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AsynchronousAsynchronous Transfer ModeTransfer Mode
(ATM)(ATM)
Issues Driving LAN ChangesIssues Driving LAN Changes
• Traffic Integration
– Voice, video and data traffic
– Multimedia became the ‘buzz word’
• One-way batch Web traffic
• Two-way batch voice messages
• One-way interactive Mbone broadcasts
• Two-way interactive video conferencing
• Quality of Service guarantees (e.g. limited jitter, non-blocking streams)
• LAN Interoperability
• Mobile and Wireless nodes
Stallings “High-Speed Networks”
Stallings “High-Speed Networks”
A/D
Voice
s1 , s2 …
Digital voice samples
A/D
Video
… Compression
compressed
frames picture frames
Data
Bursty variable-length
packets
cells
cells
cells
AAL
AAL
AAL
ATM Adaptation Layers
MUX
`
Wasted bandwidth
ATM
TDM
4 3 2 1 4 3 2 1 4 3 2 1
4 3 1 3 2 2 1
Voice
Data
packets
Images
Asynchronous Transfer Mode Asynchronous Transfer Mode (ATM)(ATM)
ATMATM • ATM standard (defined by CCITT) is widely
accepted by common carriers as mode of operation
for communication – particularly BISDN.
• ATM is a form of cell switching using small fixed-
sized packets.
Header Payload
5 Bytes 48 Bytes
Basic ATM Cell Format
ATM Conceptual ModelATM Conceptual Model Four Assumptions Four Assumptions
1. ATM network will be organized as a hierarchy.
User’s equipment connects to networks via a UNI (User-Network Interface).
Connections between provided networks are made through NNI (Network-Network Interface).
2. ATM will be connection-oriented.
A connection (an ATM channel) must be established before any cells are sent.
X
X
X
X
X
X
X
X
X
Private
UNI
Public
UNI
NNI
Private
NNI
Private ATM
network
Public
UNI B-ICI
Public ATM
network A
Public ATM
network B
Figure 9.5
ATM ConnectionsATM Connections
• two levels of ATM connections:
virtual path connections
virtual channel connections
• indicated by two fields in the cell header:
virtual path identifier VPI
virtual channel identifier VCI
Physical Link
Virtual Paths
Virtual Channels
ATM Virtual Connections ATM Virtual Connections
ATM Conceptual ModelATM Conceptual Model Assumptions Assumptions
3. Vast majority of ATM networks will run on
optical fiber networks with extremely low error rates.
4. ATM must support low cost attachments. • This decision lead to a significant decision – to
prohibit cell reordering in ATM networks.
ATM switch design is more difficult.
GFC (4 bits) VPI (4 bits)
VPI (4 bits) VCI (4 bits)
VCI (8 bits)
VCI (4 bits) PT (3 bits) CLP
(1 bit)
HEC (8 bits)
ATM cell
header
Payload
(48 bytes)
UNIUNI Cell FormatCell Format
2
3
N
1 Switch
N
1 …
5
6
video 25
video
voice
data
32
32 61
25
32
32
61
75
67
39
67
N
1
3
2
video 75
voice 67
data 39
video 67
… …
ATM Cell Switching ATM Cell Switching
c ATM
Sw
1
ATM
Sw
4
ATM
Sw
2
ATM
Sw
3
ATM
DCC
a b
d e
VP3 VP5
VP2
VP1
a
b c
d e Sw = switch
Digital Cross Connect
Only switches virtual paths
ATMATM Protocol ArchitectureProtocol Architecture
• ATM Adaptation Layer (AAL) – the protocol for packaging data into cells is collectively referred to as AAL.
• Must efficiently package higher level data such as voice samples, video frames and datagram packets into a series of cells.
Design Issue: How many adaptation layers should there be?
