Computer Networks
In 1000BASE-X, a frame of 64 bytes is fi rst block coded
with 8B/10B before transmitting. Suppose the propagation
speed is 2 × 10 8 . What is the frame “length” in
“meters”? (Suppose the cable is 500 m long.)
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Answers
The "frame length" in this case is 880 meters. This is because the frame length is 7.5 times the actual number of bits (64 bytes * 8 bits/byte = 512 bits). When the frame is block coded with 8B/10B, there are 640 bits (10 * 64 = 640), so the frame length is 7.5 * 640 = 4800 bits. Finally, the frame length in meters can be calculated from the speed of the signal, which is 2 * 10^8, and the frame length in bits, 4800. Dividing the two yields 880 meters, which is the frame length in meters for this setup. Note that this is longer than the actual cable length of 500 meters since it includes the time it takes for the signal to traverse the entire length of the cable.
