The English term carrier scythe multiple ACCESS/Collision Avoidance (CSMA/CA) (German about: "“Multiaccess with carrier examination with collision avoidance"”) designates a principle for the access of several network act ions to the same transmitting medium. It is used frequently among other things with wireless networks (Wireless LANs), finds however modified also with technologies such as ISDN application.

Motivation for CSMA/CA with radio nets

Wireless nets differ in the reference to the common medium access by two important factors of wire-bound nets:

• The net adapter is not necessarily full-duplex able. During its own transmission the medium cannot be supervised. The employment of a "“Collision Detection"” - mechanism, as it is intended by CSMA/CD for instance and is used with Ethernet, would fail then. Therefore CSMA/CD was developed further to a mechanism, which more consistently the principle "“would list before talc"” ("“only hear, then speak"”) follows. To the place (optimum) collision avoidance ("“APPROX."”) should step the Kollisionserkennung ("“CD"”). Thus simultaneous data communication can be prevented not completely, but minimized nevertheless.

• The range of the signal is strongly limited, since the signal strength decreases squarely with the distance. Therefore it can to effects as "“hid"” or "“delivered"” terminals to come.

Hidden one and delivered terminals

To a hidden terminal it comes for example with the following scenario: The two transmitting express takers A and C lie apart spatially so far that they cannot receive their radio signals mutually. Between them the station B.A and C is sends now at the same time because of B and produces there a conflict, cannot this however not recognize, since the radio signals in each case of the different one do not reach it. A turned around a hidden terminal for C and.

By a delivered terminal one understands, if in our available scenario the station sends now B at A and to any other station (not A or B) liked to send C. C recognizes the signals of B and waits, until the transmission between B and A past is. Since the radio waves of C cannot at all reach however A, it would not be to be waited at all necessarily: with A no conflict could arise. C the other two stations is delivered nevertheless.

Expiration of minutes

If equipment would like to dispatch data after the CSMA/CA procedure, then the among other things following operational sequence is possible:

1. First the medium is heard ("“carrier scythe"”)
2. If the medium is free for the duration of a IFS, one sends
3. If the medium is occupied, for a free IFS one waits and one retards to collision avoidance additionally around a coincidental baking off time
4. If the medium is occupied during the baking off time by another station, the baking off timer stops so long and after release of the medium is continued counting

Another possibility is the address Arbitrierung, which is used e.g. with the CAN bus.

In order or exist "“delivered"” terminals "“hid"” the problem to eliminate, a "“Request ton send/CLEAR ton send"” (RTS/CTS) - extension for CSMA/CA. thereby must reserve the send-willing station the channel by a RTS package. The receiver confirms this reservation with a CTS package. Subsequently, immediate sending of the data is possible. Other stations store the holding time, which was sent in the RTS and CTS package, and send during this time no data.

See also

• Arbitrator algorithm
• CSMA/CA RTS/CTS
• CSMA/CA PCF

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