The 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.
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 round-trip-time 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.
Read more about this topic: Carrier Sense Multiple Access With Collision Detection
Famous quotes containing the words jam and/or signal:
“If you hadn’t been in such a hurry to cremate me, I wouldn’t be in the jam I’m in now.”
—Seton I. Miller (1902–1974)
“By day thy warning ringing bell to sound its notes,
By night thy silent signal lamps to swing.”
—Walt Whitman (1819–1892)