Plan
e man
agem
ent
Management plane
Control plane User plane
Physical layer
ATM layer
ATM adaptation layer
Higher layers Higher layers
Lay
er man
agem
ent
AAL
ATM
User
information
User
information
AAL
ATM
PHY PHY
ATM
PHY
ATM
PHY
…
End system End system Network
OriginalOriginal ATM Architecture ATM Architecture
• CCITT envisioned four classes of
applications (A-D) requiring four distinct
adaptation layers (1-4) which would be
optimized for an application class:
A. Constant bit-rate applications CBR
B. Variable bit-rate applications VBR
C. Connection-oriented data applications
D. Connectionless data application
ATM ArchitectureATM Architecture An AAL is further divided into:
The Convergence Sublayer (CS)
manages the flow of data to and from SAR sublayer.
The Segmentation and Reassembly Sublayer
(SAR)
breaks data into cells at the sender and reassembles
cells into larger data units at the receiver.
Original ATM Architecture
Transmission
convergence
sublayer
Physical medium
dependent sublayer
Physical
medium
ATM layer
Physical layer
• The AAL interface was initially defined as classes A-D
with SAP (service access points) for AAL1-4.
• AAL3 and AAL4 were so similar that they were
merged into AAL3/4.
• The data communications community concluded that
AAL3/4 was not suitable for data communications
applications. They pushed for standardization of AAL5
(also referred to as SEAL – the Simple and Efficient
Adaptation Layer).
• AAL2 was not initially deployed.
OriginalOriginal ATM ArchitectureATM Architecture
Revised ATM Architecture
RevisedRevised ATM Service CategoriesATM Service Categories
Class Description Example
CBR Constant Bit Rate T1 circuit
RT-VBR Real Time Variable Bit
Rate
Real-time
videoconferencing
NRT-VBR Non-real-time Variable Bit
Rate
Multimedia email
ABR Available Bit Rate Browsing the Web
UBR Unspecified Bit Rate Background file
transfer
QoSQoS, , PVCPVC, and , and SVCSVC • Quality of Service (QoS) requirements are
handled at connection time and viewed as part of signaling.
• ATM provides permanent virtual connections and switched virtual connections.
– Permanent Virtual Connections (PVC)
permanent connections set up manually by network manager.
– Switched Virtual Connections (SVC)
set up and released on demand by the end user via signaling procedures.
AAL ServicesAAL Services
Connectionles
s
Date Transfer
Bit Rate
Connection
Mode
Examples
of Services
Circuit
Emulation
Constant Bit
Rate
Video
and Audio
Constant Variable
Connection Oriented Connectionle
ss
Variable Bit
Rate Video and Audio
Connection-
oriented
Data
Transfer
AAL
TYPE AAL 1 AAL 2
AAL 3/4
AAL 5
Class A Class B Class C Class D
Related Not Related
Service
Timing
between
Source and
Destination
AAL3/4
AAL 5
AAL Type 1 AAL Type 1 ProtocolProtocol
• Supports Class A traffic, i.e., constant bit rate data with specific requirements for
delay, delay jitter, and timing, e.g., PCM voice, CBR video, and emulation of T-
carrier circuits (DS1, DS3)
• Receives constant bit rate stream with a well defined clock from source and
delivers the same to the destination
• Provides for timing recovery (using SRTS), synchronization, and indication of
lost information not recovered by AAL1
• Summary of AAL 1 functions
– Segmentation and reassembly of user information
– Handling of cell delay variation
– Handling of cell payload assembly delay
– Handling lost and misinserted cells
– Recovery of sending clock frequency at receiver
– Checking and handling AAL PCI (header) error
AAL Type 1 SARAAL Type 1 SAR--PDUPDU • Consists of 1 octet header (PCI) and 47 octets of payload
• Sequence Number (SN): A 1-bit Convergence Sublayer
Indication and 3-bit sequence count to detect deletion or
misinsertion of cells
• Sequence Number Protection (SNP): 3-bit CRC with even
parity for detecting and correcting SN error
SN SNP SAR-PDU Payload
4 bits 4 bits 47 Octets
AAL Type 1 Sequence Number and Sequence Number AAL Type 1 Sequence Number and Sequence Number ProtectionProtection
• The 4 bit RTS is transferred by the CSI bit in successive
