2.5 G and 10 G.. Some RPT devices even provide Dense Wavelength
Division Multiplexing (DWDM) boards to reach the line rate of 80 G or higher.
The various high line rate and abundant tributary interfaces fit the traffic
deployment in broadband metro area networks very much.
3. Spatial reuse technology based on logic MAC
The RPT supports the Spatial Reuse Protocol (SRP), which is a MAC layer
protocol being independent of mediums. All nodes have the same control right
to bandwidth.
4. Bidirectional reverse ring, single fiber ring and linear network topology
The RPT ring is a bidirectional reverse ring: one fiber is clockwise ring and the
other fiber is counter-clockwise ring. Each fiber transfers data and control
signals in one direction. The control signal is transferred as the packet with the
highest priority. The RPT also supports the single fiber topology, bidirectional
data transmission at 1 G or 2.5 G, as well as WDM data transmission etc.
5. Medium independent
The RPT does not depend on the medium of physical layer. The medium of RPT
can be fiber or wavelength in DWDM systems.
6. Comparison of RPT and other broadband technologies
Compared with other technologies, the RPT technology has the greatest features
as follows: the economy of LAN, the reliable base to guarantee TDM
transmission and full use of network bandwidth.
● Comparing the RPT with the SDH/POS (Packet Over SDH)
Both the RPT and POS avoid the complex protocol and too many header
overheads of the ATM technology. They can transfer Gigabit IP services through
fiber in the format of resilient packet data frame (similar to Ethernet frame). It is
unnecessary to disassemble and reassemble IP packets, thus improving the
Chapter 4 Theory of RPR
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processing capability of switches greatly and decrease the cost of devices. But
the RPT provides the function of using bandwidth dynamically, increasing the
bandwidth utilization ratio greatly. Thus the RPT avoids the point-to-point
limitation of POS and decrease the number of ports.
In the network protection, the RPT protection the switching based on the source
route ring, which is different from the multiplex section protection of SDH. It is
more economical than SDH protection for network resources.
When a fiber is broken, the nodes at both ends of the fiber will send Layer 2
control signaling to each node along the fiber direction. As soon as the source
node of the traffic receives the control message, it sends the service according to
the logic MAC address of the destination node to the fiber in the other direction.
In this way, the protection is implemented. It is obvious that the protection route
selected is the best in the switching based on source route. It saves the
bandwidth resources of fiber, and the protection switching time is less than 50
ms.
To sum up, the RPT functions of bandwidth statistics multiplexing, provision of
multiple high-speed Ethernet interfaces, different level services and source route
ring protection based on different service level can not only guarantee the
transfer of TDM services, but also support bursting IP services efficiently. This
is that the POS/SDH can not provide.
● Comparing the RPT with the Dynamic Packet Transport (DPT)/Gigabit Ethernet
(GE)
Compared with the DPT/GE, the RPT has the important advantage that it
provides the capability of multi-service (including TDM services) transport
switching.
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4.2.3 RPR Network Hierarchy Model
Fig. 4.2-2 shows the hierarchy reference model of the RPR.
GFP coordinate sublayer
SDH coordinate sublayer
GFP adaptationPPP/LAPSadaptation
System packet interface-x
Application Layer
Transport Layer
Session Layer
Presentation Layer
Network Layer
Data Link Layer
Physical Layer
Logical Link Sublayer
MAC Data Channel Sublayer
Fairness algorithm
Protecion
OAM
Topology discovery
MAC Control Sublayer
Ring selection
OSI Reference Model
RPR Hierarchy
Higher layer
SONET/SDH Physical Layer
Medium
Interfaces depending on medium
MACservice
interface
Physical layerservice
interface
Fig. 4.2-2 RPR Hierarchy Reference Model
The hierarchy reference model complies with the Open System Interconnection (OSI)
reference model and corresponds to the first and second layer of the ISO model.
The purposes of RPR physical layer interface and physical layer entity are as follows:
1) Supporting RPR MAC
2) Supporting GE and 10 G Ethernet physical layer entity
3) Supporting the framing modes of GFP and byte synchronous HDLC/LAPS, and
physical layer entities running at the rate of 155 Mbit/s ~ 9.95 Gbit/s.
4) Supporting synchronous or plesiochronous network applications.
5) Only supporting full-duplex operations
In the hierarchy model of RPR, the coordinate sublayer of the physical layer at the
bottom is responsible for the mapping of information between the
Medium-Independent Interface (MII) and physical medium. The System Packet
Interface (SPI) defined by the Optical Internetworking Forum (OIF) is a kind of
interface between physical layer device and data link layer device. It separates the
synchronous layer and asynchronous layer by improving and receiving data transmitted
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at the rate independent of the actual line bit rate. The SONET/SDH adaptation layer
implements the mapping from the RPR to the SONET/SDH by taking the GFP protocol,
and HDLC series protocol (PPP/LAPS are used widely) as the data link layer mapping
protocol.
The MAC sublayer consists of two parts: the data channel sublayer and the MAC
control sublayer.
The MAC data channel sublayer transfers and receives frames on the physical medium
through the physical service interface. The data channel includes two function modules:
ringlet-independent function module and ringlet-specific function module. The
ringlet-independent functions include MAC service interface processing, ring selection,
receiving frames from the ring and data transfer to the client layer. The ringlet-specific
functions include the traffic flow adjustment of local data transmission and data
exchange through the physical service interface.
