Computer Networks 1 (CSE 231) Lecture 3

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Questions and Answers

What is the approach used to study computer networks in this course?

Practical approach

Hybrid approach

Top-down approach (correct)

Bottom-up approach

What is the main topic of discussion in this lecture?

Network architecture

Network protocols

Network performance metrics (correct)

Network security

What is the purpose of the quick review section in the lecture?

To answer student questions

To introduce new topics

To review previously taught material (correct)

To provide additional resources

What is the URL provided for students to answer questions?

<a href="https://forms.office.com/r/rFJse305Dd">https://forms.office.com/r/rFJse305Dd</a> Signup and view all the answers

What are the three metrics mentioned in the lecture?

Loss, delay, and throughput Signup and view all the answers

What is the name of the department that prepared this lecture?

Computer Science and Engineering Department Signup and view all the answers

What happens when the arrival rate to a link temporarily exceeds the output link capacity?

Queue length grows Signup and view all the answers

What is the consequence of a full buffer in a router?

Packet loss occurs Signup and view all the answers

What is the sum of in the total delay formula?

dproc + dqueue + dtrans + dprop Signup and view all the answers

What is the unit of the propagation speed?

~2x10^8 m/sec Signup and view all the answers

What is the transmission delay formula?

L/R Signup and view all the answers

What happens when the traffic intensity La/R approaches 1?

Delays become large Signup and view all the answers

What is the condition for a small average queueing delay?

La/R ~ 0 Signup and view all the answers

What happens when the traffic intensity La/R is greater than 1?

More work arrives than can be serviced Signup and view all the answers

What is the main purpose of the traceroute program?

To measure delay from source to router along end-end Internet path Signup and view all the answers

What happens when a packet arrives at a full queue?

The packet is dropped (lost) Signup and view all the answers

What is throughput in computer networks?

The rate at which bits are being sent from sender to receiver Signup and view all the answers

What is the average end-end throughput when Rs < Rc?

Rs Signup and view all the answers

What is the purpose of the time-to-live field in a packet?

To specify the packet's TTL Signup and view all the answers

What can happen to a lost packet in a network?

It may be retransmitted by previous node, source end system, or not at all Signup and view all the answers

How can you test traceroute in Windows?

By clicking start and typing 'tracert' and pressing enter Signup and view all the answers

What is the link capacity in a network?

The rate at which bits can be transmitted on a link Signup and view all the answers

What determines the average end-end throughput in a network?

The speed of the bottleneck link Signup and view all the answers

What is the formula for per-connection end-end throughput?

min(Rc,Rs,R/10) Signup and view all the answers

What happens when multiple connections share a bottleneck link?

The throughput of each connection decreases Signup and view all the answers

What is the purpose of the reference to Chapter 1, 1.4?

To provide additional information on network performance metrics Signup and view all the answers

What is the relationship between Rc and Rs in terms of end-end throughput?

Rc and Rs are often the bottleneck Signup and view all the answers

What is the significance of the bottleneck link in a network?

It is the link that constrains the end-end throughput Signup and view all the answers

Study Notes

Network Performance Metrics

Network performance metrics include packet delay, loss, and throughput.
Packet delay occurs when packets queue in router buffers, waiting for transmission, and packet loss occurs when memory to hold queued packets fills up.

Packet Delay

Packet delay has four sources: transmission, propagation, processing, and queueing.
Total delay (dtotal) is the sum of processing delay (dproc), queueing delay (dqueue), transmission delay (dtrans), and propagation delay (dprop).
Transmission delay (dtrans) depends on packet length (L) and link transmission rate (R).
Propagation delay (dprop) depends on the length of the physical link (d) and the propagation speed (s).

Queueing Delay

Traffic intensity is the ratio of packet arrival rate to link bandwidth (La/R).
If traffic intensity is less than 1, average queueing delay is small.
If traffic intensity is greater than or equal to 1, delays become large or infinite.

"Real" Internet Delays and Routes

Traceroute program measures delay from source to router along the end-end Internet path.
It works by sending three packets to each router on the path and measuring the time interval between transmission and reply.

Packet Loss

Packet loss occurs when a packet arrives at a full queue and is dropped.
Lost packets may be retransmitted by the previous node, the source end system, or not at all.

Throughput

Throughput is the rate at which bits are sent from the sender to the receiver.
Instantaneous throughput is the rate at a given point in time, while average throughput is the rate over a longer period.
Bottleneck link is the link on the end-end path that constrains end-end throughput.
Average end-end throughput is the minimum of the rates of the sender, receiver, and bottleneck link.

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Description

Introduction to computer networks and performance, covering a top-down approach to studying computer networks. Prepared by the Computer Science and Engineering Department.