1 6/15/ :56 CS575ATM Adaptation Layer1 Rivier College CS575: Advanced LANs ATM Adaptation Layer

2 6/15/ :56 CS575ATM Adaptation Layer2 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

3 6/15/ :56 CS575ATM Adaptation Layer3 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)

4 6/15/ :56 CS575ATM Adaptation Layer4 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

5 6/15/ :56 CS575ATM Adaptation Layer5 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

6 6/15/ :56 CS575ATM Adaptation Layer6 AAL Services Connectionless Date Transfer Bit Rate Connection Mode Examples of Services Circuit Emulation Constant Bit Rate Video and Audio Constant Variable Connection Oriented Connectionle ss Variable Bit Rate Video and Audio Connection- oriented Data Transfer AAL TYPE AAL 1AAL 2 AAL 3/4 AAL 5 Class A Class BClass C Class D RelatedNot Related Service Timing between Source and Destination AAL3/4 AAL 5

7 6/15/ :56 CS575ATM Adaptation Layer7 AAL Type 1 Protocol 0 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

8 6/15/ :56 CS575ATM Adaptation Layer8 AAL Type 1 SAR-PDU 0 Consists of 1 octet header (PCI) and 47 octets of payload 0 Sequence Number (SN): A 1-bit Convergence Sublayer Indication and 3-bit sequence count to detect deletion or misinsertion of cells 0 Sequence Number Protection (SNP): 3-bit CRC with even parity for detecting and correcting SN error SN SNPSAR-PDU Payload 4 bits 47 Octets

9 6/15/ :56 CS575ATM Adaptation Layer9 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 bit 3 bit 1 bit 3 bits Convergence Sublayer Indication (CSI) Sequence Number (SN)

10 6/15/ :56 CS575ATM Adaptation Layer10 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

11 6/15/ :56 CS575ATM Adaptation Layer11 AAL Type 2 Protocol 0 Designed to support Class B (VBR) traffic 0 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 transfer 0 Supports voice compression and silence suppression 0 Supports idle voice channel deletion 0 Supports multiple user channels with varying bandwidth on a single ATM connection

12 6/15/ :56 CS575ATM Adaptation Layer12 AAL Type 2 Voice Application Examples 0 PBX-to-PBX trunking for compressed voice 0 ATM trunking on public-switched telephone network 0 ATM backbone for cellular systems and personal communications services (PCS) 0 ATM backbone connectivity to packet telephone

13 6/15/ :56 CS575ATM Adaptation Layer13 PBX-to-PBX Connectivity Compression Silence Suppression Packetization PBX PCM Voice Compression Silence Suppression Packetization PBX PCM Voice AAL ATM PHY ATM Network AAL ATM PHY

14 6/15/ :56 CS575ATM Adaptation Layer14 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

15 6/15/ :56 CS575ATM Adaptation Layer15 AAL Type 2 Protocol Sublayers Service Specific Convergence Sublayer (SSCS) Common Part Sublayer (CPS)

16 6/15/ :56 CS575ATM Adaptation Layer16 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 AAL 0 Assembling/disassembling the variable payload associated with each individual user 0 Error detection and correction 0 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

17 6/15/ :56 CS575ATM Adaptation Layer17 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 Specific Convergence Sublayer (SSCS) Common Part Sublayer (CPS) ATM Layer AAL-SAP ATM-SAP

18 6/15/ :56 CS575ATM Adaptation Layer18 Format of AAL2 Packet 0 Channel Identification (CID): Uniquely identifies the individual user channel (LLC) within the AAL2, and allows up to 248 ( ) 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. CID 8 bits Information 1 to 45/64 octets LI 6 bits UUI 5 bits HEC 5 bits Packet Header (PH) Packet Payload (PP) Packet

19 6/15/ :56 CS575ATM Adaptation Layer19 Format of AAL2 CPS-PDU 0 Packets are combined into CPS-PDU payload 0 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 OSF 6 bits CPS Information SN 1 bit P 1 bit PAD 0 to 47 octets Start Field CPS-PDU Payload CPS -PDU Cell Header 5 octets

20 6/15/ :56 CS575ATM Adaptation Layer20 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 Forum 0 A null SSCS satisfies most mobile voice applications

21 6/15/ :56 CS575ATM Adaptation Layer21 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 optional 0 CPCS provides message mode or streaming mode services

22 6/15/ :56 CS575ATM Adaptation Layer22 AAL Type 3/4 Protocol Sublayers Service Specific Convergence Sublayer (SSCS) Common Part Convergence Sublayer (CPCS) Segmentation and Reassembly Sublayer (SAR)

23 6/15/ :56 CS575ATM Adaptation Layer23 AAL Type 3/4 CPCS-PDU 4-octet header and 4-octet trailer CPICommon Part Indicator BtagBeginning Tag BASizeBuffer Allocation Size PADPadding ALAlignment EtagEnd Tag LengthLength of CPCS-PDU Payload CPIBtagBASize CPCS-PDU PayloadPADAL EtagLength

24 6/15/ :56 CS575ATM Adaptation Layer24 AAL Type 3/4 SAR-PDU 2-octet header, 44-octet payload, and 2-octet trailer STSegment Type SNSequence Number MIDMultiplexing Identifier LILength Indication CRC-10CRC Code SNMIDSAR-PDU PayloadLI CRC-10ST 2 bits 4 bits10 bits352 bits 10 bits6 bits

25 6/15/ :56 CS575ATM Adaptation Layer25 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 PADPadding CPCS-UUCPCS User-to-User Indicator CPICommon Part Indicator LengthCPCS-PDU Payload Length CRC-32Cyclic Redundancy Chuck

26 6/15/ :56 CS575ATM Adaptation Layer26 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 CPCS-PDU Payload

27 6/15/ :56 CS575ATM Adaptation Layer27 AAL Type 5 Protocol (Concluded) 0 The CPCS-PDU is divided into 48 octets SAR-PDUs 0 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 trailer 0 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)

28 6/15/ :56 CS575ATM Adaptation Layer28 W. Stalling, Local and Metropolitan Area Networks, 6 th edition, Prentice Hall, 2000, Chapter 11 References W. Stalling, Data and Computer Communications, 6 th edition, Prentice Hall, 2002, Chapters A. Wu, Advanced Local Area Networks, Lectures & Slides, Rivier College, 2001.