The English term carrier scythe multiple ACCESS/Collision Detection (CSMA/CD) (German about: "„carrier frequency-sensitive multiaccess with Kollisionserkennung "“) designates a principle, which describes the access of different stations to a common transmitting medium in the time division (TDMA). Use finds CSMA/CD for example within the range of the computer networks with the Ethernet and as IEEE 802,3 was standardized there. With Wireless LANs or the CAN bus used within the automobile range a similar mechanism is used named CSMA/CA.

Collisions

At network procedures such as Ethernet a data communication in blocks (packages) takes place. No endless data stream is produced. Thus it becomes on the one hand possible that several stations use the same medium (e.g. cable), on the other hand one develops thereby the danger of collisions:

Since it is not predetermined, at which time a station has to send, can it happen that several master stations would like to send at the same time, whereby the two signals overlay and disturb thus: None of the stations can send something useful. CSMA/CD is a procedure, in order to react and prevent to arising collisions that they repeat themselves.

Realization

The pattern is relatively simple, compared to token ring or master-controlled networks. If equipment liked to send data, it adheres to the following operational sequence:

1. Carrier scythe = on signal horchen: First the medium must be supervised.
2. If the medium is a certain time long (IFS) free, begins with the transmission, otherwise far with step 5.
3. Information transfer, at the same time is continued to hear continual the medium. If here a collision is discovered, terminate the data communication and set a defined spurious signal to guarantee (jam) on the line (over that all other Transceiver likewise recognizes the collision), then far with step 5.
4. Transmission successfully finally: Success message to higher network layers, move mode leave.
5. Line is occupied: Control rooms, until the line is again free.
6. Line became free straight. Still another wait for coincidental time (baking off, see below), begin then again with step 1, if the maximum number of transmission attempts were not exceeded.
7. Maximum number of transmission attempts exceeded: Errors to higher network layers announce, move mode abandoned.

Since the signals are measurable as voltage levels, and an overlay an addition of the levels meant, is recognized a collision by all (also the momentarily indifferent) computers, since a threshold value is exceeded (Collision Detection = Kollisionserkennung). The collision must be recognized in particular by the transmitter, so that it can initiate a transmission repetition. In addition the minimum package length must be measured in such a way that the transmission duration ("“time"” slot) is sufficient, in order the signal twice over the longest distance to transfer (so mentioned "“propagation delay"”). If "“too short"” data are to be transmitted, they must be extended in addition if necessary to the minimum package length.

The baking off procedure with Ethernet

If the transmission must be broken off because of a conflict, then it would come directly to a renewed conflict, if the master stations involved would send again immediately to the abort. They must insert therefore ideally a break of different lengths, so that the stations get a transmission sequence assigned.

With Ethernet the conflict parties select for this a coincidental whole number of z from the interval [0; (2^i) - 1], whereby i stands for the number of conflicts already arisen. The master station waits now the period of z * slot delay (s.o.) off and sends to again if the medium is free. If no other station drew the same z, there is thus no more conflict.

Since the dispersion of the possible waiting periods grows exponentially with the number of arisen conflicts, the probability is very small that many conflicts arise one behind the other, since the conflict parties would have to draw regularly for this the same random number. Therefore after 16 conflicts in consequence the transmission attempt is broken off and a system error is accepted.

The disadvantage of the method is that computationally no warranty prevails that a package arrived at a certain time already. Transmission success has only a certain probability. The procedure is thus not real timable, as it is for instance with token ring the case.

Collisions prevent

Due to these arising collisions it is not possible to exhaust the theoretical transmission capacity fully. The more computer in the network take part, to more collisions arise and the information flow-rate sinks thus clearly.

Use only two stations the same transmitting medium, creates the duplex operation remedy. With some transmission methods the medium can be divided by means of a Switches or a bridge into several collision domains. Then several stations can be active in the duplex operation, if the SWITCH stored the respective MAC addresses in its address list.

Also there are completely collision-free transmission principles such as tokens Passing.

See also

• CSMA/CA
• Arbitrator algorithm

Related links

• IEEE 802,3 IEEE 802,3 CSMA/CD (ETHERNET)

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