PDF of this tutorialAs real-time voice services have been traditionally supported in the WAN via circuit-based techniques (e.g., via T1 multiplexers or circuit switching), it is natural to map these circuits to ATM CBR PVCs using circuit emulation and ATM adaptation Layer 1 (AAL1). However, there are significant disadvantages in using circuit emulation in that the bandwidth must be dedicated for this type of traffic (whether there is useful information being transmitted or not), providing a disincentive for corporate users to implement circuit emulation as a long-term strategy. For example, a T1 1.544–Mbps circuit requires 1.74 Mbps of ATM bandwidth when transmitted in circuit-emulation mode. This does not downplay its importance as a transitional strategy to address the installed base.
As technology has evolved, the inherent burstiness of voice and many real-time applications can be exploited (along with sophisticated compression schemes) to decrease the cost of transmission significantly through the use of VBR–RT connections over ATM.
VBR techniques for voice exploit the inherently bursty nature of voice communication, as there are silence periods that can result in increased efficiency. The following silence periods (in decreasing levels of importance) arise:
- when no call is up on a particular trunk; that is, the trunk is idle during off-peak hours (trunks are typically engineered for a certain call-blocking probability: at night, all the trunks could be idle)
- when the call is up, but only one person is talking at a given time
- when the call is up, and no one is talking
Work is just starting in the ATM Forum on ATM adaptation for VBR voice.
The addition of more bandwidth-effective voice coding (e.g., standard voice is coded using 64–kbps PCM) is economically attractive, particularly over long-haul circuits and T1 ATM interfaces. Various compression schemes have been standardized in the industry (e.g., G720 series of standards). Making these coding schemes dynamic provides the network operator the opportunity to free up bandwidth under network-congestion conditions. For example, with the onset of congestion, increased levels of voice compression could be dynamically invoked, thus freeing up bandwidth and potentially alleviating the congestion while diminishing the quality of the voice during these periods.
A further enhancement to the support of voice over ATM is to support voice switching over SVCs. This entails interpreting PBX signaling and routing voice calls to the appropriate destination PBX (see Figure 5). The advantage from a traffic management perspective is that connection admission controls can be applied to new voice calls; under network congestion conditions, these calls could be rerouted over the public network and therefore not cause additional levels of congestion.
Figure 5. Voice Switching over SVCs
The ATM Forum is currently focusing its efforts on voice handled on CBR SVCs. VBR–RT voice is a future standards activity.