The MAC control sublayer is responsible of necessary works related to maintain the
data channel sublayer, including RPR topology discovery protocol, RPR fairness
algorithm, RPR ring network protection and RPR OAM.
The RPR topology discovery message is transferred through the RPR control frame.
The MAC service interface is used to transmit data from the MAC client layer as well
as local messages from the MAC layer to the MAC client layer. The MAC control
sublayer establishes the data channel independent of the actual ring network, and
performs related control operations. While the MAC data channel sublayer performs
functions related to the actual ring network, such as the access control and transmission
of data.
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4.2.4 RPR MAC Data Frame Processing
The RPR MAC frame has four types: data frame, control frame, fair frame and idle
frame.
Fig. 4.2-3 RPR MAC Data Frame Processing
RPR processing on the receiving side (Ingress):
1. After the node receives the data frames from the ring, it checks the value of
Time to Live (TTL) in the data frames. If the value is not zero, the TTL field is
decremented by 1. If the TTL reaches zero, the packet is stripped from the ring.
2. Check the type of frames and the frames again, and then strip the error frames.
If the fairness algorithm is used to control the frames, then send these frames to
the fairness algorithm module directly for processing. If these frames are idle
frames, then strip them directly from the ring.
3. Compare the source address of the frame with that of the local node, and judge
whether this frame is output from this node. If the addresses are same, check
whether the ring is mismatched. If it is mismatched, determine whether to enter
the wrapping protection. If the ring has already been in the wrapping protection
situation, forward this frame directly; or else discard the frame.
4. Judge whether the destination address of the frame is same as that of the local
node. If they are same, check whether the frame is a control frame. If yes, send
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this frame to the CPU for processing. If no, input it to the egress specified in
802.3 for copy processing. If the destination does not identify the local node,
forward the frame to the transfer channel.
Note
● Drop: The local node can receive unicast or multicast frames from the ring after
Stack VLAN filtering which are transferred to this node from other nodes. Unicast
frames will be stripped from the ring and sent to corresponding user ports. Multicast
frames are sent to corresponding user ports and are transited.
● Transit: Frames received from the ring at the local node are transit to the Primary
Transit Queue (PTQ) and the Secondary Transit Queue (STQ) channel. The data frames
in the PTQ and STQ are inserted to the transfer ports of the source ring directly.
● Strip: The local node receives the frames from the ring, which will no be
forwarded. The frames are terminated at this node.
RPR processing on the transmitting side (Egress):
The data frames at the transmitting side include data to be forwarded and data frames
and control frames inserted at the node. For the inserted data frame, determine its
destination address and ringlet selection through topology discovery and routing table.
Then send it to the corresponding inserted queue (A, B, or C) according to the priority.
The important point of the processing at the transmitting side is the dispatching of
queues. The dispatching priority sequence is as follows.
PTQ over threshold > STQ close to limit threshold > CTL > PTQ > STQ over
threshold > A > B > eB > C > STQ
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4.2.5 RPR Fairness Algorithm
Bandwidth management is an important part of the RPR. Three classes of service are
defined for the RPR MAC -- A, B and C.
Class A – This class of service allocates and ensures the data rate, and requires
point-to-point low delay and jitter guarantee. It has the bandwidth reservation
mechanism, and is not controlled by the fairness algorithm. Class A services usually are
used to provide voice and video flow.
Class B -- This class of service allocates and ensures the data rate, which is optional.
For optional data rate, there is no allocation and guarantee. For ensured data rate, class
B services provide the point-to-point delay and jitter guarantee. For class B services,
the processing of the data rate allocation and guarantee is same as that of class A
services. The processing of those exceeding the allocated rate range is same as that of
class C services.
Class C -- This class of services is dispatched fairly and requires no bandwidth
guarantee. It usually used to transfer general IP services. Class A services can be
considered as “best effort” services, and they are always be restricted by the fairness
algorithm.
The fair dispatching function is implemented through the fairness algorithm for the
purpose of dynamical bandwidth adjustment and sharing.
The RPR fairness algorithm has the following features:
● Providing a mechanism to divide the available bandwidth fairly between nodes
on the ring
● Only applicable to low-priority services and excessive medium-priority
services, which are Excess Information Rate (EIR) data frames in the
medium-priority services
● The RPR fairness algorithm can control the bandwidth of two sub-ringlets
respectively. That is, there are two fairness protocols on a RPR ring to control
the bandwidth respectively.
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1. Fairness algorithm module block diagram
Fig. 4.2-4 shows the fairness algorithm modules.
Fig. 4.2-4 Fairness Algorithm Module Block Diagram
The functions of fairness algorithm modules are as follows:
● Receiving and processing fair frames
● Calculating the fair allowed rate of the local node
● Controlling the fair traffic rate with the shaper
● Determining the fair rate to be propagated
● Generating and sending fairness control messages
2. Bandwidth adjustment technique
The RPR fairness algorithm initiates the bandwidth adjustment by detecting
congestion.
