AAL-2 — A new ATM adaptation layer for small packet encapsulation and multiplexing

Asynchronous transfer mode (ATM) networks carry fixed-size cells within the network irrespective of the applications being supported. At the network edge or at the end equipment, an ATM adaptation layer (AAL) maps the services offered by the ATM network to the services required by the application. Many trunking applications that have voice compression and silence suppression require transmission of small delay-sensitive packets. Existing AALs are very inefficient for this purpose. In this paper, we discuss a new AAL called the AAL Type 2 (AAL-2), which allows very high efficiency for carrying small packets. We describe the basic principles and compare several alternatives with respect to transmission error performance, bandwidth efficiency, and delay/jitter performance. The results show that the AAL-2 adds significant value to packet telephony applications over ATM networks. We discuss the desirability of additional rebundling in the network and the need for a signaling protocol to communicate changes in native connections (voice calls) within the same ATM connection. We also describe how the principles of the AAL-2 are being used to define multiplexing protocols over the Internet and frame-relay networks.