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Rivier College CS575: Advanced LANs ATM Adaptation Layer

Rivier College CS575: Advanced LANs ATM Adaptation Layer. ATM Adaptation Layer Functions. 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

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Rivier College CS575: Advanced LANs ATM Adaptation Layer

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  1. Rivier CollegeCS575: Advanced LANsATM Adaptation Layer ATM Adaptation Layer

  2. ATM Adaptation Layer Functions • 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 • 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 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 ATM Adaptation Layer

  3. Higher Layers Convergence Sublayer (CS) ATM Adaptation Layer (AAL) Segmentation and Reassembly Sublayer (SAR) Virtual Channel (VC) Virtual Path (VP) ATM Layer (ATM) Transmission Convergence Sublayer (TC) Physical Layer (PL) Physical Medium Sublayer (PM) ATM Adaptation Sublayers • Convergence Sublayer (CS): • Handling lost/misdelivered cells • Timing recovery • Interleaving • Segmentation and Reassembly Sublayer (SAR): • Split frames/bit stream into cells for transmission • Reassemble frames/bit stream for receiving • Support multiple protocols ATM Adaptation Layer

  4. AAL Protocols and Service Classes • To minimize the number of AAL protocols, ITU-T proposed a service classification 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 ATM Adaptation Layer

  5. AAL Protocols and Service Classes (Concluded) • 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 ATM Adaptation Layer

  6. Class A Service Class B Class C Class D Related Not Related AAL Services Timing between Sourceand Destination BitRate Constant Variable Connection Connectionless ConnectionOriented Mode Circuit Connection- Emulation VariableBit Examples Connectionless oriented RateVideo of Date Transfer Data ConstantBit andAudio Services Transfer RateVideo andAudio AAL 3/4 AAL3/4AAL 5 AAL AAL 1 AAL 2 TYPE AAL 5 ATM Adaptation Layer

  7. AAL Type 1 Protocol • 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) • 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 ATM Adaptation Layer

  8. 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 4 bits 4 bits 47 Octets SN SNP SAR-PDU Payload ATM Adaptation Layer

  9. 1 bit 3 bits Convergence Sublayer Indication (CSI) Sequence Number (SN) 1 bit 3 bit Cyclic Redundancy Check Even Parity AAL Type 1 Sequence Number and Sequence Number Protection • The 4 bit RTS is transferred by the CSI bit in successive SAR-PDU headers 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 ATM Adaptation Layer

  10. 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 ATM Adaptation Layer

  11. 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 and silence suppression • Supports idle voice channel deletion • Supports multiple user channels with varying bandwidth on a single ATM connection ATM Adaptation Layer

  12. AAL Type 2 Voice Application Examples • PBX-to-PBX trunking for compressed voice • ATM trunking on public-switched telephone network • ATM backbone for cellular systems and personal communications services (PCS) • ATM backbone connectivity to packet telephone ATM Adaptation Layer

  13. PBX PCM Voice PBX PCM Voice Compression Silence Suppression Packetization Compression Silence Suppression Packetization ATM Network AAL ATM PHY PBX-to-PBX Connectivity AAL ATM PHY ATM Adaptation Layer

  14. 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 ATM Adaptation Layer

  15. AAL Type 2 Protocol Sublayers Service Specific Convergence Sublayer (SSCS) Common Part Sublayer (CPS) ATM Adaptation Layer

  16. AAL 2 Common Part Sublayer • Defines an end-to-end AAL connection as a concatenation of AAL2 channels • Each AAL2 channel is a bi-directional virtual channel, with the same channel 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 each individual user • Error detection and correction • Multiplexing multiple AAL channels (merging multiple streams of CPS packets) onto a single ATM connection • Provides QoS through the choice of AAL-SAP for data transfer ATM Adaptation Layer

  17. SSCS-PDU Trailer (if present) SSCS-PDU Header (if present) SSCS-PDU Payload Packet Payload (PP) Packet Header (PH) Start Field (STF) CPS-PDU Payload Cell Payload Cell Header AAL 2 Structure AAL-SAP Service Specific Convergence Sublayer (SSCS) SSCS-PDU Packet Common Part Sublayer (CPS) CPS-PDU ATM-SAP ATM Layer ATM Adaptation Layer

  18. UUI 5 bits HEC 5 bits CID 8 bits Information 1 to 45/64 octets LI 6 bits Packet Payload (PP) Packet Header (PH) Packet 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. ATM Adaptation Layer

  19. Format of AAL2 CPS-PDU • Packets are combined into CPS-PDU payload • 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 • 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 OSF 6 bits Cell Header 5 octets SN 1 bit PAD 0 to 47 octets P 1 bit CPS Information CPS-PDU Payload Start Field CPS -PDU ATM Adaptation Layer

  20. AAL 2 Service Specific Convergence Sublayer • SSCS is the link between the AAL2 CPS and the higher layer applications 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 ATM Adaptation Layer

  21. 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 ATM Adaptation Layer

  22. AAL Type 3/4 Protocol Sublayers Service Specific Convergence Sublayer (SSCS) Common Part Convergence Sublayer (CPCS) Segmentation and Reassembly Sublayer (SAR) ATM Adaptation Layer

  23. 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 1 1 0 - 3 1 1 2 2 Etag Length CPCS-PDU Payload PAD AL CPI Btag BASize ATM Adaptation Layer

  24. AAL Type 3/4 SAR-PDU 2-octet header, 44-octet payload, and 2-octet trailer ST Segment Type SN Sequence Number MID Multiplexing Identifier LI Length Indication CRC-10 CRC Code 6 bits 10 bits 4 bits 10 bits 352 bits 2 bits ST CRC-10 SN MID SAR-PDU Payload LI ATM Adaptation Layer

  25. 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 ATM Adaptation Layer

  26. AAL Type 5 SAR-PDU CPCS-SDU CPCS-PDU CPCS-PDU Trailer CPCS-PDU Payload PAD SAR-PDU Payload SAR-PDU Payload SAR-PDU Payload SAR-PDU Payload SAR-PDU Payload SAR-PDU SAR-PDU SAR-PDU SAR-PDU SAR-PDU ATM Adaptation Layer

  27. AAL Type 5 Protocol (Concluded) • 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) ATM Adaptation Layer

  28. 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. ATM Adaptation Layer

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