AAL Types

Atm Adaptation Layer

The ATM Adaptation Layer (AAL) performs functions required by the user, control and management planes and supports the mapping between the ATM layer and the next higher layer. The functions performed in the AAL depend upon the higher layer requirements. In short, the AAL supports all of the functions required to map information between the ATM network, and the non-ATM application that may be using it.  

Different adaptation layers exist to carry traffic as diverse as packet-based or isochronous (T1 or E1) over the ATM backbone. AALs are standardized in the ITU-T I.363.x series of Recommendations. The two most commonly implemented are AAL1, which supports isochronous transmission — circuit emulation, for example — and AAL5, which supports carrying packet data, such as Frame Relay, over ATM.

The following five types of ATM Adaptation Layer (AAL) capabilities comply with ITU-TSS Recommendations 1.361, 1.362, and 1.363, and ITU-TSS Q.SAAL for signaling.

SAR (Segmentation And Reassembly) PDU (Protocol Data Unit) - this is a term given to the payload of each ATM cell.  Since ATM cells are so small, virtually all applications must encapsulate larger data units within the cells.  This requires the larger units to be segmented into smaller units (cells) at the transmitting end, and then reassembled at the receiving end.  The term SAR is used so often to describe the AAL types and their associated ATM cell fomats, that it requires a definition to be inserted here.

IMPORTANT - today, ATM networks only use AAL1 (CBR), AAL2 (VBRrt), or AAL5 (VBRnrt and VBRrt).  AAL2 is rarely used.

• AAL TYPE 1 - for CBR ATM - 53-byte cells - (47 bytes of payload, 6 bytes overhead).  Unlike VBR, CBR uses an extra byte in it's header for sequence numbers (4 bits for sequence numbers and 4-bits for sequence number protection).  This is necessary to insure a constant bit-rate.

• AAL Type 2 - for VBR rt - 53-byte cells  -  48 bytes payload (5 bytes overhead) - used for VoATM (Voice over ATM).  Click Here for a comprehensive AAL2 guide. 

• AAL TYPE 3  - not used - for VBR - 48-byte cells - 44 bytes payload (4 bytes overhead - 2byte header and 2byte trailer)

• AAL TYPE 4  - not used - for connectionless VBR which supports SVC's

• AAL Type 3/4 - not used - AAL3 and AAL4 are so similar that they were merged to AAL3/4 in 1992.  48-byte cells - 44 bytes payload (4 bytes overhead - 2byte header and 2byte trailer).  But it is not used today

• AAL Type 5 - for VBRnrt and VBRrt - 53-byte cells (48 bytes payload , 5 bytes overhead) - used for ATM DS1 with HEC mapping

NOTE:  customer has a choice in running AAL2 or AAL5 with VBR rt.  However, AAL2 is preferred for voice  as it supports silence suppression and compression schemes for rt apps.  

Example - AAL1 CBR ATM Cell

AAL2 CPS (Common Part Sublayer)

AAL2 is extremely complex (Click Here for a comprehensive AAL2 guide. ), because it is designed to support applications directly through delivery of encapsulated CPS packets (such as ADPCM VoATM = Adaptive Differential Pulse Code Modulated Voice over ATM)).  The AAL2 cells must support encapsulation of data units called CPS (Common Part Sublayer).  A headerless CPS SDU (Service Data Unit) is enclosed in a CPS packet with a 3-byte header, which is enclosed in a CPS PDU with a 1-byte header, which is enclosed in the 48-byte payload of the ATM cell with a 5-byte header.  Therefore, actually, the payload is less than 48 bytes if you consider the CPS headers.  If the SDU is small, there can be several of them within one ATM cell, and since each CPS packet headers = 3 bytes each, and CPS PDU header = 1 byte, the total overhead can be a high percentage.  However, keep in mind that other applications running over other AAL type cells, have their own associated overhead (for example, IP over ATM AAL5 has the additional overhead of the layer 3 IP packet headers).

Here is an example of 6 CPS packets, encapsulated into 3 AAL2 cells.  Cell 3 has leftover space, and since all cells = 53 bytes, it must pad the rest of the cell with zeros :