As covered in Unit 1, a fault on a Token Ring network,
or on networks with long underground cable runs, can be
difficult to trace. A typically device which is used to
determine the location of a fault is a Time Domain Reflectometer
(TDR). With this, a pulse is sent down the line, and the
amount of time that it takes to reflect from the fault and
back is measured. As it is possible to estimate the speed
of propagation of the pulse, it is possible to estimate
the position of the fault.
For example, in Figure Ch.1, a pulse is sent down the
line. It will propagate along the line until it reaches
an open or a short circuit. If we assume that there is a
distance d to the fault, and that it takes a time
(t) to reach the fault and propagate back to the
source. Thus if we assume that the pulse travels at a speed
s, then the distance to the fault will given the
speed multiplied by half the time taken:
For example if we assume that the pulse travels at half
the speed of light (3E8 m/s) then:
Figure Ch.1: Pulse reflecting from a break in a cable
Challenge 1
A fault has occurred on a token ring network with a radius
of 30m. It uses fibre optic cables which propagate the pulses
at one-third of the speed of light. If it takes 1 µs
(1E-6) for the pulse to be sent from Node A, and be received
back, estimate the sector that contains the fault for the
ring given in Figure Ch.2. Also estimate the total time
that the pulse will take to go round the ring.