SAR-PDU headers with an odd SN (SN = 1, 3, 5, 7)
• For P format operations, the CSI value in SAR-PDU headers
with an even SN (SN = 0, 2, 4, 6) is set to 1
Cyclic Redundancy Check Even Parity
1 bit 3 bit
1 bit 3 bits
Convergence Sublayer
Indication (CSI) Sequence Number (SN)
… Higher layer User data stream
Convergence
sublayer
SAR sublayer
ATM layer
CS PDUs
SAR PDUs
ATM Cells
47 47 47
1 47 1 47 1 47
H H H
5 48
H
5 48
H
5 48
H
b1 b2 b3
AAL 1
AAL Type 1 CS PDUAAL Type 1 CS PDU
• Two CS PDU formats
– Non-P format: No CS header (CS PCI), 47 octet user
information for transfer of unstructured data such as
circuit emulation of full DS1 or DS3
– P format: 1 octet header (Structure Pointer SP), and 46
octet user information for transfer of octet-aligned data
such as N x 64 kbps (e.g., fractional DS1) services
47 Octets AAL User Information
CS-PDU Non-P Format
CS-PDU P Format
46 Octets AAL User Information1-Octet SP
AAL Type 2 Protocol AAL Type 2 Protocol
• Designed to support Class B (VBR) traffic
• Supports variable bit rate data where a strong timing
relationship between source and destination is required, e.g.,
VBR audio and video
• Data passed to AAL2 from higher layers at the source at
fixed intervals and must be passed to the destination at the
same rate
• The amount of data passed to AAL2 may vary with each
transfer
• Supports voice compression and silence suppression
• Supports idle voice channel deletion
• Supports multiple user channels with varying bandwidth on a
single ATM connection
AAL Type 2 Voice Application ExamplesAAL Type 2 Voice Application Examples
• PBX-to-PBX trunking for compressed voice
• ATM trunking on public-switched telephone network
• ATM backbone for cellular systems and personal
communications services (PCS)
• ATM backbone connectivity to packet telephone
PBXPBX--toto--PBX Connectivity PBX Connectivity
Compression
Silence Suppression
Packetization
PBX
PCM Voice
Compression
Silence Suppression
Packetization
PBX
PCM Voice
AAL
ATM
PHY
ATM Network
AAL
ATM
PHY
AAL Type 2 PacketsAAL Type 2 Packets
• AAL2 provides bandwidth-efficient transmission of low-
rate, short, and variable-size packets for delay sensitive
applications
• AAL2 uses one ATM connection between two points to
carry packets from multiple native connections
• The ATM payloads from successive cells of the ATM
connection are used as a byte stream on which packets
from different native channels, called logical link channels
(LLCs), are packed without regard to the cell boundaries
• A channel identification (CID) field is used in the packet
header to identify the LLC to which a packet belongs
• A length indicator (LI) field is used to identify the
boundaries of variable-length LLC packets
AAL Type 2 Protocol AAL Type 2 Protocol SublayersSublayers
Service Specific Convergence Sublayer
(SSCS)
Common Part Sublayer (CPS)
AAL 2 Common Part AAL 2 Common Part SublayerSublayer
• Defines an end-to-end AAL connection as a concatenation of
AAL2 channels
• Each AAL2 channel is a bi-directional virtual channel, with the
same channel identifier value for both directions
• AAL2 channels are established over an ATM layer PVC, SPVC,
or SVC
• Provides basic structure for identifying the users of the AAL
• Assembling/disassembling the variable payload associated with
each individual user
• Error detection and correction
• Multiplexing multiple AAL channels (merging multiple streams of
CPS packets) onto a single ATM connection
• Provides QoS through the choice of AAL-SAP for data transfer
AAL 2 StructureAAL 2 Structure
SSCS-PDU Header
(if present)
SSCS-PDU Trailer
(if present) SSCS-PDU Payload
SSCS-PDU
Packet Header
(PH)
Packet Payload
(PP)
Packet
Start Field
(STF) CPS-PDU Payload
CPS-PDU
Cell Header Cell Payload
Service Specific
Convergence
Sublayer (SSCS)
Common Part
Sublayer (CPS)
ATM Layer
AAL-SAP
ATM-SAP
Format of AAL2 PacketFormat of AAL2 Packet
• Channel Identification (CID): Uniquely identifies the individual user
channel (LLC) within the AAL2, and allows up to 248 (8 - 255)
individual users within each AAL2 structure.
