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Size of PDU (protocol data unit)

How big is a physical PDU (protocol data unit)? The answer to this question determines the number of simultaneous transmissions for many network architectures.

If a signal propagates through copper wire at the rate of 2*10^8 meters per second, then on a carrier running at 10 Mbps the length of each bit pulse is given by:

Speed of propagation / Speed of bus = 2*10^8 m/s / 10*10^6 b/s = 20 meters/bit

If a data frame is 512 bits long, then the entire frame occupies:

(Length of one bit) x (Frame size) = 20 x 512 = 10240 meters.

a. How big is a 1024-bit packet if the network runs at 100 Mbps?
b. How big is it if the network speed is increased to 155 Mbps?
c. At 100 Mbps, how much time elapses as one of these frames passes a particular point in the network?

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Solution Preview

As per the given formula,

Size of packet (in meters) = Size of packet (in bits) * Speed of propagation (in m/s) / Speed of bus (in bits/sec)

Given that the speed of ...

Solution Summary

Simple calculations as per the given formulas.