1、Whenever the transmission capacity of a medium linking two devices is greater than the transmission needs of the devices, the link can be shared, much as a large water pipe can carry water to several separate houses, at once. Multiplexing is the set of techniques that allows the simultaneous transmi
2、ssion of multiple signals across a single data link.As data and telecommunications usage increases, so does traffic. We can accommodate this increase by continuing to add individual lines each time a new channel is needed, or we can install higher capacity links and use each to carry multiple signal
3、s. As described in Chapter 5, todays technology includes high-bandwidth media such as coaxial cable, optical fiber, and terrestrial and satellite microwaves. Each of these has a carrying capacity far in excess of that needed for the average transmission signal. If the transmission capacity of a link
4、 is greater than the transmission needs of the devices connected to it, the excess capacity is wasted. An efficient system maximizes the utilization of all facilities. In addition, the expensive technology involved often becomes cost-effective only when links are shared.Figure 6.1 shows two possible
5、 ways of linking four pairs of devices. In Figure 6.la, each pair has its own link. If the full capacity of each link is not being utilized, a portion of that capacity is being wasted. In Figure 6.lb, transmissions between the pairs are multiplexed; the same four pairs share the capacity of a Single
6、 link.6.1 Many to one/ one to manyIn a multiplexed system, n devices share the capacity of one link. Figure 6.lb shows the basic format of a multiplexed system. The four devices on the left direct their transmission streams to a multiplexer (MUX) which combines them into a single stream (many to one
7、). At the receiving end, that stream is fed into a de-multiplexer (DEMUX), which separates the stream back into its component transmissions (one to many) and directs them to their intended receiving devices.Figure 6.1 Multiplexing versus no multiplexingIn Figure 6.l b the word path refers to the phy
8、sical link. The word channel refers to a portion of a path that carries a transmission between a .given pair of devices. One path can have many (n) channels.8.2 Type of multiplexingSignals are multiplexed using three basic techniques: frequency-division multiplexing (FDM), time-division multiplexing
9、 (TDM) and wavelength-division multiplexing (WDM). TDM is further subdivided into synchronous TDM (usually just called TDM) and asynchronous TDM also called statistical TDM or concentrator.Frequency-Division Multiplexing (FDM)Frequency-division multiplexing (FDM) is an analog technique that can be a
10、pplied when the bandwidth of a link is greater than the combined bandwidths of the signals to be transmitted. In FDM, signals generated by each sending device modulate different carrier frequencies. These modulated signals are then combined into a single composite signal that can be transported by t
11、he link. Carrier frequencies are separated by enough bandwidth to accommodate the modulated signal. These bandwidth ranges are the channels through which the various signals travel. Channels must be separated by strips of unused bandwidth (guard bands) to prevent signals from overlapping. In additio
12、n, carrier frequencies must not interfere with the original data frequencies. Failure to adhere to either condition can result in un-recoverability of the original signals.Figure 6.2 gives a conceptual view of FDM. In this illustration, the transmission path is divided into three parts, each represe
13、nting a channel to carry one transmission. As an analogy, imagine a point where three narrow streets merge to form a three-lane highway. Each of the three streets corresponds to a lane of the highway. Each car merging onto the highway from one of the streets still has its own lane and can travel wit
14、hout interfering with cars in other lanes.Figure 6.2 FDMKeep in mind that although Figure 6.2 shows the path as divided spatially into separate channels, actual channel divisions are achieved by frequency rather than by space.The FDM ProcessFigure 6.3 is a conceptual time-domain illustration of the
15、multiplexing process. FDM is an analog process and we show it here using telephones as the input and output devices. Each telephone generates a signal of a similar frequency range. Inside the multiplexer, these similar signals are modulated onto different carrier frequencies (f1, f2, and f3). The re
16、sulting modulated signals are then combined into a single composite signal that is sent out over a media link that has enough bandwidth to accommodate it.Figure 6.3 FDM multiplexing process, time-domainFigure 6.4 is the frequency-domain illustration for the same concept. (Note that the horizontal axis of this figure denotes frequency, not time. All three carrier frequencies exist at the same time within the bandwidt
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