ATM Networks

1

ATM Networks

• Introduction

• ATM Cells

• Virtual-circuit switching

2

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

ATM Multiplexing

3

Cell Switching (ATM)

• Connection-oriented packet-switched network

• Used in both WAN and LAN settings

• Signaling (connection setup) Protocol: Q.2931

• Specified by ATM forum

• Packets are called cells

– 5-byte header + 48-byte payload

• Commonly transmitted over SONET

– other physical layers possible

4

Variable vs Fixed-Length Packets

• No Optimal Length

– if small: high header-to-data overhead

– if large: low utilization for small messages

• Fixed-Length Easier to Switch in Hardware

– simpler

– enables parallelism

5

Big vs Small Packets

• Small Improves Queue behavior

– finer-grained pre-emption point for scheduling link• maximum packet = 4KB• link speed = 100Mbps• transmission time = 4096 x 8/100 = 327.68us• high priority packet may sit in the queue 327.68us• in contrast, 53 x 8/100 = 4.24us for ATM

– near cut-through behavior • two 4KB packets arrive at same time• link idle for 327.68us while both arrive• at end of 327.68us, still have 8KB to transmit • in contrast, can transmit first cell after 4.24us• at end of 327.68us, just over 4KB left in queue

6

Big vs Small (cont)

• Small Improves Latency (for voice)

– voice digitally encoded at 64KBps (8-bit samples at 8KHz)

– need full cell’s worth of samples before sending cell

– example: 1000-byte cells implies 125ms per cell (too long)

– smaller latency implies no need for echo cancellors

• ATM Compromise: 48 bytes = (32+64)/2

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Header Payload

5 Bytes 48 Bytes

The ATM Cell

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Cell Format

• User-Network Interface (UNI)

– host-to-switch format – GFC: Generic Flow Control (still being defined)– VCI: Virtual Circuit Identifier– VPI: Virtual Path Identifier– Type: management, congestion control, AAL5 (later)– CLPL Cell Loss Priority – HEC: Header Error Check (CRC-8)

• Network-Network Interface (NNI)– switch-to-switch format– GFC becomes part of VPI field

GFC HEC (CRC-8)

4 16 3 18

VPI VCI CLPType Payload

384 (48 bytes)8

9

2

3

N

1Switch

N

1…

5

6

video 25

video

voice

data

32

32 61

25

32

3261

75

67

39

67

N

1

32

video 75

voice 67

data 39

video 67

…

…

ATM Switching

10

c ATMSw1

ATMSw4

ATMSw2

ATMSw3

ATMDCC

ab

de

VP3 VP5

VP2

VP1

a

bc

deSw = switch

Virtual Paths in an ATM Network

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Physical Link

Virtual Paths

Virtual Channels

ATM Virtual Connections

12

Packet

Packet

Virtual circuit Switching

13

t

t

t

t

31 2

31 2

321

Release

Connect request

CR

CR Connect confirm

CC

CC

VC Setup Delays

14

SW 1

SW 2

SW n

Connect request

Connect request

Connect request

Connect confirm

Connect confirm

…

VC Signaling

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Identifier Outputport

15 15

58

13

13

7

27

12

Nextidentifier

44

23

16

34

Entry for packetswith identifier 15

VC Routing Table

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Plane managem

entManagement plane

Control plane User plane

Physical layer

ATM layer

ATM adaptation layer

Higher layers Higher layers

Layer m

anagement

Broadband ISDN reference model

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AAL

ATM

User information

User information

AAL

ATM

PHYPHY

ATM

PHY

ATM

PHY

…

End system End systemNetwork

User Plane Layers

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X

X

X

X

X

X

X

X

X

Private UNI

Public UNI

NNI

Private NNI

Private ATM network

Public UNI

B-ICI

Public UNIPublic ATM network A

Public ATM network B

ATM Network Interfaces

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ATM QOS Parameters

• Non-negotiable parameters: characteristics of networks

• Cell error ratio

• Cell misinsertion ratio: undetected header errors.

• Severely-errored cell block ratio: Bursty errors

• Negotiable parameters: User can ask during setup

• Cell loss ratio

• Cell transfer delay

• Cell delay variation

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ATM Service Categories

• Constant bit rate (CBR): Rigorous timing requirements, voice, circuit emulation.

• Real-time Variable Bit Rate: VBR traffic such as video.

• Non-real-time VBR: Bursty data sources, no rigorous timing requirments

• Available bit rate (ABR): Sources that can adapt to available bandwidth in the network. Low cell loss ratio.

