Network Latency Calculation: Store-and-Forward vs. Cut-Through Switching

(a) Latency for a single store-and-forward switch:

• Time to transmit packet = 1500 bytes * 8 bits/byte / 100 Mbps = 120 μs
• Propagation delay for one link = 10 μs
• Switch processing delay = 0 μs (since it begins retransmitting immediately) Total latency = Time to transmit + (number of links * propagation delay) + switch processing delay = 120 μs + (1 * 10 μs) + 0 μs = 130 μs

(b) Latency for three store-and-forward switches:

• Time to transmit packet = 1500 bytes * 8 bits/byte / 100 Mbps = 120 μs
• Propagation delay for one link = 10 μs
• Switch processing delay = 0 μs (since they begin retransmitting immediately) Total latency = Time to transmit + (number of links * propagation delay) + switch processing delay = 120 μs + (3 * 10 μs) + 0 μs = 150 μs

(c) Latency for three cut-through switches:

• Time to transmit first 200 bits = 200 bits / 100 Mbps = 2 μs
• Time to transmit remaining 1300 bytes = 1300 bytes * 8 bits/byte / 100 Mbps = 104 μs
• Propagation delay for one link = 10 μs
• Switch processing delay = 0 μs (since they begin retransmitting after receiving first 200 bits) Total latency = Time to transmit first 200 bits + Time to transmit remaining bytes + (number of links * propagation delay) + switch processing delay = 2 μs + 104 μs + (3 * 10 μs) + 0 μs = 142 μs