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Welcome to ITS323
by Steven Gordon - Tuesday, 12 August 2014, 8:20 AM
 
Welcome to ITS323 Introduction to Data Communications! The lectures start this week:

Section 2 (CS): Wednesday 14:40 in BKD 2605
Section 1 (IT): Friday 13:00 in BKD 2601

Note that IT section has a make-up class this week on Friday 15th August at 1pm to 4pm. This is to cover the lectures missed on Monday 11th and Tuesday 12th due to holidays, and to ensure both sections are aligned each week.

Printed handouts are available in the copy centre. The main course website is available via:
http://ict.siit.tu.ac.th/moodle/

We will use Moodle for course content and online quizzes. This will be explained in the first lecture. You will need your username/password for the ICT server (check your junk/trash mail for an email from root@ict.siit.tu.ac.th). If you don't have it or want to change the password, then I will set it for you in the first lecture. Details about the ICT server are at: http://ict.siit.tu.ac.th/help/

Most people registered for ITS323 will receive this email. Please let your friends who haven't yet registered know that lectures for CS start tomorrow, while IT students have a makeup this Friday afternoon.

See you soon,

Steve

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Homework for Week 1
by Steven Gordon - Saturday, 16 August 2014, 3:26 PM
 
Your homework for ITS323 this week is to explore the course website. Some thing that I will assume you will know next week include:

- How to log in to Moodle and access the quizzes (although there is not quiz yet, you can try Practice 1 Units and Prefixes).

- How many percent each assessment item is worth, and how the quizzes are graded.

- What is the course textbook, and what other online resources are available for the course.

- Where to find the exams (and answers) from previous years of this course.

- Where to find the lecture notes (PDF), screencasts and notes calculations performed in class.

As a hint for the last point, check my YouTube channel:

http://www.youtube.com/StevesLectures

Finally, if you want further practice for this course and others I teach, you can try the activities in the "Networking" course:

http://ict.siit.tu.ac.th/moodle/course/view.php?id=12

Some parts of this "Networking" course are very similar to ITS323, so if you can complete the quizzes in "Networking", then it will be easy for ITS323. I've added all students to the "Networking" course, so once you login to Moodle you can access the quizzes. I'll explain this further in the next lectures.

Steve

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Make-up Lecture for CS Wednesday
by Steven Gordon - Monday, 18 August 2014, 5:07 PM

This Wednesday 20 Aug I have an unexpected appointment which means I must cancel the lecture for CS students (section 2) on Wednesday 14:40 to 16:00. I will schedule a make-up lecture for this Friday 22 August at 13:00 to 14:20. As many students have a class Friday morning at SIIT Rangsit, I will hold the make-up class at Rangsit in RS3204. 

If you have any problems with the make-up on Friday or the being held in Rangsit, let me know.

Steve

PS This does not apply for IT section, your normal lecture tomorrow (Tuesday) morning at 10:40 is as scheduled.

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Quiz 1 is Open
by Steven Gordon - Thursday, 21 August 2014, 5:10 PM
 
The first assessed quiz is now available. It is due 10am Tuesday 26 August. That means you must make any of your attempts before that time. You are allowed a maximum of 4 attempts. Your highest score of all of your attempts will be considered; less than 50% is a fail.

Please try the quiz on your own (not with the help of your friends). If after the first attempt you find it very hard, then study some more before attempting the 2nd time. To study you should review the lecture notes, the examples performed in class, discuss with other students and talk to me.

Let me know if you have any problems with the quiz. If there is an error or it doesn't accept an answer that you are confident is correct, then I can manually mark it.

Steve

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Quiz 2 on Throughput and Delay
by Steven Gordon - Thursday, 28 August 2014, 4:38 PM
 
Quiz 2 is available and due Tuesday 2 Sep, 10am. There are 4 questions on throughput and delay.

