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binary exponential backoff (Ethernet) (BEB)

Binary Exponential Backoff (BEB) is a congestion resolution mechanism in the Ethernet according to IEEE 802.3. Before a station that is willing to transmit in the Ethernet starts to transmit, it listens to the cable to see if another station is carrying data traffic. If this is not the case, it begins to transmit, otherwise it waits until the cable is free again and then begins to transmit its data.

Because of the finite propagation time of the data on the cable, it may happen that a distant station also starts transmitting data and feeds the signals into the transmission medium. The signals from both stations collide and their levels overlap. These collisions create an increased signal level on the transmission medium, the collision level, which is detected by the stations, whereupon they immediately abort their transmission. They then attempt to restart their transmission, and do so either immediately or after the round-trip delay of 51.2 µs. This may result in a collision again if both stations happen to make the same choice.

In the next attempt, each of the two stations will now again randomly choose a new start time, but this time from four possibilities: 0, 1, 2 or 3 slot times, i.e. `2^2 = 4`.

In case of another collision, there are then `2^3 = 8` possibilities, then 16, 32, 64, 128, 256, 512 and finally 1024. After 10 doublings and the increase to 1024 slot times, which corresponds to a time interval of about 52 ms, the slot time is not increased any further (truncated). After a total of 16 unsuccessful transmission attempts with collision, it is aborted with an error message from the Ethernet controller.

Englisch: binary exponential backoff (Ethernet) - BEB
Updated at: 22.01.2022
#Words: 256
Links: resolution, Ethernet, IEEE 802.3, cable, data
Translations: DE

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