If we break down a generic communication system into its simplest parts, we can distinguish four elements: a sender, a receiver, a message, and a medium or channel for communication. One of the problems that arises once communication is established is medium access.

When we have a communication channel shared among several users or stations, it is necessary to have access control. This prevents multiple stations from starting to transmit at the same time, which causes interference and will likely make communication impossible. Carrier Sense Multiple Access (CSMA) is a protocol designed to solve this problem.

CSMA allows a station to perform a “listen” to ensure that the medium is free before transmitting information. If it is free, it performs the transmission; otherwise, it waits. The behavior when the channel is busy determines the “persistence” of the CSMA:

• 1-persistent CSMA: Attempts to transmit as soon as the channel becomes free.

• Non-persistent or 0-persistent CSMA: Waits for a random amount of time determined by a probability distribution before trying again.

• p-persistent CSMA: Behaves persistently with a probability $p$, and non-persistently with a probability $1 – p$.

Variants of the CSMA Protocol

In general, when there is a high load, CSMA offers better performance in its non-persistent form. There are two main variations of CSMA:

• CSMA/CD – Carrier Sense Multiple Access with Collision Detection.

• CSMA/CA – Collision Avoidance.

CSMA/CD is a variant that can be used in situations where it is possible to listen and transmit simultaneously. This allows the system to detect interference and interrupt the transmission immediately, first sending a jam signal to notify other stations sharing the medium of the collision so they wait before sending. Once the medium is cleared, transmission resumes, allowing for an equitable distribution of the channel without any station monopolizing communication. This protocol is not as widespread today due to the use of data link layer devices like switches in Ethernet networks and the use of other protocols in different types of networks.

The other main variant is CSMA/CA. In this protocol, if the medium is free after listening, the station waits for an additional random amount of time before transmitting, which reduces the number of collisions.

These protocols, however, have their drawbacks, as they do not solve problems such as the hidden node problem. In this case, there are two stations, A and B, which are not within each other’s range, and a station C located where both overlap, to which both want to transmit. If B is transmitting to C and A wants to do the same, if A uses the CSMA protocol, it will check the medium and not detect B. Consequently, it will assume the medium is free and transmit. This produces interference in the area where both stations converge.

Other Alternative Protocols

MACA (Multiple Access with Collision Avoidance) is a protocol that solves this problem by sending additional frames: a “Request to Send” (RTS) from the sender, and a response from the receiver with a “Clear to Send” (CTS). Only then does transmission begin. Evolutions of MACA, such as MACAW, solve similar problems by adding improvements like sending ACK confirmations from the receiver upon receiving data frames. Currently, the IEEE 802.11 standard, which defines what is commonly known as Wi-Fi, uses a combination of CSMA/CA and MACAW.

There are other medium access protocols such as ALOHA and its variants, as well as methods based on token passing or the master/slave system used, for example, in Bluetooth (IEEE 802.15). However, CSMA is becoming increasingly important given the massive use of Wi-Fi networks among all users.