To combine multiple signals (analog or digital) for transmission over a single line or media. A common type of multiplexing combines several low-speed signals for transmission over a single high-speed connection. As in "It's possible to multiplex 24 phone conversations over a single T1 Line simultaneously. Multiplexing is used heavily in the Broadband Telcom Industry.


Source: Last Updated 05.14.03

Just to elaborate on Pedro's writeup, the Multiplex technology was invented around the end of the 19th century, by a fellow by the name of Alexander Graham Bell. It was based on a system of separate electrical pulse frequencies that ran along one telegraph wire, allowing for many communications at once. It was multiplexing technology that made the invention of things like the phone and then the internet possible. Otherwise, at most about 4 people (that is, after the invention of the quadruplex telegraph by Edison) could use one cable (and by Morse code only), which would terribly impractical given maintenance costs.

Multiplex is also a term coined by Arthur C. Clarke, refering to the next in the series of: Simplex...Complex...Multiplex.

A good description of multiplex can be found in The Science of Discword by Terry Pratchett, Jack Cohen and Ian Stewart.

An all-too-common kind of movie theater. The classic movie theaters usually included only one screen in an ornately decorated building -- the setting you watched the movie in was often just as big a spectacle as the movie you went to see. But they were expensive, and they could only show one film at a time, so the multiplex was born. Instead of one screen, the multiplex includes many screens -- sometimes three or four, sometimes a dozen or more. To accomodate so many screens in one building, the individual screening rooms are reduced in size and most decorative frills are removed.

The good thing about multiplexes is that you have all those movies in one place. You can, in theory, show up for the first showing of the day and watch different movies all day long. The bad thing about multiplexes is that many of them are dull, joyless places, given over to zero-frills and slavery to the bottom line. Who wants to watch movies in a featureless box? On the bright side, more and more multiplexes are being built that try to put some of the spectacle back into going to the movies, with a lot more gilt and glitz (some of 'em even have curtains in front of the screen that part right before the film starts! Dude! Just like in the Old Days!), as well as better material comforts, like stadium seating, rocking seats, cupholders in the armrests, and meals, appetizers, and alcoholic beverages served during the movie. So maybe things aren't so bad after all...
In highway jargon, multiplexing is using the same physical road for multiple route numbers, as mentioned in neil's WU at How the United States highway system works. It's relatively common in small amounts (2-3 route numbers), particularly for an interstate and the US route number that section road carried prior to its upgrade (e.g. Interstate 81 and US 11) or for a N/S interstate to share with an E/W interstate on a relatively diagonal stretch (e.g. Virginia's section of I-64/81).

I have spent a somewhat significant amount of time riding on two rather unusual multiplexed highways.

  • Interstate 81 (I seem to be using this a lot) runs nearly straight east-west with a very slight southwest lean through much of Southwest Virginia. Interstate 77 crosses 81 at Wytheville, VA, and is multiplexed with it for about nine miles. The strange part is that the two roads, both nominally north-south interstates, are labeled as running in opposite directions -- 77N is 81S, and vice versa. Surprisingly, very few people get lost on this stretch, due to the excellent signage.
  • The biggest stack of signs I've ever seen on one highway was in Greensboro, NC. One approximately 2-mile stretch of road carries the markers:
    • I-40 East
    • I-85 North
    • US 29 North
    • US 52 South
    • US 74 East
    • US 220 North

Multiplexing is a name for any system which allows multiple signals to be mixed and seperated. Specifically, a system in which signals pass between two units in both directions is known as a duplex system.

The simplest multiplexing, perhaps, is time multiplexing (also called time-division multplexing). The same medium is used for all signals, but signals may only go in one direction at any particular time. Time-division duplex is known as half-duplex, while all other types of duplex are known as full-duplex.

Regular time multiplexing would be like a system with a 3-second period, one second being allocated to signals from unit A, one second being allocated to signals from unit B, and one second being allocated to signals from unit C. If unit A has a message to transmit (to either B or C), it must wait for it's allocated time (the maximum waiting time, then, would be 2 seconds). Of course, during the two seconds that unit A is not allowed to transmit, it must still monitor the bus for messages from one of the other units, in case there is a message that e.g. unit C needs to send to unit A. One problem with this kind of time-multiplexing is that all units must be synchronized all the time in order to agree not to use the medium at the same time. At least some digital cellular phone systems use regular time multiplexing.

Irregular time multiplexing is used, for example, on the I2C bus. A master may wish to transmit a command to one of several slaves, and receive a response from that slave. When the bus is idle, the bus is allocated to signals from the master. When the master transmits a command to a slave that requires an acknowledge or a response, the bus will be allocated to signals from that slave once the command is complete. The bus remains allocated to the slave until some event: the slave's response is complete, or the slave asks for an acknowledge or for more data from the master. (A timeout may also be used in an irregular time multiplexing system, but timeouts are not used in I2C.) Again, all units must agree not to use the medium at the same time - in this case, it is accomplished by monitoring bus traffic for commands, rather than by keeping track of time. (The I2C protocol is even more complex than this, with provisions for many masters on one bus. One of the things which allows for this is that signals may actually be sent simultaneously by more than one unit, because all outputs are open drain, and there are pull-up resistors on the bus lines. If, at any time the bus is allocated to masters, any master detects a signal on that line that it is not transmitting, it must concede arbitration, i.e. give up control of the bus to whatever master was sending the signal.)

Frequency-division multiplexing is how radio stations may all transmit their own signals continuously without interfering with each other. The medium must have a large bandwidth in order for this multiplexing to work. Many signals may be sent at the same time, all at different frequencies. The signals are seperated with filters.

Medium-division multiplexing is a name for a system with a different medium for different signals. An RS-232 bus uses medium-division - there is one wire carrying data signals from the DTE, there is a different wire carrying data signals from the DCE.

Note that multiplexing methods may be combined. For example, a medium may be frequency-multiplexed into two 'channels' X and Y. Channel X itself may be regularly time-multiplexed. Channel Y may be irregularly time-multiplexed.

Mul"ti*plex (?), a. [L. multiplex, -plicis. See Multiply.]

Manifold; multiple.


© Webster 1913.

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