网络chapter 6.docx

1、网络chapter 6Chapter 6 Multiplexing 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 th

2、at allows the simultaneous transmission 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

3、 use each to carry multiple signals. 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 t

4、he transmission capacity of a link 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 sh

5、ared.Figure 6.1 shows two possible 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 pa

6、irs share the capacity of a Single 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

7、 into a single stream (many to one). 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

8、 b the word path refers to the physical 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

9、 (FDM), time-division multiplexing (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

10、 an analog technique that can be applied 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 s

11、ignal that can be transported by the 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 si

12、gnals from overlapping. In addition, 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 divi

13、ded into three parts, each representing 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 h

14、as its own lane and can travel without 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 conceptua

15、l time-domain illustration of the 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 fr

16、equencies (f1, f2, and f3). The resulting 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