The classification is based on whether a timing relationship must be maintained between source and destination, whether the application requires a constant bit rate, and whether the transfer is connection oriented or connectionless.
AAL Type 0 (also referred as raw cells) consists of 48 bytes of payload without any reservation for special fields.
Bits are fed in by the application at constant rate and must be delivered to other end with minimum delay, jitter or overhead.
For this traffic, error detection protocols cannot be used since timeouts and retransmission causes delay but the missing cells are reported to the application, that must take its own action to recover from them.
AAL2 is also widely used in wireless applications due to the capability of multiplexing voice packets from different users on a single ATM connection.