GATE | GATE-CS-2015 (Set 3) | Question 65

Consider a CSMA/CD network that transmits data at a rate of 100 Mbps (10⁸ bits per second) over a 1 km (kilometre) cable with no repeaters. If the minimum frame size required for this network is 1250 bytes, what is the signal speed (km/sec) in the cable?
(A) 8000
(B) 10000
(C) 16000
(D) 20000


Answer: (D)

Explanation:

Data should be transmitted at the rate of 100 Mbps.
Transmission Time >= 2*Propagation Time
=> 1250*8 / (100 * 106) <= 2*length/signal_speed
=> signal_speed  <= (2 * 103 * 100 * 106) / (1250 * 8)
                 <= 2 * 10 * (103) km/sec 
                 <= 20000

Refer http://pet.ece.iisc.ernet.in/course/E2223/Problems.pdf for more details.
Option (D) is correct.

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