 T-1 or an E-1 from a network operator, or you might be leasing dark fiber from a railroad company. Between some locations, you might deploy a privately owned digital microwave system. By whatever means you choose, you Chapter 7 Wide Area Networking can integrate what used to be four separate networks into one cohesive backbone (see Figure 7.6). In terms of the capacities and components that would then be used within that backbone, the majority of customers today would rely on the dimen- sions deliverable from the T-, E-, and J-carrier infrastructure, although those with very large data centers and high levels of multimedia and visual applications have already migrated to the optical carrier levels of the SDH/SONET hierarchy. A customer needs the following equipment to manage transmission facilities (refer to Figure 4.5 in Chapter 4): Transmission media-The customer might be using copper twisted-pairs with T-1 or E-1 and might be using higher-bandwidth media such as coax, microwave, or fiber with T-3 or E-3 capacities. These are four-wire circuits operating in full-duplex, which means that you can communicate in both directions simultaneously. CSUs-A CSU terminates each end of the T-1/E-1/J-1 carrier facility. A CSU equalizes the received signal, filters the transmitted and received wave- forms, and interacts with the customer's and the carrier's test facilities so that diagnostics can be performed. Essentially, the CSU is used to set up the San Francisco London Video- Video- conferencing conferencing Leased OC-3 San Francisco UK Mux Router Router Web Server Leased OC-3 Leased T-1/E-1/J-=1 Beijing Hong Kong Fiber Dubai Dubai Beijing Hong Kong Private Microwave Switch Router Switch Video- conferencing Figure 7.6 An enterprise backbone network Circuit-Switched Networks T-1/E-1/J-1 line with a customer-owned PBX, channel banks as standalone devices, intelligent multiplexers (e.g., T-, E-, or J-carrier multiplexers), and any other DS-x/CEPT-x-compliant DTE, such as digital cross-connects. Multiplexers-A channel bank 