In telecommunications, a jam signal is a signal that carries a 32-bit binary pattern sent by a data station to inform the other stations that they must not transmit.
In CSMA/CD the maximum jam-time is calculated as follows: The maximum allowed diameter of an Ethernet installation is limited to 232 bits. This makes a round-trip-time of 464 bits. As the slot time in Ethernet is 512 bits, the difference between slot time and RTD is 48 bits (6 bytes), which is the maximum "jam-time".
This in turn means: A station noting a collision has occurred is sending a 4 to 6 byte long pattern composed of 16 1-0 bit combinations. Note: The size of this jam signal is clearly beyond the minimum allowed frame-size of 64 bytes.
The purpose of this is to ensure that any other node which may currently be receiving a frame will receive the jam signal in place of the correct 32-bit MAC CRC, this causes the other receivers to discard the frame due to a CRC error.
Some notes about jam signals include the following:
- In carrier sense multiple access with collision detection (CSMA/CD) networks, the jam signal indicates that a collision has occurred.
- In carrier sense multiple access with collision avoidance (CSMA/CA) networks, the jam signal indicates that the sending station intends to transmit.
This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C".
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