Computer Networking

Transport Layer

Channelization Protocols (Multiplexing) 

In channelization Protocols, the available bandwidth of the medium is shared in time, or shared in frequency, or shared in code for multiple stations to access the shared medium at a time.

Important: Channelization protocols are achieved through Multiplexing. Let explain channelization protocols through multiplexing.

  • Time division Multiplexing (TDM) channelize the channel through different time slots
  • Frequency division Multiplexing (FDM) channelize the channel through different frequency
  • Wavelength division Multiplexing (WDM) channelize the channel through different wavelength
  • Code division Multiplexing (CDM) channelize the channel through different codes

Let explain multiplexing for batter understanding of channelization.

Multiplexing And It’s Types

“Multiplexer combine multiple signals into one signal and DE-Multiplexer separate the multiplexed signals. So, multiplexing (combination of multiplexer and de-multiplexer)  allows multiple signals from multiple users to share a common communication channel.”

In simple words Multiplexing can defined as,

  • When multiple senders try to send digital or analog signal over a single medium, then a device called Multiplexer allocates the physical channel to each.
  • On the receiving end a De-multiplexer receives data from a single medium, identifies each, and sends to different receivers.

The following figurer explain the technique of multiplexing.

Purpose of Multiplexing

Basic purpose of multiplexing is to share a single medium between multiple signals of different devices. 

Note: Combination of different signals over the single medium does not mix with each other due to Modulation.

Types of Multiplexing

Signals are of two types analog and digital, so we required multiplexing for both types of signals.

1. Frequency Division Multiplexing (FDM)

FDM is an analog technology which is used in radio and TV transmission. The every input signal is modulated with a carrier frequency to change its frequency (characteristics).  Changed frequency signal is called frequency modulated. As explained under,

Following figure shows that, Modulated signals are multiplexed and transmitted through common transmission media.

Note: Channels (different stations i.e. mobile phones) are separated by guard bands. Guard band is a frequency which is not use by either any other channel.

2. Wavelength Division Multiplexing

Wavelength Division Multiplexing (WDM) is just like the frequency division multiplexing (FDM) except

  • WDF Use fiber optical modulated signals instead of frequency modulated signals.
  • And it use Fiber optic cable instead of any other medium.

As we know, the light has different wavelength (colors). For the purpose of modulation in fiber optic medium, multiple optical carrier signals are multiplexed with different input wavelengths.

  • Multiplexing and De-multiplexing is achieved through prism.
  • Prism can perform a role of multiplexer by combining the different optical signals to form a composite signal, and the composite signal is transmitted through a fiber optical cable.
  • Prism can also perform de-multiplexing operation by separating different optical signals.

Note: To utilize the high data rate capability of fiber optic cable, WDM is used.

Keep In Mind: The relationship between frequency and wavelength  is  λ = c/f. were C is velocity of light.

3. Time Division Multiplexing (TDM)

In TDM every transmitter is assigned a specific time slot to transmit it signals. Each user can transmit data within the provided time slot only.

Types of TDM

There are two major types of TDM.

I. Synchronous TDM:

The time slots are fixed assign. If the sender (station) is not ready to send data in it’s time-slot then empty slot is transmitted.

Note: In Synchronous TDM, Both ends, i.e. Multiplexer and De-multiplexer are timely synchronized and both switch to next channel simultaneously.

II. Asynchronous TDM: It cannot send empty slot in his time if no signal has to transmit.

The slots are allocated dynamically.  If the sender (station) is not ready to send data in it’s time-slot then the data of that station is transmitted which is ready at that time. In this this way empty slots cannot send. So it save channel capacity.

In Asynchronous TDM, sending of empty slots means, there are no signals remaining to transmit.

Important: In synchronous TDM, if we have X  input lines then there must  equal no of  slots (Y) in one frame So X=Y. but In asynchronous TDM, if we have X input lines then the frame contains not more than slots, with less than K (Y K).                                             

4. Code Division Multiplexing (CDM)

As we use carrier frequency signals to uniquely identify the signals in modulation. The alternative of this method is to use code division multiplexing where every input signal is assigned a unique code.

After assigning the unique code to every input signal, multiplexing and de-multiplexing is used over a single medium to transmits data successfully

 Note: Code division multiplexing is a mathematical technique which does not use the physical properties of signal for data transmission over a single medium.


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