Carrier Sense Multi-Access (CSMA) is a network access method used on shared network topologies such as Ethernet for a node to verify the absence of other traffic before transmitting on a shared physical medium. CSMA Devices attached to the network cable listen (carrier sense) before transmitting. If the channel is in use, devices wait before transmitting. Multiple Access(MA) indicates that many devices can connect to and share the same network. All devices have equal access to use the network when it is clear. Even though devices attempt to sense whether the network is in use, there is a good chance that two stations will attempt to access it at the same time. There are two methods for avoiding these so-called collisions: Carrier Sense Multi-Access/Collision Avoidance and Carrier Sense Multi-Access/Collision Detection.

Collisions Concurrent transmission by multiple nodes results in frame collisions. The multiple transmissions interfere with each other so that all are garbled and receivers are unable to distinguish the overlapping received signals from each other. It is impossible to entirely prevent collisions in CSMA networks, but there are three ways to address them:

In pure CSMA, only the carrier sense is used to avoid collisions. If two nodes try to send a frame at nearly the same time, neither detects a carrier so both begin transmitting at the same time. The transmitters do not detect collisions, so transmit the entire frame (thus wasting the bandwidth used). Receivers cannot distinguish between collisions and other sources of frame errors, so collision recovery relies on the ability of the communicating nodes to detect frame errors and invoke an error recovery procedure. For example, the receiver may not send a required ACK, causing transmitters to time out and retry.

In Carrier Sense Multiple Access With Collision Avoidance (CSMA/CA), each node must inform other nodes of an intent to transmit. When the other nodes have been notified, the information is transmitted. This arrangement prevents collision because all nodes are aware of a transmission before it occurs. However, collisions are still possible, and are not detected so have the same consequences as in pure CSMA.

In Carrier Sense Multiple Access With Collision Detection (CSMA/CD), sending nodes are able to detect when a collision occurs and stop transmitting immediately, backing off for a random amount of time before trying again. This results in much more efficient use of the media since the bandwidth of transmitting the entire frame is not wasted. However, it is not possible with all media (e.g., radio), and requires extra electronics (not really an issue with today's technology, but one reason Apple used CSMA/CA-based LocalTalk instead of the then much more expensive Ethernet).

In Carrier Sense Multiple Access with Bitwise Arbitration (CSMA/BA), all of the nodes on the bus are assigned an identification number or priority code. When a collision occurs, one of the nodes that are attempting to send at the same time will be given priority to transmit according to its identification number or priority code (as opposed to waiting a random amount of time and then retransmitting, as in CSMA/CD). There are three types of collisions

Local collision Remote collision Late collision