Aal Present

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� DEFINITION OF AAL

� SERVICES BY AAL� APPLICATIONS OF AAL� AAL TYPES

� AAL SUBLAYERS� TYPES EXPLAINED



ATM Adaptation Layer (AAL)� ATM Adaptation Layer (AAL): ³adapts´ upper layers (IP

or native ATM applications) to ATM layer below

� AAL present only in end systems , not in switches� AAL layer segment (header/trailer fields, data) fragmentedacross multiple ATM cells ± analogy: TCP segment in many IP packets



ATM Adaptation Layer (AAL)� Support higher-level protocols and/or native

applications ± e.g., PCM voice, LAPF, IP

� AAL Services ± Handle transmission errors ± Segmentation/reassembly ( SAR ) ± Handle lost and misinserted cell conditions ± Flow control and timing control



Applications of AAL and ATM� Circuit emulation (e.g., T-1 synchronous

TDM circuits)� VBR voice and video� General data services� IP over ATM� Multiprotocol encapsulation over ATM

(MPOA)� LAN emulation (LANE)



A/D

Voice

s1 , s2 «

Digital voice samples

A/D

Video

« Compressioncompressed

frames picture frames

DataBursty variable-length

packets

cells

cells

cells

Figure 9.3Leon-Garcia & Widjaja: Communication NetworksCopyright ©2000 The McGraw Hill Companies

AAL

AAL

AAL

ATM Adaptation Layers



Service classes and AAL� Organization of AAL is based on concept of

service classes (defines the characteristics of traffic offered by an application to the n/w)

� Parameters used to characterize traffic are: ± B it rate ± rate of traffic offered by an application to the

n/w. constant bit rate or variable bit rate ± Timing relationship - Some applications require a

strict timing relationship between source & destinationsuch as voice and video. ± Nature of connection

� Connection-oriented (e.g. voice applications)� Connectionless (e.g. FTP applications)



� Based on these 3 parameters five service classes are defined (Class A, Class B,Class C, Class D and Class X)

� AAL0 doesn¶t perform any functions. Data is sent as such to ATM layer withoutany processing. Also called null AAL.� Following figure shows Service classes, Parameters and Relationship with AALs:



AAL Types AAL Types



� Convergence Sub-layer (CS) ± Again divided into 2 sub-layers

± CPCS (Common Part Convergence Sub-layer) ± SSCS (Service Specific Convergence Sub-layer)

± Interfaces directly with the user ± May add header/trailer to data and passes to SAR

� Segmentation And Reassembling Sub-layer (SAR) ± Segments data received from CS into smaller data units.

± May add header/trailer to these smaller data units to form a48-byte SAR-PDU (Protocol data unit)

AAL SU B LAYERSAAL SU B LAYERS



AAL AAL



AAL Protocols and PDUs



ATM Adaptation Layer (AAL)

McGraw-Hill ©The McGraw-Hill Companies, Inc., 2001

F igure 19-22

AAL Types



AAL Type 1� Constant-bit-rate source� SAR simply packs bits into cells and

unpacks them at destination� One-octet header contains 3-bit SC field to

provide an 8-cell frame structure� No CS PDU structure is defined since CS

sublayer primarily for clocking andsynchronization



AAL Type 1

M c r - ill T M c r - ill i s, I c.,

F ig re 19-23

1



AAL 1 AAL 1



«Higher layer User data stream

Convergencesublayer

SAR sublayer

ATM layer

CS PDUs

SAR PDUs

ATM Cells

47 4747

1 47 1 47 1 47

H H H

5 48

H

5 48

H

5 48

H

b1 b2 b3

AAL 1



AAL Type 2

� Intended for variable bit rate applications thatgenerate bursty data and demand low loss

� Originally, connectionless (AAL4) or connection(AAL3) oriented, now combined into single format(AAL 3/4)

� Provides comprehensive sequencing and error control mechanisms

AAL Type 3/4 AAL Type 3/4

Intended for use with applications withIntended for use with applications withvariable bitvariable bit- -rate service on multiplerate service on multiplechannels (multiplexing), or low bit rate,channels (multiplexing), or low bit rate,shortshort--frame trafficframe traffic



AAL 3/4 AAL 3/4� AAL3/4 supports both connectionAAL3/4 supports both connection- -oriented and connectionless data. Itoriented and connectionless data. It

was designed for network service providers and is closely aligned withwas designed for network service providers and is closely aligned withSwitched Multimegabit Data Service (SMDS). AAL3/4 is used toSwitched Multimegabit Data Service (SMDS). AAL3/4 is used to

transmit SMDS packets over an ATM network.transmit SMDS packets over an ATM network.

