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Digital Multiplexers Chap Chapter26 DigitalMultiplexers In the same way that various analog systems are used to transmit analog signals of different bandwidths, there are various digital trans­ mission facilities designed to transmit digital signals of different rates. These facilities must be interconnected into a network that allows a digital signal to reach its destination using one or more of these facilities. Interconnections must be flexible enough to provide for alternate transmission paths in case of equipment failures, chang­ ing traffic patterns, and routine maintenance. Interconnection of fa­ cilities of the same digital rate involves either manual patching or automatic switching. Interconnection of different digital rates re­ quires multiplexers that combine several signals to share a higher speed digital facility. Time division multiplexing of several digital signals to produce a higher speed stream can be accomplished by a selector switch that takes a pulse from each incoming line in turn and applies it to the higher speed line. The receiving end will do the inverse of separating the higher speed pulse stream into its component parts and thus recover the several lower speed digital signals. The main problems involved are the synchronization of the several pulse streams so that they can be properly interleaved and the framing of the high-speed signal so that the component parts can be identified at the receiver end. Both of these operations require elastic stores, which constitute important parts of a multiplexer. Elastic stores are also called data buffers. Information pulses arriving at the multiplexer must await their turn to be applied to a higher speed transmission system. Due to delay variations of the incoming lines and to the framing and synchronization operation of the multiplexer terminal, this wait is variable in time. 608 Methods of Synchronization 609 The framing problem is similar to that of digital terminals, and the same techniques are available. For reasons of flexibility, added bit framing is chosen for all multiplexers. This choice assumes no given input signal statistics and leaves the digits intact so that a multiplexer can handle any digital signal regardless of its source. 26. l METHODS OF SYNCHRONIZATION The major problem of multiplexer system design is synchroniza­ tion. Digital signals cannot be directly interleaved in a way that allows for their eventual identification unless their pulse rates are locked to a common clock. Because the sources of these digital sig­ nals are often separated by large distances, their synchronization is difficult. Synchronization methods which can be used are: (1) master clock, (2) mutual synchronization, (3) stable clocks, and (4) pulse stuffing. Master Clock An obvious method of synchronization is the use of a master clock for timing the entire system [1]. One outstanding difficulty of this approach is the vulnerability of the system to failures of either the master clock or the transmission links. A system with enough re­ dundancy and automatic protection against failures presents a diffi­ cult design problem and is expensive to establish. Mutual Synchronization In this method of synchronization (referred to also :as phase averaging), each station or central office has its own clock whose frequency is the average of all the incoming frequencies and a local standard [2]. It can be shown that all stations will approach a com­ mon steady-state frequency. This method avoids one aspect of the master clock reliability problem since now no one clock or transmis­ sion path is essential. Further studies are necessary, however, to determine the optimum averaging algorithm for each station such that the network can grow gracefully from a few nodes to a large network and that minor disturbances such as protection switch­ ing of a transmission link will not cause the system frequency to swing beyond the design limits [3]. Stable Clocks A third method of synchronization is the use of very stable clocks at each office containing digital terminals. Elastic stores are then used 610 Digital Multiplexers Chap. 26 to absorb the very slowly varying phase errors. Since their capacity is finite, these stores must be reset pedodically with some loss of information. With atomic clocks stable to one part in 10 12 and with large enough elastic stores, loss of information will be acceptably infrequent. A combination of the preceding three methods is likely to be used in the future. Each region will have stable clocks supple­ mented by mutual synchronization while the master clock scheme will be used within each region. Pulse Stuffing The final method of synchronization is pulse stuffing, presently being used in the design of multiplexers [4]. The concept is to have the outgoing digital rate of a multiplexer higher than the sum of the incoming rates by stuffing in additional