When the congestion occurs at a node, it dispatches a fair rate to the upstream
node on the reverse ring, which is calculated with the add_rate in this node and
the normalized weight. The upstream node adjusts its sending rate and keeps it
not exceeding the fair rate after it receives the fair rate. Nodes receiving the fair
rate may respond in two ways:
If the node is congested, it will select the minimum one from its own fair rate
and the received fair rate, and then dispatch the rate to its upstream node.
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If the node is not congested, it will forward the fair rate to its upstream node.
3. Example of bandwidth adjustment based on fairness algorithm
Fig. 4.2-5 Fairness Algorithm Bandwidth Adjustment Example
As shown in Fig. 4.2-5, the non-reserved bandwidth of the RPR (ring bandwidth
– reserved bandwidth for A0 services) is 500 M, that is, the maximum ring
bandwidth which can be controlled by the fairness algorithm is 500 M. There are
convergence services between site 1, 2 and 3. Services of site 1 and site 2 are
converged at the site 3.
4.2.6 RPR Topology Discovery
1. Purpose of topology discovery
The purpose of topology discovery is to provide decision-making principles for
ring selection, fairness algorithm and protection units by making each site know
the whole structure of the ring, the hop count to itself and the capacity of each
site on the ring etc.
RPR topology discovery is a periodical activity. It can also be initiated by a node
which wants to know the topology. That is, a node can generate a topology
information frame when necessary, such as, when the node is just accessed to
the RPR ring, when the node receives a protection switching requirement
information, or when the node detects a fiber link error.
The topology information generation cycle can be configured as the value
between 50 ms and 10 s. The minimum resolution is 50 ms. And the default
configuration is 100 ms.
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2. Process of topology discovery
In the case of ring initiation, access of new nodes, ring protection switching or
startup of PRP auto-reorganization mode, some node generates a topology
discovery packet, which contains the MAC address and state information of this
node. When other nodes receive the packet, they insert their own MAC
addresses and state information in this packet and forward it to downstream
nodes. In this way, every node gets the knowledge of the node number on the
ring and the queue information, and then forms the topology mapping.
RPR topology discovery can process various topology changes, such as
adding/deleting nodes on the ring and broken link etc. Every node can discover
the topology automatically. The process of topology discovery is similar to the
link state protocol of Open Shortest Path First (OSPF), which transfers
information with corresponding control message and trigger the node to dispatch
the message with the trigger.
Fig. 4.2-6 illustrates the basic procedures of topology discovery.
Fig. 4.2-6 Topology Discovery Procedures
As shown in Fig. 4.2-6, the line between S7 and S1 of the closed loop has been
damaged. The node S1 and S7 broadcasts topology (TP) frames to indicate the
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network boundary. Such TP frame will trigger all nodes, which support the
Steering protection mode, to change or keep the direction of data to be
transferred (the principle is to avoid wrong paths). However, this frame will not
change those nodes which support the Wrapping protection mode. When the
new topology becomes stable, each node will check the topology with its
adjacent nodes. If the topology is right, there is an open-loop topology structure
in the topology database of each node.
Although the RPR topology discovery is a kind of periodical activity, it still
should be initiated by a node which knows the topology structure. That is, some
node on the ring can generate a topology frame when necessary.
4.2.7 RPR Protection
The RPR has a perfect protection mechanism. RPR MAC layer supports the Steering
protection or Wrapping protection.
1. Steering protection
When the sending node transfers unicast services, the steering protection will
select the ringlet 0 or ringlet 1 according to actual situations. Which ringlet will
be selected depends on the avoidance of wrong paths. Multicast services will be
sent to the ringlet 0 and ringlet 1 at the same time.
Fig. 4.2-7 RPR Steering Protection of Unicast Services
Normally, the S2 sends data to S6 along the ringlet 0. When some line failures,
the S2 will send data to S6 along the ringlet 1 to avoid the wrong path.
Chapter 4 Theory of RPR
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2. Wrapping protection
Fig. 4.2-8 RPR Wrapping Protection
Under the wrapping protection mode, the sending node will keep using the
original ringlet instead of considering the avoidance of wrong path. The
protection action occurs at the boundary of the ring, as shown in Fig. 4.2-8.
In normal state, the sending node S2 transmits data to S6 along the ringlet 0. If
the fiber between S3 and S4 is broken, the S2 will send data to S3 still along the
ringlet 0. The node S3 will send the data to S6 along the ringlet 1. During the
process, the data packet will not be stripped to avoid frame out of sequence in
the protection. Only when the data reaches the S6, can it be stripped from the
ring.
When the topology becomes stable, select “re-steer” to optimize the path, that is,
send data through the shortest path S2 > S1 > S7 > S6.
3. Implementation of RPR protection
The key of RPR protection is to know which path has problems. The topology
structure should be known at any moment, and then process according to
corresponding configurations.
The RPR network supervises the network topology continuously while it
transferring data. Once any topology change is found, it will carry out the
steering protection or wrapping protection according to corresponding
configurations.
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For MSTP devices with embedded RPR, users can assign whether to adopt both
the RPR MAC layer protection and SDH physical layer protection. If both of
them are enabled, the action of delaying the RPR protection switching can be
taken to support switching between layers and avoid the overlapping of two
kinds of protection switching.
