Uni-directional distribution systems or broadcast systems are an extreme version of asymmetric communications systems .Quite often ,bandwidth limitations ,differences in transmission power ,or cost factors prevent a communication system from being symmetrical.Symmetrical communication systems offer the same transmission capabilities in both communication directions ,i.e the channel characteristics form A to B are as same as from B to A .
Examples of symmetric communications services are the plain old telephone service (POTS) or GSM ,if end-to-end communication is considered .In this case ,it does not matter if one mobile station calls the other or the other way round,bandwidth and delay are the same in both services .
This symmetry is necessary for a telephone service ,but many other applications do not require the same characteristics for both directions of information transfer.Consider a typical client /server environment.Typically ,the clients needs more data from server than the server needs from the client .Today's prominent example is world wide web .Millions of users download the data using there browsers form the web servers .A user only returns information to server from time to time .Single requests for new page with typical size of several hundreds bytes result in responsese of upto some 10 kbytes on average.
A television with a set-top box represents more extreme scenario .While high resolution video stream requires several mbits/s, a typical user returns some bytes from time to time to switch between channels or return some information for television shopping
Finally ,todays pagers and radio work completely one-way.These devices can only reveive information backto e.g,the radio station .Typically the telephone system is used for this purpose.
A special case of asymetrical communication systems are unidirectional broadcast systems where typically a high bandwidth data stream exists from one sender to many receivers.The problem arising from this is that the sender can only optimize transmitted data for the whole group of the recivers and not fro individual users .

Multiplexing is not only a fundamental mechanism in communication systems but also in everyday life.Multiplexing describes how several users can share a medium with minimum or no interferance.One example ,is highways with several lanes.Many users (car drivers)use the same medium (highways) with hopefully no interferance(accidents).Thid is possible due to provision of several lanes (space division multiplexing )seperate traffic .In addition ,different cars use the same medium (i.e the same lane) at different points in time(time division multiplexing).
While this sample example illustrates our everday use of multiplexing ,the following examples deal with the use of multiplexing and assignment of a medium to users (traffic regulations)

In extc, computer-networks multiplexing (short muxing) is a term used to refer to a Process where multiple analog message signals or digital data streams are combined into one signal over a shared medium. The aim is to share an expensive resource. For example, in electronics, multiplexing allows several analog signals to be processed by one analog-to-digital converter (ADC), and in telecommunications, several phone calls may be transferred using one wire. In communications, the multiplexed signal is transmitted over a communication channel, which may be a physical transmission medium. The multiplexing divides the capacity of the low-level communication channel into several higher-level logical channels, one for each message signal or data stream to be transferred. A reverse process, known as demultiplexing, can extract the original channels on the receiver side.
A device that performs the multiplexing is called a Multiplxer (MUX), and a device that performs the reverse process is called a De-multiplexer (DEMUX).
The two most basic forms of multiplexing are time division multiplexing (TDM) andfrequency division multiplexing (FDM).
In optical, FDM is referred to as wave Division multiplexing (WDM).
Variable bit rate digital bit streams may be transferred efficiently over a fixed bandwidth channel by means of statisics, for example packet mode communication. Packet mode communication is an asynchronousmode time-domain multiplexing, which resembles but should not be considered as time-division multiplexing.
Digital bit streams can be transferred over an analog channel by means of code division multiplexing.
In wireless communication, multiplexing can also be accomplished through alternating Polarization (horizontal/vertical or clockwise/counterclockwise) on each adjacent channel and satellite, or through phase multiarray antennacombined with a multiple input multiple output comm (MIMO) scheme.