• This is one of the group of protocol for media access control.
• There is no schedule time for a station to transmit, transmission can happen by any station at any point of time.
• All the station compete for the shared media. So it is also called contention method.
• If multiple station transmit at the same time, it can lead to collision and the frames will be either be destroyed or modified.

In this group, there are four protocols:

• ALOHA
• CSMA
• CSMA/CD
• CSMA/CA

ALOHA:

In this multiple station can transmit data at the same time and hence lead to collision and data can be damaged.

Again this is of two types:

• Pure Aloha
• Slotted Aloha

Pure Aloha:

• In this each station sends a data and wait for an acknowledgement, if an acknowledgment is not received within a specific time, data is considered to be lost and need to be retransmitted.
• To reduce the chances of collision, each station waits randomly before retransmitting the data.
• Since each waits for different amount of time, the probability of collision decreases.

Slotted Aloha:

• In this case total time is divided into n number of slots, where n is the total number of stations.
• Each station can send only at the beginning of its slot and one frame per slot
• If the slots is missed, it must wait for its next slot.
• There is still a possibility of collision if two stations try to send at the beginning of the same time slot.

Collision Sense Multiple Access(CSMA):

• In this method each station has to sense the media(idle or busy) before sending the data.
• Still there are chances for collision because of propagation delay that is may be a station sense the media and finds it to be idle but before first station can send, even the other station finds the media to be idle and sends the data leading to collision.

Collision Sense Multiple Access/ Collision Detection(CSMA/CD): In this method, station monitors the medium after sending the frame to see if the transmission is successful. If so station is finished else it is retransmitted again.

Collision Sense Multiple Access/ Collision Avoidance(CSMA/CA):

In this method, collision is avoided by three strategies:

• Interframe space:

In this case even if the channel is found idle, data is not send immediately(to avoid collision due to propagation delay) rather it waits for a period of time called interframe space. After this time, it again check the channel for being idle. If so data is send.

• Contention Window:

In this case time is divided into slots, and random number of slot is chosen as wait time before sending data. If the channel is not free, wait time is doubled.

• Acknowledgment:

The sender re-transmits the data if acknowledgement is not received before time-out.

Categories: Networking