4.3 RPR Implementation Scheme
4.3.1 Three Implementation Schemes of RPR
The implementation scheme of RPR can be divided into three classes. For each of these
schemes, there are vendors to provide corresponding products.
● Independent Layer2-based RPR scheme
This scheme is applicable to the access layer and convergence layer of IP metro
area networks. Some vendors provide broadband multi-service solutions, which
optimize IP and support TDM services, by combining the Layer2-based scheme
with the MPLS technology, synchronization technology, Coarse Wavelength
Division Multiplexing (CWDM) technology and the TV video broadcast
technology.
In addition, Layer2-based RPR products provided by some vendors have great
networking capability. They can support linear networking, tangent networking
and dual ring internetworking topology structures as well as Dual Node
Interconnection (DNI) protection etc. The RPR products with these enhanced
functions can also be used on the core layer of the IP metro area network in
small cities. But the independent Layer2-based RPR scheme is seldom used
because the construction cost of single RPR technology scheme is very high for
the moment.
● Router-based single-board RPR scheme
This scheme is mainly applicable to the core layer and convergence layer of IP
metro area networks. Most vendors implement RPR functions by adding boards
based on existing routers. The router-based scheme can be regarded as the
optimization of existing router networking. It can improve the protection
performance greatly and achieve the 50 ms ring protection function as well as
save fiber resources.
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● MSTP-based RPR scheme
The MSTP-based RPR scheme separates an independent path from the MSTP ring
bandwidth to support the RPR technology. Compared with the traditional SDH,
the MSTP adopts the Layer 2 switching technology to implement the Ethernet
services bandwidth sharing, completes the mapping from Ethernet frames to SDH
VCs through the GFP encapsulation, and improves the flexibility and reliability of
the virtual container bandwidth allocation through the virtual concatenation and
LCAS technology. But because of the inherent shortcomings of Ethernet
application in ring networks, many vendors are considering adopting the RPR
technology into the new generation MSTP for the purpose of providing integrated
solutions to support data services.
At present, TDM services occupy the dominant status, and the MSTP-based RPR
scheme will be the best multi-service transport platform. However, the commercial
application of corresponding products still need more time.
When data services take the place of TDM services as the dominant one, the
independent Layer2-based RPR scheme will be the best multi-service transport
platform. And currently there are already some mature products used widely
corresponding to this scheme. Because there are always data services to be processed
in IP metro area networks, it can be expected that the independent Layer2-based RPR
scheme and router-based RPR scheme will be used widely in IP metro area network
constructions as good optimization solutions.
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4.3.2 System Architecture of RPR-Embedded MSTP
Fig. 4.3-1 Architecture of RPR-Embedded MSTP Ring and Nodes
As shown in Fig. 4.3-1, the node architecture basically represents the RPR protocol
reference model. In the RPR protocol reference model, the RPR is located at the data
link layer, including the logical link control sublayer, MAC control sublayer and the
MAC data channel sublayer. The logical link control layer transfers data to one or more
remote logical link control layers same as it through the MAC service interface. The
MAC data channel layer performs the access control and data transmission between
itself and some special ringlet. Between the MAC control sublayer and MAC data
channel sublayer, RPR MAC frames are transferred or received.
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5 Theory of MPLS
Key points
Architecture of MPLS
Basic working mode of MPLS
Advantages of MPLS
5.1 Introduction to MPLS
With the rapid development of Internet, traditional routers became the bottlenect due to
their inherent limitations. ITU-T has accepeted the ATM technology as the final
solution for Broadband Integrated Services Digital Networks (B-ISDN), and developed
countries already implemented the pilot network plan and commercial service plan.
Since the middle of 90s in 20th century, the ATM technology has been adopted in the
construction of most Internet backbone networks and high-speed LANs. And IP over
ATM has been the most popular field in these years cross the telecommunication
industry and computer industry. Multiple technologies appeared one after another, such
as overlapping Classic IP over ATM (CIPOA), LAN Emulation (LANE), Multiple
Protocol over ATM, integrated IP switch and label switch etc.
The Multiple Protocol Label Switching (MPLS) provides the better solution for IP over
ATM on the base of combining technologies metioned above. The Internet Engineering
Task Force (IETF) issued a series of recommendation drafts about the MPLS, and
several main drafts of them were released formally at the meeting in Mar. 1999. RFC
codes were applied for them and have been approved.
Early the MPLS mainly works in IP protocol (IPv4 IPv6) at the network layer.
However, its core technology can also be applicable to other network layer protocols
such as IPX, Appletalk, Dcnet and CLWP. Although the MPLS is not limited to some
specific link layer, the main work focuses on the ATM.
With the development of IP networks, especially for Gigabit-rate routing switches, the
MPLS technology must be applied and developed while developing the
IP/SDH/OPTICS mode to the IP/OPTICS (DWDM) directly. For from the point of
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layer, there is a link layer between the IP and DWDM, which is used for transfer,
switching and transmitting. There are only two kinds of link layer transport technology
applicable to IP packet now: SDH with the Synchronous Transfer Mode (STM) and
cells with the Asynchronous Transfer Mode (ATM). The MPLS is the technology
applicable to both the SDH and ATM, and it can be developed in the future as the
technology for any special link layer. The MPLS also has the functions to support the
network management, traffic engineering, QoS and COS. IP services can be transferred
on the OPTICS directly with the MPLS (other corresponding modes can also be used).
