ATM Adaptation Layer

The ATM adaptation layer, which resides atop the ATM layer, is responsible for mapping the requirements of higher layer protocols onto the ATM network (1). It operates in ATM devices at the edge of the ATM network and is totally absent in ATM switches. The adaptation layer is divided into two sublayers: the convergence sublayer (CS), which performs error detection and handling, timing, and clock recovery; and the segmentation and reassembly (SAR) sublayer, which performs segmentation of convergence sublayer protocol data units (PDUs) into ATM cellsized SAR sublayer service data units (SDUs) and vice versa.

In order to support different service requirements, the ITU-T has proposed four AAL-specific service classes. Figure 5 depicts the four service classes defined in recommendation I.362 (1). Note that even though these AAL service classes are similar in many ways to the ATM layer service categories defined in the previous section, they are not the same; each exists at a different layer of the protocol reference model, and each requires a different set of functions.

Figure 5. Service classification for AAL

AAL service class A corresponds to constant bit rate services with a timing relation required between source and destination. The connection mode is connection- oriented. The CBR audio and video belong to this class. Class B corresponds to variable bit rate (VBR) services. This class also requires timing between source and destination, and its mode is connection-oriented. The VBR audio and video are examples of class B services.

Class C also corresponds to VBR connection-oriented services, but the timing between source and destination needs not be related. Class C includes connection-oriented data transfer such as X.25, signaling, and future high-speed data services. Class D corresponds to connectionless services. Connectionless data services such as those supported by LANs and MANs are examples of class D services.

Four AAL types (Types 1, 2, 3/4, and 5), each with a unique SAR sublayer and CS sublayer, are defined to support the four service classes. AAL Type 1 supports constant bit rate services (class A), and AAL Type 2 supports variable bit rate services with a timing relation between source and destination (class B). AAL Type 3/4 was originally specified as two different AAL types (Type 3 and Type 4), but because of their inherent similarities, they were eventually merged to support both class C and class D services. AAL Type 5 also supports class C and class D services.

AAL Type 5. Currently, the most widely used adaptation layer is AAL Type 5. AAL Type 5 supports connection-oriented and connectionless services in which there is no timing relation between source and destination (classes C and D). Its functionality was intentionally made simple in order to support high-speed data transfer. AAL Type 5 assumes that the layers above the ATM adaptation layer can perform error recovery, retransmission, and sequence numbering when required, and thus, it does not provide these functions. Therefore, only nonassured operation is provided; lost or corrupted AAL Type 5 packets will not be corrected by retransmission.

Figure 6 depicts the SAR-SDU format for AAL Type 5 (5,6). The SAR sublayer of AAL Type 5 performs segmentation of a CS-PDU into a size suitable for the SAR-SDU payload. Unlike other AAL types, Type 5 devotes the entire 48-octet payload of the ATM cell to the SAR-SDU; there is no overhead. An AAL specific flag (end-of-frame) in the ATM PT field of the cell header is set when the last cell of a CS-PDU is sent. The reassembly of CS-PDU frames at the destination is controlled by using this flag.

Figure 6. SAR-SDU format for AAL Type 5

Figure 7 depicts the CS-PDU format for AAL Type 5 (5,6). It contains the user data payload, along with any necessary padding bits (PAD) and a CS-PDU trailer, which are added by the CS sublayer when it receives the user information from the higher layer. The CS-PDU is padded using 0 to 47 bytes of PAD field to make the length of the CS- PDU an integral multiple of 48 bytes (the size of the SAR- SDU payload). At the receiving end, a reassembled PDU is passed to the CS sublayer from the SAR sublayer, and CRC values are then calculated and compared.

Figure 7. CS-PDU format, segmentation and reassembly of AAL Type 5

If there is no error, the PAD field is removed by using the value of length field (LF) in the CS-PDU trailer, and user data is passed to the higher layer. If an error is detected, the erroneous information is either delivered to the user or discarded according to the user’s choice. The use of the CF field is for further study.

AAL Type 1. AAL Type 1 supports constant bit rate services with a fixed timing relation between source and destination users (class A). At the SAR sublayer, it defines a 48-octet service data unit (SDU), which contains 47 octets of user payload, 4 bits for a sequence number, and a 4-bit CRC value to detect errors in the sequence number field.

AAL Type 1 performs the following services at the CS sublayer: forward error correction to ensure high quality of audio and video applications, clock recovery by monitoring the buffer filling, explicit time indication by inserting a time stamp in the CS-PDU, and handling of lost and misinserted cells that are recognized by the SAR. At the time of writing, the CS- PDU format has not been decided.

AAL Type 2. AAL Type 2 supports variable bit rate services with a timing relation between source and destination (class B). AAL Type 2 is nearly identical to AAL Type 1, except that it transfers service data units at a variable bit rate, not at a constant bit rate. Furthermore, AAL Type 2 accepts variable length CS-PDUs, and thus, there may exist some SAR-SDUs that are not completely filled with user data.

The CS sublayer for AAL Type 2 performs the following functions: forward error correction for audio and video services, clock recovery by inserting a time stamp in the CS-PDU, and handling of lost and misinserted cells. At the time of writing, both the SAR-SDU and CS-PDU formats for AAL Type 2 are still under discussion.

AAL Type 3/4. AAL Type 3/4 mainly supports services that require no timing relation between the source and destination (classes C and D). At the SAR sublayer, it defines a 48-octet service data unit, with 44 octets of user payload; a 2-bit payload type field to indicate whether the SDU is at the beginning, middle, or end of a CS-PDU; a 4-bit cell sequence number; a 10-bit multiplexing identifier that allows several CS-PDUs to be multiplexed over a single VC; a 6-bit cell payload length indicator; and a 10-bit CRC code that covers the payload. The CS-PDU format allows for up to 65535 octets of user payload and contains a header and trailer to delineate the PDU.

The functions that AAL Type 3/4 performs include segmentation and reassembly of variable-length user data and error handling. It supports message mode (for framed data transfer) as well as streaming mode (for streamed data transfer). Because Type 3/4 is mainly intended for data services, it provides a retransmission mechanism if necessary.

ATM Signaling. ATM follows the principle of out-of-band signaling that was established for N-ISDN. In other words, signaling and data channels are separate. The main purposes of signaling are (1) to establish, maintain, and release ATM virtual connections and (2) to negotiate (or renegotiate) the traffic parameters of new (or existing) connections (7).

The ATM signaling standards support the creation of point-to-point as well as multicast connections. Typically, certain VCI and VPI values are reserved by ATM networks for signaling messages. If additional signaling VCs are required, they may be established through the process of metasignaling.