GATE Overflow Test Series | Computer Networks | Test 1 | Question: 11

Question

A network with CSMA/CD protocol in the MAC layer is running at $3\;\text{Gbps}$ over a $2\;\text{km}$ cable with no repeaters. The signal speed in the cable is $2\times 10^8\; \text{m/sec}$. The minimum frame size for this network should be?

• A. $30000\; \text{bytes}$
• B. $7500\; \text{bytes}$
• C. $15000\; \text{bytes}$
• D. $60000\; \text{bytes}$

Answer 1

The frame transmission time must be at least two times the maximum propagation time $T_t \geq 2 \times T_p$, for collision detection.
    
    Propagation time $ = \dfrac{d}{v} = \dfrac{2\times 10^3}{2\times 10^8} = 10\mu\;s$
    
    Transmission time $ = \dfrac{L}{B} = \dfrac{L}{3\times 10^9}$
    
    Frame size$(L)  \geq 2\times 10\mu s \times 3\times 10^9 = 60000\;\text{bits or}\; 7500\; \text{bytes}$.