Actually, the MPLS is not only a technology applied in IP over ATM, but also an
“interlayer”network technology between Layer 3 and Layer 2. It is researched and
developed as architecture. Currently, the MPLS can be used in ATM networks and FR
networks. Furthermore, it becomes the focus as the preferred technology during the
research and development of IP over OPTICS. Some people even say that the MPLS is
the terminator of ATM.
In any case, the MPLS and ATM can not take the place of each other, for their function
positioning does not cover each other. The ATM implements the ATM cell layer and
AAL layer in the four-layer reference model of B-ISDN, corresponding to the functions
of the second layer (data link layer) in the ISO-OSI seven-layer reference model. The
MPLS implements the funcions of the comparatively-independent interlayer between
the third layer (network layer) and the second layer (data link layer) in the seven-layer
reference model. It does not have the integrated functions of the data link layer.
Therefore, the MPLS can only implement the actual transfer function of the data link
layer depending on a special link layer, such as the ATM cell layer or FR-SDH layer of
frame relay. Fig. 5.1-1 shows the function positioning of MPLS.
Chapter 5 Theory of MPLS
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Fig. 5.1-1 Function Positioning of MPLS, IP Network and ATM Network Layers
As shown in Fig. 5.1-1, the MPLS interlayer simplifies and specifies the transform
protocol between L2 and L3 greatly. However, each IP packet in the IP network can
arrive at the special link layer only after being processed with multiple corresponding
intermediate protocols. Although there is only one protocol suite at the data link layer
of the ATM network, the network layer needs multiple corresponding interworking
protocols for every kind of service from various networks. The MPLS interlayer is
absolutely necessary for IP over OPTICS. A link layer is needed from the IP at the
network layer to the OPTICS at the physical layer. The MPLS interlayer can satisfy the
requirements of existing FR-SDH and ATM link layer. In the future, it can also be
adaptive to any new link layer technology.
The MPLS with powful capacity can implement many functions and performances
which are difficult to realize in common route networks, such as explicit routes, traffic
engineering, QoS and COS. Moreover, the problems caused by the restriction of IP
over ATM and IP over FR can be solved, such as flexibility, generality and SVC
contention etc. Although the requirements of various users and services on the Internet
become more and more complicated as well as the classification of Forwarding
Equivalence Class (FEC), all of them can be processed in one time after they entering
the MPLS domain. In the domain, the route, which performs the label exchange and
forwarding, may not be influenced, and the highest working capability of transport
switching is still required based on the 0 (n) traffic. Therefore, the routing protocol and
interconnecting network architecture of the MPLS has great flexibility. And MPLS can
guarantee the security and long-term reliability of MPLS networks.
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5.2 Architecture of MPLS
The basic purpose of MPLS is to integrate the label switching and forwarding
technology and the network layer routing technology. The core is the label meaning,
label forwarding, and distribution of labels.
The MPLS terms are as follows:
● Forwarding Equivalence Class (FEC): a group of IP packet which can be
processed with the same mode. It can also be regarded as the same path, or the
same forwarding processing.
● Label (L): a short fixed-length identifier used to identify the FEC in the
forwarding packet group. It is valid locally.
● Label Switch Path (LSP): on the peer layer, it corresponds to a special FEC
mapped by a group of IP packet through a path of one or more LSRS.
● Lable Switch Router (LSR): a device with MPLS node functions and the
function of forwarding IP packets on the pure Layer 3.
● MPLS Domain: an adjacent aggregation of nodes running the MPLS protocol,
as an automanous system or an LSR management domain.
● MPLS Node: a node running the MPLS protocol. It can be discovered, abutted
and conversed by the MPLS control protocol, performing one or more routing
protocols. The MPLS node has the function of label switching and forwarding
as well as the processing of IP packets on the pure Layer 3.
● MPLS Edge Node: an MPLS node to connect the MPLS domain to a node
outside of the domain, which can not perform MPLS.
● MPLS Ingress: an MPLS node to process IP packet traffics input to the MPLS
domain.
● MPLS Egress: an MPLS edge node to process IP packet traffic output from the
MPLS domain.
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5.2.1 Basic Working Mode of MPLS
1. Label switching and forwarding process
In a common route network, an IP packet is forwarded by hop along the routers.
Each router should recognize the header of the IP packet independently, analyzes
it and then runs the routing algorithm to select the next hop. Actually, the IP
packet header contains more information than that needed for selecting the next
hop. The selection of next hop can be summarized as the combination of two
functions. The first function is specifying the IP packet to be forwardes as the
Forwarding Equivalence Classes (FECS) according to the forwarding direction.
The second function is mapping each special FEC to the next hop.
MPLS specifies that when each special IP packet enters the MPLS domain, it is
mapped to a special FEC once, which is divided based on the destination
addreee of the IP packet. And a label switch path (LSP) mapped to the special
FEC is established between the MPLS Ingress and Egress. The FEC is coded as
a short fixed-length label (L) on two adjacent label switch routers (LSR) along
the LSP and on the link between them. The label is forwarded together with the
IP packet. The IP packet with the label is called as labeled packet.
