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 several problems as it requires an I/O port for each device, a need line for each device, and also a large amount of wiring needed if on different floors. But instead, if we use a multiplexer approach then all devices are connected to MUX and one line to the host, the link carries multiple channels of information and a number of lines equal to the number of lines out. 

Why Multiplexing?

Multiplexing is the sharing of a medium or bandwidth. It is the process in which multiple signals coming from multiple sources are combined and transmitted over a single communication/physical line.

Multiplexing

The ‘n’ input lines shown above are transmitted via a multiplexer and it combines the signals to form a composite signal. Now, these signals are passed through DEMUX and it transfers those signals to their respective destinations.

Types of Multiplexing

Multiplexing can be classified as:

1. Frequency Division Multiplexing (FDM)
2. Time-Division Multiplexing (TDM)
3. Wavelength Division Multiplexing (WDM)

Types of Multiplexer

1. 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. 

Frequency Division Multiplexing (FDM)

A multiplexer accepts inputs and assigns frequencies to each device. The multiplexer is attached to the high-speed communication line. 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. 

Advantages of FDM

• The process is simple and easy to modulate.
• A corresponding multiplexer or de-multiplexer is on the end of the high-speed line and separates the multiplexed signals.
• For frequency division multiplexing analog signaling is used in order to transmit the signals.

Disadvantages 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.
• 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.

2. 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. 

Time Division Multiplexing

There are two types of TDMs which are as follows: 

1. Synchronous Time Division Multiplexing
2. Statistical Time Division Multiplexing
3. Asynchronous Time Division Multiplexing
4. Interleaving Time Division Multiplexing

1. Synchronous Time Division Multiplexing: It is synchronous because the multiplexer and the de-multiplexer have to agree on the time slots. The original time-division multiplexing. The multiplexer 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. 

Synchronous TDM

2. Statistical Time Division Multiplexing: It is a time-division but on-demand rather than fixed, rescheduling links 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 multiplexer 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.

Statistical TDM

3. Asynchronous Time Division Multiplexing: It is a type of multiplexing, where the rate of sampling is different and also does not require a general clock, it is called Asynchronous Time Division Multiplexing. Asynchronous TDMs have generally low bandwidth. In case when there is nothing to transmit, this type of TDM gives its time slot to other devices.

4. Interleaving Time Division Multiplexing: Interleaving TDM can be taken as two switches rotating at high speed, one on the multiplexing and the other on the demultiplexing surface. These switches can also be rotated in the reverse direction. At once, when it leaves the surface of multiplexing and releases at the surface of de-multiplexer, it is called Interleaving.

3. Wavelength Division Multiplexing(WDM)

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. 

Wavelength Division Multiplexing(WDM)

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. 

1. Dense wavelength division multiplexing: It combines many (30, 40, 50, or more) channels onto one fiber. DWDM channels have a very high capacity and it keeps on improving.
2. Coarse wavelength division multiplexing: It combines only a few lambdas. In this, channels are more widely spaced and are a cheaper version of DWDM.

Some Other Types of Multiplexing

There are some more types of multiplexing that are discussed here.

1. Code Division Multiplexing

It is a type of multiplexing where each code is distributed among different spectrums so that each one of them can work at the same time simultaneously. It is also called Code Division Multiple Access (CDMA). CDMA makes use of spectrum technology along with analog-to-digital conversion(ADC). It is thus used by various radio communication technologies. Mainly, it is used for mobile communication. 

Advantages of Code Division Multiplexing

• It provides enhanced data communication capabilities.
• It does not compress the bandwidth of other frequencies.

2. Orthogonal Frequency Division Multiplexing

Orthogonal frequency-division multiplexing (OFDM) is a digital communication technique initially developed for use in cable television systems. OFDM is similar to the broadcasting technique known as frequency division multiplexing (also known as FDM), which uses a multitude of transmitters and receivers to send information on different frequencies over a single wire, such as an electrical power cable.

OFDM vs FDM

Uses of OFDM

• OFDM is used in Digital radio, Digital Radio Mondiale, digital audio broadcasting, and satellite radio.
• OFDM is used in Wired data transmission.

3. Space Division Multiplexing

Space Division Multiplexing (SDM) is the combination of Frequency Division Multiplexing (FDM) and Time Division Multiplexing (TDM). It basically means that a particular channel is used against a particular frequency band for some time. 

Advantages of Space Division Multiplexing

• It provides a high data transmission rate.
• Some Passive Optical Network (POV) implementations use SDM.