pulses. All incoming digital signals are stuffed with a sufficient number of pulses to raise each of their rates to that of the locally generated clock signal, Fig. 26-1. ~~,~ l l \\Original signal l ! \ \ \ i ! ! \ * Stuffed signal to bt, * • Represents stuffed pulses multiplexed Destuffed signol after smoothing Fm. 26-1. Pulse stuffing synchronization. Pulse stuffing is the least complex of the four methods proposed because it needs the least amount of buff er storage. In each of the other methods, even assuming that the clocks are perfectly synchron­ ous, propagation delay variations may cause a surplus or deficit of pulses at any one location. For example, a 1000-kilometer coaxial cable carry 3 X 10 8 pulses per second will have about one million pulses in transit, each pulse occupying about one meter of the cable. A 0.01 per cent variation of propagation delay, as would be produced Multiplexer System Design 611 by a 1 °F decrease in temperature will result in 100 fewer pulses in the cable; these must be absorbed by elastic stores in the multiplexer. With pulse stuffing, a deficit of one pulse is immediately made up by stuffing so that one cell of storage is all that is needed to handle such variations. Pulse stuffing is done independently for. each multiplexer, and this contributes to system reliability. The pqlse rate on a particular trans­ mission line is determined locally and will not influence any other clock in the system. Failure of one multiplexer or line will only affect those signals passing through that multiplexer or line. By choosing pulse stuffing, network synchronization is not pre­ cluded. Asynchronous low-speed digital signals are stuffed in order to use a higher speed transmission line; they are destuffed and the original rate restored before they leave the transmission system. If digital switching is established in the future, the lower speed signals can be made synchronous and the higher speed transmission lines can still be used. Flexibility is thus preserved. 26.2 MULTIPLEXER SYSTEM DESIGN In addition to pulse stuffing, other design choices must be made to characterize the family of digital multiplexers. When a pulse is stuffed, an additional communication channel is used to inform the receiving terminal of the location of the stuffed pulse. Either this channel ·can be provided separately for each signal being stuffed or a common data channel can be shared. Separate stuff channels would be more flexible but because shared equipment is more economical, stuffing information for all signals entering a multiplexer is processed and transmitted on one channel. This common data channel is multi­ plexed along with the information pulses for transmission over a higher speed digita1 line. Another choice concerning the system design of the family of multiplexers is the structure of the digital hierarchy. Certain digital rates are designated as belonging to the hierarchy, e.g., 1.544 Mb/s is Tl rate, 6.312 Mb/s is T2, and 46.304 Mb/s is T3. Multiplexers accept signals of one rate and multiplex them only to the next higher rate. To combine .~everal Tl-speed digital signals into a TB rate, it is necessary to go through two multiplexers. Multiplexers are named for the rates they bridge. Multiplexer M12, for example, is designed to combine several Tl signals into a T2 signal. 612 Digital Multiplexers Chap. 26 Signal Format These design choices fix the general structure of the line formats of all multiplexers. Figure 26-2 illustrates a typical format, that of the M12 multiplexer [5]. Digit-by-digit interleaving of four signals at the Tl speed forms the fine structure of the format. After every 48 information time slots, 12 from each of the four Tl signals, a control digit is inserted by the multipllexer. Control digits labeled F are the main framing digits. Between F digits are control digits labeled M and C. Three successive C digits denote the presence or absence of a single stuffed pulse, and the corresponding M digit identifies in which of the four multiplexed Tl signals the stuff occurs. The M digits thus form secondary framing digits and iden­ tify four subframes. The subscripts of the M and F digits identify the digit as a O or a 1. Thus, F1 is always a 1 and the next control digit is either an M1 or Mo. The three C digits in the subframe follow­ ing Mo are stuffing indicators for the first T1 signal, three l's for the presence of a stuffed pulse and three O's for no-stuff. If the C digits indicate a stuff, the location of the stuffed pulse is the first informa­ tion pulse position associated with the first Tl signal following the next F1 pulse. The other sequences of C digits denote stuffing in the second, third, and fourth Tl signal. The use of three digits for a stuff indication provides a single digit error correction code. The demultiplexer at the receiving M12 first searches for the FoF1FoF1 sequence.
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