pling rate that is 8,000 times in a second. The frame structure shown in Figure 4.23 is used in North America and known as T1 or DS1 frame [5]. The North American PCM system accomplishes frame alignment dif- ferently than does the European 2-Mbps system. Like its European counter- part, it uses eight-bit time slots, but each frame contains 24 channels. To each frame, one-bit frame, a frame alignment, or synchronization bit (S-bit) is added, and we get a 1.544-Mbps data rate as shown in Figure 4.23. A mul- tiframe is constructed from 12 subsequent frames and their 12 S-bits make up the 6-bit frame and 6-bit multiframe synchronization words [5]. In T1 there is no reserved time slot for CAS information as we have in the 2-Mbps frame structure. Instead of that, the least significant bit of each channel in every sixth frame is used for signaling. As a consequence, only seven bits in each time slot are transparently carried through the network and the basic user data rate is 56 Kbps instead of the 64 Kbps in the European systems. For frame synchronization and for demultiplexing of signaling infor- mation, frames make up a multiframe structure with two alternative lengths, a superframe containing 12 frames or an extended superframe (ESF) contain- ing 24 frames. The framing bits of ESF, one in each frame, carry frame syn- chronization information including CRC code and data channel for network management messages. The detailed structure of the multiframe is explained, for example, in [5]. Each time slot (TS) of a frame contains 8 bits. 125 microseconds, 196 bits Channel number Time slot number Framing bit Frame is repeated 8,000 times in a second which is the same as PCM sampling rate. Each frame contains one sample of 24 different speech signals. To each frame 1 bit, called a framing bit, is added. (24 time slots 8 bits + 1 bit) *8,000 = 1,544 Kbps. One bit in each slot in every sixth frame is replaced by signaling information. As a consequence, only 7 out of 8 bits can be used transparently through the network. Therefo