rminals (RTs). Thus, the second method used to connect a large number of subscribers to a central office is referred to as digital loop carrier remote terminal (DLC RT), which is illustrated in the lower portion of Figure 7.2. Depending on the type of copper-based line routed to the DLC RT, either 24 or 30 voice channels can be supported. The T1 line supports 24 voice channels, while the E1 supports 30. The use of a DLC considerably reduces the cost associated with sup- porting subscribers since it enables groups of 24 or 30 subscribers to be serviced via a common T1 or E1 transmission facility. Unfortunately, the use of a DLC has a major disadvantage. To understand this disadvantage, we must briefly review the method by which T1 and E1 transmission facilities operate. That method is based on time division multiplexing LIMITATIONS OF TIME DIVISION MULTIPLEXING (TDM) Figure 7.3 illustrates the use of a T1 multiplexer to transport 24-voice conversations over a common twisted-pair wire. In examining this figure, Chapter Seven one T1 frame Figure 7.3 Using a T1 multiplexer. Channel 24 Channel 2 Channel 1 Legend: Coder/decoder Framing bit note that the coder portion of the coder/decoder represents a module that converts an analog voice signal into a pulse-code modulation (PCM) digitized voice signal. The PCM digitized signal results from samples of the analog voice signal occurring 8000 times per second and each sample being converted into an 8-bit value. Thus, the data transmission rate of each digitized voice conversation represents a 64-kbit/s data stream. In the opposite direction, the decoder portion of the coder/decoder converts the digital signal back into its equivalent analog signal. In examining the use of the T1 multiplexer shown in Figure 7.3, note that 24 voice inputs are digitized. A basic T1 frame consists of one 8-bit sample from each of the 24 voice channels plus a 1-bit framing bit that provides synchronization between T1 multiplexers. Thus, the oper- ating rate of the T1 circuit becomes 193 bits/fra