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I am preparing for an exam in networking.

In one of the previous exams this question was given:

Assume you're sending a packet of length 4000 bit
through a cable of length 1000 km.
The signal in the cable is moving at 200000 km/s.
The signal bandwidth is 10 Mbit/s.
Calculate how much time it would take for the packet to arrive.

If I would have done this with a car, considering road length and car speed, it would probably take 200 seconds. Though I am not sure how to apply the mbit/s and bits in the calculation.

Is this a correct way of doing it?

(10 mbit/s / 4000 bit) * (200000 km/s / 1000 km) = seconds packet needs to arrive
Horse SMith
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1 Answers1

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The transfer time equals to SEND_TIME + EXPANSION_TIME

(I use Mbit as 10^6 bit for simplicity instead of 2^20, the principle remains the same)

SEND_TIME = #bits / #bits_per_sec = 4000 / 10*10^6 = 4*10^-4
EXPANSION_TIME = length / expansion_speed = 1000 / 200000 = 5 * 10^-3

total is then 0.0054 seconds

Bonus:

A good practice is to look at the units and make sure you get to the correct units at the end, so the above is actually:

SEND_TIME = #bits / #bits_per_sec = 4000[bit] / 10*10^6[bit/sec] = 4*10^-4 [bit/bit * sec] = 0.0004 [sec]
EXPANSION_TIME = length / speed = 1000 [km] / 200000 [km/sec] = 5 * 10^-3 [km / km * sec] = 0.005 [sec]
amit
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