NETE0510: Communication Media and Data Communications 1 NETE0510 ATM Dr. Supakorn Kungpisdan [email protected]

NETE0510: Communication Media and Data Communications

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NETE0510ATM

Dr. Supakorn [email protected]


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Outline

Overview Cells Segmentation and Reassembly Virtual Paths Physical Layers for ATM ATM in the LAN


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ATM Overview

Asynchronous Transfer Mode ATM is a competing technology with Ethernet switching,

but the areas of application for these two technologies only partially overlap

ATM is a connection-oriented, packet-switched technology Use virtual circuits

Connection phase is called “signaling” The main signaling protocol is known as Q.2931

Q.2931 Discover a suitable route across an ATM network Responsible for allocating resources at the switches among the

circuit This is to ensure the circuit a particular quality of service (QoS),

one of the greatest ATM strengths


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ATM Overview (cont’d)

ATM has specific format of identifying addresses: E.164 and NSAP (network service access point), different from MAC address in LANs

Packets that are switched in ATM has fixed length: 53 bytes 5 bytes of header followed by 48 bytes of payload

To distinguish between fixed-length packets and variable-length packets, they are given a special name “cells”


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Outline



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Cells

Variable-length packets are normally constrained to fall within some bounds The lower bound is set by the minimum amount of

information that needs to be contained in the packet Typically a header with no optional extensions

The upper bound may be set by a variety of factors Maximum FDDI packet size

• How long each station is allowed to transmit without passing on the token, and thus determine how long a station might have to wait for the token to reach it

Cell are both fixed-length and small in size


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Cell Size

Cell has fixed length in order to facilitate the implementation of hardware switches

When ATM was being created in the mid- and late 1980s, 10-Mbps was the cutting-edge technology in terms of speed.

To go much faster, most people thought in terms of hardware

Fixed-length packets turn out to be very helpful thing if want to build fast, highly scalable switches


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Cell Size (cont’d)

Two main reasons: Easier to build hardwa5re to do simple jobs,

and the job of processing packets is simple when you already know how long each one will be

If all packets are the same length, then you can have lots of switching elements all doing much the same thing in parallel, each of them taking the same time to do its job


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The second reason improves scalability of switch designs Ease the task of building such hardware and that there

was a lot of knowledge available about how to build cell switches in hardware at the time the ATM standards were being defined

Fixed-length cell is also good for queuing Queues build up in a switch when traffic from several

inputs may be heading for a single output Once you extract a packet from a queue and start

transmitting it, you need to continue until the whole packet is transmitted


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The longest time that a queue output can be tied up is equal to the time it takes to transmit a maximum-sized packet Tqueue = Ttrans-max-size-packet

Fixed-length cell means that a queue output is never tied up for more than the time it takes to transmit one cell, which is almost certainly shorter than the maximum-sized packet on a variable-length packet network Tqueue-cell ≤ Ttrans-max-size-packet

If tight control over the latency that is being experienced by cells is important, cells provide some advantage


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A network with variable-length packets, Maximum packet size is 4kB Link speed is 100 Mbps

Thus the time to transmit a max-size packet is 4096 x 8 /100M = 327.68 µs

A high-priority packet that arrives just after the switch starts to transmit a 4-kB packet will have to wait in a queue 327.68 µs

In contrast, if the switch were forwarding 53-byte cells, the longest wait would be 53 x 8 /100M = 4.24 µs

Also queues of cells also tend to be a little shorter than queues of packets When a packet begins to arrive in an empty queue, switch has to wait

for the whole packet to arrive before it can start transmitting the packet on an outgoing link

The link sits idle when the packet arrives Large-size packet VS small-size cell


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How large is a cell size?

If the size is too short, the amount of header relative to the amount of data gets larger

Percentage of link bandwidth used to carry data goes down If you build up a device that processes cells at some max number of

cells per second, then as cells get shorter, the total data rate drops in direct proportion to cell size E.g. network adapter that reassembles cells into larger units before

handling them up to the host The performance of such device depends on cell size

If the cell is too big, there is a problem of waste bandwidth caused by the need to pad transmitted data to fill a complete cell If the payload size is 48 bytes and you want to transmit 1 byte of data

need to pad 47 bytes This lowers link utilization

53-byte size were chosen, 5 bits of header and 48 bits of payload


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Cell Format

Two different cell formats UNI (user-network interface): between a telephone company and

its user NNI (network-network interface): between a pair of telephone

companies The only significant difference in cell formats is that the NNI format

replaces the GFC fielded with 4 extra bits of VPI Understanding all the three-letter acronyms (TLAs) is a key part of

understanding ATM

ATM Cell Format at the UNI


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ATM Cell Format (cont’d)

