Notes and Study Materials

Random Access Protocols in DCN:

 

In random access or contention methods, no station is superior to another station and none is assigned the control over another. No station permits, or does not permit, another station to send. At each instance, a station that has data to send uses a procedure defined by the protocol to make a decision on whether or not to send. This decision depends on the state of the medium (idle or busy).

 

Two features give this method its name. First, there is no scheduled time for a station to transmit. Transmission is random among the stations. That is why these methods are called random access. Second, no rules specify which station should send next. Stations compete with one another to access the medium. That is why these methods are also called contention methods.

 

 

 

However, if more than one station tries to send, there is an access conflict-collision-and the frames will be either destroyed or modified. To avoid access conflict or to resolve it when it happens, each station follows a procedure that answers the following questions:

 

Aloha Protocols

Carrier Sense Multiple Access Protocol

Carrier Sense Multiple Access with Collision Detection

 

    When can the station access the medium?
    What can the station do if the medium is busy?
    How can the station determine the success or failure of the transmission?
    What can the station do if there is an access conflict?

 

The different random access methods are as follows:

 

    ALOHA
    CSMA
    CSMA/CD
    CSMA/CA

 

For Further Reading:

Carrier Sense Multiple Access with Collision Avoidance
Controlled Access Protocols
Channelization Protocols
Back to DCN Questions and Answers