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Rivier College

CS575: Advanced LANs

ATM Adaptation Layer

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ATM Adaptation Layer Functions

0 The ATM Adaptation Layer (AAL) is designed to support different types of applications and different types of traffic, such as voice, video, imagery, and data

0 Its basic function is the enhanced adaptation of services provided by the ATM layer to the requirements of the higher layer

0 It maps higher layer PDUs into the information field of the ATM cell

0 AAL is divided into the Convergence Sublayer (CS) and the Segmentation and Reassembly (SAR) Sublayer

0 The Convergence Sublayer is service dependent and provides the AAL services at the AAL-SAP

0 The functions of the SAR sublayer are segmentation of higher layer PDUs into a suitable size for the information field of the ATM cell (48 octets) at the transmitting end and reassembly of the information fields into higher layer PDUs at the receiving end

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ATM Adaptation Sublayers

0 Convergence Sublayer (CS):

- Handling lost/misdelivered cells

- Timing recovery

- Interleaving

0 Segmentation and Reassembly Sublayer (SAR):

- Split frames/bit stream into cells for transmission

- Reassemble frames/bit stream for receiving

- Support multiple protocols

Higher Layers

ATM Adaptation Layer (AAL) Convergence Sublayer (CS)

Segmentation and Reassembly Sublayer (SAR)

ATM Layer (ATM) Virtual Channel (VC) Virtual Path (VP)

Physical Layer (PL) Transmission Convergence Sublayer (TC)

Physical Medium Sublayer (PM)

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AAL Protocols and Service Classes

0 To minimize the number of AAL protocols, ITU-T proposed a service classification specific to the AAL

0 The classification was made with respect to the following parameters:

- Timing relationship between sender and receiver

=Related

=Not related

- Bit rate

=Constant bit rate

=Variable bit rate

- Connection mode

=Connection-oriented

=Connectionless

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AAL Protocols and Service Classes (Concluded)

0 Four AAL protocols have been defined to support four AAL service classes

0 AAL service classes

- Class A

- Class B

- Class C

- Class D

0 AAL protocols

- AAL 1

- AAL 2

- AAL 3/4

- AAL 5

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AAL Services

Connectionless Date Transfer

Bit Rate

Connection Mode

Examples of Services

CircuitEmulationConstant Bit Rate Video and Audio

Constant

Variable

Connection Oriented Connectionless

Variable Bit Rate Video

and Audio

Connection- oriented Data Transfer

AALTYPE AAL 1 AAL 2

AAL 3/4AAL 5

Class A Class B Class C Class D

Related Not Related

Service

Timing betweenSource andDestination

AAL3/4AAL 5

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AAL Type 1 Protocol0 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)

0 Receives constant bit rate stream with a well defined clock from source and delivers the same to the destination

0 Provides for timing recovery (using SRTS), synchronization, and indication of lost information not recovered by AAL1

0 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

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AAL Type 1 SAR-PDU

0 Consists of 1 octet header (PCI) and 47 octets of payload0 Sequence Number (SN): A 1-bit Convergence Sublayer Indication

and 3-bit sequence count to detect deletion or misinsertion of cells0 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

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AAL Type 1 Sequence Number and Sequence Number Protection

0 The 4 bit RTS is transferred by the CSI bit in successive SAR-PDU headers with an odd SN (SN = 1, 3, 5, 7)

0 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 bit3 bit

1 bit 3 bits

Convergence Sublayer Indication (CSI)

Sequence Number (SN)

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AAL Type 1 CS PDU

0 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

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AAL Type 2 Protocol

0 Designed to support Class B (VBR) traffic0 Supports variable bit rate data where a strong timing relationship

between source and destination is required, e.g., VBR audio and video

0 Data passed to AAL2 from higher layers at the source at fixed intervals and must be passed to the destination at the same rate

0 The amount of data passed to AAL2 may vary with each transfer0 Supports voice compression and silence suppression0 Supports idle voice channel deletion0 Supports multiple user channels with varying bandwidth on a

single ATM connection

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AAL Type 2 Voice Application Examples

0 PBX-to-PBX trunking for compressed voice0 ATM trunking on public-switched telephone network0 ATM backbone for cellular systems and personal communications

services (PCS)0 ATM backbone connectivity to packet telephone

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PBX-to-PBX Connectivity

CompressionSilence Suppression

Packetization

PBXPCM Voice

CompressionSilence Suppression

Packetization

PBXPCM Voice

AALATMPHY

ATM NetworkAALATMPHY

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AAL Type 2 Packets

0 AAL2 provides bandwidth-efficient transmission of low-rate, short, and variable-size packets for delay sensitive applications

