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Time Division Multiplexing | Access Control

Access Control in Networking-


Before you go through this article, make sure that you have gone through the previous article on Access Control.


We have discussed-

  • Access Control is a mechanism that controls the access of stations to the transmission link.
  • Broadcast links require the access control mechanism.
  • There are various access control methods-



  1. Time Division Multiplexing
  2. Polling
  3. CSMA / CD
  4. Token Passing
  5. Aloha


In this article, we will discuss about Time Division Multiplexing (TDM).


Time Division Multiplexing-


In Time Division Multiplexing (TDM),

  • Time of the link is divided into fixed size intervals called as time slots or time slices.
  • Time slots are allocated to the stations in Round Robin manner.
  • Each station transmit its data during the time slot allocated to it.
  • In case, station does not have any data to send, its time slot goes waste.





Size Of Time Slots-


The size of each time slot is kept such that each station gets sufficient time for the following tasks-

  • To put its data packet on to the transmission link
  • Last bit of the packet is able to get out of the transmission link




Size of each time slot = Tt + Tp



  • Tt = Transmission delay
  • Tp = Propagation delay




To keep the size of time slots constant,

  • We have assumed that all the stations want to send the packets of same size.
  • This keeps Tt constant for all the stations.
  • We have considered the worst case when both the stations are present at the two extreme ends.
  • This ensures Tp will be maximum and all the stations will get sufficient time to propagate their data.




Efficiency (η) = Useful Time / Total Time


  • Useful time = Transmission delay of data packet = Tt
  • Useless time = Propagation delay of data packet = Tp





Important Formulas-


  • Size of each time slot in Time Division Multiplexing = Tt + Tp
  • Efficiency (η) = 1 / (1+a) where a = Tp / Tt
  • Effective Bandwidth / Bandwidth Utilization / Throughput = Efficiency(η) x Bandwidth
  • Maximum Available Effective Bandwidth = Total number of stations x Bandwidth requirement of 1 station




  • If any station does not have the data to send during its time slot, then its time slot goes waste.
  • This reduces the efficiency.
  • This time slot could have been allotted to some other station willing to send data.






If transmission delay and propagation delay of a packet in Time Division Multiplexing is 1 msec each at 4 Mbps bandwidth, then-

  1. Find the efficiency.
  2. Find the effective bandwidth.
  3. How many maximum stations can be connected to the network if each station requires 2 Kbps bandwidth?





  • Transmission delay (Tt) = 1msec
  • Propagation delay (Tp) = 1msec
  • Bandwidth = 4 Mbps




For a TDM Network,


Efficiency (η) = 1 / 1+a where a = Tp / Tt


Calculating Value Of ‘a’-


a = Tp / Tt

a = 1 msec / 1 msec

a = 1


Calculating Efficiency-


Efficiency (η)

= 1 / (1+a)

= 1 / (1 + 1)

= 1 / 2

= 0.5

= 50%




We know-


Effective Bandwidth = Efficiency (η) x Bandwidth



Effective Bandwidth

= 0.5 x 4 Mbps

= 2 Mbps




We know-


Maximum Effective Bandwidth

= Total number of stations x Bandwidth requirement of 1 station


Let the total number of stations that can be connected be N.

Then, we have-

2 Mbps = N x 2 Kbps

N = 1000

Thus, maximum 1000 stations can be connected.


To gain better understanding about Time Division Multiplexing (TDM),

Watch this Video Lecture


Next Article- Polling | Access Control Method


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