Discussion

A 100 km long cable runs at the T1 data rate. The propagation speed in the cable is two-thirds the speed of light. How many bits are in the cable?

Answer: C

Correct answer: 772

First the propagation delay:

\[ t = \frac{100\times 10^{3}}{2\times 10^{8}} = 500\ \mu\text{s} \]

Then the bits in flight at the T1 rate of 1.544 Mbps:

\[ 1.544\times 10^{6}\times 500\times 10^{-6} \approx 772\ \text{bits} \]

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