Multiple access protocols are a set of protocols operating in the Medium Access Control sublayer (MAC sublayer) of the Open Systems Interconnection (OSI) model. These protocols allow a number of nodes or users to access a shared network channel. Several data streams originating from several nodes are transferred through the multi-point transmission channel.

The main objectives of multiple access protocols are optimization of transmission time, minimization of collisions and avoidance of crosstalks.

Classification of Multiple Access Protocol:

1. Random Access Protocol: Random access protocols assign uniform priority to all connected nodes. Any node can send data if the transmission channel is idle. No fixed time or fixed sequence is given for data transmission in random access protocol.

Four types of Random Access Protocols:

• ALOHA
• Carrier sense multiple access (CMSA)
• Carrier sense multiple access with collision detection (CMSA/CD)
• Carrier sense multiple access with collision avoidance (CMSA/CA)

2) Controlled Access Protocols: Controlled access protocols allow only one node to send data at a given time. Before initiating transmission, a node seeks information from other nodes to determine which station has the right to send. This avoids collision of messages on the shared channel.

Three methods of Controlled Access Protocols are as follows:

• Reservation

• Polling

• Token Passing

3) Channelization Protocols: Channelization Protocols allows the total usable bandwidth in a shared channel to be shared across multiple stations based on their time, distance and codes. It can access all the stations at the same time to send the data frames to the channel.

Three channelization methods are as follows:

• Frequency division multiple access (FDMA)

• Time division multiple access (TDMA)

• Code division multiple access (CDMA)

Explain the CSMA/ CD:

CSMA/CD is a carrier sense multiple access/ collision detection network protocol to transmit data frames. The CSMA/CD protocol works with a medium access control layer. Therefore, it first senses the shared channel before broadcasting the frames, and if the channel is idle, it transmits a frame to check whether the transmission was successful. If the frame is successfully received, the station sends another frame. When collision condition is detected, the station stops transmitting that frame, transmits a jam signal, and then waits for a random time interval before trying to resend the frame.

CSMA/CD is a modification of pure carrier-sense multiple access (CSMA). CSMA/CD is used to improve CSMA performance by terminating transmission as soon as a collision is detected, thus shortening the time required before a retry can be attempted.

CSMA/CD was used in now-obsolete shared media Ethernet variants (10BASE5, 10BASE2) and in the early versions of twisted-pair Ethernet which used repeater hubs. CSMA/CD is supported for backwards compatibility and for half-duplex connections.