he actual number of voice channels in use. Although minimal bandwidth and burstiness are rou- (An EI is a wide-area digital transmission scheme used tinely contracted, latency and jitter are often not in- predominantly in Europe, which carries data at a rate of cluded in service level agreements (SLAs) with service 2.048 Mbps.) providers. As a result, voice performance can vary. Structured AALI contains a pointer in the payload Even if quality is good at first, voice quality can de- that allows the digital signal level 0 (DSO) structure to be grade over time as a service provider's network be- maintained in subsequent cells. This allows network comes saturated with more traffic. For this reason, efficiencies to be gained by not using bandwidth for many large enterprise customers are beginning to unused DSOs. (A DSO is a framing specification used in specify latency and jitter, as well as overall packet transmitting digital signals over a single channel at 64 throughput from carriers. In these situations, VoFR can kbps on a T1 facility.) provide excellent service. The remapping option allows the ATM network to VoIP has begun to be deployed in recent years as terminate structured AAL1 cells and remap DSOs to the well. Unlike VoFR and VoATM, VoIP is a Layer 3 so- proper destinations. This eliminates the need for per- lution. It offers much more value and utility because IP manent virtual circuits (PVCs) between every possible goes all the way to the desktop. This means that in source/destination combination. The major difference addition to providing basic tie trunk and tandem- from the previous approach is that a PVC is not built switching functions to PBXs, VoIP can actually begin to across the network from edge to edge. Network infrastructure CHAPTER 3.2 At no time is the ATM network participating in the interpretation of the signaling that takes place between end stations. However, as a value-added feature, some products are capable of understanding CAS and can Telephone prevent the sending of empty voice cells 