HTTP and the web

Next class reading is section 2.2 of the book.

The inner workings of the World Wide Web service will be presented.

Homework

Two end-systems (A& B) communicate through a packet switching network. There are two routers (R1 & R2) between the two end-systems. All links are 1Mbps. Host A sends 1 MByte of data to B using 520 bits packets. Each packet has a header of 20 bits.

1. How long does it take since transmission starts in Host A till the last bit has been received by Host B? (Consider propagation delay, processing and queueing delays negligible).
2. What is the end-to-end throughput?
3. Repeat questions 1&2 if propagation delay is 1 millisecond on each link.

Sample use of traceroute -T

With the required privileges, the traceroute command may give us some of the information that was missing before: http://pastebin.com/hU2siqwZ

You can find this tool useful too.

More work to do

Next week lecture will be based on Section 2.1 of the textbook. Let me know if I should focus on a particular concept you've found difficult to understand.

Please bring the following problems solved for the next class in written form:

1)Suppose users share a 2 Mbps link. Also suppose each user transmits continuosly at 1 Mbps when transmitting, but each user transmits only 20 percent of the time. (See the discussion of statistical multiplexing in Section 1.3.)
a. When circuit switching is used, how many users can be supported?
b. For the remainder of this problem, suppose packet switching is used. Why will there be essentially no queuing delay before the link if two or fewer users transmit at the same time? Why will there be a queuing delay if three users transmit at the same time?
c. Find the probability that a given user is transmitting.
d. Suppose now there are three users. Find the probability that at any given time, all three users are transmitting simultaneously. Find the fraction of time during which the queue grows.

2)Suppose Host A wants to send a large file to Host B. The path from Host A to
Host B has three links, of rates R1 = 500 kbps, R2 = 2 Mbps, and R3 = 1 Mbps.
a. Assuming no other traffic in the network, what is the throughput for the file transfer.
b. Suppose the file is 4 million bytes. Dividing the file size by the throughput, roughly how long will it take to transfer the file to Host B?
c. Repeat (a) and (b), but now with R2 reduced to 100 kbps.

Reading and homework

For the next class please read sections 1.3, 1.4 & 1.5. You may also enjoy reading 1.6 & 1.7 but these won't be covered in the subject.

As I mentioned in the classroom, please let me know those topics we want me to elaborate a bit futher in next Friday's lecture, where we will focus on the core network: what's inside the "cloud" of my drawing and how it works.

I failed to mention in the classroom, but I would like you to respond to the following questions in written form. Please hand it over tom me in the next class:

1. What is the difference between a host and an end system? List the types of end systems. Is a Web server an end system?
2. What is a client program? What is a server program? Does a server program request and receive services from a client program?
3. Dial-up modems, HFC, DSL and FTTH are all used for residential access. For each of these access technologies, provide a range of transmission rates and comment on whether the transmission rate is shared or dedicated.

Don't forget to write down your name too. Feel free to use the Internet to answer any of these questions but try to provide your own answer instead of just pasting someone else's. 

Welcome to Computer Networks 101 (2013-14)

One more year I welcome new students (and maybe some not so new ones) to this subject that as you know will be entirely taught in English. Not only the language will be different but so will be the on-line platform, as the only thing you'll find from me in PoliformaT is a link to this blog.

Contrary to PoliformaT, this blog is open, so you can read it even if you are not a student or you are not enrolled in this subject. Not that I think this will bring in many students to the subject but I do not see the point of being closed and I do not particularly enjoy typing my password away, plus my students doing the same everytime they want to access any information about the subject. It makes me think of the many hours of our lives we will be typing passwords.

This is not an easy subject, so at least I will try to keep the access to the subject's information as easy as possible. By the way the blog's name matches the subject code, in case you wondered.

In the first class next Friday 13th I will present the subject and I will give the first lecture about the Internet. If you can read Sections 1.1.1, 1.1.2 and 1.1.3 of the textbook you will have a better understanding of the topics. You can read it from the free sample chapter provided by the publisher or borrowing the book from the library.