When a collision is detected in CSMA/CD, other devices on the network may not know a collision occurred and might start transmitting, causing more collisions. A way to quickly notify everyone is needed.
A special signal sent immediately after a collision is detected, ensuring all devices on the network become aware of the collision and stop transmitting.
- Device detects collision (signal strength change on wire)
- Device immediately stops transmitting its data
- Device sends a jam signal (a predetermined pattern of high-amplitude signal)
- All devices hearing the jam signal know a collision occurred
- Devices enter backoff algorithm to wait before retrying
- Short, high-intensity signal designed to be detected by all devices
- Sent by all devices that detected the collision
- Duration is at least the slot time (time to detect collision)
- Part of CSMA/CD protocol
- Built from: CD — jam signal is part of this protocol
- Related: Collision Detection — triggers jam signal
- Related: Binary Exponential Backoff — follows jam signal
- Related: Collision — what triggers the jam
- Jam signal itself could theoretically collide (rare, handled by backoff)
- Only works on wired shared media (not in switched/full-duplex networks)
- Slot time determines minimum jam signal duration