 of consecutive frames. This feature is referred to as the system's flywheel. The minimum number of failed FAWs that can lose the alignment of the system is called momentum of the flywheel. The transmission errors appear as short bursts, and therefore the momentum of the flywheel should be long enough to accommodate the longest possible burst. Higher-Order Multiplexing The principles of PCM were discussed in the previous section. As mentioned there, the input voice data is fed into a bandpass filter with the range from 300 to 3400 Hz. The filter output is then sampled at 8 kHz, giving the sample duration of 125 us. Then each sample is quan- tized using 8 bits. This means that the bitrate will be equal to (8/125) bits/ us or 64 kbit/s. This is also referred to as the basic PCM rate. There are basically two worldwide transmission systems: one with 24 and the other with 32 channels. While the former is used in Japan and North America, the latter is employed by Europe and the rest of the world. The American system is based on the PCM system, originally developed by AT&T. The name T1 is derived from the identification number of the committee set up by the American National Standards Institute (ANSI). The European system, on the other hand, is referred to as E1, which is based on the CEPT (Conference of Post and Telecommunication) recommendation. The T1 carrier framing was designed to carry 24 channels of speech in each frame. This was based on one of the earliest framing formats called D4 framing. A T1 frame consists of Multiplexing 193 bits, with 192 bits (8 x 24) dedicated to the voice samples and the very last bit to framing and signaling. Twelve frames are combined to form a superframe. This means that each superframe contains 12-bit words, composed of individual bits coming from each of the 12 frames. The framing bits are the odd-number framing/signaling bits while the signal- ing bits are the even-numbered bits. The framing bits alternate every other frame. On the other hand, in an E1 connection, out of 32 slots