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 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 CS575 ATM Adaptation Layer

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 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) The AAL functions are divided into two sublayers: segmentation and reassembly sublayer (SAR) and convergence sublayer (CS). On transmission, the SAR sublayer performs segmentation of higher layer PDUs into a suitable size for the information field of the ATM cell. On receiving, it reassembles the information field of the ATM cells into higher layer PDUs. The functions performed in the CS sublayer are service dependent. It performs mapping of the higher layer PDUs into the information field of an ATM cell. Four AAL service classes have been specified depending on the timing relationship, bit rate, and connection mode. CS575 ATM Adaptation Layer

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 CS575 ATM Adaptation Layer

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 CS575 ATM Adaptation Layer

AAL Services CS575 ATM Adaptation Layer Class A Class B Class C Class D Related Not Related Service Timing between Source and Destination Bit Rate Constant Variable Connection Mode Connection Oriented Connectionless Circuit Emulation Connection- Examples Variable Bit oriented Connectionless of Rate Video Constant Bit Data Date Transfer Services and Audio Rate Video Transfer and Audio AAL 3/4 To minimize the number of AAL protocols, ITU-T (CCITT) proposed a service classification which is specific to the AAL This classification was made with respect to timing relation, bit rate, and connection mode. Four classes of services have been defined: class A, class B, class C, and class D. Four AAL protocols have also been developed: AAL 1, AAL 2, AAL 3/4, and AAL 5. AAL 1 was designed to provide class A service. AAL 2 was designed for class B traffic. AAL3 and AAL 4 were combined into a single AAL 3/4 protocol to provide class C and class D services. A more efficient protocol AAL 5 was developed for class C traffic. However, no strict relationship between the the AAL service classes and the AAL protocol types is requested. In some applications, the AAL layer may be empty and the AAL functions are just the reception/delivery of the ATM-SDUs. AAL AAL3/4 AAL 5 AAL 1 AAL 2 TYPE AAL 5 CS575 ATM Adaptation Layer

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 AAL 1 supports Class A traffic. Timing information is transferred between source and destination. If necessary, information about the data structure can also be conveyed. Indication of lost or errored information is sent to the higher layer if these failures cannot be recovered within the AAL. The functions performed by the AAL are as follows: • Segmentation and reassembly of user information • Handling of cell delay variation • Handling of cell payload assembly delay • Handling of lost and misinserted cells • Source clock frequency recovery at the receiver • Recovery of the source data structure at the receiver • Monitoring of AAL-PCI for bit errors as well as handling of those errors • Minitoring of the user information field for bit errors and possible corrective action In the case of circuit emulation, monitoring of the end-to-end QoS is necessary, which will be located in the CS. For this purpose, a CRC may be calculated for the information carried in one or more cells. The result is transferred to the receiver within the information field of a cell or in a special OAM cell. CS575 ATM Adaptation Layer

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 The SAR-PDU consists of 48 octets. the PCI is subdivided into a 4-bit sequence number (SN) and a 4-bit sequence number protection (SNP) field. The SN consists of a convergence sublayer indication (CSI) bit and a 3-bit sequence count. The CSI bit can be used to transfer timing information and/or information about the data structure. The SN field makes it possible to detect the loss or misinsertion of cells. The SNP field provides error detection and correction capabilities. The following two step approach will be used to allow the correction of all 1 bit errors and the detection of all 2 bit errors: 1. The SN is protected by the polynomial G(x) = x3 + x + 1. 2. The resulting 7 bit code word is protected by an even parity check. CS575 ATM Adaptation Layer

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 1 bit 3 bits Convergence Sublayer Indication (CSI) Sequence Number (SN) Cyclic Redundancy Check Even Parity 1 bit 3 bit CS575 ATM Adaptation Layer

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 The functions of the CS depend strongly on the service to be supported. Some of these functions are listed below: • Handling of cell delay variation. A buffer is used to support this function. • Handling of lost and misinserted cells. • Source clock frequency recovery. The 4-bit residual time stamp (RTS) is transferred by the CSI bit in successive SAR-PDU headers with an odd number of sequence count. • Transfer of structure information between source and destination. For the P format an additional pointer field (1 octet) is provided in the SAR-PDU payload. This pointer indicates the start of a structured block. The CSI in SAR-PDU header with an even SN is set to 1. • Forward error correction may be used to ensure high quality for some video and audio applications. CS575 ATM Adaptation Layer

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 AAL type 2 is proposed for VBR services with a timing relation between source and destination (class B traffic, e.g., VBR audio and video). AAL type 2 is not well defined yet. The functions performed by the AAL 2 are as follows: • Segmentation and reassembly of user information • Handling of cell delay variation • Handling of lost and misinserted cells • Recovery of the source clock at the receiver • Monitoring of AAL-PCI for bit errors as well as handling of these errors • Minitoring of the user information field for bit errors and possible corrective action CS575 ATM Adaptation Layer

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 CS575 ATM Adaptation Layer

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 CS575 ATM Adaptation Layer

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 CS575 ATM Adaptation Layer

AAL Type 2 Protocol Sublayers Service Specific Convergence Sublayer (SSCS) Common Part Sublayer (CPS) CS575 ATM Adaptation Layer

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 CS575 ATM Adaptation Layer

AAL 2 Structure Service Specific Convergence Sublayer (SSCS) AAL-SAP Service Specific Convergence Sublayer (SSCS) SSCS-PDU Header (if present) SSCS-PDU Trailer SSCS-PDU Payload SSCS-PDU Packet Header (PH) Packet Payload (PP) Packet Common Part Sublayer (CPS) Start Field (STF) CPS-PDU Payload CPS-PDU ATM-SAP ATM Layer Cell Header Cell Payload CS575 ATM Adaptation Layer

