Carrier-sense multiple access is a media access control protocol in which a node verifies the absence of other traffic before transmitting on a shared transmission medium, such as an electrical bus or a band of the electromagnetic spectrum. A transmitter attempts to determine whether another transmission is in progress before initiating a transmission using a carrier-sense mechanism. That is, it tries to detect the presence of a carrier signal from another node before attempting to transmit. If a carrier is sensed, the node waits for the transmission in progress to end before initiating its own transmission. Using CSMA, multiple nodes may, in turn, send and receive on the same medium. Transmissions by one node are generally received by all other nodes connected to the medium. Variations on basic CSMA include addition of collision-avoidance, collision-detection and collision-resolution techniques.
Access modes
Variations of CSMA use different algorithms to determine when to initiate transmission onto the shared medium. A key distinguishing feature of these algorithms is how aggressive or persistent they are in initiating transmission. A more aggressive algorithm may begin transmission more quickly and utilize a greater percentage of available bandwidth of the medium. This is typically at the expense of increased likelihood of collision with other transmitters. ; 1-persistent : 1-persistent CSMA is an aggressive transmission algorithm. When the transmitting node is ready to transmit, it senses the transmission medium for idle or busy. If idle, then it transmits immediately. If busy, then it senses the transmission medium continuously until it becomes idle, then transmits the message unconditionally. In case of a collision, the sender waits for a random period of time and attempts the same procedure again. 1-persistent CSMA is used in CSMA/CD systems including Ethernet. ; Non-persistent : Non persistent CSMA is a non aggressive transmission algorithm. When the transmitting node is ready to transmit data, it senses the transmission medium for idle or busy. If idle, then it transmits immediately. If busy, then it waits for a random period of time before repeating the whole logic cycle again. This approach reduces collision, results in overall higher medium throughput but with a penalty of longer initial delay compared to 1–persistent. ; P-persistent : This is an approach between 1-persistent and non-persistent CSMA access modes. When the transmitting node is ready to transmit data, it senses the transmission medium for idle or busy. If idle, then it transmits immediately. If busy, then it senses the transmission medium continuously until it becomes idle, then transmits with probability p. If the node does not transmit, it waits until the next available time slot. If the transmission medium is not busy, it transmits again with the same probability p. This probabilistic hold-off repeats until the frame is finally transmitted or when the medium is found to become busy again. In the latter case the node repeats the whole logic cycle again. p-persistent CSMA is used in CSMA/CA systems including Wi-Fi and other packet radio systems. ; O-persistent : Each node is assigned a transmission order by a supervisory node. When the transmission medium goes idle, nodes wait for their time slot in accordance with their assigned transmission order. The node assigned to transmit first transmits immediately. The node assigned to transmit second waits one time slot. Nodes monitor the medium for transmissions from other nodes and update their assigned order with each detected transmission. O-persistent CSMA is used by CobraNet, LonWorks and the controller area network.
Protocol modifications
When broadcasting over vehicular ad hoc networks, the original 1-persistence and p-persistence strategies often cause the broadcast storm problem. To improve performance, engineers developed three modified techniques: weighted p-persistence, slotted 1-persistence, and slotted p-persistence. ;Carrier-sense multiple access with collision detection ;Carrier-sense multiple access with collision avoidance ;CSMA with Collision Resolution ;Virtual time CSMA
