en long-distance telephone service. Some companies have enhanced this functionality by having the PBX use the internal lines to serve certain outgoing calls, as well as intercompany calls. 2 This approach has the advantage making very good use of available capacity, but it requires the existence of a reliable al- ternate route. Sharing physical transport media and the physical layer of the Open Systems Inter- connect (OSI) model does have some cost-saving advantages. The techniques described earlier make good use of capacity within the limits of the technology. However, they also illustrate the limitations of this approach. New approaches allow dynamic sharing of a single pipe defined at the data link layer or network layer for a mix of voice and high-speed data. For reasons explained in Chap. 23, high-quality voice networks almost always have unused capacity. Technologies that dynamically allocate channels to voice or data as needed can make use of this capacity, reducing transport costs. However, this requires that voice and data both be carried over the same equipment, as well as being carried over the same physical transport medium. In the remaining sections of this chapter we will look at several solutions to this problem. 19.3 Voice and Data Sharing ATM ATM, with its defined QoS levels, its ATM adaptive layers (AALs), and its high-speed cell-based transport service, provides a data-link layer that can carry a mixture of voice and data services. The various AAL protocols translate the service being offered. If the customer has asked for frame relay, the customer sees frame relay, and the same is true for customers who want voice channels or other services. Major corporations use ATM for this function. One cellular carrier built an ATM net- work that provided data management services, allowing its network operations center (NOC) to monitor the entire computer network as well as the switch status at its mo- bile switching centers. The company also routed customer-service calls from cellular subscribers who dia