CN: CSMA JAMMING SIGNAL

The jam is sent to ensure that all devices on an Ethernet segment know that a collision has occurred and that errors detected by other devices receiving the signal do not mistake this as noise. When a collision occurs both transmitters will detect the collision. Receivers will only detect the collision by receiving the jam signal. Transmitters detect the collision by comparing what is being sent, by what is being received (this is why it is called a "transceiver" because it transmits and receives at the same time). If the received signal differs from the transmit, a collision has occurred, and it sends a jam to notify the receivers to drop the frame and mark it as a collision. The collision is what triggers the backoff timer on the transmitter, not the jam signal. Both transmitters will always detect the collision.

The standard diameter of a 10Mbps Ethernet is 232 bit delay or 23.2 us. Thus the round trip time is 464 bit delay or 46.4 us. The transmitting station, say A, keeps sending the data frame for this period of time. Simultaneously it also receives that data frame. If for the entire duration of 46.4 us it receives what it had sent, then it means that all stations in the network are aware that it (station A) is sending a data frame, and thus hold their transmissions. Thus it is now ensured that no other station will transmit for a next slot time period, and so the station A sends the remaining data of the frame (which includes a 32 bit CRC) in continuation after 46.4 us.

But if station A received, at some point of time, something different from what it was sending then it means that collision has occurred coz some other station also started its transmission before it could have sensed station A’s frame on the channel. Thus the station A now must some how intimate the receiver about the corrupt data it will receive. This intimation is done with the help of JAMMING SIGNAL.

As soon as collision is detected (at worst at the 46.4^th us), the station A aborts transmission and sends a 48 bit jamming signal instead of the remaining data. A jamming signal is a special sequence of repeated “10” bit combination which acts as a CRC not confirming to any valid data possible. And thus the receiver will discard the data.

NOTE: The slot time and round trip time differ by the maximum JAM TIME. For a 48 bit jamming signal,

Slot Time = Round Trip Time + 4.8 us

And since, Round Trip Time = 46.4 us, therefore, Slot time = 51.2 us