Carrier Sense Multiple Access

from Wikipedia, the free encyclopedia

The English term Carrier Sense Multiple Access (CSMA) (German as: multiple access with carrier sense ) designated in the field of telecommunications and computer networks a distributed, asynchronous method for obtaining the access right ( arbitration ) by the competition method on bus lines such. B. the fieldbus , or on a radio channel .


Carrier check or carrier sense means that all participants observe the status of the bus line and only send their messages when no other participant is sending, i.e. the channel is free. If the medium is not occupied for a certain period of time (9.6 µs for 10 Mbps Ethernet, 960 ns for 100 Mbps Fast Ethernet and 96 ns for 1000 Mbps), it is considered free.

CSMA, in turn, is divided into various procedures for handling or avoiding a collision on the bus . Collisions can occur if two or more participants start transmitting at the same time.


There are three types of CSMA:

  • 1-Persistent: If the medium is recognized as occupied, a check is made until it becomes free; if free, it is sent (with probability 1, ie always).
  • P-Persistent: There is a probability P, after the channel has been identified as free, to begin sending. In the event that the station does not transmit (1-P), it waits a specified period of time before checking again.
  • Non-persistent: After the medium is recognized as occupied, a random time interval is waited and then checked again; if free, it will be sent.


Main article: Carrier Sense Multiple Access / Collision Avoidance

Carrier Sense Multiple Access / Collision Avoidance avoids collisions through a random waiting time after channel-free detection. However, CSMA / CA should not be confused with the multiplex method CDMA , which is used in mobile radio technology. The main area of application is radio networks .


Main article: Carrier Sense Multiple Access / Collision Detection

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


Main article: Carrier Sense Multiple Access / Collision Resolution

Carrier Sense Multiple Access / Collision Resolution detects collisions and resolves the competitive situation through priority analysis when transmissions start at the same time. CAN implements the procedure through bit arbitration .


  • Ralf Gessler, Thomas Krause: Wireless networks for the local area. 2nd edition, Springer Verlag, Berlin / Heidelberg 2015, ISBN 978-3-8348-1239-1 .
  • Magnus Jonsson, Alexey Vinel, Boris Bellalta, Marina Ninoslav, Dimitrova Desislava, Dieter Fiems (eds.): Multiple Access Communications. Springer Science + Business Media Inc, London 1998, ISBN 978-3-319-03870-4 .
  • Jeffrey S. Beasley, Piyasat Nilkaew: Networking Essentials. Third Edition, Pearson Education Inc, 2012, ISBN 978-0-7897-4903-1 .
  • Gurdeep S. Hura, Mukesh Singhal: Data and Computer Communications. Networking and Internetworking, CRC Press LLC, Boca Raton 2001, ISBN 0-8493-0928-X .
  • Martin Werner: Information and Coding. Basics and Applications, 2nd edition, Vieweg + Teubner, Wiesbaden 2008, ISBN 978-3-8348-0232-3 .

Individual evidence

  1. Example: IEEE 802.15.4-2006, Chapter CSMA-CA algorithm
  2. data sheet from intel and freescale (PDF; 418 kB)

See also

Web links