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Module 5
Lesson 5
The token passing access method

Objective
Assess the characteristics of the token passing access method.

What is token passing?

The token passing access method is a non-contention method that works very differently from the contention methods previously discussed. Token passing is a more orderly way for a network to conduct its business. A signal called a token goes from one computer to the next. In a Token Ring network, the token goes around the ring; in a token bus network, it goes down the line of the bus. If a computer has data to transmit, it must wait until the token reaches it; then that computer can capture the token and transmit data.

Token passing is the second most popular access method in use on LANs today, after CSMA/CD. The FlipBook below shows how this method differs from the contention methods you learned about in the previous lessons.
 



Transcript



 

TIP:Specifications for token passing are defined in the IEEE 802.5 standard.



 
There is no contention involved, and there can be no collisions on a token passing network.

There are some variations on the process above. Continue reading for more information on Token passing variations.
 


TIP:Different token passing architectures work differently. Token Ring networks allow only one token on the cable at a time. FDDI networks can have multiple tokens circulating simultaneously.



 
The token passing access method has both advantages and disadvantages.
 
Advantages of token passing Disadvantages of token passing
  • Non-contention method; Computers do not compete for access to the cable. Each computer will get its "turn" as the token comes around the network.

  • Effective; Collisions are prevented altogether.

  • Reliable; The maximum amount of time before a given computer will be able to transmit can be calculated.
  • Slow; A large amount of network bandwidth is consumed in the process.

  • Costly; Implementation is expensive due to the media and equipment used.


 

 
Next lesson

In the next lesson, you will learn about the demand priority access method.
 
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