Jam Signal

Definition of jam signal in The Network Encyclopedia.

What is Jam Signal?

A signal sent by a device on an Ethernet network to indicate that a collision has occurred on the network.

Collisions occur on Ethernet networks because access to media (usually a cable) is based on contention - that is, on a first-come, first-served basis. If two stations attempt to take control of the medium at the same time and begin transmitting, both stations will detect each other’s signal and realize that a collision has occurred.

The two stations then issue a jam signal, which notifies all other stations on the network of the collision. They all must wait a short period of time before attempting to transmit again.

The length of time is random for each station so that the retransmissions won’t cause more collisions. The jam signal sent by one transmitting station must start with a 62-bit pattern of alternating 0s and 1s, followed by a 32-bit sequence that provides a dummy checksum value for the other transmitting station. This 32-bit sequence cannot be equal to the cyclical redundancy check (CRC) value for the frame preceding the jam.