Till now we have focused upon the various methods of flow control. We learnt about the Stop and wait ARQ, Selective Repeat and the Go Back N methods. Those tutorials gave us a fare idea of how the flow of information occurs in the computer networks. From this tutorial onwards, we will begin with the introduction to what are access control methods, we will glimpse through what are the various Access Control methods and explore the Time Division Multiplexing (TDM) method.

What do we mean by the access control method?

In order to send the data from one point to another the sender and the receiver make use of some links. Since a lot of computers may be sending the data at the same time, collision might occur which might lead to loss of data. Thus, there is a high possibility that the data never reaches its destination. To ensure that there is no collision while various computers are sending the data, we need some protocols or methods. Such methods used to access the data links are called access control methods. Before we begin to know the various kinds of such methods we look at a few points for a better understanding.

Introduction

• There are two types of links:
  (a) Broadcast Link: Many stations are connected to a common channel.
  (b) Point to point link: Source and destination are connected through a single wire.
• If more than one station is transmitting the data at the same time, there will be a collision. Thus, the access to the link has to be controlled in such a way that at any time only one station has to transmit data or if more than one station is transmitting, no collision should occur.
• Methods used to control the link are called access control methods.

We now move ahead to know the various access control methods in use. Different types of access Control methods are:

• Time Division Multiplexing
• Polling
• CSMA/CD
• Token Passing
• Pure Aloha
• Slotted Aloha

In this tutorial, we will look at time division multiplexing in greater detail.

Time Division Multiplexing

What is TDM?

It is a digital multiplexing technique of communication which allows transmission of two or more streaming digital signals over a common channel.

Time Division Multiplexing

• Timeline is divided into slots based on time and not frequency. Each slot is assigned to stations in a round robin manner.
• The users get a part of the available time and are allotted a time slot. The user can transmit the data only during that time and use the entire bandwidth in that allotted time.
• For an efficient transmission of data, the data rate capacity of the medium should be greater than the data rate required by sending or receiving devices.
• The signals to be transmitted are transmitted one by one and not simultaneously and hence each signal is transmitted for a very short duration of time. When all the signals are transmitted once on the transmission channel we say one cycle frame is complete.
• The Time Division Multiplexing system can be used to multiplex analog or digital signals but it is mostly preferred for digital signal multiplexing.
• The TDM Signal is transmitted on the common communication medium in the form of frames.

After multiplexing, the signals are transmitted over shared medium and reassembled into their original format after de-multiplexing.

Some Formulas of TDM

• Size of each slot = TT + TP.
  Note: This is the time taken by each station to transmit a packet onto the link and for packet to get out of the link.
• Efficiency of TDM = $ ( \frac {TT}{TT+TP} ) $

Assumption:
– Size of each packet is same for all stations. Thus, TT is same. Thus, at any time, only one link is accessing the link.
– Once a station is done transmitting, another station will get the slot.
– Data rate capacity of the transmission medium has to be greater than the data rate
required by sender and receiver in Time Division multiplexing

Questions

1. Answer the question given below –

Find the efficiency if 
 - TT = 1 ms,
 - TP = 1 ms

2. Answer the below question.

If N stations are connected to the above configuration and if each station needs 2kbps bandwidth, 
how many maximum stations can be connected to this channel?

Disadvantages of TDM

1. When we reserve a slot for a station, it may not have some data to transmit. Thus, the allotted slot is wasted.
2. Synchronisation is required in time division multiplexing.
3. It is complex to implement.

Applications of TDM

• It is used for some telephone system.

Hope that clears your concepts about Time division Multiplexing. To continue our learning of access control methods, we will learn about the Polling method in our next tutorial. Stay tuned for more knowledge.

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