• Length Indicator (LI): Identifies the length of the LLC packet
associated with each individual user, and assures conveyance of
variable payload.
• User-to User Indication (UUI): Provides a link between CPS and an
appropriate SSCS that satisfies the higher layer application. Different
SSCS protocols may be defined to support specific AAL2 user
services, or groups of services. The SSCS may also be null.
CID
8 bits
Information
1 to 45/64 octets
LI
6 bits
UUI
5 bits
HEC
5 bits
Packet Header (PH) Packet Payload (PP)
Packet
Format of AAL2 CPSFormat of AAL2 CPS--PDUPDU
• Packets are combined into CPS-PDU payload
• The Offset Field (OSF) identifies the location of the start of the
remaining length of the packet that possibly started in the
preceding cell and is continuing in the current cell
• Data integrity is protected by the Sequence Number (SN)
• The Start Field is protected from error by a Parity bit (P)
• When it is necessary to transmit a partially filled cell to limit
packet emission delay, the remainder of the cell is padded with
all zero octets
OSF
6 bits CPS Information
SN
1 bit P
1 bit
PAD
0 to 47 octets
Start Field CPS-PDU Payload
CPS -PDU
Cell Header
5 octets
AAL 2 Service Specific Convergence AAL 2 Service Specific Convergence SublayerSublayer
• SSCS is the link between the AAL2 CPS and the
higher layer applications of the individual AAL2
users
• Standards for SSCS are being developed in ITU-T
and ATM Forum
• A null SSCS satisfies most mobile voice
applications
AAL Type 3/4 ProtocolAAL Type 3/4 Protocol
• Supports variable bit rate data where there is no timing relationship
between source and destination, e.g., X.25, frame relay, and
TCP/IP data
• Supports Class C (connection-oriented) and Class D
(connectionless) traffic
• Convergence sublayer divided into two parts:
– Common Part Convergence Sublayer (CPCS)
– Service Specific Convergence Sublayer (SSCS)
• SSCS layer may provide assured or non-assured services, or may
be null
– Assured service provides retransmission of missing or corrupted
SSCS-PDUs and flow control is mandatory
– AAL-SDUs may be lost or corrupted for non-assured service
and flow control is optional
• CPCS provides message mode or streaming mode services
AAL Type 3/4 Protocol AAL Type 3/4 Protocol SublayersSublayers
Service Specific Convergence Sublayer (SSCS)
Common Part Convergence Sublayer (CPCS)
Segmentation and Reassembly Sublayer (SAR)
AAL Type 3/4 CPCSAAL Type 3/4 CPCS--PDUPDU
4-octet header and 4-octet trailer
CPI Common Part Indicator
Btag Beginning Tag
BASize Buffer Allocation Size
PAD Padding
AL Alignment
Etag End Tag
Length Length of CPCS-PDU Payload
CPI Btag BASize CPCS-PDU Payload PAD AL Etag Length
0 - 3 1
1
2
1
1
2
AAL Type 3/4 SARAAL Type 3/4 SAR--PDUPDU
2-octet header, 44-octet payload, and 2-octet trailer
ST Segment Type
SN Sequence Number
MID Multiplexing Identifier
LI Length Indication
CRC-10 CRC Code
SN MID SAR-PDU Payload LI CRC-10 ST
