ATM Suitability for VoiceTraffic

Sometimes mistakenly associated with VoIP, VoATM is a completely separate technology that predates VoIP. In contrast to IP and frame relay, ATM uses small, fixed-length data packets of 53 bytes each that fill more quickly are sent immediately, and are much less susceptible to network delays. (Delays experienced by voice in a framerelay or IP packet network can typically be ten times higher than for ATM andincrease on slower links.)

ATM's packet characteristics make it by far the best-suited packet technology for guaranteeing the same QoS found in toll-quality voice connections.

The ATM Adaptation Layer (AAL), the part of ATM responsible for converting voice and data into ATM cells,enables various traffic types to have data converted to and from the ATM cell and translates high-layer services (such as TCP/IP)into the size and format of the ATM protocol layer. A number of AAL definitions exist to accommodate the various types of network traffic. Those AAL types most commonly used for voice traffic are AAL1, AAL2, and AAL5.

VoATM with AAL1 is the traditional approach for constant bit rate (CBR), time-dependent traffic such as voice and video and provides circuit emulation for trunking applications. ATM with AAL1 is still suitable for voice traffic, but is not the ideal solution for voice services in the local loop because its design for fixed bandwidth allocation means network resources are consumed even when no voice traffic is present. Some equipment manufacturers use AAL5 to provide VoATM and support for variable bit rate (VBR) applications. They also use AAL5 because it is a better choice over AAL1 in terms of bandwidth used. However, the means for carrying voice traffic over AAL5 is not yet fully standardized or widely deployed, and implementations are usually proprietary. ATM with AAL2 is the newest approach to VoATM.

The above picture shows how AAL2 provides a number of important improvements over AAL1 and AAL5, including support for CBR and VBR applications, dynamic bandwidth allocation, and support for multiple voice calls over a single ATM permanent virtual circuit (PVC). An additional and significant advantage of AAL2 isthat cells carry content information. This feature provides traffic prioritization for packets (cells) and is the key to dynamic bandwidth allocation and efficient network use.