1.4 Delay packet loss and throughput in packet switched networks

In the last section, we mentioned the concept of packet loss, the loss of data in transit which is unacceptable at any time. To do this, we must introduce delay and limit the throughput of the network, which is inevitable. In this section, we will begin to study and quantify the problems of packet loss, delay, and throughput in the network.

1.4 1 Overview of Latency in Packet Switched Networks

Packets are sent from one host to another, passing through a series of routers on their way to another host. Caused by routers store forward transmission mechanism transmission delay and queuing delay and caused massive grouped into the way of node processing delay and transmission delay of the inevitable delay added together to node is always delay, delay effect on the performance is very huge, and then we will discuss this several time delay.

Type of delay

1. Processing delay Processing delay mainly includes the time required by the router to check the packet head and select the link for the packet according to the forwarding table, as well as the time required by the router to check for bit-level errors and other factors. The delay order of high-speed routers is usually microseconds or less.
2. Queuing delay The queuing delay for a particular packet depends on the number of packets that arrive early in the output cache waiting to be transmitted to the link. If the queue is empty, the delay reaches 0. Conversely, if there are a large number of packets, the delay increases to the millisecond level. If it exceeds a certain amount, packet loss occurs.
3. Transmission delay In the previous section we discussed the calculation of transmission delay a little bit. The main reason is that the packet switch will not send the packet again until it has received the whole packet.
4. Propagation delay The propagation delay is determined by the physical media (optical fiber, twix-pair copper wire, etc.). The speed order is 28−382^{8} -3 ^{8}28−38, which is nearly the speed of light propagation. The propagation delay in the link meets the physical formula T = D /s (distance divided by speed).

Propagation delay and transmission delay

Let’s make a distinction between transmission delay and transmission delay: using cars on highways as a category, transmission delay is the time you spend at service tollbooths, and transmission delay is the time you spend on roads.

1.4.2 Queuing delay and Packet Loss

Queuing delay

Among the four different kinds of delay, the most complicated one is queue delay, which is full of uncertainty and depends on three factors

1. R: Link bandwidth (BPS)
2. L: Packet size (bits)
3. A: Average arrival time of packets

Then the traffic intensity K=L* A /R, and there is a delay of (N-1)L/R for the NTH arriving packet. When K is greater than 1 and the input rate is greater than the output rate, packets will be accumulated in the router output cache. Assuming that the router cache grows infinitely, the delay required by the infinitesimal packet will grow infinitely

Packet loss

In real life, the value of K is not greater than 1. The reason is that packets are discarded when the router cache is full. Therefore, the performance of a router is measured not only by the delay, but also by the probability of packet loss.

1.4.3 End-to-end delay

We are talking about the delay of a single router node. Of course, there are more than one router in front of each other, and they add up to the delay of the end-to-end path. End-to-end, however, may have latency associated with end systems and other applications, which we’ll discuss in more detail in Chapter 6.

1.4.4 Throughput in computer network

Throughput is the speed at which files can be transferred end-to-endSatisfies the barrel principle, that is, depending on the smaller router bandwidth, if two routers A and B, then the file transfer rate is min{Ra, Rb}, also known asBottleneck link transmission rateCan be likened to the flow of water in a pipe, the natural water output depends on the smaller one.

Homework problems

R16. Consider sending a packet from a source host across a fixed route to a destination host. List the end-to-end delay components. Which of these delays are fixed and which are variable?

Answer: End-to-end delay components are: node processing delay, queuing delay, transmission delay, transmission delay. The fixed part of the delay is: transmission delay; the variable part of the transmission delay is: node processing delay queuing delay (different from other answers, I think the processing time of the router is not fixed)

R18. How long does it take for a packet with a length of 1000 bytes to travel over a link with a distance of 2500km at a rate of 2.5×10^8m/s and a transmission rate of 2Mbps? More generally, how long does it take for a packet of length L to travel over a link of distance D at s and Rbps? Does the delay depend on the transmission rate?

Answer: (1) Transmission delay: 8Kb/2Mb = 4ms Transmission delay: 2500km/2.5×10^8m/s = 10ms The required time is 14ms (2) Transmission delay: 8*L/R Transmission delay: D /s 8 x L/R + D /s The delay depends on the transmission rate.

R19. Suppose host A wants to send A large file to host B. There are three links from host A to host B, with rates R1 = 500kbps, R2 = 2 mbit/s, and R3 = 1 mbit/s respectively.

A. Assuming there is no other traffic on the network, what is the throughput of the file transfer?

B. Assume that the file size is 4MB. How long will it take to transfer the file to host B?

C. Repeat (a) and (b), except that R2 is reduced to 100kbp

A. throughput of 500kbps B. approximately 64s C. Throughput is 100kbps, which takes roughly 320s

R20. Suppose that end system A wants to send A large file to end system B. At a very high level, describes how the end system generates groups from this file. When one of these packets reaches a packet switch, what information in the packet does the switch use to decide which path to forward the packet to? Why is packet switching on the Internet comparable to driving from one city to another asking for directions along the way?

Answer: End systems use the application layer to generate groupings. In this case, the packet switch refers to the router. The router uses the IP address in the packet to determine which route to forward the packet to. Each router has a forwarding table, and when a packet reaches a router, the router checks the IP address and searches the forwarding table with this address to find the appropriate output link. This is similar to asking for directions along the way.