The bandwidth is 1 Gbps or 109 bits ,
1 bit = 1/ 109 sec = 1/ 103 microsecond
RTT = 30 microsecond
1/ 103 microsec for 1 bit , 1 microsec for 103 bits
30 microsec 30, 000 bit
data frme size 2000 bit
link utilization = (2,000 / 32,000) * 100 = 2/32 = 0.0625 * 100 = 6.25
in this question
1 bit delay to make round trip time 30 microseconds
means
Transmission delay for 1 bit t = 1/(109) = 1/ 103 microsecond .
we calculate RTT by using RTT= 2 * Tp + Tt ..
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Another way of solving :
the efficiency formula for Stop&Wait ,
1/1+2a .... (i)
a = Tp / Tt
Tp = 15 Microseconds and Tt = 2 microseconds
now put them in (i) , a = 15/2 , 2a = 15 .
1/1+15 = 1/16 = 0.0625
0.0625 * 100 = 6.25 %
we get link utilization or efficiency is 0.0625 or 6.25%