1.4 Delay, Loss, and Throughput in Packet-Switched Networks

 

1.4.1 Overview of Delay in Packet-Switched Networks

 

• The most important of these delays are the nodal processing delay, queuing delay, transmission delay, and propagation delay; together, these delays accumulate to give a total nodal delay.

 

Processing Delay

• The time required to examine the packet’s header and determine where to direct the packet is part of the processing delay.

 

Queuing Delay

• At the queue, the packet experiences a queuing delay as it waits to be transmitted onto the link. The length of the queuing delay of a specific packet will depend on the number of earlier-arriving packets that are queued and waiting for transmission onto the link.

 

Transmission Delay

• Denote the length of the packet by L bits, and denote the transmission rate of the link from router A to router B by R bits/sec.

• The transmission delay is L/R

 

Propagation Delay

• The propagation speed depends on the physical medium of the link (that is, fiber optics, twisted-pair copper wire, and so on) and is in the range of 2⋅108 meters/sec to 3⋅108 meters/sec
• the propagation delay is d/s, where d is the distance between router A and router B and s is the propagation speed of the link
• The transmission delay is the amount of time required for the router to push out the packet; it is a function of the packet’s length and the transmission rate of the link, but has nothing to do with the distance between the two routers.
• The propagation delay, on the other hand, is the time it takes a bit to propagate from one router to the next; it is a function of the distance between the two routers, but has nothing to do with the packet’s length or the transmission rate of the link.

 

1.4.2 Queuing Delay and Packet Loss

• Unlike the other three delays (namely, dproc, dtrans, and dprop), the queuing delay can vary from packet to packet.
• when characterizing queuing delay, one typically uses statistical measures, such as average queuing delay, variance of queuing delay, and the probability that the queuing delay exceeds some specified value.
• When is the queuing delay large and when is it insignificant? The answer to this question depends on the rate at which traffic arrives at the queue,
• R is the transmission rate; that is, it is the rate (in bits/sec) at which bits are pushed out of the queue. Also suppose, for simplicity, that all packets consist of L bits. Then the average rate at which bits arrive at the queue is La bits/sec
• The ratio La/R, called the traffic intensity,