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.

 

Example-

 

 

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

 

Thus,

 

Size of each time slot = Tt + Tp

 

where-

  • Tt = Transmission delay
  • Tp = Propagation delay

 

NOTE-

 

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-

 

Efficiency (η) = Useful Time / Total Time

 

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

 

Thus,

 

 

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

 

Disadvantage-

 

  • 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.

 

PRACTICE PROBLEM BASED ON TIME DIVISION MULTIPLEXING (TDM)-

 

Problem-

 

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?

 

Solution-

 

Given-

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

 

Part-01:

 

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%

 

Part-02:

 

We know-

 

Effective Bandwidth = Efficiency (η) x Bandwidth

 

Thus,

Effective Bandwidth

= 0.5 x 4 Mbps

= 2 Mbps

 

Part-03:

 

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

 

Get more notes and other study material of Computer Networks.

Watch video lectures by visiting our YouTube channel LearnVidFun.

Summary
Time Division Multiplexing | Access Control
Article Name
Time Division Multiplexing | Access Control
Description
Time Division Multiplexing is an access control method. Access Control in Networking controls the access of stations to shared link. Time Division Multiplexing divides the link time into fixed time slots and distributes them in Round Robin order.
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Publisher Name
Gate Vidyalay
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