0 AAL2 uses one ATM connection between two points to carry packets from multiple native connections

0 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

0 A channel identification (CID) field is used in the packet header to identify the LLC to which a packet belongs

0 A length indicator (LI) field is used to identify the boundaries of variable-length LLC packets

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AAL Type 2 Protocol Sublayers

Service Specific Convergence Sublayer (SSCS)

Common Part Sublayer (CPS)

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AAL 2 Common Part Sublayer

0 Defines an end-to-end AAL connection as a concatenation of AAL2 channels

0 Each AAL2 channel is a bi-directional virtual channel, with the same channel identifier value for both directions

0 AAL2 channels are established over an ATM layer PVC, SPVC, or SVC

0 Provides basic structure for identifying the users of the AAL0 Assembling/disassembling the variable payload associated with each

individual user0 Error detection and correction0 Multiplexing multiple AAL channels (merging multiple streams of

CPS packets) onto a single ATM connection 0 Provides QoS through the choice of AAL-SAP for data transfer

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AAL 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 SpecificConvergence

Sublayer (SSCS)

Common PartSublayer (CPS)

ATM Layer

AAL-SAP

ATM-SAP

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Format of AAL2 Packet

0 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.

0 Length Indicator (LI): Identifies the length of the LLC packet associated with each individual user, and assures conveyance of variable payload.

0 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.

CID8 bits

Information 1 to 45/64 octets

LI6 bits

UUI5

bits

HEC5

bitsPacket Header (PH) Packet Payload (PP)

Packet

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Format of AAL2 CPS-PDU

0 Packets are combined into CPS-PDU payload0 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

0 Data integrity is protected by the Sequence Number (SN)0 The Start Field is protected from error by a Parity bit (P)0 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

OSF6 bits CPS Information SN

1 bitP

1 bitPAD

0 to 47 octets

Start Field CPS-PDU Payload

CPS -PDU

Cell Header5 octets

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AAL 2 Service Specific Convergence Sublayer

0 SSCS is the link between the AAL2 CPS and the higher layer applications of the individual AAL2 users

0 Standards for SSCS are being developed in ITU-T and ATM Forum0 A null SSCS satisfies most mobile voice applications

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AAL Type 3/4 Protocol

0 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

0 Supports Class C (connection-oriented) and Class D (connectionless) traffic

0 Convergence sublayer divided into two parts:- Common Part Convergence Sublayer (CPCS)- Service Specific Convergence Sublayer (SSCS)

0 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 optional0 CPCS provides message mode or streaming mode services

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AAL Type 3/4 Protocol Sublayers

Service Specific Convergence Sublayer (SSCS)

Common Part Convergence Sublayer (CPCS)

Segmentation and Reassembly Sublayer (SAR)

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AAL Type 3/4 CPCS-PDU

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 - 31

1

2

1

1

2

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AAL Type 3/4 SAR-PDU

2-octet header, 44-octet payload, and 2-octet trailer

ST Segment TypeSN Sequence NumberMID Multiplexing IdentifierLI Length IndicationCRC-10 CRC Code

SN MID SAR-PDU Payload LI CRC-10ST

2 bits 4 bits 10 bits 352 bits 10 bits6 bits

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AAL Type 5 Protocol

0 AAL5 is a simple and efficient AAL (SEAL) to perform a subset of the functions of AAL3/4

0 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

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AAL Type 5 SAR-PDU

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

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AAL Type 5 Protocol (Concluded)

0 The CPCS-PDU is divided into 48 octets SAR-PDUs0 Since CPCS-PDU is 48-octet aligned, there is no need for a length

field in the SAR-PDU 0 The AAL5 SAR-PDU is 48 octets of data with no overhead of SAR-

PDU header or trailer0 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)

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W. Stalling, Local and Metropolitan Area Networks, 6th edition, Prentice Hall, 2000, Chapter 11

References

W. Stalling, Data and Computer Communications, 6th edition, Prentice Hall, 2002, Chapters 11-12

A. Wu, Advanced Local Area Networks, Lectures & Slides, Rivier College, 2001.