Format of AAL2 Packet CID 8 bits Information 1 to 45/64 octets LI 6 bits UUI 5 bits HEC Packet Header (PH) Packet Payload (PP) 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. CS575 ATM Adaptation Layer

Format of AAL2 CPS-PDU Packets are combined into CPS-PDU payload Cell Header 5 octets OSF 6 bits SN 1 bit P 1 bit PAD 0 to 47 octets CPS Information Start Field CPS-PDU Payload 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 CS575 ATM Adaptation Layer

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 CS575 ATM Adaptation Layer

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 There are two ways to transport messages in data communications: connection-oriented and connectionless. AAL3 was developed for connection-oriented communications and AAL4 was developed for connectionless communications. Since many functions performed by AAL 3 and 4 are common, the CS sublayer was further divided by ITU-T into two more sublayers, the common part CS (CPCS) and service specific CS (SSCS) so that AAL3 and AAL4 could share the same SAR and CPCS coding. With this layering structure, SSCS would provide AAL3 with connection management, flow control, error recovery, etc., while AAL4 would have no need for this service. In 1992, ITU-T merged AAL3 and 4 into a single AAL3/4 protocol. SSCS sublayer may provide assured or non-assured services. CPCS supports multiple SSCS services. Two modes of service have been defined for CPCS: message mode service and stream mode service. The message mode can be used for framed data transfer (e.g., HDLC frames). A single AAL-SDU is transported in one or more CS-PDUs. The stream mode is suitable for the transfer of low speed data with low delay requirements. One or more fixed-size AAL-SDUs are transported in one CS-PDU. CS575 ATM Adaptation Layer

AAL Type 3/4 Protocol Sublayers Service Specific Convergence Sublayer (SSCS) Common Part Convergence Sublayer (CPCS) Segmentation and Reassembly Sublayer (SAR) CS575 ATM Adaptation Layer

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 2 0 - 3 1 1 2 CPI Btag BASize CPCS-PDU Payload PAD AL Etag Length The Common Part Indicator (CPI) field is used to indicate the usage of other CPCS-PDU header and trail fields. It varies depending on the modes of operation. The Beginning Tag (Btag) field allows association of header and trailer of the CPCS-PDU, to ensure that all SAR-PDUs have been received correctly. This is done by putting the same value in both Btag and Etag. The receiving AAL will check whether they are still the same. The Buffer Allocation Size Indication (BASize) field indicates to the receiving AAL the maximum buffering requirements to receive the CPCS-PDU. For message mode, it is set to the CPCS-PDU length. For stream mode, it is set to equal or greater than CPCS-PDU since the length of the CPCS-SDU (payload of CPCS-PDU) being transmitted is not known to the sending AAL. The maximum CPCS-PDU payload size supported is 65,535 octets. The Padding (PAD) field is from 0 to 3 octets to make the CPCS-PDU payload as an integral multiple of 4 octets. The Alignment (AL) field is used to make 32 bit alignment for the CPCS-PDU trailer. The Length field indicates the length of the CPCS-PDU payload. CS575 ATM Adaptation Layer

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 2 bits 4 bits 10 bits 352 bits 6 bits 10 bits ST SN MID SAR-PDU Payload LI CRC-10 AAL3/4 SAR split CPCS-PDU into SAR-PDUs. The Segment Type (ST) field indicates whether the SAR-PDU is the Beginning of Message (BOM, ST = 10), Continuation of Message (COM, ST = 00), End of Message (EOM, ST = 01), or a Single Segment Message (SSM, ST = 11). The Sequence Number (SN) field is used to detect the loss or misordering of a SAR-PDU. The Multiplexing Identification (MID) field is used to identify SAR-PDUs belonging to a particular SAR-SDU. MID allows the multiplexing of SAR-PDUs from different CPCS-PDUs and reassembly of them correctly at the receiving AAL. The SAR-PDU payload field is 44 octets long. The Length Indicator (LI) indicates how many of the 44 octets of payload actually carrying user data. An EOM or SSM SAR-PDU may not contain 44 octets of data. The value of LI must be a multiple of 4 octets. The CRC field is used to detect bit errors for the entire SAR-PDU. CS575 ATM Adaptation Layer

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 The AAL5 CPCS-PDU payload may vary from 0 to 65,535 octets. The CPCS User-to-User field is used to support future AAL functions. The AAL5 SAR splits CPCS-PDU into SAR-PDUs and passes them to the ATM layer. All 48 octets are the SAR-PDU payload. No SAR-PDU header or trailer, no MID; therefore, SAR-PDUs are reassembled on VCI only. Use Payload Type field in the cell header to indicate whether the SAR-PDU belongs to BOM (PT = 0X0), COM (PT = 0X0), or EOM (PT = 0X1). The EOM carries the 8 octets trailer of the CPCS-PDU. CS575 ATM Adaptation Layer

AAL Type 5 SAR-PDU CS575 ATM Adaptation Layer CPCS-SDU CPCS-PDU CPCS-PDU Payload PAD CPCS-PDU Trailer 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 CS575 ATM Adaptation Layer

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) CS575 ATM Adaptation Layer

References W. Stalling, Local and Metropolitan Area Networks, 6th edition, Prentice Hall, 2000, Chapter 11 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. CS575 ATM Adaptation Layer