2 bits 4 bits 10 bits 352 bits 10 bits 6 bits
AAL Type 5 ProtocolAAL Type 5 Protocol
• AAL5 is a simple and efficient AAL (SEAL) to perform a
subset of the functions of AAL3/4
• The CPCS-PDU payload length can be up to 65,535 octets
and must use PAD (0 to 47 octets) to align CPCS-PDU length
to a multiple of 48 octets
PAD Padding
CPCS-UU CPCS User-to-User Indicator
CPI Common Part Indicator
Length CPCS-PDU Payload Length
CRC-32 Cyclic Redundancy Chuck
CPCS-PDU Payload PAD CPI Length
1 2 40 - 47
CPCS
UU
1
CRC-32
AAL Type 5 SARAAL Type 5 SAR--PDUPDU
PAD CPCS-PDU
Trailer
CPCS-PDU
SAR-PDU
Payload
CPCS-SDU
SAR-PDU
Payload
SAR-PDU
Payload
SAR-PDU
Payload SAR-PDU
Payload
SAR-PDU SAR-PDU SAR-PDU SAR-PDU SAR-PDU
CPCS-PDU Payload
AAL Type 5 ProtocolAAL Type 5 Protocol
• The CPCS-PDU is divided into 48 octets SAR-PDUs
• Since CPCS-PDU is 48-octet aligned, there is no need for a
length field in the SAR-PDU
• The AAL5 SAR-PDU is 48 octets of data with no overhead of
SAR-PDU header or trailer
• The PTI field of the cell header identifies the beginning or end of
the CPCS-PDU
– PTI = 0X1: End-of-Message (EOM)
– PTI = 0X0: Beginning-of-Message (B0M), or Continuation-
of-Message (COM)
5050
ATM NETWORKSATM NETWORKS
• Public ATM Network:
– Provided by public telecommunications carriers (e.g., AT&T, MCI WorldCom, and Sprint)
– Interconnects private ATM networks
– Interconnects remote non-ATM LANs
– Interconnects individual users
• Private ATM Network:
– Owned by private organizations
– Interconnects low speed/shared medium LANs (e.g., Ethernet, Token Ring, FDDI) as a backbone network
– Interconnects individual users as the front-end LAN for high performance or multimedia applications
5151
Switches in
the middle
End systems
of ATM
5252
Public
ATM Network
Token Ring
Token Ring
FDDI
FDDI
Mainframe Computer
Video
Video
Video
Ethernet
Ethernet
Mainframe Computer
Edge Switch
Ethernet
File Server
Private ATM
Switch
Edge Switch
Edge Switch
Edge Switch
PBX
PBX
Voice
Voice
Private ATM
Network
FDDI
5353
ATM InterfacesATM Interfaces
•
Private UNI
Private
ATM LAN
Public UNI
Public
ATM Network
Public
ATM Network
B-ICI
Private
ATM WAN
P-NNI
5454
How ATM Works?How ATM Works?
• ATM is connection-oriented -- an end-to-end connection must be established and routing tables setup prior to cell transmission
• Once a connection is established, the ATM network will provide end-to-end Quality of Service (QoS) to the end users
• All traffic, whether voice, video, image, or data is divided into 53-byte cells and routed in sequence across the ATM network
• Routing information is carried in the header of each cell
• Routing decisions and switching are performed by hardware in ATM switches
• Cells are reassembled into voice, video, image, or data at the destination
5555
ATM Network
H
H
H H H
H
H
H
Voice Video Data Voice Video Data
BISDN Services
BISDN Services
Reassembly
User Applications User Applications
Workstation Workstation
Multiplexing Demultiplexing
H H H H H H
Segmentation