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MULTIPLEXERS (MUXs or MUXes)

The term multiplex has its roots in the Latin words multi (many) and plex (fold). Multiplexers (MUXs) act as both concentrators and contention devices to allow multiple, relatively low-speed terminal devices to share a single, high-capacity circuit (physical path) between two points in a network. The benefit of multiplexers is that they allow carriers and end users to take advantage of the economies of scale. Just as a multilane highway can carry increased volumes of traffic in multiple lanes at higher speeds and at relatively low incremental cost, a high capacity circuit can carry multiple conversations in multiple channels at relatively low incremental cost.

The modern saying, “Time is Money,” is indeed most of all true when applied to telegraphic signalling; and many endeavours have been made, not only to transmit signals with celerity, but also to transmit more than one communication at the same time along the same wire. This has been successfully done in the duplex system—by which a message is sent from either end of the same wire simultaneously; in the diplex system—in which two messages can be sent simultaneously in one direction; and in the quadruplex system, which combines the two former methods, and by which it is possible to convey four signals along the same wire at the same moment. This last method was invented by Mr. Edison….

Wonders of the Universe. 1899. [2-5]

Contemporary multiplexers rely on four-wire circuits that permit multiple logical channels to be derived from a single physical circuit, and that permit high-speed transmission simultaneously in both directions. In this manner, multiple communications (either unidirectional or bidirectional) can be supported. Multiplexing is used commonly across all transmission media, including twisted pair, coaxial and fiber optic cables; and microwave, satellite and other radio systems.

Traditional multiplexing comes in several varieties, presented in chronological order of development and evolution. Included are Frequency Division Multiplexing (FDM), Time Division Multiplexing (TDM), and Statistical Time Division Multiplexing (STDM). Wavelength Division Multiplexing (WDM), although still in development, is discussed here in brief. WDM will be used in fiber optic cable systems.

Frequency Division Multiplexing (FDM)

Frequency Division Multiplexing (FDM) takes advantage of the fact that a single twisted pair, copper circuit can carry much more than the 4 kHz guaranteed for individual voice conversations. Even in the early days of vacuum tube technology, up to 96 kHz could be supported over a set of 2 copper pairs (a 4-wire circuit, with 2 wires in each direction), thereby enabling the carrying of up to 24 individual voice channels, separated by frequency bands [2-2]. In terms of a commonly understood analogy, multiple frequencies can be supported over a single, four-wire electrical circuit much as can multiple radio stations, and TV channels be supported over the airwaves through frequency separation.

Through a FDM MUX (Figure 2.10), conversation #1 might be supported over frequencies 0 Hz–4,000 Hz; conversation #2 over frequencies 4,000 Hz–8,000 Hz; conversation #3 over frequencies 8,000 Hz–12,000 Hz; and so on. Additionally, small slices of frequency are designated as subchannels, or guard bands, which separate the carrier channels used for information transmission. The guard bands serve to minimize the likelihood of interference between conversations riding in adjacent information channels over the same physical circuit.


Figure 2.10  Frequency Division Multiplexing (FDM) in a data communications application.

Frequency Division Multiplexers, however, typically are not particularly intelligent. Specific devices or groups of devices often are tuned to using designated frequency bands for communications. As noted in Figure 2.10, the bandwidth associated with those devices is unused if the communication is inactive for some reason, even though other devices could perhaps make effective use of it.

FDM served its purpose well, at the time, for long-haul voice transmission. Data communications over FDM, however, requires sets of special low-speed modems, one for each channel, with one set at each end of the facility. FDM currently is used in broadband Local Area Networks (LANs), which support multiple simultaneous transmissions. FDM also is used in cellular radio networks and in certain digitized voice applications. As we have noted, however, (all things being equal) digital generally is better. Digital is especially better when data traffic is involved, and this rapidly is becoming a data world.

Time Division Multiplexing (TDM)

Time Division Multiplexing (TDM) offers all of the advantages of digital transmission, namely improved bandwidth utilization, enhanced error performance, improved security and upgradeability.

At the transmitting end of the connection, the TDM scans the ports to which individual devices are attached, allocating each device port a channel, or time slot, for transmission of data. Device #1 transmits through Port #1 and over Time Slot #1, Device #2 transmits through Port #2 and over Time Slot #2, and so on, in a serial fashion. The transmitting TDM typically accepts an 8-bit set of data from each port and interleaves those sets of data into a single, composite digital output. As the MUX completes a scan of the ports and transmits a set of such data, it will separate one set of data from another by inserting some number (usually one) of framing bits, which serve to frame, or package the data set.

At the receiving end, the process is reversed, with the channels being identified serially and the individual conversations being transmitted by the MUX over individual ports to the individual, intended receiving terminal devices. Clearly, the MUXs must be carefully synchronized in time, so as to allow the receiving MUX to determine the proper separation of frames and channels of data.

The primary constraint of a basic TDM is one of static configuration. In other words, Channel #1 is always reserved for Port #1, over which Terminal #1 always transmits. Terminals which are idle, turned off, unplugged or on fire still are allocated valuable bandwidth, thereby having a deleterious effect on the cost-effectiveness of the facility (see Figure 2.11). As a result, TDMs are no longer held in favor.


Figure 2.11  Time Division Multiplexing (TDM) in a data communications application.


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