� Originally 2 separate AALs:Originally 2 separate AALs:

± ± AAL3: ConnectionAAL3: Connection- -oriented packet svcs (X.25)oriented packet svcs (X.25) ± ± AAL4: Connectionless svcs (IP)AAL4: Connectionless svcs (IP)

� Eventually combined into a single type for all data service� Eventually combined into a single type for all data service



AAL 3/4

M c r - ill T M c r - ill i s, I c.,

F ig re 19-253/4



AAL3/4 CS PDU AAL3/4 CS PDU



AAL3/4 SAR PDU AAL3/4 SAR PDU



Higher layer

Common partconvergence

sublayer

SAR sublayer

ATM layer

Service specificconvergence

sublayer

Information

Assume null

TPAD

User message

Pad message to multipleof 4 bytes. Add header and trailer.

Each SAR-PDU consistsof 2-byte header, 2-bytetrailer, and 44-byte

payload.

H

4 4

2 44 2 2 44 22 44 2

«

«

Information

AAL 3/4



AAL3/4 Operation AAL3/4 Operation



AAL Type 5� Streamlined transport for connection

oriented protocols ± Reduce protocol processing overhead ± Reduce transmission overhead ± Ensure adaptability to existing transport

protocols ± primary function is segmentation and

reassembly of higher-level PDUs



AAL5AAL5� AAL 5 is used for the transfer of data. Due to its simplicity, it is the mostAAL 5 is used for the transfer of data. Due to its simplicity, it is the most popular adaptation layer. popular adaptation layer.

� AAL5 is a Simple Efficient Adaptation Layer (SEAL). The Common PartAAL5 is a Simple Efficient Adaptation Layer (SEAL). The Common Part(CP) AAL5 supports Variable Bit Rate (VBR) traffic, both connection(CP) AAL5 supports Variable Bit Rate (VBR) traffic, both connection- -

oriented and connectionless.oriented and connectionless.

� It is used to transfer most nonIt is used to transfer most non- -SMDS data, such as classical IP over ATMSMDS data, such as classical IP over ATMand LAN Emulation (LANE).and LAN Emulation (LANE).

Eff iciency: Eff iciency:AAL3/4: 4 bytes per message + 4 bytes per cell => 44 User Data BytesAAL3/4: 4 bytes per message + 4 bytes per cell => 44 User Data Bytes/ Cell/ CellAAL5: 8 bytes per message => 48 User Data Bytes / Cell, 8%AAL5: 8 bytes per message => 48 User Data Bytes / Cell, 8%improvementimprovement



AAL5

McGraw-Hill ©The McGraw-Hill Companies, Inc., 2001

F igure 19-26

AAL5



AAL5 CS PDU AAL5 CS PDU



AAL5 CS PDU AAL5 CS PDU P adding ( P ad) P adding ( P ad) : It can be between 0 and 47 bytes long, and is added so that: It can be between 0 and 47 bytes long, and is added so thatthe entire CPSthe entire CPS- -PDU including the padding and the remaining fields in thePDU including the padding and the remaining fields in thetrailer becomes an integer multiple of 48 bytes.trailer becomes an integer multiple of 48 bytes.

C PS user C PS user--toto--user indication (C PS user indication (C PS --UU)UU) : A 1: A 1--byte field used to transfer byte field used to transfer

transparently CPS user transparently CPS user- -toto--user information.user information.

Common part indicator (C PI )Common part indicator (C PI ): A 1: A 1--byte field to support future AAL 5 byte field to support future AAL 5functions.functions.

Length Length : A 2: A 2--byte field used to indicate the length in bytes of the CPS byte field used to indicate the length in bytes of the CPS- -PDUPDU payload . payload .

CRC CRC--3232: This 4: This 4--byte field contains the FCS calculated by the transmitting byte field contains the FCS calculated by the transmittingCPS over the entire contents of the CPSCPS over the entire contents of the CPS- -PDU The pattern used is:PDU The pattern used is: x x32 +32 +

x x26 +26 + x x23 +23 + x x22 +22 + x x16 +16 + x x12 +12 + x x11 +11 + x x10 +10 + x x8 +8 + x x7 +7 + x x5 +5 + x x4 +4 + x x2 +2 + x x + 1.+ 1.



