Random access methods

Random access methods are also known as contention methods where no station is assigned any priority, i.e. no station is superior to another station and none is assigned any 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 its procedure defined by the protocol to make a decision on whether or not to send. This decision does not depend on other stations rather depends on the medium is idle or busy. In other words, each station can transmit when it desires upon the testing of the state of the medium.

Why this methods are called Random access?

1. There is no scheduled time for a station to transmit. Transmission is random among the stations.
2. No rules specify which station should send next. Stations compete with each other to access the medium. That is why these methods are also called contention methods.

What are basic features of random access methods?

1. In a random access method, each station has the right to the medium without being controlled by any other station. However, if more than one station tries to send, there is an access conflict-collision-and the frames will be either destroyed or modified.
2. So basically the protocol solves:
   • When can particular 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?

Different Random access methods

1. Aloha: It uses a very simple procedure called multiple access (MA).
   Aloha is further of two types:
   • Pure Aloha
   • Slotted Aloha
2. CSMA: Aloha is improved with the addition of a procedure that forces the station to sense the medium before transmitting. This is called carrier sense multiple access.
   CSMA is further of two types:
   • CSMA/CD
   • CSMA/CA