GFC (Generic Flow Control) 4 bits, not widely used, were intended to have local

significance at a site and could be overwritten in the network and could be overwritten in the network

To provide a means to arbitrate access to the link if the local site used some shared medium to connect to ATM

VPI (Virtual Path Identifier) and VCI (Virtual Circuit Identifier) 8-bit VPI and 16-bit VCI Both are used to identify a virtual connection


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Type 3 bits 8 possible values 4 of them, where the 1st bit is set, relate to

management functions If the 1st bit is not set, the cell contains user data The 2nd bit is the “explicit forward congestion

indication” (EFCI) bit Used for congestion control in conjunction with the available

bit rate (ABR) service set by the congested switch to tell an end node that it is congested

The 3rd bit is the “user signaling” bit Used in conjunction with ATM adaptation Layer 5 to

delineate frames


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CLP (cell loss priority) A user or network element may set this bit to indicate cells that

should be dropped preferentially in the event of overload E.g. a video coding application could set this bit for cells that

would not dramatically degrade the quality of the video if dropped

A network element might set this bit for cells that have been transmitted by a user in excess of the amount that was negotiated

CRC Known as header error check (HEC), use CRC-8 Provide error detection and single-bit error correction capability

on the cell header only Important because error in the VCI will cause the cell to be

misdelivered


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Outline



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Segmentation and Reassembly

The packet handed down from high-level protocol are often larger than 48 bytes need fragmentation and reassembly In ATM, often called segmentation and reassembly (SAR)

To deal with this, a protocol was added in between ATM and the variable-length packet protocols that might use ATP, such as IP This layer is called ATM Adaptation Layer (AAL)

AAL header contains the information needed by the destination to reassemble the individual cells back into the original message


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AAL and ATM

Segmentation and reassembly in ATM


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AALs

ATM supports all sorts of services, including voice, video, and data Need different AALs

4 adaptation layers was originally defined: 1 and 2 designed to support applications, like voice, that require

guaranteed bit rates 3 and 4 support packet data running over ATM

AAL3 used by connection-oriented packet services (e.g. X.25) AAL4 used by connectionless services (e.g. IP)

Later 3 and 4 were combined into AAL3/4 Some perceived shortcomings in AAL3/4 caused AAL5

Now there are 4 AALs: 1, 2, 3/4, and 5


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ATM Adapter Layer 3/4

Main function of AAL3/4 is to provide enough information to allow variable-length packets to be transported across the ATM network as a series of fixed-length cells

A packet is this layer is called “protocol data unit (PDU)”

The task of segmentation/reassembly involves two different packet formats: Convergence sublayer PDU (CS-PDU): defines a way of

encapsulating variable-length PDUs prior to segmenting them into cells

The PDU passed down to the AAL is encapsulated by adding a header and a trailer, and the resultant CS-PDU is segmented into ATM cells


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AAL3/4 CS-PDU Packet Format

CPI indicates which version of the CS-PDU format is in use Only the value 0 is currently defined

Beginning tag (Btag) is supposed to match the end tag (Etag) for a given PDU Protect against the situation in which the loss of the last cell of one PDU

and the first cell of another causes two PDUs to be inadvertently joined into a single PDU and passed up to the next layer in the protocol stack

Buffer allocation size (BASize) is not necessarily the length of the PDU Supposed to be a hint to the reassembly process as to how much buffer

space to allocate for the reassembly

ATM AAL3/4 packet format


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AAL3/4 Packet Format (cont’d)

Before adding the CS-PDU trailer, the user data is padded to one byte less than a multiple of 4 bytes, by adding up to 3 bytes of padding Ensure that the trailer is aligned on a 32-bit boundary, making for

more efficient processing The CS-PDU trailer contains the Etag and the real length of the

PDU (Len) AAL3/4 specifies a header and trailer that are carried in each cell

The CS-PDU is segmented into 44-byte chunks plus 4 bytes of its header and trailer bring up to 48 bytes

Then carried as the payload of an ATM cell

ATM cell format for AAL3/4


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Type The first two bits of the AAL3/4 header contain Type field

indicating is this is the first cell of a CS-PDU, the last cell of a CS-PDU, a cell in the middle of a CS-PDU, or a single-cell PDU (in which case it is both first and last)


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SEQ (sequence number) Detect cell loss or misordering so that reassembly can be

aborted MID (multiplexing identifier)

