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Challenge 1 - Network Topology

Back | Challenge 1 | Challenge 2  | Challenge 3  |[CNDS] 

PDF Version

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Background

As covered in Unit 2, 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:

See PDF Version

and that it takes 1 µs (1E-6) to travel to the fault and back again, then the distance to the fault will be:

See PDF Version

 

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.

 

Figure Ch.2: Token ring network

 

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