**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-

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 = T_{t} + T_{p} |

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-

- Find the efficiency.
- Find the effective bandwidth.
- 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 = T_{p} / T_{t} |

**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),

**Next Article-****Polling | Access Control Method**

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