In other words, devices are in listen-before-transmit carrier sense mode. When a device wants to place data on the wire for transmission, it will "sense" the wire to find whether there is a signal already on the wire. If there is traffic already in the medium, the device will wait a random period of time and "sense" again. If there is no traffic no signal in wire , the device will place the data in the medium. At the same time, the device is listening from the media, because the device wants to ensure no other devices are transmitting at the same time.
But, if two devices place data on the medium at the same instance, the signals will collide with each other, destroying the data. If the data signal is destroyed during transmission because of collision and the lost data signal need to be retransmitted. When a collision occurs and detected, the devices involved in collision will transmit "JAM signal" for a short time, so that all devices in the network detect that the collision had happened.
However, if another host begins transmitting at the same time and a collision occur, the transmitting host sends a jam signal that causes all hosts on the network segment to stop sending data. After a random period of time, hosts retransmit their packets. Consider the following example:. In the picture above we have a network of four hosts connected to a hub. Host A detects that there are no other signals on the network and decides to send a packet.
However, Host B also assumes that no other station is transmitting and sends a packet as well. As long as this is occupied, the monitoring will continue. Only when the medium is free and for a certain time in interframe spacing , will the station send a data packet. Meanwhile, the transmitter continues to monitor the transmission medium to see if it detects any data collisions.
If no other participant tries to send its data via the medium by the end of transmission, and no collision occurs, the transmission has been a success.
Collisions result in a voltage displacement on the cable. This makes it possible for the stations to detect the collisions. If, however, a collision is detected, the participant immediately interrupts the transmission and instead sends an interference signal JAM signal so that all other stations can also detect the collision.
Now the participant waits for a random amount of time backoff before trying the transmission again. Since both stations select a random value, the probability that both of them will start a transport attempt at the same time is quite low.
The transmission attempts are counted. If the following attempts also fail and the maximum number of attempts 16 is reached, the station reports the error to the next higher network layer and then terminates the transmission permanently.
Since it is highly unlikely that a network participant will reach the maximum number of attempts during a normal process, it can be assumed that a system error has occurred. The Ethernet collision resolution mechanism binary exponential backoff is used to mitigate permanent collisions.
The participant must wait a while before starting a new attempt after a collision. This duration is always a random multiple of the so-called slot time.
The slot time depends on the transmission rate and is a few microseconds long. The range of possible multiples doubles with each failed attempt so that the probability of simultaneous transmission becomes exponentially lower. To detect whether a collision has occurred, the station simultaneously checks whether the transmitted signal is identical to that on the transmission medium.
If this is not the case, another station carries out a transmission simultaneously and falsifies the signal on the bus. Collisions like these occur regularly and are part of the normal operation of a local area network LAN. However, the size of the network also increases the probability of collisions. In this case, size refers not only to the number of participants, but also to the length of the transmission paths.
This also leads to more collisions and slows down the network overall. A collision domain is the area of a network in which participants share a transmission medium. However, the collision domain must not exceed a certain size : Networks cannot be enlarged indefinitely if the collision procedure is to remain effective. The reason for this is the speed of the transmission and the distance it must travel. Network protocols can be divided into seven abstract layers according to the OSI reference model.
The top layer the application layer provides features for applications, allowing users to enter and output data. These include layers for transport where you can find TCP, for example and switching where the internet protocol is located. In the lowest layer the physical layer , the task is to translate the binary digits of the computer into a physical signal that matches the transmission medium.
The collision domain is also located at this level.
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