nd leases services to a third company. Some large companies may lease dark fiber directly from the owner of the physical transport and maintain their own networks as well. The same approach is used in wireless carriers. If a company has capacity on a mi- crowave channel, it can use that capacity for a mix of voice telephone calls and data service. The company also can lease some of the line's capacity to other customers. In the cellular network, the current air interface is used primarily for voice calls, but there is some use for low-speed data and fax services. With the advent of third-gener- ation (3G) data services, the cellular network will become a hybrid channel with high- speed data capacity. When this is done using data-only (DO) carriers, the approach is analogous to sharing a single pipe, with some optical fibers used for data and some for voice. Let us take a look at the way in which hybrid voice-data networks are evolving over time. The earliest approach was discussed earlier: Using a single physical trans- port with different data-link-layer protocols has the advantages of making use of ex- listing capacity and reducing the need to install new physical facilities. However, this approach also has two significant limitations: The terminating and routing equipment for voice and data is different and separate. This increases the equipment cost substantially. Once the carriers are set up, each carrier is locked in for use for only voice or only data. There is no possibility for dynamic allocation of bandwidth across types of service. This second point is worth elaborating on. A flood of telephone calls could create block- ing, even while the data stream has open channels. Similarly, if the data capacity is used up, the data transport network cannot make use of excess capacity on the trans- port dedicated to voice service. Because the pipes using data equipment are not suit- able for voice, the full capacity of the pipes cannot be flexibly allocated on demand. There are two common ways of managing around 