TYPES OF MULTIPLE ACCESS PROTOCOL

CSMA/CD Carrier Sense Multiple Access /With collision detection

Transmission time (Tt) > Propagation Time (Tp)

After collision occurs, a collision signal is generated and sent to both A & B to inform the stations about collision. Since the collision happened midway, the collision signal also takes 30 minutes to reach A& B

This collision occurs just before the data reaches B. Now the collision signal takes 59:59 minutes again to reach A. Hence, A receives the collision information approximately after 2 hours, that is, after 2 * Tp. Tt >>= 2 * Tp

CSMA/CA CARRIER SENSE MULTIPLE ACCESS /WITH COLLISION AVIODANCE

CSMA/CD (Carrier Sense Multiple Access/Collision Detection) that deals with collisions after their occurrence, CSMA/CA prevents collisions prior to their occurrence.

These are three types of strategies:

• InterFrame Space (IFS)- When a station finds the channel busy, it waits for a period of time called IFS time. IFS can also be used to define the priority of a station or a frame. Higher the IFS lower is the priority.
• Contention Window - It is the amount of time divided into slots. A station which is ready to send frames chooses random number of slots as wait time.
• Acknowledgements - The positive acknowledgements and time-out timer can help guarantee a successful transmission of the frame.

Algorithm

The algorithm of CSMA/CA is:

• When a frame is ready, the transmitting station checks whether the channel is idle or busy.
• If the channel is busy, the station waits until the channel becomes idle.
• If the channel is idle, the station waits for an Inter-frame gap (IFG) amount of time and then sends the frame.
• After sending the frame, it sets a timer.
• The station then waits for acknowledgement from the receiver. If it receives the acknowledgement before expiry of timer, it marks a successful transmission.
• Otherwise, it waits for a back-off time period and restarts the algorithm.