Multiplexing (Channel Sharing) in Computer Network
Multiplexing means multiple sources but one link. An alternative approach to it is Direct Point to Point Connection but it has a number of problems as it requires an I/O port for each device, a need line for each device, and also a large amount of wiring is needed if on different floors. But instead, if we use a multiplexer approach then all devices are connected to MUX and one line to host, the link carries multiple channels of information and a number of lines equal to the number of lines out.
Types of Multiplexers:
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- Frequency Division Multiplexing (FDM) –
The frequency spectrum is divided among the logical channels and each user has exclusive access to his channel. It sends signals in several distinct frequency ranges and carries multiple video channels on a single cable. Each signal is modulated onto a different carrier frequency and carrier frequencies are separated by guard bands. The bandwidth of the transmission medium exceeds the required bandwidth of all the signals. Usually, for frequency division multiplexing analog signaling is used in order to transmit the signals, i.e. more susceptible to noise. Assignment of non-overlapping frequency ranges to each user or signal on a medium. Thus, all signals are transmitted at the same time, each using different frequencies.
A multiplexer accepts inputs and assigns frequencies to each device. The multiplexor is attached to the high-speed communication line. A corresponding multiplexor or de-multiplexor is on the end of the high-speed line and separates the multiplexed signals. The frequency spectrum is divided up among the logical channels where each user hangs onto a particular frequency. The radio spectrum is an example of the media and the mechanism for extracting information from the medium.
The disadvantage of FDM:
One problem with FDM is that it cannot utilize the full capacity of the cable. It is important that the frequency bands do not overlap. Indeed, there must be a considerable gap between the frequency bands in order to ensure that signals from one band do not affect signals in another band.
- Time Division Multiplexing (TDM) –
Each user periodically gets the entire bandwidth for a small burst of time, i.e. entire channel is dedicated to one user but only for a short period of time. It is very extensively used in computer communication and telecommunication. Sharing of the channel is accomplished by dividing available transmission time on a medium among users. It exclusively uses Digital Signaling instead of dividing the cable into frequency bands. TDM splits cable usage into time slots. The data rate of transmission media exceeds the data rate of signals. Uses a frame and one slot for each slice of time and the time slots are transmitted whether the source has data or not.
There are two types of TDMs which are as follows:
- Synchronous Time Division Multiplexing:
It is synchronous because the multiplexer and the de-multiplexer have to agree about the time slots. The original time-division multiplexing. The multiplexor accepts input from attached devices in a round-robin fashion and transmits the data in a never-ending pattern. Some common examples of this are T-1 and ISDN telephone lines. If one device generates data at a faster rate than other devices, then the multiplexor must either sample the incoming data stream from that device more often than it samples the other devices or buffers the faster incoming stream. If a device has nothing to transmit, the multiplexor must still insert a piece of data from that device into the multiplexed stream.
- Statistical Time Division Multiplexing:
Time-division but on-demand rather than fixed, reschedule link on a per-packet basis and packets from different sources interleaved on the link. It allows the connection of more nodes to the circuit than the capacity of the circuit. Works on the premise that not all the nodes will transmit at full capacity at all times. It must transmit a terminal identification i.e destination id no. and may require storage. A statistical multiplexor transmits only the data from active workstations. If a workstation is not active, no space is wasted on the multiplexed stream. It accepts the incoming data streams and creates a frame containing only the data to be transmitted.
- Synchronous Time Division Multiplexing:
- Wavelength Division Multiplexing –
It is the same as FDM but applied to fibers, only the difference is that here the operating frequencies are much higher actually they are in the optical range. There’s great potential for fibers since the bandwidth is so huge. Fibers with different energy bands are passed through a diffraction grating prism. Combined on the long-distance link and then split at the destination. It has got high reliability and very high capacity.
It multiplexes multiple data streams onto a single fiber optic line. Different wavelength lasers(called lambdas) transmit multiple signals. Each signal carried on the fiber can be transmitted at a different rate from the other signals.
- Dense wavelength division multiplexing combines many (30, 40, 50, or more) channels onto one fiber. DWDM channels have a very high capacity and it keeps on improving.
- Coarse wavelength division multiplexing combines only a few lambdas. In this, channels are more widely spaced and are a cheaper version of DWDM.