Polling in Networking | Access Control Method

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 polling.

 

Polling-

 

In this access control method,

• A polling is conducted in which all the stations willing to send data participates.
• The polling algorithm chooses one of the stations to send the data.
• The chosen station sends the data to the destination.
• After the chosen station has sent the data, the cycle repeats.

 

Example-

 

 

Here-

• T_poll = Time taken for polling
• T_send = Time taken for sending the data = Transmission delay + Propagation delay = T_t + T_p

 

Efficiency-

 

Efficiency (η) = Useful Time / Total Time

 

• Useful time = Transmission delay of data packet = T_t
• Useless time = Time wasted during polling + Propagation delay of data packet = T_poll + T_p

 

Thus,

 

 

Advantages-

 

• Unlike in Time Division Multiplexing, no slot is ever wasted.
• It leads to maximum efficiency and bandwidth utilization.

 

Disadvantages-

 

• Time is wasted during polling.
• Link sharing is not fair since each station has the equal probability of winning in each round.
• Few stations might starve for sending the data.

 

Important Formulas-

 

• Efficiency (η) = T_t / (T_poll + T_t + T_p)
• Effective Bandwidth / Bandwidth Utilization / Throughput = Efficiency(η) x Bandwidth
• Maximum Available Effective Bandwidth = Total number of stations x Bandwidth requirement of 1 station

 

To gain better understanding about Polling Access Control Method,

Watch this Video Lecture

 

Next Article- CSMA / CD | Access Control Method

 

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