In order for ATM to support many kinds of services with different traffic characteristics and system requirements, it is necessary to adapt the different classes of applications to the ATM layer. This function is performed by the AAL, which is service-dependent. Four types of AAL were originally recommended by CCITT. Two of these have now been merged into one. Also, within the past year a fifth type of AAL has been proposed.
Briefly the four ATM adaptation layers (AAL) have/are being defined:
AAL1 :
Supports connection-oriented services that require constant bit rates and have specific timing and delay requirements. Example are constant bit rate services like DS1 or DS3 transport.
AAL2 :
Supports connection-oriented services that do not require constant bit rates. In other words, variable bit rate applications like some video schemes.
AAL3/4 :
This AAL is intended for both connectionless and connection oriented variable bit rate services. Originally two distinct adaptation layers AAL3 and 4, they have been merged into a single AAL which name is AAL3/4 for historical reasons.
AAL5 :
Supports connection-oriented variable bit rate data services. It is a substantially lean AAL compaired with AAL3/4 at the expense of error recovery and built in retransmission. This tradeoff provides a smaller bandwidth overhead, simpler processing requirements, and reduced implementation complexity. Some organizations have proposed AAL5 for use with both connection-oriented and connectionless services.