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 T_t constant for all the stations.
• We have considered the worst case when both the stations are present at the two extreme ends.
• This ensures T_p 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 = T_t
• Useless time = Propagation delay of data packet = T_p

 

Thus,

 

 

Important Formulas-

 

• Size of each time slot in Time Division Multiplexing = T_t + T_p
• Efficiency (η) = 1 / (1+a) where a = T_p / T_t
• 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 (T_t) = 1msec
• Propagation delay (T_p) = 1msec
• Bandwidth = 4 Mbps

 

Part-01:

 

For a TDM Network,

 

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

 

Calculating Value Of ‘a’-

 

a = T_p / T_t

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

 

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