Multiplex several PDUs onto a single connection Length

Show the number of bytes of PDU that are contained in the cell

Must equal 44 for BOM and COM cells CRC

Detect errors anywhere in the 48-byte cell payload


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Encapsulation and Segmentation for AAL3/4


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ATM Adaptation Layer 5

AAL3/4 seems to take a lot of fields and thus a lot of overhead to perform conceptually simple function of SAR

Could just have 1 bit in the ATM header (as opposed to the AAL header) to delineate the end of a frame, them SAR could be accomplished without using any of the 48-byte ATM payload for SAR information

This led to AAL5


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AAL5 (cont’d)

Replace 2-bit Type filed of AAL3/4 with 1 bit of framing information in the ATM cell header Set this bit to identify the last cell of a PDU The next cell is assumed to be the first cell of the next

PDU Subsequent cells are assumed to be COM cells until

another cell is received with this bit set. Other features of AAL3/4 are provided in AAL5


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AAL5 CS-PDU

AAL5 CS-PDU consists of data portion and 8-byte trailer To make sure that the trailer always falls at the tail end

of an ATM cell, there may be up to 47 bytes of padding between the data and trailer

The first 2 bytes of trailer is Reserve and must be 00 Len is the number of bytes carried in the PDU, not

including the trailer or any padding before trailer

ATM AAL5 packet format


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Encapsulation and Segmentation for AAL5


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Encapsulation and Segmentation for AAL5 (cont’d) AAL5 provides almost the same functionality as AAL3/4

without using an extra 4 bytes out of every cell The main feature missing from AAL% is the ability to

provide an additional layer of multiplexing onto one virtual circuit using the MID

AAL5 is preferred in the IETF form transmitting IP datagrams over ATM

More efficient use of bandwidth and simple design are the main features that make it more appealing than AAL3/4


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Outline



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Virtual Paths

ATM uses 24-bit identifier for virtual circuits 8-bit virtual path identifier (VPI) 16-bit virtual circuit identifier (VCI)

This creates two levels of virtual connections A corporation has two sites connecting to a public ATM

network and each site the corporation has a network of ATM switches

We can establish a virtual path between two sites using only VPI field The switch in the public network would use the VPI as the

only field on which to make forwarding decisions It is a virtual circuit network with 8-bit circuit identifier The 16-bit VCI has no interest to these public switches


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Virtual Paths (cont’d)

Within corporate sites, the full 24-bit space is used for switching

Traffic flowing between two sites is routed to a switch that has an connection to the public network, and its top (most significant bits) 8 bits VPI are mapped onto the appropriate value to get the data to the other site


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Virtual Paths (cont’d)

Advantages Support many virtual connections across the public

network The switches in the public network behave as if there is

only one connection There needs to be much less connection-state

information stored on the switches, avoiding the need for big, expensive tables of per-VCI information


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Outline



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Physical Layers for ATM

From early in the process of standardizing ATM, it was assumed that ATM would run on top of a SONET physical layer

Standard ways of carrying ATM cells inside a SONET frame have been defined. Can buy ATM-over-SONET products However, they are entirely separable. Can lease a SONET link from a telephone company to carry

variable-length packets Can send ATM cells over many other physical layers instead of

SONET Notable early physical layers for ATM was TAXI, the physical

layer used in FDDI Wireless physical layers for ATM are also being defined


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How to find boundaries of the ATM cells

With SONET, find ATM cell boundaries: One of the overhead bytes in the SONET frame can

be used as a pointer into the SONET payload to start of an ATM cell Having found the start of one cell, it is known that the

next cell starts 53 bytes further on in the SONET payload

In theory, need to read this pointer only once, but in practice, may read it every time the SONET overhead goes by to detect errors or resynchronize if needed


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Outline



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ATM in the LAN

ATM can be used in LANs a replacement for Ethernet and 802.5

Its popularity can be attributed to two main factors: ATM is a switched technology, whereas Ethernet and

802.5 were originally envisioned as shared-media technologies

ATM was designed to operate on links with speeds of 155 Mbps and above, compared to the original 10 Mbps of Ethernet and 4 or 16 Mbps of token rings


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ATM in the LAN (cont’d)

Switched networks have a big performance advantage over shared-media networks A single shared-media network has a fixed total bandwidth

that must be shared among all hosts, whereas each host gets its own dedicated link to the switch in a switched network

However, bridge that connects a number of shared-media network together is also a switch, giving dedicated access

Also when ATM were launched, high-speed Ethernet became available. It speed began to approach that of ATM


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One advantage of ATM over Ethernet that remains is the lack of distance limitation for ATM links. Also high-speed ATM links (e.g. 622 Mbps) became available.

