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 objectives of multiple access protocols are optimization of transmission time, minimization of collisions and avoidance of crosstalks.
Multiple access protocols can be broadly classified into three categories - random access protocols, controlled access protocols and channelization protocols.
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.
The four random access protocols are−
Carrier sense multiple access (CMSA)
Carrier sense multiple access with collision detection (CMSA/CD)
Carrier sense multiple access with collision avoidance (CMSA/CA)
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.
The station can be assigned the right to send by the following three methods−
Channelization are a set of methods by which the available bandwidth is divided among the different nodes for simultaneous data transfer.
The three channelization methods are−
Frequency division multiple access (FDMA)
Time division multiple access (TDMA)
Code division multiple access (CDMA)