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FIGURE 3-3. An asynchronous bit stream
In telecommunications, and particularly in ADSL terminology, asynchronous refers to the differing data rates sent from and received by an end point of the transmission path, such as customer premises equipment (CPE). If the CPE receives data at one frequency and either does not transmit any data back to the source or transmits back at a much slower rate, then the link between the sending and receiving points is considered an asynchronous link. As an example, ADSL sending equipment can transmit 1.5 Mbps to a user who can return up to 640 Kbps.
Multiplexing is the framework of the ATM and ADSL foundations. It allows multiple data signals to be combined for transport. Multiplexing promotes efficient use of bandwidth as the process maximizes the number of data signals combined into a single signal suitable for transmission. Current telephone installations do not use multiplexing from the home to the central office (CO). Multiplexing occurs at the CO where multiple voice circuits are combined into large bundles of circuits.
Multiplexing is the business of placing data signals from multiple sources into a known sequence on the same transmission medium for transport from source to destination. Two or more data signals are combined in the transmission equipment in such a manner that neither signal disturbs the fundamental relationships (level and timing) of the other signals and each independent data signal is recovered intact by the receiving equipment. The technique of multiplexing signals is made possible due to increased integrated circuit speeds and increased transmission speeds.
Take voice circuits as an example. To digitize a voice circuit, or channel as they are sometimes called, the voice signal itself is periodically sampled. The Nyquist rate is two samples per hertz of frequency. Telephone circuit voice bandwidth is about 4 KHz. So, the Nyquist sampling rate is 8 KHz x 8 bits / Hz = 64 Kbps. Now the transmission rate for ISDN is 1.544 Mbps. Somehow, multiple voice circuits, 24 of them to be exact, must be combined to fully utilize the 1.544 Mbps ISDN rate. Well, multiplexing is the technique that gets the job done.
Figure 3-4. A lightly loaded (multiplexed) and very inefficient train
Figure 3-5. A fully loaded (multiplexed) train
In a simplified explanation, each voice circuit is continuously digitized at the 8 Kbps sampling rate. Each group of 8 bits from each circuit must wait its turn to board the ISDN train. Think of each group of 8 bits as a tour group occupying exactly one passenger car in the train. If the train leaves the station with only a single voice circuit occupying one car of the train, as shown in Figure 3-4, the train does not operate very efficiently (wastes space or bandwidth). But, if the train is fully loaded, as shown in Figure 3-5, then the train operates at maximum efficiency (utilizes all the bandwidth to get the data to the destination). If you want to understand modern communications technology, and particularly ATM and ADSL, you must understand the concept of bandwidth and come to grasp the idea that bandwidth is gold, green gold, the color of money. Service providers charge for the amount of bandwidth used by networks. And it is not cheap.
Now, each voice circuit has a specific order, like a seating arrangement, to load its bits into its car in the train. The order the cars are in and the seating arrangement of each car is known to both the departing station (source multiplexer) and the arriving station (destination demultiplexer). If any circuit gets out of order, either loading onto the wrong car or not placing its bits into the correct seats when loading, then the destination will unload (demultiplex) the signals incorrectly.
Kind of like multiple tour groups milling about a train station, some members will certainly become separated from their tour group. Yet every tour group must immediately exit the station in groups of eight people (bits). Another train is immediately behind and there is no time to waste. So the mean stationmaster forces every group to immediately vacate the premises, whether or not the original group member structure is intact.
Figure 3-6. Frequency division multiplexing
There are two flavors of multiplexing of interest in ATM and ADSL. One is called frequency division multiplexing (FDM) and the other is time division multiplexing (TDM). The difference between the two flavors is indicated by their names. One multiplexes signals in the frequency spectrum, also called frequency domain, and the other multiplexes signals in the time domain.
Frequency multiplexing combines signals of differing frequencies into a single signal that is composed of the differing signals. As an example, two original voice signals, channel one of 4 KHz bandwidth and channel two of 4 KHz bandwidth, are combined into a single signal that is composed of the two original signals plus some carrier signal. See Figure 3-6. The technique involves adding (modulating) the two signals onto the carrier at differing frequencies so the two original signals do not interfere with each other. In the figure, 2,140 KHz is chosen for one voice channel and 2,150 KHz is chosen for the other voice channel. The two resulting signals are applied to a multiplexer circuit which is essentially a filter combiner that just combines the two signals onto the same transmission path. Note if the two original voice channels are combined onto the same transmission path while still original 4 KHz signals, the resultant combined signal is gibberish.
Figure 3-7. Frequency division demultiplexing
Figure 3-8. Time division multiplexing
To restore the two original voice signals, the multiplexed signal must undergo a reverse multiplexing process called demultiplexing. See Figure 3-7. Figure 3-7 is essentially the same as Figure 3-6 except the arrows are turned around as the demultiplexer signal flow is the reverse of a multiplexer. Also, instead of an adder circuit to mix the carriers with the original voice signals, now there is a subtractor circuit to remove the original voice signals from the carriers.
Time division multiplexing (TDM) is a simple concept. Divide some increment of time, say one second, into multiple slots and assign specific slots to individual voice circuits. If the sending multiplexer and receiving demultiplexer agree upon the assignment of slots, then the recovered data is identical to the transmitted data.
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