This made ATM fairly popular for the high-performance “backbone” of larger LANs

One common configuration is to connect hosts to Ethernet switches, which in turn could be interconnected by ATM switches

High-performance servers might also be connected directly to the ATM switch


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ATM in the LAN


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ATM in the LAN

One significant problem with running ATM in a LAN is that it doesn’t look like a “traditional” LAN

Because most LANs are shared-media networks (i.e. every node on the LAN is connected to the same link), it is easy to implement broadcast and multicast

Many protocols that people depend on in their LANs, e.g. ARP, depend in turn on the ability of the LAN to support broadcast and multicast

However, ATM is not a shared-media network.

How do we can broadcast to all nodes on an ATM LAN if don’t know all their addresses and set up VCs to all of them?


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Two possible solutions Redesign the protocols that make assumption about

LANs that are not in face true for ATM ATMARP does not depend on broadcast

Make ATM behave more like a shared-media LAN supporting broadcast and multicast without losing the performance advantages of a switched network LAN Emulation or LANE: aim to add functionality to ATM

LANs so that anything that runs over a shared-media LAN can operate over an ATM LAN


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One aspect of LANE that can be confusing is the variety of different addresses and identifiers that are used

ATM devices must have ATM address used when signaling to establish a VC Different from IEEE 802 MAC address used in Ethernet and

token rings LANE does not actually change functionality of ATM

switches, but add functionality to the network through the additional of a number of servers. Devices connecting to the ATM network – hosts, bridges, routers

– are referred to as LAN emulation clients (LECs).


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LAN Emulation

The servers that are required to build an emulated LAN are: The LAN emulation configuration server (LECS) The LAN emulation server (LES) The broadcast and unknown server (BUS)

These servers can be physically located in one or more devices

LECS and LES primarily perform configuration functions BUS has a central role in making data transfer in an

ATM network resemble that of a shared-media LAN


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LAN Emulation (cont’d)

LES BUS

H1 H2


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Broadcasting in LAN Emulation

1. LECS enables a newly attached or rebooted LAN emulation client to get some essential information

2. First, the client must find the LECS, which may use a well-known, predefined VC that is always set up. Alternatively, the client must have prior knowledge of the ATM address of the LECS so it can set up a VC to it

3. Once connected, the client provides the LECS with its ATM address4. LECS responds by telling the client

what type of LAN is being emulated (Ethernet or token ring) What the maximum packet size is The ATM address of the LES. One LES might support many separate

emulated LANs5. The client signals for a connection to the LES whose ATM address

is just learned.6. Once connected to the LES, the client registers its MAC and ATM

addresses with the LES. The LES provides the client with the ATM address of the BUS


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Broadcasting in LAN Emulation (cont’d)

7. The BUS maintains a single point-to-multipoint VC that connects it to all registered clients. BUS and multipoint VC are crucial to LANE:

Enable the broadcast capability of traditional LAN to be emulated in a VC environment

8. Once LEC has the ATM address of the BUS, it signals for a connection to the BUS

9. The BUS adds the LEC to the point-to-multipoint VC Now LEC is ready to participate in data transfer

BUS is the place to send any packet that needs to be broadcast to all clients on the LAN, not efficient to deliver unicast packets


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Unicasting in LANE

Assume that a host has a packet to send to a particular MAC address

In traditional LAN, simply send packets over on the wire. It will be picked up by intended recipient

But in LANE, packet needs to be delivered over a VC. The sending host also does not know the ATM address of the recipient, which is required to set up a VC

The host performs the following steps:1. It sends the packet to the BUS, which it knows can deliver the

packet to the destination using its point-to-multipoint VC

2. It sends an “address resolution” request to the LES, of the form “what ATM address correspond to this MAC address?”


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Unicasting in LANE (cont’d)

Since all clients should have registered their MAC and ATM addresses with the LES, the LES should be able to answer the query and provide an ATM address to the client

The client now can signal for a VC to the recipient, which may use to forward subsequent frames to the destination

The reason for using BUS to send the first packet is to minimize delay, since it may take some time to get a response from the LES and establish a VC


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Questions?

Next Lecture

Wireless LANs and

Cellular Networks

Documents

NETE0510: Communication Media and Data Communications 1 NETE0510 ATM Dr. Supakorn Kungpisdan [email protected]