1、CS575,ATM Adaptation Layer,1,Rivier CollegeCS575: Advanced LANs ATM Adaptation Layer,CS575,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 da

2、ta 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

3、 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

4、into higher layer PDUs at the receiving end,CS575,ATM Adaptation Layer,3,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

5、/bit stream for receiving Support multiple protocols,CS575,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 relati

6、onship between sender and receiver Related Not related Bit rate Constant bit rate Variable bit rate Connection mode Connection-oriented Connectionless,CS575,ATM Adaptation Layer,5,AAL Protocols and Service Classes (Concluded),Four AAL protocols have been defined to support four AAL service classes A

7、AL 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,6,AAL Services,Connectionless,Date Transfer,Bit Rate,Connection,Mode,Examples,of,Services,Circuit,Emulation,Constant Bit,Rate Video,and Audio,Constant,Variable,Connection Oriented,Co

8、nnectionless,Variable Bit,Rate Video,and Audio,Connection-,oriented,Data,Transfer,AAL,TYPE,AAL 1,AAL 2,AAL 3/4,AAL 5,Timing,between,Source and,Destination,AAL3/4AAL 5,CS575,ATM Adaptation Layer,7,AAL Type 1 Protocol,Supports Class A traffic, i.e., constant bit rate data with specific requirements fo

9、r 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

10、 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 PC

11、I (header) error,CS575,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 wi

12、th even parity for detecting and correcting SN error,SN,SNP,SAR-PDU Payload,4 bits,4 bits,47 Octets,CS575,ATM Adaptation Layer,9,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

13、format operations, the CSI value in SAR-PDU headers with an even SN (SN = 0, 2, 4, 6) is set to 1,CS575,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

14、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,CS575,ATM Adaptation Layer,11,AAL Type 2 Protocol,Designed to support Class B (VBR) traffic Supports variable bit rate data whe

15、re 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

16、voice compression and silence suppression Supports idle voice channel deletion Supports multiple user channels with varying bandwidth on a single ATM connection,CS575,ATM Adaptation Layer,12,AAL Type 2 Voice Application Examples,PBX-to-PBX trunking for compressed voice ATM trunking on public-switche

17、d telephone network ATM backbone for cellular systems and personal communications services (PCS) ATM backbone connectivity to packet telephone,CS575,ATM Adaptation Layer,13,PBX-to-PBX Connectivity,AAL ATM PHY,CS575,ATM Adaptation Layer,14,AAL Type 2 Packets,AAL2 provides bandwidth-efficient transmis

18、sion 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-leng