lexing All T carrier signals (for example, T1, E1, E3, and T3) are based on Time-Division Multiplexing (TDM). Each device that communicates over a T1 line is assigned a time slot. If there are eight telephones contending for a T1 circuit, a time slot is saved for each telephone for the duration of the particular telephone call. Telephone 1 might be assigned slot A; telephone 2, slot B; and SO forth. Figure 5-7 shows an example TDM on T1 circuits. During pauses in voice conversations, the slot is not assigned to another computer. The assigned time slot is transmitted without any information. This is why TDM is not efficient on a Wide Area Network (WAN). Pauses in data T1 and T3: The First Broadband Services for Businesses Router Router A time slot is assigened to each device Telephone at site A puts person at site B on hold Slot 5 is transmitted with no data Devices 1-5 at site A Devices 1-5 at site B Figure 5-7 Wasted network capacity on the Time-Division Multiplexing (TDM) used in T1/E1 and T3/E3. TDM multiplexers are located in the routers. transmission result in idle time slots. In a network with millions of time slots, this can result in many idle time slots and wasted bandwidth. Channel Service Units and Digital Service Units When customers order any type of T1/E1 or T3/E3 service, they also need to order a Channel Service Unit/Digital Service Unit (CSU/DSU). These devices act like modems and are situated between the outside network and the customer's equipment. They serve as a point to troubleshoot service and ensure that the T1 service is compatible with the outside network. CSU/DSUs are generally cards within multiplexers, routers, and Private Branch Exchanges (PBXs) rather than stand-alone, external devices. Maintenance and performance tests are done from the CSU/DSU to determine if a repair problem is in the equipment, the CSU/DSU, or the telephone line. The CSU also provides clocking and signal reshaping. The clocking function is responsible for sending out bits in an evenly timed fashion. If the clock