a. Its pliant and efficient packet switching the available bandwidth. The Internet is a classic exam- provides flexible transfer of other forms of data. In ple of a communications network in which the volume of a relatively short period of time, ATM has gained CHAPTER 3.2 Network infrastructure a worldwide reputation as the ultimate means of solving replace those PBXs as an application. As a Layer 3 so- end-to-end networking problems. The popularity of lution, VoIP is routable and can be carried transparently ATM has grown such that virtually every LAN equip- over any type of network infrastructure. This includes ment vendor and service provider is racing to develop both Frame Relay and ATM. Of all the packet voice ATM-based products. technologies, VoIP has perhaps the most difficult time Voice over ATM (VoATM) can be supported as stan- supporting voice quality because QoS cannot be guar- dard pulse code modulated (PCM) voice via circuit anteed. Normal applications such as TCP running on IP emulation (AAL1, described later) or as VBR voice in are insensitive to latency but must retransmit lost ATM cells as AAL2 (also described later). ATM offers packets due to collisions or congestion. Voice is much many advantages for transport and switching of voice. more sensitive to packet delay than packet loss. In ad- First, quality of service (QoS) guarantees can be specified dition to normal traffic congestion, QoS for VoIP is by service provisioning or on a per-call basis. In addition, often dependent on lower layers that are ignorant to the call setup signaling for ATM switched virtual circuits voice traffic mingled with the data traffic. (SVCs), Q.2931, is based on call setup signaling for voice The ATM Forum and the ITU have specified different ISDN. Administration is similar to circuit-based voice classes of services to represent different possible traffic networks. types for VoATM. Designed primarily for voice com- However, VoATM suffers from the burden of addi- munications, constant bit rate (CBR) and VBR classes t