 the world. The PBXs for the most part rely on the PSTN, but there is a leased line between San Francisco and London because of the volume of traffic and security requirements. The videoconferencing systems between San Francisco and London use a specially provisioned satellite link. In essence, there are four separate networks, you're paying for four different infrastructures, and you don't have complete connectivity between all the loca- tions, users, and resources. You therefore want to build an enterprise backbone net- work that ties together everybody with one common infrastructure and that provides for more fluid connectivity between the various locations and applica- tions. This requires intelligent equipment at each location, to manage the transmis- Circuit-Switched Networks San Francisco London Video- Video- conferencing conferencing Leased T-1/E-1/J-1 Router Leased T-3/E-3/J-3 Router Web Server Leased T-1/E-1/J-1 Leased OC-3 Leased T-1/E-1/J-1 Beijing Dubai Router Hong Kong T-1/E-1/J-1 Private Router Microwave Router Remote Location Figure 7.5 Single-purpose leased-line networks sion resource and to properly allocate the capacity to voice, data, image, video, fax, or other forms of transmissions. Fractional Services Some locations may be too small to justify a full T-1 or E-1 facility. In those cases, frac- tional services can be used; they allow you to subscribe to capacity in small bundles of 56Kbps or 64Kbps channels, which are generally provided in increments of four chan- nels. These services are referred to as fractional T-1 (FT-1) and fractional E-1 (FE-1) ser- vices. However, even though fractional services are technically available, many places do not have them tarriffed, and they are therefore unavailable from a regulatory perspective. The facilities used to transport the information on this revised network can be combinations of privately owned and leased facilities, but you, the customer, own and are responsible for the equipment to manage those transmission facilities. You might be leasing a