Information

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

UU CPI Length CRCPad

AAL 5

Convergent Sublayer Format

SAR Format

48 bytes of DataATMHeader

1-bit end-of-datagram field (PTI)



Higher layer

Common partconvergence

sublayer

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



AAL5 SAR AAL5 SAR� Simply breaks CS PDU into 48Simply breaks CS PDU into 48- -byte chunks and passes them to ATM byte chunks and passes them to ATM

Layer.Layer.� No overhead bytes added. No overhead bytes added.



AAL5 Operation AAL5 Operation



AAL 2 AAL 2� Designed to support Variable Bit Rate (³Bandwidth on Demand´)Designed to support Variable Bit Rate (³Bandwidth on Demand´)� Provides for partial payloads to support low rate dataProvides for partial payloads to support low rate data� Error protection over full PDUError protection over full PDU� Simple flag to indicate position in messageSimple flag to indicate position in message

� Also AAL 2 was designed to multiplex a number of such low variableAlso AAL 2 was designed to multiplex a number of such low variable bit rate data streams on to a single ATM connection. bit rate data streams on to a single ATM connection.



AAL2 Operation AAL2 OperationCPS packet



CPS packet

CPS packetChannel identi f ier (C ID )Channel identi f ier (C ID ): CPS can multiplex several streams onto a single: CPS can multiplex several streams onto a single

ATM connection. The CID identifies each channel. CID values areATM connection. The CID identifies each channel. CID values areallocated as follows: the 0 value is used as padding, and the 8 to 255 valuesallocated as follows: the 0 value is used as padding, and the 8 to 255 valuesare valid CID values used to identify channels.are valid CID values used to identify channels.

Length indicator (L I ) Length indicator (L I ): Its value is one less than the number of bytes in the: Its value is one less than the number of bytes in theCPSpacket payload. The default maximum length of the CPSCPSpacket payload. The default maximum length of the CPS- -packet packet payload is 45 bytes. payload is 45 bytes.

Header error control (H E C) Header error control (H E C): It use the pattern: It use the pattern x x5 +5 + x x2 + 1. The receiver 2 + 1. The receiver uses the contents of the HEC to detect errors in the header.uses the contents of the HEC to detect errors in the header.

User User--toto--user user--indication (UU I )indication (UU I ): used for transferring information between: used for transferring information betweenthe peer CPS users. The CPS transports this information transparently.the peer CPS users. The CPS transports this information transparently.



CPSCPS--PDUPDUP arity ( P ) P arity ( P ): A 1: A 1--bit field used to detect errors in the STF. bit field used to detect errors in the STF.

S equence numbers (

S N)

S equence numbers (

S N): A 1: A 1--bit field used to number modulo 2 the bit field used to number modulo 2 thesuccessive CPSPDUs.successive CPSPDUs.

O ff set f ield (O SF )O ff set f ield (O SF ): The CPS: The CPS--PDU payload can carry CPS packets in aPDU payload can carry CPS packets in avariety of different arrangements. To extract the CPSvariety of different arrangements. To extract the CPS- -packets from the packets from theCPSCPS--PDU payload, a 6PDU payload, a 6- -bit bit o ff set f ield (O SF )o ff set f ield (O SF ) is used to indicate theis used to indicate thestart of a new CPSstart of a new CPS- -packet in the CPS packet in the CPS- -PDU payload. Specifically, OSFPDU payload. Specifically, OSFgives the number of bytes between the end of the STF and the start of gives the number of bytes between the end of the STF and the start of the first CPSthe first CPS- -packet in the CPS packet in the CPS- -PDU payload.PDU payload.



Thank you for your Thank you for your attentionattention



1 Byte1 Byte

5 Byte5 ByteHeader Header

47 Byte47 BytePayloadPayload

11± ± 4848BytesBytes


11± ± 47 Byte47 BytePayloadPayload


44 Byte44 BytePayloadPayload 4 Bytes4 Bytes


48 Byte48 BytePayloadPayload

nonotaxtax

AAL- 1 Cell Tax AAL-2 Cell Tax

AAL-3/4 Cell Tax AAL-5 Cell Tax

AAL Cell Tax

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