Carrier sense multiple access with collision avoidance

The English term Carrier Sense Multiple Access / Collision Avoidance (CSMA / CA ) ( to German as: " multiple access with carrier sense and collision avoidance " ) refers to a principle for collision avoidance when accessing multiple network stations on the same transmission channel. It is, inter alia, often in wireless networks (wireless LANs) used is also varied but in communication methods such as ISDN application, or in many communications networks, in which a plurality of clients store data on a bus, and it may not cause collisions. In centrally coordinated communication networks, the problem does not occur.

Motivation for CSMA / CA in wireless networks

Wireless networks differ in terms of the common media access by two important factors of wired networks:

  • The AC adapter is not necessarily full - duplex capability. While its own transmission, the medium can not be monitored. The use of a " collision detection " mechanism, as envisaged about CSMA / CD used in Ethernet and would then fail. Therefore, CD was CSMA / evolved into a mechanism consistent with the principle "listen before talk " ( " listen first, then speak " ) follows. In place of the collision detection ( "CD") should be the ( best possible) collision avoidance ( "CA" ) occur. This concurrent data transfers can not completely prevent, but minimized.
  • The range of the signal is severely limited, since the reception level decreases with the square of distance. Therefore, it can be " hidden" or come to effects such as " delivered " terminals.

Hidden station problem

A Hidden station or a hidden terminal ( engl. "hidden terminal " ) called in asynchronous and not centrally coordinated communication networks, radio networks or computer networks the undesirable fact that at a transmission between two nodes (A and B) another potential channels (C, the hidden terminal) in the vicinity of the receiver ( B), which can not be seen from the actual transmitter (A ) (see as used herein means through carrier sense to be detected). This potential channel (C), the communication of the other two nodes (A and B) interfere by also sends a message to the nodes in the center ( B), this may lead to a collision at the receiver (B).

The picture is the typical situation of the hidden station problem in wireless networks with only one modulation method on a radio frequency dar. network node A does not send data to node B. reached The signal B, because of the limited range of the radio link but the more distant node C. C gets so from the current data exchange with nothing and gains the impression that the communication medium is free to send data to node B. Heralds C with sending so the signals at B. superimpose The result is a data collision due to which B neither A nor the message of successfully receives from C.

Specific methods can not exclude collision, but they create with stochastic process, a deterministic behavior of the participants involved. By the use of RTS / CTS, the hidden station problem is tried to be avoided. If B responds to a request - to-send from A with a clear-to -send, hears this and waits for C with the time of transmission between A and B. This may be the problem but not completely prevent it.

Exposed station problem

Under an Exposed Exposed station or terminal ( to German: exposed terminal ) is defined as when in our present scenario, station B sends to A and then C wants to send to any other station that is not in the transmission range of B. C detects the signals from B and waits until the communication between B and A is over. However, since the radio waves from C can not reach station A, it would not be necessary to wait at A could no conflict occur. However, C is the mission of the other two stations dependent ( delivered ).

Protocol sequence

Want a device send data according to the CSMA / CA procedure, including, inter alia, the following sequence is possible:

In addition, methods are defined which provide a systematic solution for the problem cases of Hidden and Exposed station without restricting the transmission level in addition. The prerequisite in this process is that the receiver is listening to both stations, in the process engaged by sending own:

  • A transmission is not recorded as long as a program is running. Each transmitter sends only a limited time.
  • The transmission is canceled when the transmitter detects through a different station receiving a collision. The next release will be delayed by a randomly determined interval.
  • The receiver, which detects only the collision itself sends a signal to the expectation that both colliding transmitters detect this and then both take the break routine.

RTS / CTS coordination

To alleviate the problem of the hidden station, an extension is defined, which is referred to as CSMA / CA, RTS / CTS ( Request To Send / Clear To Send). Synonym is the term for MACA Multiple Access with Collision Avoidance.

The transmitting station tries to occupy the channel with an RTS packet for a particular, specified in the packet has to wait for DIFS time. The receiver confirms this has to wait for SIFS, with a CTS packet, which also contains an allocation of time for the channel.

All stations located in the transmission range that receive this RTS, silent until the coming back from the receiver CTS response ( clear to send, contains the length of the data frame copied from the RTS) has been received free of conflict and the transmitting station has sent the data. According wait all recipients of CTS according to the length standing in the CTS.

An advantage of this method is that collisions are possible only during the transmission of RTS and CTS packets as a disadvantage applies the high cost of replacement of the reservation messages. RTS / CTS is particularly suited to the problem of hidden station partially solved. However, it is still two for each other hidden devices simultaneously send RTS to a common receiver. These collide there and lost. In this case, the sender wait for a random amount of time and re-send RTS '. The algorithm used is the binary exponential backoff.

An experimental comparison shows that the RTS / CTS packets worth in each traffic class. Even with short audio frames, which produce a high overhead on RTS / CTS frames, the use of RTS / CTS mechanism profitable. In the test environment, the following classes of traffic were used: data ( not time critical ), Data ( time-critical), video, audio. For example, means ( 0 | 0 | 0 | 2): 2 audio stations; ( 1 | 1 | 2 | 0): 1 data station ( not time critical ), 1 data station ( time-critical), 2 video stations.

The problem of Exposed Station is not solved by this standard, but is not as dramatic rate as the hidden station problem, as it only leads to lower throughput.

A further development of ad -hoc networks is the PAMAS protocol (Power Aware Multi- Access Protocol with Signaling Ad Hoc Networks).

The difference MACA is that recipients who hear the RTS / CTS signal and shut off for the duration of the transmission. This is possible, because the RTS / CTS each include the packet size. Furthermore, this saves energy that the node itself off at regular intervals.

Compared with MACA has PAMAS an energy saving of up to 70 %. This depends on the degree of crosslinking. The downside to MACA and PAMAS is the low throughput rate - caused by the restriction of transfers to only one in a neighborhood of mutually reachable nodes.

PCF coordination

CSMA / CA PCF ( Point Coordination Function ) is another method of avoiding collisions in wireless networks (wireless LANs), in which the access point controls the media access center.

Do this, set the access point in addition to the competition period (CP, Contention Period) a contention free period ( CFP Contention Free Period) a. In the CFP, the AP requests after waiting PIFS from all stations to transmit a data frame. A station wishing to transmit can begin sending has to wait for SIFS, then moves the AP after waiting SIFS by sending the Poll packets continue.

CSMA / CA PCF is optional and is therefore rarely implemented.