Carrier Sense Multiple Access

The English term Carrier Sense Multiple Access (CSMA ) ( German as: multiple access with carrier sense ) denotes the field of telecommunications and computer networks a distributed, asynchronous method for obtaining the access right ( Arbitration) after competing processes on bus lines such as the fieldbus, or on a radio channel.

Operation

Carrier testing or Carrier Sense means that all participants observe the status of the bus line and only send their messages when not sending a other users, the channel is free. Is the media for a certain period of time ( 9.6 microseconds for 10 Mbps Ethernet, 960 ns for 100Mbps Fast Ethernet, and 96 ns at 1000 Mbps) is not occupied, it shall be considered free.

CSMA is in turn divided in various methods for treating or preventing a collision on the bus. Collisions can occur when two or more participants simultaneously start sending.

Species

There are three kinds of CSMA:

CSMA / CA

Carrier Sense Multiple Access / Collision Avoidance avoids collisions by a random waiting time after the channel free detection. However, CSMA / CA should not be using the CDMA multiplexing method, which finds application in the mobile technology, confused. Main field of use are wireless networks.

CSMA / CD

Carrier Sense Multiple Access / Collision Detection detects collisions and tries to avoid the competitive situation by terminating the current program and subsequent different transmission delay. Since the transmission delays are selected usually random, it is a stochastic process. CSMA / CD is the access technology in local computer networks and is often equated with the term Ethernet.

CSMA / CR

Carrier Sense Multiple Access / Collision Resolution detects collisions and resolves the contention priority by analyzing the simultaneous launch of transmissions. CAN is the procedure to carry bit arbitration.