On each router in the next hops, it is unnecessary to recognize and analyze the
IP packet header. Just take the label in the packet as a pointer to direct it to a
new label and reach the output port of the next hop. The labeled packet becomes
a new one by replaceing the old label in the packet with a new label. And then
the output port will send the packet to the next hop.
Actually the label switching and forwarding process is similar to the forwarding
process based on Data Link Connection Identifier (DICI) in FR networks and
the forwarding process based on VPI/VCI in ATM networks. The difference
between them is that the DLCI in FR networks is the identifier of link and the
VPI/VCI in ATM networks is the identifier of cell, while the FEC in MPLS is
more complicated than link and cell. The FEC is a concept intergrated and
extracted from various independent objects such as data flow, link and port etc.
Because the MPLS forwarding is based on labels, the packets can be forwarded
by switches. Generally, switches can not forward IP packets directly for they can
not recognize, analyze and process headers of IP packets, or are not provided
with the proper speed to do so.
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Fig. 5.2-1 MPLS Running Diagram
Chapter 5 Theory of MPLS
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2. MPLS running
MPLS runs in the MPLS domain, and it can also run between MPLS domains at
the same time. The MPLS is allowed running in the mixed networks of MPLS
and non-MPLS.
Fig. 5.2-1 a) shows the configuration of MPLS domain. The edge node close to
users, the edge label switch router (ELSR), which is connected to extra-domain
nodes, has complex processing functions. The inner node in the domain, the
inner label switch router (ILSR), which is not connected to extra-domain nodes,
performs the label switching and forwarding functions as simple as possible.
The MPLS running can be divided into two phases: the first one is generation of
automatic routing table, and the second is the forwarding of IP packets. In actual
running, these two phases are carried out alternately.
● Phase One Generation of automatic routing table
Step 1: Establish topology routes between nodes in the MPLS domain with the
mode same as the autonomous system of common route networks. Then run the
OSPF routing protocol (other routing protocols can also be runned at the same
time) to make all nodes clear about the topology information of the domain. The
MPLS can allocate flows equally in the whole domain and optimize the
transmission performance of the network with the participation of the
management layer. The Border Gateway Protocol (BGP) is runned mainly
between domains to provide and achieve accessible information for adjacent
domains and the backbone core network.
Step 2: Run the Label Distribution Protocol (LDP) to establish abutting
connections between nodes in the MPLS domain. Classify the FECS according
to accessible destination addresses and establish the Label Switch Path (LSP).
Allocate labels (L) to FEC along the LSP and generate the forwarding routing
table on each Label Switch Router (LSR).
Step 3: Maintain and update routing tables.
● Phase Two Forwarding IP packets in the MPLS domain
Step 1: After the IP packet entering the edge node of MPLS domain, the ELSR
recognizes the IP packet header, checks up corresponding FEC F and the LSP
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mapped, and then insert the label into the packet. As a labeled packet, it is
output to the specified port.
Step 2: The next top ILSR in the MPLS domain recevices the labeled packet
from the input port. By taking the label in the packet as the pointer, the ILSR
checks up the forwarding routing table. It takes out the new label, and then
replaces the old one in the packet with it. The newly labeled packet is output to
the next hop from the specified port.
When the IP packet arrives at the hop before the MPLS Egress, the second hop
counting backwards, the label in the packet is not switched; it is just popped
out of the packet. Then the packet without label is forwarded. For the Egress is
the output port for the destination address; and it is unnecessary to forward the
packet according to the label. The Egress reads out the header of packet directly
and forwards the packet to the final destination address. This processing mode
ensures that all LSRs observe and process the packet to be dealt with only once
during the whole MPLS running procedure, and facilitate the layered
processing of forwarding function.
Step 3: After receiving the IP packet without label, the Egress LSR in the
MPLS domain reads out the packet header, and outputs the IP packet from the
specified port according to the final destination address.
The explanations of the example in Fig. 5.2-1 are as follows.
The terminal I is connected to ELSR A, and the terminal II connected to ELSR
B. There is a label switch path LSP (A > R1 > R2 > R4 > R6 > B) from A to B.
The IP packet from the terminal I to II is mapped into the special FEC BA.
The label allocation along the LSP is:
A FEC B
R1 = LA, R1 FEC B
R2 = L1, R2 FEC B
R4 = L2, R4 FEC B
R6 = L4, R4 FEC B
B = Null (no lable)
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The label allocation completes in the Phase One, which can be intervened by
the management layer. Corresponding forwarding routing tables forms on each
LSR, as shown in Fig. 5.2-2.
ILabel FECS
0Port
0Label
FECBA 1 L
ILabel
IPort FECS
0Port
L 1 FECBA 2
0Label
L
A R1
R6 B
ILabel
IPort FECS
0Port
L 4 FECBA 1
0Label
Perform L3IP routing table
1 1
4 1 1
2
Fig. 5.2-2 MPLS Forwarding
The forwarding of IP packet from I to II is carried out in three steps:
Step 1: The IP packet being sent from the terminal I to A is a pure packet
without label. The ELSR A reads out the IP packet header and analyzes it, then
look up the FEC BA to which the packet is mapped. After that, the ELSR A
reads out the label LA and the output port 1, and then encapsulates the label LA
and the IP packet as a labeled packet. The labeled IP packet is sent out from the
output port 1 of the ELSR A.
