Kurose and Ross Edition 6 Exercise 1 Question P12 (Page No 73)

Question

A packet switch receives a packet and determines the outbound link to which the packet should be forwarded. When the packet arrives, one other packet is halfway done being transmitted on this outbound link and four other packets are waiting to be transmitted. Packets are transmitted in order of arrival. Suppose all packets are 1,500 bytes and the link rate is 2 Mbps. What is the queuing delay for the packet? More generally, what is the queuing delay when all packets have length L, the transmission rate is R, x bits of the currently-being-transmitted packet have been transmitted, and n packets are already in the queue?

Answer 1

QUEUING DELAY = (TOTAL NO. OF BITS NOT YET TRANSMITTED)/ RATE OF TRANSMISSION(in bits)

general formula: → 

• length of each packet = L,
• the transmission rate = R,
• no. of bits of the currently-being-transmitted packet that has already been transmitted = x
• packets already in the queue = n

Tq = (n*L + L-x)/R

The first packet is halfway through→ half of its bits are already transmitted, therefore half of the bits of that packet are still in the queue.

packet size = 1500

transmitted bits (x) =  1500/2 = 750

remaining bits (L-x) = 1500-750 = 750

Tq = (4*(1500)+ 750)*8 / (2*10^(6))

Tq = 27 msec = 0.027 sec