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Controlled Access Protocols in Computer Network

In controlled access, the stations seek information from one another to find which station has the right to send. It allows only one node to send at a time, to avoid the collision of messages on a shared medium. The three controlled-access methods are:

  1. Reservation
  2. Polling
  3. Token Passing

Reservation

The following figure shows a situation with five stations and a five-slot reservation frame. In the first interval, only stations 1, 3, and 4 have made reservations. In the second interval, only station 1 has made a reservation.

Advantages of Reservation:

Disadvantages of Reservation:

Polling

 

Advantages of Polling:

Disadvantages of Polling:

Efficiency Let Tpoll be the time for polling and Tt be the time required for transmission of data. Then,

 Efficiency = Tt/(Tt + Tpoll)

Token Passing

 

Performance of token ring can be concluded by 2 parameters:-

  1. Delay, is a measure of time between when a packet is ready and when it is delivered. So, the average time (delay) required to send a token to the next station = a/N.
  2. Throughput, which is a measure of successful traffic.
 Throughput, S = 1/(1 + a/N) for a<1 

and

           S = 1/{a(1 + 1/N)} for a>1.  
       where N = number of stations
             a = Tp/Tt 
(Tp = propagation delay and Tt = transmission delay)

Advantages of Token passing:

Disadvantages of Token passing:

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