AAL1, AAL3/4 and AAL5 are ATM Adaptive Layer protocols already defined by ITU-T. They can deal with the needs of some applications, but seem to be inadequate for low-speed voice and data applications. For this reason, new AAL protocol-AAL2 emerges.

• 目前ITU-T已经定义的ATM适配层规程有AAL1；AAL3/4和AAL5；已满足某些应用的需要；但对于低速话音或数据的应用尚嫌不足；为此；新型ATM适配层规程AAL2应运而生.