ional complexity, and incomplete support and in- have provisions for passing real-time traffic and are teroperability among vendors. It also tends to be more suitable for guaranteeing a certain level of service. CBR, expensive because it is oriented toward all-optical in particular, allows the amount of bandwidth, end-to-end networks. Most importantly, ATM is typically deployed delay and delay variation to be specified during the as a WAN Layer 2 protocol and therefore does not call setup. Designed principally for bursty traffic, extend all the way to the desktop. Nevertheless, ATM unspecified bit rate (UBR) and available bit rate (ABR) is quite effective for providing trunking and tandem are more suitable for data applications. UBR, in partic- switching services between existing voice switches and ular, makes no guarantees about the delivery of the data PBXs. traffic. Voice over Frame Relay (VoFR) has become widely The method of transporting voice channels through an deployed across many networks. Like VoATM, it is ATM network depends on the nature of the traffic. typically employed as a tie trunk or tandem-switching Different ATM adaptation types have been developed function between remote PBXs. It benefits from much for these different traffic types, each with its own ben- simpler administration and relatively lower costs than efits and detriments. ATM adaptation layer 1 (AAL1) is VoATM, especially when deployed over a private WAN the most common adaptation layer used with CBR network. It also scales more economically than services. VoATM, supporting links from T1 down to 56 kbps. Unstructured AAL1 takes a continuous bit stream and When deployed over a carefully engineered Frame places it within ATM cells. This is a common method of Relay network, VoFR works very well and provides supporting a full E1 byte stream from end to end. The good quality. However, voice quality over Frame Relay problem with this approach is that a full E1 may be sent, can suffer depending on network latency and jitter. regardless of t