General definition

UNIT 3 Time Division Multiplexing

Термокомпенсация.

Последовательно (или параллельно) включается какой-нибудь элемент, параметры которого изменяются при изменении температуры. VT2 должен быть таким же, как VT1, но включен как диод.

PART 1 (21.1 – 21.2)

In telephone systems a time division multiplexer, better known as a TDM, is generally defined as a device that distributes a number of channels periodically in time through the intermediary of pulse modulation. Each pulse corresponds to a channel and is interleaved between those of other channels. Hence, a time division multiplexed signal is always composed by means of synchronous sampling of the channels, with pulses shifted with respect to each other. This is illustrated in Figure 21.1. The interleaved channels form one frame of a duration corresponding to the sampling period T_s. The pulse modulation used can be analog (e.g. РАМ, PPM) or digital (PCM).

With rime division multiplexing time is a relative value, i.e. TDM requires a reference point in the frame cycle to which the receiver must be synchronised in order to correctly demultiplex the stream of pulses it receives and extract the signals concerning each channel individually. Since the receiver must be continuously synchronised in both frequency and phase this reference point is repeated peri­odically. The easiest way to understand this is to cyclically represent the time division multiplexing process as illustrated in Figure 21.2.

Figure 21.2 depicts a rotating switch S scanning a number of channels connected to its inputs. The channels are scanned one by one in the order they are encountered by the switch: channel 1 is sampled at time tj, channel 2 at time X_y and finally channel n at time t, after which the process is repeated. The resulting output signal is composed of the samples of the different channels, shifted over a time period given by Equation 21.1.

(21.1)

If the channels presented at the input are pulse amplitude modu­lated signals, we obtain a signal as depicted in Figure 21.1. At the other side of the connection we find an identical switch executing the opposite operation, i.e. scanning the composite input signal at exactly the same frequency and phase, it demultiplexes the input signal and distributes the extracted signals over the respective channels. Both systems synchronise each time they pass the refer­ence point.

Time division multiplexing has become very important, not only in transmission but also in switching, particularly in connection to the digital systems.

In the early days of electrical communications a medium such as copper wire carried a single information channel. For economic reasons, in terms of both cost and equipment, it was necessary to find ways of packing multiple channels onto one physical link. The resulting system is referred to as a carrier. Digital signals are now transmitted from one location to another by transmission facilities or systems using a multitude of media (paired cable, coaxial cable, analog or digital radio systems, optical fibres, satellite communica­tion).

The synchronous time division multiplexers typically are used as termination equipment of such carrier systems. These multiplexers are better known as Tl or El multiplexers, depending on the multiplexing scheme they use. Tl is generally used in America and is based on the PCM system that was originally designed by AT&T. The name Tl was derived from the identification number of the committee set up by the American National Standards Institute (ANSI). Europe, on the other hand, uses the El system, which is based on CEPT (Conference of Post and Telecommunications) recommendations. The delay with which CEPT has undertaken the definition of a primary digital PCM system has allowed it to profit from the experience of the American systems.

On the international level, the two types of system coexist al­though they are incompatible, and are a subject of two recommen­dations of the CCITT. Although both systems will be discussed later in this chapter, Table 21.1 compares their main characteristics. The meaning of the different items will become clear throughout the rest of this chapter.

Before discussing the Tl and El technologies in detail, consider first the general structure of a digital time division multiplex.

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