Multiplexing – Data Communication and Networking

  • August 14, 2021
  • Aashish Mishra
  • Multiplexing which means multiple sources but one link.
  • Multiplexing is a technique used to combine and send the multiple data streams over a single medium.
  • The process of combining the data streams is known as multiplexing and hardware used for multiplexing is known as a multiplexer.
  • Multiplexing is achieved by using a device called Multiplexer (MUX) that combines n input lines to generate a single output line. Multiplexing follows many-to-one, i.e., n input lines and one output line.
  • Demultiplexing is achieved by using a device called Demultiplexer (DEMUX) available at the receiving end. DEMUX separates a signal into its component signals (one input and n outputs). Therefore, we can say that demultiplexing follows the one-to-many approach.
  • The ‘n’ input lines are transmitted through a multiplexer and multiplexer combines the signals to form a composite signal.
  • The composite signal is passed through a Demultiplexer and demultiplexer separates a signal to component signals and transfers them to their respective destinations.

Advantages of Multiplexing:

  • More than one signal can be sent over a single medium.
  • The bandwidth of a medium can be utilized effectively.

Multiplexing Techniques

1. Frequency Division Multiplexing(FDM)

  • It is an analog technique.
  • Available bandwidth is divided into bands that are separated by guard band.
  • FDM divides the spectrum or carrier bandwidth in logical channels and allocates one user to each channel.
  • Each user can use the channel frequency independently and has exclusive access of it.
  • All channels are divided in such a way that they do not overlap with each other.
  • Channels are separated by guard bands.
  • More than one station (even all) can send data at the same time.

Applications of FDM

  • FDM is commonly used in TV networks. It is used in FM and AM broadcasting. Each FM radio station has different frequencies, and they are multiplexed to form a composite signal.
  • The multiplexed signal is transmitted in the air.



2. Time Division Multiplexing

  • TDM is applied primarily on digital signals but can be applied on analog signals as well.
  • In TDM the shared channel is divided among its user by means of time slot.
  • Each user can transmit data within the provided time slot only.
  • Digital signals are divided in frames, equivalent to time slot i.e. frame of an optimal size which can be transmitted in given time slot.
  • Channel is time shared between the stations.
  • At one time only one station will use the entire channel.

3. Code Division Multiplexing

  • One channel carries all transmissions simultaneously.
  • CDM differ from FDM because only one channel occupies the entire bandwidth.
  • CDM differ from TDM because all stations can send data simultaneously ;there is no time sharing.
  • CDM allows its users to full bandwidth and transmit signals all the time using a unique code. CDM uses orthogonal codes to spread signals.
  • Each station is assigned with a unique code, called chip. Signals travel with these codes independently, inside the whole bandwidth.
  • The receiver knows in advance the chip code signal it has to receive.
  • The assigned codes have two properties: If we multiply any code (c1) to another (c2) = 0. If we multiply any code (c1) to itself (c1) = total no. of stations.

Example-
data: (d1.c1 + d2.c2 +d3.c3 + d4.c4 * c1
= d1.c1.c1 + d2.c2.c1 + d3.c3.c1 + d4.c4.c1
= 4d1 + 0 + 0 + 0 = 4.d1