Data Acquisition

This jam signal continues to propagate along the cable, as a contention signal, since it is‘mixed’ with the signal still being transmitted from node 1. Eventually, node 1 recognizesthe collision, and goes through the same jam process as node 2. You can see from thisthat the frame from node 1 must be at least twice the end-to-end propagation delay of thenetwork, or else the collision detection will not work correctly. The jam signal from node1 will continue to propagate across the network until absorbed at the far end terminator,meaning that the system vulnerable period is three times the end to end propagationdelay.After the jam sequence has been sent, the transmission is halted. The node thenschedules a retransmission attempt after a random delay controlled by a process known asthe truncated binary exponential back-off algorithm. The length of the delay is chosen sothat it is a compromise between reducing the probability of another collision and delayingthe retransmission for an unacceptable length of time. The delay is always an integermultiple of the slot time. In the first attempt, the node will choose, at random, either 1 or0 slot-times delay. If another collision occurs, the delay will be chosen at random from 0,1, 2 or 3 slot-times, thus reducing the probability that a further collision will occur. Thisprocess can continue for up to 10 attempts, with a doubling of the range of slot timesavailable for the node to delay transmission at each attempt. After ten attempts, the nodewill attempt 6 more retries, but the slot times available for the delay period will remain asthey were at the tenth attempt. After 16 attempts, it is likely that there is a problem on thenetwork and the node will cease attempting to retransmit.9.4 MAC frame formatThe basic frame format for an 802.3 network is shown below. There are eight fields ineach frame, and they will be described in detail.Figure 9.9MAC frame format