Step 2: The packet forwarded from R1 to B is the labeled one. The next hop of
A ILSR R1 receives the labeled packet from the input port 1 and reads out the
label LA as the pointer. The R1 finds out the new label L1 and output port 2
from its forwarding routing table, and then replaces the lable LA with the L1.
The packet with the new label is sent out from the output port 2 of the R1.
The processing procedure on the ILSR R2 and R4 is same as that of the R1.
When the IP packets reaches the R6, the old label L4 is popped out and no new
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label will be inserted in. The R6 sends out the null-label IP packet from the
output port 1.
Step 3: The packet forwarded from B to the terminal II is a pure packet. After
receiving the null-label IP packet from the input port 1, the ELSR B reads out
the packet header directly and sends the IP packet to the terminal II according
to the destination address.
5.2.2 Advantages of MPLS
1. Compared with common route networks, the MPLS has the following
advantages:
● Simplifying the forwarding procedure
The MPLS can forward IP packets directly according to labels, while IP packets
are forwarded through the longest match lookup algorithm in route networks.
Obviously, the MPLS forwarding mechanism is simpler, which means the
MPLS can be used to implement the routing forwarding more efficiently with
lower cost.
● High-efficiency explicit route
The explicit route is a path specified by the source host which leads to the
destination address over the Internet. The explicit route is also called as the
source route. It is a technology with powerful functions which can be used for
various purposes. When the source route is used for network testing in common
route networks, it is forbidden for IP packets to carry the whole explicit route
information while transferring pure data packets. In MPLS, the LSP is allowed
carrying the whole explicit route information when it is established, and it is
unnecessary for each IP packet to carry it. That means the MPLS can employ
the explicit route in practice and make full advantages of the advanced
characteristics of explicit route.
● Traffic engineering
Traffic engineering is a selection procedure of traffic flow routing. It is used to
balance the traffic flow on various links, routers and switches in the network
based on a special rule. The traffic engineering is very important when there are
multiple parallel or alternate pathes available between any pair of nodes in the
network. In recent years, the rapid development of Internet, especially the
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increasing demand of bandwidth, make some core networks have to adapt
themselves to more and more fork networks. Therefore, the traffic engineering
becomes more important.
Today, the IP over ATM is implemented through Permanent Virtual Circuits
(PVCs), which are always configured manually. Therefore the typical mode of
traffic engineering is manual allocation in IP over ATM networks.
The traffic engineering is difficult to carry out in common route networks. The
load equilibrium can be achieved to a certain extent through adjusting
measurements related to links in the network. Using this method to meet the
requirements of traffic engineering is resticted in many aspects. Because there
are lots of alternate pathes between nodes in the network, it is difficult to get
equilibrium traffic flow through adjusting the data packet route measurements
of each hop.
The MPLS provides a direct mechanism of measure for each pair of input and
output node. It allows labeling the data flow respectively from the special input
node to the special output node. Besides, the MPLS allows establishing the
high-efficiency explicit route of LSP, which ensures some special data flow can
be forwarded through the optimal path directly. The most difficult part of the
traffic engineering implementation is the selection of each LSP route. The
MPLS can get over it through configuring routes manually, and recalculating by
using routing protocols to notify passing routes according to traffic flow load in
the network and then allocating the traffic flow.
● Quality of Service (QoS)
The QoS route refers to a routing method which is used to select the route for a
special data flow. The selected route should satisfy the QoS requirements of the
special data flow. In many cases, the QoS route adopts the explicit route for the
most important item of QoS route is bandwidth guarantee, which is same as
that of traffic engineering.
● Various service classes
The demand of some users for more special services on the Internet increases
day by day. For example, the source address, destination address, input
interface and other characteristics of the IP packet being forwarded should be
known for services provided by some Internet Service Providers (ISPs). It is
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impossible for a medium ISP to get all information needed from routers on the
network. Besides, it is difficult to get some information on routers, such as the
information of input interface, except on the ingress node on the network. The
best method to configure the COS and QoS is mapping IP packets to the most
proper COS and QoS class on the network and ingress node, and identifying
these IP packets with some mode.
MPLS can provide an effective method to identify any special IP packet related
to COS and QoS. The mappling from the IP packet to a special FEC is
completed on the ingress node in MPLS domain. The MPLS makes it easy to
mapping IP packets to proper COS and QoS classes, which is difficult for other
modes.
● Function division
MPLS must support the convergence and forwarding of data flow. The label has
the granularity characteristic. It can identify one original user data flow at least,
while identify one data flow converged from all data flows from switches or
routers at most. In this way, the route processing functions can be classified and
allocated to different network units. For example, the edge node close to users
in the network is configured with complex processing functions; while the
configuration of the core part in the network should be as simple as possible,
adopting the forwarding mode with pure label.
● Unified forwarding mode for different service types
MPLS can provide various service types on the same network with the unified
forwarding mode, such as IP services, FR services, ATM services, Tunning
Protocol (TP) services and VPN services etc.
