Sender and receiver are separated by two 1-Gigabit/s links and a single switch. The packet size is 5000 bits, and each link introduces a propagation delay of 10 microseconds. Assume that the switch begins forwarding immediately after it has received the last bit of the packet and the queues are empty.
For each link, it takes 1 Gigabits/5 Kbits = 5 microseconds to transmit the packet on the link, after which it takes an additional 10 microseconds for the last bit to propagate across the link. Thus, with only one switch that starts forwarding only after receiving the whole packet, the total transfer delay is two transmit delays + two propagation delays = 30 microseconds.