ATM Adaptation Layer

There are 4 different AAL service attributes defined by the ITU-T standards:

• Class A - Constant bit rate with end-to-end timing, connection-oriented.
• Class B - Variable bit rate with end-to-end timing, connection-oriented.
• Class C - VBR, no timing, connection-oriented.
• Class D - VBR, no timing, connectionless.

ATM Forum standards define AAL1-4 mapping exactly to A-D, with a few extras.

AAL1 specifies how TDM-type circuits can be emulated over ATM networks by Synchronous Residual Timestamp or Structured Data Transfer.

AAL2 specifies ATM transport of connection-oriented circuits and VBR high bit-rate packetized audio and video. AAL2 is not very well defined at this time. However, this layer holds the most promise for the future of ATM, as MPEG2 encoding supports VBR operation.

AAL3/4 are defined as one combined standard in support of VBR connection-oriented and connectionless support, such as IEE80.26 L3_PDU's.