by Abhijit S. Pandya; Ercan Sen CRC Press, CRC Press LLC
ISBN: 0849331390 Pub Date: 110198
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IV. ATM Adaptation Layer AAL
ATM networks are designed for providing transport services to applications which use a wide variety of formats, and information types Data, Video, Voice, etc.. The majority of these applications and their
related protocols generate data units of variable sizes before sending the applications data across the ATM network as they have to be adapted to the ATM network. At the transmitter the variable-length
data must be segmented into cells. At the receiver these cells must be reassembled back into original variable length data. This entire preparation is carried out by the ATM Adaptation Layer AAL which is
the highest level of the ATM protocol stack.
A. AAL Structure
The structure of the AAL and its interfaces are illustrated in Figure 5-14. The various sub-layers within the AAL are shown along with the primitives regarding their respective protocol Data Units PDUs that
are passed between them. The AAL is divided into the segmentation and Reassembly SAR sub-layer and the convergence sub-layer CS. The ATM cell payload, also called the SAR-PDU primitives, is
passed to and from the ATM layer through the ATM Service Access Point ATM-SAP.
The CS is further divided into Common Part CP and service specific SS components. The primitives regarding the AAL PDUs are passed through the AAL-SAP shown in Figure 5-14 at the top to the
higher layers.
The SSCS is responsible for a series of actions that must be taken for providing the services demanded by the quality of services QOS parameters. In some cases the SSCS is further divided into Service
Specific Coordination Function SSCF and Service Specific Connection Oriented Protocol SSCOP components.
Figure 5-12 Special cells.
Figure 5-13 Some more special cells.
Figure 5-14 A generic model for the AAL protocol sub-layer.
Figure 5-15 ATM service classes.
B. Classification Based on AAL Service Attributes
1. Timing relationship between source and destination 2. Bit rate
3. Connection Mode
Figure 5-15 illustrates the four ITU-T defined service classes labeled A through D. Class A is constant bit rate CBR while C through D are variable bit rate VBR. Classes X and Y are the additional two
proposed by the ATM forum. In class X cells are transported as they are presented to the ATM layer; thus it is defined as an Unassigned Bit Rate UBR Service. In the case of class Y the cells are
transported across the ATM network if there is capacity available for them. As a result, class Y is defined as Available Bit Rate ABR Service.
The various service classes shown in Figure 5-15 result in six types of services. The null type of AAL AAL-0 is used for cell relay services that are inherently cell based and need no adaptation. The rest are
labeled AAL-1, AAL-2, and so forth.
C. ATM Adaptation Layer Functions
ATM cells consist of 5 byte headers and 48 byte payloads. ATM cell headers are used for multiplexing and demultiplexing as well as switching based on the connection number at the ATM layer. The AAL
only deals with payloads so there are no cells at this layer. The cell header as discussed in Section II of this chapter is generated at the transmitter to the ATM layer and stripped at the receiver.
As seen in Figure 5-15 ATM supports various service classes. Thus AAL is composed of service classes. Thus AAL is composed of a series of compartments and it is modular in structure. Each compartment
contains a service dependent function and some other functions that are common to all ATM network applications. At the transmitter, based on the required user service the corresponding active AAL passes
on the payloads to the ATM layer. At the receiver the ATM layer is responsible for demultiplexing the cells based on the connection number VPIVCI and passing on the payloads to the proper AALs.
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