Practical_modern_SCADA_protocols_-_dnp3,_60870-5_and_Related_Systems

Ethernet and TCP/IP networks 327
Assume that both node 1 and node 2 are in listen mode and node 1 has frames queued
to transmit. All previous traffic on the medium has ceased ie there is no carrier, and the
interframe gap from the last transmission has timed out. Node 1 now commences to
transmit its preamble signal, which immediately commences to propagate both left and
right on the cable. At the left end, the termination resistance absorbs the transmission,
but the signal continues to propagate to the right. However, the MAC sublayer in node
2 has also been given a frame to transmit from the LLC sublayer, and since the node
‘sees’ a free cable, it too commences to transmit its preamble. Again, the signals propagate
on to the cable, and some short time later they ‘collide’. Almost immediately, node
2’s transceiver recognizes that the signals on the cable are corrupted, and the logic
incorporated on the NIC asserts a collision detect signal. This causes node 2 to send
a jam signal of 32 bits of random data, and then stop transmitting. In fact, the standard
allows any data to be sent as long as, by design, it is not the value of the CRC field of
the frame. It appears that most nodes will send the next 32 bits of the data frame as
a jam, since that is instantly available.
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 recognizes
the collision, and goes through the same jam process as node 2. You can see from
this that the frame from node 1 must be at least twice the end-to-end propagation delay
of the network, or else the collision detection will not work correctly. The jam signal
from node 1 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
propagation delay.
After the jam sequence has been sent, the transmission is halted. The node then
schedules a retransmission attempt after a random delay controlled by a process known
as the truncated binary exponential backoff algorithm. The length of the delay is chosen
so that it is a compromise between reducing the probability of another collision and
delaying the retransmission for an unacceptable length of time. The delay is always an
integer multiple of the slot time. In the first attempt, the node will choose, at random,
either one or zero 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. This process can continue for up to 10 attempts, with a doubling
of the range of slot times available for the node to delay transmission at each attempt.
After ten attempts, the node will attempt 6 more retries, but the slot times available for
the delay period will remain as they were at the tenth attempt. After 16 attempts, it is
likely that there is a problem on the network and the node will cease attempting to
retransmit.