Jam signal

In telecommunication, a jam signal is a signal that carries a bit pattern sent by a data station to inform the other stations that they must not transmit.

Note 1: In carrier-sense multiple access with collision detection (CSMA/CD) networks, the jam signal indicates that a collision has occurred.

Note 2: In carrier-sense multiple access with collision avoidance (CSMA/CA) networks, the jam signal indicates that the sending station intends to transmit.

Note 3: "Jam signal" should not be confused with "electronic jamming."

Source: from Federal Standard 1037C