• Unspecified bit rate (UBR): No QOS guarantees.

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Segmentation and Reassembly

Sublayer

AAL Layer

Convergence Sublayer

Common Part

Service Specific Convergence

Sublayer

AAL Sublayers

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Segmentation and Reassembly

• ATM Adaptation Layer (AAL)– AAL 1 and 2 designed for applications that need

guaranteed rate (e.g., voice, video)– AAL 3/4 designed for packet data– AAL 5 is an alternative standard for packet data

AAL

ATM

AAL

ATM

… …

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…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

AAL1 Process

For Constant rate transfers, e.g., 64kbps PCM voice call

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(a) SAR PDU header

(b) CS PDU with pointer in structured data transfer

CSI SNP

AAL 1 Pointer

1 Byte 46 Bytes

47 Bytes

Seq. Count

1 bit 3 bits 4 bits

AAL1 PDUs

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AAL 2 ATM cells

Low bit rate Short voice packets

Mobile switching

office

Application scenario for AAL2

Originally, for variable bit rate (VBR) applications with end-to-end delay requirements, e.g., compressed video.

Now, BW-efficient transfer of low-bit-rate short-packet traffic with low-delay requirements. Allows multiplexing of multiple users on a single ATM connection, e.g., cell phone traffic, compressed audio.

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Higher layer This example assumes 24 byte packets

Common part convergence

sublayer

SAR sublayer

ATM layer

1 47

5 48

H

P3

Service specific convergence

sublayer

P2P1

Assume null

1 47

3 243 243 24

PAD

5 48

H

Add 3-byte header to each user packet

Segment into SAR PDUs

H H H

AAL2 Process

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Higher layer


sublayer

SAR sublayer

ATM layer


sublayer

Information

Assume null

TPAD

User message

Pad message to multiple of 4 bytes. Add header and trailer.

Each SAR-PDU consists of 2-byte header, 2-byte trailer, and 44-byte payload.

H

4 4

2 44 2 2 44 2 2 44 2

…

…

Information

AAL 3/4 Process

AAL3 for connection-oriented bursty data with low loss, no delay requirements. AAL4 for connection-less data transfers.

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(a) CPCS-PDU format

(b) SAR PDU format

CPI Btag BASize CPCS - PDU Payload

1 1 2 1 - 65,535 0-3 1 1 2 (bytes) (bytes) (bytes)

AL Etag LengthPad

Header Trailer

ST SN MID SAR - PDU Payload

2 4 10 44 6 10 (bits) (bytes)

(bits)

LI CRC

Header (2 bytes)

Trailer (2 bytes)

AAL 3/4 CPCS and SAR formats

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Higher layer Assume two packets from different users

Common part convergence and SAR sublayers

ATM layer


sublayer

P2P1

Each packet is segmented separately. SAR PDUs identified by MID.

Cells from two packets are interleaved.

Interleaved cells

MID = bMID = a

CPCS SAR

CPCS SAR

Interleaver

SPDUA2

SPDUA1

SPDUB2

SPDUB1

Multiplexing in AAL 3/4

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AAL 3/4

• Convergence Sublayer Protocol Data Unit (CS-PDU)

– CPI: commerce part indicator (version field)

– Btag/Etag:beginning and ending tag

– BAsize: hint on amount of buffer space to allocate

– Length: size of whole PDU

CPI Btag BASize Pad 0 Etag Len

8 16 0– 24 8 8 16< 64 KB8

User data

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Cell Format

– Type• BOM: beginning of message

• COM: continuation of message

• EOM end of message

– SEQ: sequence of number

– MID: message id

– Length: number of bytes of PDU in this cell

ATM header Length CRC-10

40 2 4

SEQ MIDType Payload

352 (44 bytes)10 6 10

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AAL5

• CS-PDU Format

– pad so trailer always falls at end of ATM cell

– Length: size of PDU (data only)

– CRC-32 (detects missing or misordered cells)

• Cell Format

– end-of-PDU bit in Type field of ATM header

• A more efficient alternative to AAL3/4

– message and stream modes

– assured and nonassured delivery

CRC-32

< 64 KB 0– 47 bytes 16 16

ReservedPad Len

32

Data

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Higher layer


sublayer

SAR sublayer

ATM layerPTI = 0


sublayerAssume null

48 (1)

Information

TPAD

…

…

Information

48 (0)

48 (0)

PTI = 0 PTI = 1

AAL 5 Process

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Information

0 - 65,535 0-47 1 1 2 4 (bytes) (bytes)

UU CPI Length CRCPad

AAL 5 PDU

ATM Networks

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