Telecommunication

Networks and SystemsNetworks and Systems

AAL (ATM Adaptation Layer)

Krzysztof Wajda

Department of Telecommunications, AGH-USTNovember, 2018

Outline

• Motivation to introduce so many AAL versions

• Description of AAL1-AAL5

• Interworking between AAL and • Interworking between AAL and applications (also: higher layers)

Concepts:

• application requirements

• traffic classes

•

Lecture scope

• low-level support for service

differentiation

AAL LayerAAL Layer(ATM Adaptation layer)

This is an interim layer between higherlayers (applications) and ATM layer,

main functions: error detection and

AAL Layer

main functions: error detection and reaction, recognition of lost and wrongpackets, flow control, bit timing.

class A class B class C class D

Synchroniza-tion betweenterminals

required Not required

bitstream constant variable

Service classes

bitstream constant variable

Connectionmode

Connection-oriented Conn.less

AAL type 1 2 3/4 i 5 type 5

• Matching the size and features of application PDUs with transport features of ATM networks,

• Error correction,

• Synchronization.

CS functions

• Synchronization.

• Transformation of higher layers PDUs into SDU (and in opposite way)

n+1 layer PDUn+1

SAR functions

n layer

n-1 layer

SDUnPCIn

+

PDUn

SDUn-1

AAL1

• Transfer of data in CBR (connection-oriented CBR service),

• Decreasing the effect of cell jitter,

• Reaction for lost or out-of-order cells,

• Clock frequency recovery inside receiver,• Clock frequency recovery inside receiver,

• Monitoring and error recovery service for control field of AAL called SNP.

cell header SN SNP SAR-PDU payload

SAR-PDU header

SAR-PDU for AAL1

SAR-PDU

SN

SNP

= Sequence Number

= Sequence Number Protection

(4 bits)

(4 bits)

• The most significant bit of SN field is used for special purposes

• 3 other bits of SN field constitute cell counter modulo 8

SAR-PDU header

counter modulo 8

• Pole SNP field secures whole SAR-PDU header

SN-field SNP-field

SAR-PDU header for AAL1

SpecialPurposeIndication

Sequence NumberCount

CRCParityBit(Even)

AAL1 - summary

Single bits, bytes

47 bytes

Higher layers

Convergence sublayer

(CS)

Segmentation and

48 bytes

53 bytes

Segmentation and

reassembly sublayer

(SAR)

ATM layer

• Transfer of data in VBR (connection-oriented real-time service),

• CS-PDUs with variable length,

• For long time without precise specification for AAL2,

AAL2

for AAL2,

• For voice services, mobile systems,

• Error correction for audio and video services

• Synchronization of terminals by sending time stamps

CS sublayer functions for AAL2

sending time stamps

• Service for lost and out-of-sequence cells

cell header SN IT SAR-PDU payload

SAR-PDU header

SAR-PDU

SAR-PDU trailer

LI CRC

SAR-PDU structure for AAL2

SAR-PDU

SN = Sequence number

= Information Type

LI

CRC

= Length Indicator

= Cyclic Redundancy Check

IT

• Transfer of data in VBR (connection-oriented real-time service),

• Synchronization between sender and receiver is not necessary,

• Two modes of operation:

AAL3

• Two modes of operation: – framed mode, message mode,

– streaming mode,

• Monitoring and service of errors in AAL (CRC field in SAR-PDU).

AAL3/4 - summary

SN SNPSAR-PDU

Informacja użyteczna

nagłówek1-bajtowy

47 bajtów

b) Struktura danych podwarstwy SAR dla AAL typ 2

ST IT LI CRCSAR-PDU

Informacja użyteczna

nagłówek1-bajtowy

45 bajtów

a) Struktura danych podwarstwy SAR dla AAL typ 1

zakończenie2-bajtowe

AAL1 - 4 - summary

nagłówek1-bajtowy

ST SN MIDSAR-PDU

Informacja użyteczna

44 bajty

c) Struktura danych podwarstwy SAR dla AAL typ 3

d) Struktura danych podwarstwy SAR dla AAL typ 4

zakończenie2-bajtowe

ST SN ResSAR-PDU

Informacja użyteczna

nagłówek2-bajtowy

44 bajty zakończenie2-bajtowe

SN ( - 4 bity) - numer kolejnySNP ( - 4 bity) - ochrona SNIT ( - 4 bity) - typ informacjiLI ( - 6 bitów) - wskaźnik, długościCRC ( - 10 bitów) - ciąg kontrolny kodu cyklicznegoST ( - 2 bity) typ segmentuRes ( - 10 bitów) - rezerwaMID ( - 10 bitów) -identyfikator multipleksowania

Sequence NumberSequence Number Protection

Information TypeLength Indicator

Cyclic-Redundancy-CheckSegment TypeReservedMultiplexing IDentification

LI CRC

LI CRC

• Universal version of AAL, covering functions of AAL3/4 and more. VBR mode, connection-oriented, without synchronization,

AAL5 - summary

• Without numbering of subsequent cells,

• No retransmission,

• SEAL (Simple and Efficient Adaptation Layer),

ATM cell header SAR-PDU payload

SAR-PDU for AAL5

SAR-PDU

CS-PDU trailer

CRCLFCFPADUser data

CS-PDU

CS sublayer

Split of CS-PDU into cells

= Cyclic Redundancy CheckCRC

PAD = Pad field (0 to 47 bytes)

LF = Length Field (2 bytes)CF = Control Field (2 bytes)

(4 bytes)

ATM cell(AAL5)

SAR sublayer

ATM cell(AAL5)

ATM cell(AAL5)

ATM cell(AAL5)

PT

Summary

• Flexible but complicated concept of AAL,

• Extensive and explicit service specification,

• AAL5 mostly used.• AAL5 mostly used.

• ATM Forum standards• ITU-T Recommendations

• D. Ginsburg, „ATM. Solutions for enterprise networking”, Addison-Wesley,

Bibliography

enterprise networking”, Addison-Wesley, 1999• Lecture notes: http://www.kt.agh.edu.pl/~wajda/students/telecommunication-networks-and-

systems/

Thank you!