CSMA/CD
There are certain phases of media access:
Carrier Sense
Stations that want to transmit frames must perform a carrier sense check. If the media is idle for about two bit times the station is allowed to start transmission.
In case the media isn't idle because a frame is passing, transmission must not start immediately if the passing frame is just over. There must be an inter frame gap of 9,6 µs.
Multiple Access
Multiple stations may perform carrier sense and access the media at the same time: multiple access = multiple transmissions.
The physical signals of these frames will be corrupted by collisions; the frames cannot be received properly any more. Ethernet transceivers of transmitting stations will detect this collision.
Collision Detection
A transmitting transceiver always compare the bits being sent to those that can be received simultanously. If incoming/outgoing signals are different, this may (or may not) be caused by a collision.
If a collision condition had been detected, the transceiver will do as follows:
1. Jam Sequence
The transceiver will stop transmission of the current frame; this runt frame will be extended by a so-called 'jam sequence' which is a 32-bit extension. This jam sequence is a dummy checksum with an always invalid FCS value.
Collisions can be detected by transceivers because of invalid voltage levels. But this can only be done by transceivers that are connected to the same cable segment (local collisions); this cannot be done by stations behind repeaters (remote collisions): If a collision passed a repeater, voltage levels are valid again because the transmitting repeater port is unable to repeat the bad voltage levels. As a result, invalid voltage levels can only be detected on the local segment where the collision had happened. If a transceiver detects (a) 'frame short' and (b) 'checksum error' this will prove that the frame was corrupted by a collision; these frames are called runts.
This way of collision detection depends on some specific conventions:
2. Truncated Binary Exponential Backoff
Each transceiver has a collision counter. If a collision is detected during frame transmission, the collision counter is incremented by one. (If the transceiver is in listening mode, collisions won't cause the counter to be incremented.) Each time a transmission was successfull (without collisions), the collision counter is set to zero. If the collision counter is incremented to 16, the transceiver will send a SQE signal to the controller indicating 'carrier lost'. This mechanism results in the autopartitioning feature of repeaters; reconnection means that the port's transceiver indicates 'carrier available' again.
After each collision the transceiver has to wait for certain time before retransmission (backoff time). The backoff time is a multiple of the signal's maximum round trip delay (max. RDT 51,2 µs); the absolute value is randomized:
n = value of collision counter / restricted to value of 10.
backoff time = 51,2 µs * 2 0..(n-1)
The collision counter may exceed a value of 10, the variable n may not (truncated binary exponential backoff).
This will cause:
A. The more collisions, the more backoff time.
B. If two stations are waiting because their frames were corrupted by collisions, a third station can transmit a frame because the media is idle.
3. Retransmission
After the backoff time had passed, the concerned transceivers will perform carrier sense again.