Voice over ATM using AAL2 and bit dropping: performance and call admission control

We consider an asynchronous transfer mode (ATM) adaptation layer 2 (AAL2) system similar to that described by Baldwin, Bharucha, Doshi, Dravida and Nanda (see Bell Labs Technical Journal, vol.2, no.2, p.111-31, 1997), but with the additional possibility of selectively dropping the less significant bits of voice packets during periods of congestion. Dropping less significant bits of voice for relieving congestion depends on the ability of the transport protocol to handle variable length packets, and is currently used in frame-relay networks. Because of the fixed ATM cell size, bit dropping on voice was not viable in ATM networks prior to the development of the AAL2 protocol. By allowing variable length packets within the ATM cell payload, the AAL2 protocol makes it possible to use bit dropping. AAL2 also makes it possible to take advantage of voice silence periods. We show that the combination of silence elimination and bit dropping enables high overall statistical multiplexing gain, and makes voice transmission over ATM bandwidth very efficient and economical.