The ATM Adaption Layer, (AAL), makes the ATM layer services more adaptable to specific services. The specific services may include user services, control services and management services. The AAL is the layer above the ATM layer and it is responsible for converting the information from the higher layers into 48 byte lengths so that the ATM layer can add the 5 byte header to make the 53 byte cell. The two main functions of this AAL are to provide functions needed to support applications and to break up information into units that will fit into cells. The AAL layer is thus divided into two sublayers: the convergence sublayer (CS) and segmentation and reassembly sublayer (SAR). The convergence sublayer provides the functions needed to support specific applications, such as handling the cell delay variation and keeping a track of the clock. Each application accesses the AAL at a service access point (SAP), which is the address of the application. The SAR sublayer packs the information from the CS into cells and unpacks the information at the destination. The SAR maps SAR headers plus CS information into 48 byte cells.
The AAL accommodates all services and in particular adapts both packet switched and circuit switched services. The CCITT service classification is based upon the timing relation, bit rate, and connection mode. Figure 3 depicts the CCITT service classification according to these parameters. There are five AAL types that correspond approximately to the CCITT service classes as shown in Figure 3.
Figure 3: CCITT Service Classification
Class A is a constant bit rate connection with a timing relationship between source and destination and is often called circuit emulation. This could be used to carry voice of 64 kb/s or constant bit rate video. This could also be used for intelligent multiplexing equipment that needs what is essentially a circuit. The adaption layer that deals with this type of traffic is called AAL 1. AAL 1 operates by placing a 1 byte header on 47 bytes of user data and then transferring the 48 bytes to the ATM layer. The SAR of the AAL 1 will be notified of the extistance of the CS sublayer by the CS indicator, (CSI). A sequence number, (SN), is passed from the CS sublayer to the SAR and this SN can be used to detect lost of missing SAR loads. Finally the header of the SAR is protected by a sequence number protection, (SNP), field which can inform the CS sublayer of bit errors. The layout of the SAR is shown in Figure 4.
Figure 4: AAL 1 Cell Format
Class A is most appropriate for voice transmission that does not incorporate time assignment speech interpolation (TASI). In TASI speech is only transmitted when the speaker is active. To incorporate efficiencies that can be achieved with coding and compression techniques on real time services there is a second class called Class B. Class B is used for services similar to Class A but which are not constant bit rate. Examples of these would be variable bit rate audio and video. AAL 2 is the AAL layer responsible for providing these type of services from the ATM layer to the higher layers. AAL 2 is not yet fully specified but there is some indication as to the format of the protocol. As the intended use is for compressed voice and video there will likely be strict bounds on the bit error rate. To help the system there is likely to be a CRC in the SAR to protect all the data being sent. As the user field may not be full it is likely that the user amount of information will be variable length and this will be indicated by the length indicator, (LI).
The remaining CCITT classes of service and AAL's are used for services which have no relationship of timing between source and destination. These services are esentially variable bit rate data services and can be differentiated by whether they are connection oriented or not. Class C is connection oriented data transfer while Class D is connectionless. The distinction between the conectionless and connection orientated AAL's has been lessened to such an extent that they now share the same AAL called AAL 3/4. Initially AAL 3 was for Class C and AAL 4 was for Class B services. The AAL 3/4 takes information from the higher layer and after the CS sublayer operates on it the SAR breaks the data up into 44 byte sizes and adds 4 bytes of header fields to make a 48 byte information load for the ATM layer cell. The four bytes of header are made up of a 10 bit CRC, a LI of 6 bits and an SN of 4 bits. There is also a 10 bit field reserved for either multiplexing or else are reserved for future use. There is also a field called the segment type, (ST), which indicated whether the SAR is the start, middle or end of a message. The CS sublayer also adds a header and trailer to the data coming from the higher layers. As yet that is not fully defined. Because of the high overhead of the AAL 3/4, 4 bytes for every 48 bytes of ATM user information, and because of the complexity of the protocol there has been a simplified AAL proposed called AAL 5 for data transfer. The AAL 5 basically puts the headers and trailers onto the CS-PDU rather than the SAR-PDU. This has a large number of advantages like improved efficiency and better error correction and detection . The format of the AAL 5 is shown in Figure 5.
Figure 5: AAL 5 Cell Format
There is also the possibility of designing an AAL specific to a particular application service if there is a need. Even if a new AAL is designed to work with an application the overall system still adheres to ATM standards.