ATM Adaptation Layer

5/27/2018 ATM Adaptation Layer

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ATM Adaptation Layer

ATM Adaptation Layer1


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ATM Adaptation Layer Functions


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

data

Its basic function is the enhanced adaptation of services provided by the ATM

layer to the requirements of the higher layer

It maps higher layer PDUs into the information field of the ATM cell AAL is divided into the Convergence Sublayer (CS) and the Segmentation and

Reassembly (SAR) Sublayer

The Convergence Sublayer is service dependent and provides the AAL services

at the AAL-SAP

The functions of the SAR sublayer are segmentation of higher layer PDUs intoa 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


To minimize the number of AAL protocols, ITU-T proposed a serviceclassification specific to the AAL

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


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

AAL service classes

Class A

Class B

Class C

Class D

AAL protocols

AAL 1

AAL 2

AAL 3/4

AAL 5


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AAL Services


ConnectionlessDate Transfer

BitRate

ConnectionMode

Examplesof

Services

CircuitEmulation

ConstantBit

RateVideo

Constant Variable

ConnectionOriented Connectionless

VariableBitRateVideo

andAudio

Connection-oriented

DataTransfer

AALTYPE

AAL 1 AAL 2AAL 3/4

AAL 5

Class A Class B Class C Class D

Related Not Related

Service

Timingbetween

SourceandDestination

AAL3/4AAL 5


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AAL Types

Five AALs have been accepted for consideration by the

CCITT.

AAL 1 is meant for constant-bit-rate services (voice).

AAL 2 is meant for variable-bit-rate services with a required

timing relationship between source and destination (audio

and video).

AAL 3 was originally meant for connection-oriented variable -

bit-rate services without a required timing relationship; it has

now been merged with AAL 4.

AAL 3/4 and 5 are meant for connectionless services (e.g.

connectionless data).

Only AALs 3/4 and 5 are of interest for IP networking.


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AAL Type 1

Protocol


Supports Class A traffic, i.e., constant bit rate data with specific requirements fordelay, 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


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9

Higher layer User data stream

Convergencesublayer

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


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AAL Type 1 SAR-PDU


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


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AAL Type 1 Sequence Number and

Sequence Number Protection


The 4 bit RTS is transferred by the CSI bit in successive SAR-PDUheaders 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 bit3 bit

1 bit 3 bits

Convergence SublayerIndication (CSI)

Sequence Number (SN)


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AAL 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 For mat

CS-PDU P Format

46 Octets AAL User Information1-Octet SP


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AAL1 Supports Circuit Emulation Synchronous Residual Time Stamp (SRTS)

DS1, DS3

Require accurate frequency clock

4-bit Residual Time Stamp (RTS) for clock aligning

RTS is generated once every 8 cell times, carried in CSIbit of odd cells

Structured Data Transfer (SDT)

nxDS0 (64kbps)

1-octect pointer carried in payload once every two cells

(even cells) indicates the offset into the current payload of

the first octect of an nxDS0 payload

The octect contains 1 reserved bit and 7-bit offset field

which points to start of up to 93 octect structure

(47+46=93)


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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

Supports idle voice channel deletion

Supports multiple user channels with varying bandwidth on a single

ATM connection


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AAL 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


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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


Defines an end-to-end AAL connection as a concatenation of AAL2channels

Each AAL2 channel is a bi-directional virtual channel, with the samechannel 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 eachindividual user

Error detection and correction

Multiplexing multiple AAL channels (merging multiple streams ofCPS packets) onto a single ATM connection

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


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.

CID8 bits

Information1 to 45/64 octets

LI6 bits

UUI5 bits

HEC5 bits

Packet Header (PH) Packet Payload (PP)

Packet


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Format of AAL2 CPS-PDU


Packets are combined into CPS-PDU payload

The Offset Field (OSF) identifies the location of the start of the remaininglength of the packet that possibly started in the preceding cell and iscontinuing 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

OSF6 bits

CPS InformationSN1 bit

P1 bit

PAD0 to 47 octets

Start Field CPS-PDU Payload

CPS -PDU

Cell Header5 octets


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AAL 2 Service Specific Convergence

Sublayer


SSCS is the link between the AAL2 CPS and the higher layerapplications 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


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AAL 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


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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)

AAL 3/4


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Higher layer

Common part

convergencesublayer

SAR sublayer

ATM layer

Service specificconvergence

sublayer

Information

Assume null

TPAD

User message

Pad message to multiple

of 4 bytes. Add headerand trailer.

Each SAR-PDU consistsof 2-byte header, 2-byte

trailer, and 44-bytepayload.

H

4 4

2 44 2 2 44 2 2 44 2

Information

AAL 3/4

AAL 3/4


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25

(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 3/4CS and SAR PDUs


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AAL Type 3/4 CPCS-PDU


4-octet header and 4-octet trailer

CPI Btag BASize CPCS-PDU Payload PAD AL Etag Length

0 - 31

1

2

1

1

2

CPI:00000000 Btag/Etag: Beginning/Ending Tag -- 256 increment counters BAsize: receiving side maximum buffering requirement (>= CPCS-PD Pad: make CPCS-PDU on 32-bit boundary AL(Alignment): make trailer 32-bit aligned

Length: CPCS-PDU size


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AAL Type 3/4 SAR-PDU

ATM Adaptation Layer

27

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

ST: COM(00),BOM(10),EOM(01),SSM(11)

SN: Modulo 16 sequence counter

MID (Multiplexing ID) -- Multiplexing multiple CPCS connections on a

single ATM connection

LI: Length


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AAL 5 PDU Structure

The Simple and Efficient Adaptation Layer (SEAL),

attempts to reduce the complexity and overhead of AAL

3/4.

It eliminates most of the protocol overhead of AAL 3/4.

AAL 5 comprises a convergence sublayer and a SARsublayer, although the SAR is essentially null.

CS-PDU User Information


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AAL 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 5


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Higher layer

Common part

convergencesublayer

SAR sublayer

ATM layer

PTI = 0

Service specificconvergence

sublayer Assume null

48

(1)

Information

TPAD

Information

48

(0)

48

(0)

PTI = 0PTI = 1

AAL 5


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AAL Type 5 SAR-PDU


PAD CPCS-PDUTrailer

CPCS-PDU

SAR-PDUPayload

CPCS-SDU

SAR-PDUPayload

SAR-PDUPayload

SAR-PDUPayload

SAR-PDUPayload

SAR-PDU

SAR-PDU

SAR-PDU

SAR-PDU

SAR-PDU

CPCS-PDU Payload


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AAL 5

When a network node has a user datagram to transmit, itfirst converts it to a CS-PDU by adding the pad (ifnecessary) and trailer.

Then it breaks the CS-PDU into 48-octet SAR-PDUs andtransmits each in an ATM cell on the same virtual channel.

Since there is no AAL 5 SAR header, an end-of-frameindication in the ATM cell header is required: SDU type of1 (binary value 0X1) in the PTI field.

The receiver simply concatenates cells as they arereceived, watching for the end-of-frame indication.

The higher layer is responsible for ignoring PDUs with

CRC errors. Some applications may discard PDUs with errors; others

may choose to use them.


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AAL 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)

Documents

ATM Adaptation Layer