Calculate the latency (from first bit sent to last bit received) for the following:

(a) 100-Mbps Ethernet with a single store-and-forward switch in the path and a packet size of 12,000 bits. Assume that each link introduces a propagation delay of 10 µs and that the switch begins retransmitting immediately after it has finished receiving the packet.
(b) Same as (a) but with three switches.
(c) Same as (a), but assume the switch implements "cutthrough" switching; it is able to begin retransmitting the packet after the first 200 bits have been received.

Given Information:

Network speed = 100 Mbits/s

packet size = 12 kbits

Propagation delay = 10 µs

(a) Latency = 260 µs

(b) Latency = 520 µs

(c) Latency = 142 µs

Explanation:

Latency is the time taken for the data to travel to its destination over some network.

The total time taken is the sum of transmission delay and propagation delay

Latency = Ttrans + Tprop

Latency = (packet size)/(Network speed) + Tprop

(a) 100-Mbps Ethernet with a single store-and-forward switch in the path and a packet size of 12,000 bits.

Since single switch has two links, each link will take a time of

Latency = (12x10³ bits)/(100x10⁶ bits/s) + 10 µs

Latency = 120 µs + 10 µs

Latency = 130 µs

For two links,

Latency = 130*2

Latency = 260 µs

(b) Same as (a) but with three switches.

Three switches corresponds to a total of 4 links and each link takes a time of 130 µs

Latency = 130*4

Latency = 520 µs

(c) Same as (a), but assume the switch implements "cutthrough" switching; it is able to begin retransmitting the packet after the first 200 bits have been received.

The Latency in this case would be

Latency = [(200)/(100x10⁶) + 10 µs] + [(12x10³)/(100x10⁶) + 10 µs]

Latency = [ 2 µs + 10 µs] + [ 120 µs + 10 µs]

Latency = [ 12 µs ] + [ 130 µs]

Latency = 142 µs