2. Compared with ATM networks and FR networks, the MPLS has the following
advantages:
● Flexibiligy of routing protocols
In the core network of IP over ATM, n2 logical links should be established
while connecting routers on the peer layer. In MPLS, the necessary
communication of each router on the peer layer decreases to that of the router
connected to it directly. The highest capacity needed for transport and switching
processing is required according to the 0 (n) flow in the whole network.
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● General operations on data packets and cell medium
MPLS adopts the unified method for the routing and forwarding on packet and
cell medium, which allows using unified methods for traffic engineering, QoS,
COS and other performance and function requirements. That means that the
same label can be used on ATM, FR and other link layer mediums.
● Easy management
The management of MPLS networks is expected to be simplified by using
general routing protocols and label allocation methods for various mediums.
● Elimination of routing storm
MPLS eliminates the necessity of using Next-Hop Resolution Protocol (NHRP)
and establishing Switched Virtual Circuit (SVC) directly according to demands,
and therefore solves the SVC contentation problem caused by updating routes.
It also solves the delay problem related to direct establishment of SVC.
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Appendix A Abbreviations
Abbreviation Full Name
AFR Absolute Frequency Reference
AFEC Advanced FEC
AIS Alarm Indication Signal
APR Automatic Power Reduction
APS Automatic Protection Switching
APSD Automatic Power Shutdown
APSF Automatic Protection Switching for Fast Ethernet
ASE Amplified Spontaneous Emission
AWG Array Waveguide Grating
BER Bit Error Ratio
BLSR Bidirectional Line Switching Ring
BSHR Bidirectional Self-Healing Ring
CDR Clock and Data Recovery
CMI Code Mark Inversion
CODEC Code and Decode
CPU Center Process Unit
CRC Cyclic Redundancy Check
DBMS Database Management System
DCC Data Communications Channel
DCF Dispersion Compensation Fiber
DCG Dispersion Compensation Grating
DCN Data Communications Network
DCM Dispersion Compensation Module
DCF Dispersion Compensating Fiber
DDI Double Defect Indication
DFB-LD Distributed Feedback Laser Diode
DSF Dispersion Shifted Fiber
DGD Differential Group Delay
DTMF Dual Tone Multi-Frequence
DWDM Dense Wavelength Division Multiplexing
DXC Digital Cross-connect
EAM Electrical Absorption Modulation
ECC Embedded Control Channel
EDFA Erbium Doped Fiber Amplifier
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Abbreviation Full Name
EFEC Enhanced FEC
EX Extinction Ratio
FDI Forward Defection Indication
FEC Forward Error Correction
FPDC Fiber Passive Dispersion Compensator
FWM Four Wave Mixing
GbE Gigabits Ethernet
GUI Graphical User Interfaces
IP Internet Protocol
LD Laser Diode
MDI Multiple Document Interface
MCU Management and Control Unit
MOADM Metro Optical Add/Drop Multiplexer Equipment
MBOTU Sub-rack backplane for OTU
MQW Multiple Quantum Well
MSP Multiplex Section Protection
MST Multiplex Section Termination
NCP Net Control Processor
NDSF None Dispersion Shift Fiber
NE Network Element
NNI Network Node Interface
NMCC Network Manage Control Center
NRZ Non Return to Zero
NT Network Termination
NZDSF Non-Zero Dispersion Shifted Fiber
OA Optical Amplifier
OADM Optical Add/Drop Multiplexer
OBA Optical Booster Amplifier
Och Optical Channel
ODF Optical fiber Distribution Frame
ODU Optical Demultiplexer Unit
OGMD Optical Group Mux/DeMux Board
OHP Order wire
OHPF Overhead Processing Board for Fast Ethernet
OLA Optical Line Amplifier
OLT Optical Line Termination
OMU Optical Multiplexer Unit
ONU Optical Network Unit
OP Optical Protection Unit
Appendix A Abbreviations
79
Abbreviation Full Name
OPA Optical Preamplifier Amplifier
OPM Optical Performance Monitor
OPMSN Optical Protect for Mux Section(without preventing resonance switch)
OPMSS Optical Protect for Mux Section(with preventing resonance switch)
OSC Optical Supervisory Channel
OSCF Optical Supervision channel for Fast Ethernet
OSNR Optical Signal-Noise Ratio
OTM Optical Terminal
OTN Optical Transport Network
OTU Optical Transponder Unit
OXC Optical Cross-connect
PDC Passive Dispersion Compensator
PMD Polarization Mode Dispersion
PDL Polarization Dependent Loss
RZ Return to Zero
SBS Stimulated Brillouin Scattering
SDH Synchronous Digital Hierarchy
SDM Supervision add/drop multiplexing board
SEF Severely Error Frame
SES Severely Error Block Second
SFP Small Form Factor Pluggable
SLIC Subscriber Line Interface Circuit
SMCC Sub-network Management Control Center
SMT Surface Mount
SNMP Simple Network Management Protocol
SPM Self-Phase Modulation
SRS Stimulated Raman Scattering
STM Synchronous Transfer Mode
SWE Electrical Switching Board
TCP Transmission Control Protocol
TFF Thin Film Filter
TMN Telecommunications Management Network
VOA Variable Optical Attenuator
WDM Wavelength Division Multiplexing
XPM Cross-Phase Modulation