Multiple Access Protocol – ALOHA, CSMA, CSMA/CA and CSMA/CD
Multiple access protocols are a set of protocols that work in the Medium Access Control sublayer (MAC sublayer) of the Open Systems Interconnection (OSI) model.
These protocols allow a number of users to access a shared network channel. Several data streams originating from several nodes are transferred through the multi-point transmission channel.
The purpose of multiple access protocols are optimization of transmission time, minimization of collisions and avoidance of cross-talks.
What is a multiple access protocol?
When a sender and receiver have a dedicated link to transmit data packets, the data link control is enough to handle the channel. Suppose there is no dedicated path to communicate or transfer the data between two devices.
In that case, multiple stations access the channel and simultaneously transmits the data over the channel. It may create collision and cross talk.
Hence, the multiple access protocol is required to reduce the collision and avoid crosstalk between the channels.
For example-
Suppose that there is a classroom full of students. When a teacher asks a question, all the students (small channels) in the class start answering the question at the same time (transferring the data simultaneously).
All the students respond at the same time due to which data is overlap or data lost. Therefore it is the responsibility of a teacher (multiple access protocol) to manage the students and make them one answer.
Types of Multiple Access Protocols
Multiple access protocols can be broadly classified into three categories –
- Random access protocols,
- Controlled access protocols and
- Channelization protocols.
Random Access Protocol
Random Access Protocol
In this protocol, all the station has the equal priority to send the data over a channel. In random access protocol, one or more stations cannot depend on another station nor any station control another station.
Depending on the channel’s state (idle or busy), each station transmits the data frame. However, if more than one station sends the data over a channel, there may be a collision or data conflict. Due to the collision, the data frame packets may be lost or changed. And hence, it does not receive by the receiver end.
Following are the different methods of random-access protocols for broadcasting frames on the channel.
- Aloha
- CSMA
- CSMA/CD
- CSMA/CA
Controlled access protocols
It is a method of reducing data frame collision on a shared channel. In the controlled access method, each station interacts and decides to send a data frame by a particular station approved by all other stations.
It means that a single station cannot send the data frames unless all other stations are not approved. It has three types of controlled access: Reservation, Polling, and Token Passing.
Channelization Protocols
It is a channelization protocol that 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.
Following are the various methods to access the channel based on their time, distance and codes:
- FDMA (Frequency Division Multiple Access)
- TDMA (Time Division Multiple Access)
- CDMA (Code Division Multiple Access)