Some people still get confused about prefixes and units, e.g. Mb, kB, ms, m/s. I recommend you try Practice 1 - Units and Prefixes:

http://ict.siit.tu.ac.th/moodle/mod/lesson/view.php?id=630

I've also added some instructions on writing answers in Moodle quizzes.

http://ict.siit.tu.ac.th/moodle/mod/page/view.php?id=636

Finally, the last lecture we introduced signals as a summation of sine waves. I created plots using Octave in the lecture. If you are interested, the code used and plots produced are on the course website:

http://ict.siit.tu.ac.th/moodle/mod/page/view.php?id=600

Steve

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Quiz 3 on Signals
by Steven Gordon - Friday, 5 September 2014, 10:36 AM
 
Quiz 3 is available now and has several questions on communication signals. As there will be power/Internet disruption at Bangkadi on Sunday, ICT server including Moodle will most likely be unavailable from Saturday afternoon until Monday morning. Therefore I have extended the deadline for Quiz 3 to 10am Thursday 11 September.

I recommend you also try Practice 2 on Logarithms and Decibels. This is not a quiz, but will introduce/remind you of some basic math that will be used in the upcoming lectures.

Steve

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Quiz 4 and Practice Lessons
by Steven Gordon - Sunday, 14 September 2014, 1:24 PM
 
A reminder to complete Quiz 4, which has 3 questions on Nyquist and Shannon capacity, before 10am Tuesday 16 Sep.

Also, there are several practice lessons available. These are NOT marked, but I recommend you try them, especially if you have trouble with some of the mathematics (e.g. units, logarithms, dB). I've just added a 3rd practice that takes a detailed look at the tradeoffs between frequency, bandwidth, components, accuracy and data rate of signals.

http://ict.siit.tu.ac.th/moodle/mod/quiz/view.php?id=639

If you attempt this, make sure you read and complete the questions in the order presented. You should "Check" your answers as you go, as each new page of questions depends on the previous pages. This practice lesson is open until the mid-term exam.

Steve

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Quiz 5 on Decibels and Antennas
by Steven Gordon - Sunday, 21 September 2014, 8:55 AM
 
Quiz 5 has 6 questions about dB and antennas (especially antenna gain). Although there are concepts in some of the questions that have not been mentioned in the lectures, they are explained in the question (e.g. "receiver sensitivity", antenna gain equation). You should be able to answer all questions without any further study. However to give students a chance to ask me questions, I have extended the quiz deadline until Thursday 25 Sep, 10am.

A reminder that some equations, especially Shannon capacity equation and antenna gain equation (G = ...), use gain as a factor. For example in Shannon capacity equation, SNR is a factor, not in dB. If you have the gain in dB (e.g. 20dB) then you must first convert it into a factor (e.g. 100) before using it in the equation. And the antenna gain equation in question 6 of the quiz (and slide 18 of Transmission Media) calculates gain as a factor, not in dBi.

Steve

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Midterm Exam Preparation
by Steven Gordon - Friday, 26 September 2014, 11:10 AM

Here are some resources for preparing for the Midterm Exam:

1. Hints about the exam, including the front page and directions to find past exams:

http://ict.siit.tu.ac.th/moodle/mod/page/view.php?id=643

2. Practice 4 (will not be marked), which is random questions from Quizzes 1 to 5:

http://ict.siit.tu.ac.th/moodle/mod/quiz/view.php?id=644

3. You can still do practices 1, 2 and 3.

4. The handwritten examples in lectures are available on the course website for each topic. I've also added a PDF of the examples for each topic, e.g.:

http://ict.siit.tu.ac.th/~sgordon/its323y14s1/notes/Data_Transmission.pdf

Explore the course website to see the PDF for the other topics.

I will be available for questions about the exam in or nearby my office most of next week EXCEPT: Mon 29 Sep, 9am-12noon and Wed 1 Oct all day. Or you can contact me by email at any time.

Steve

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Midterm Exam Results
by Steven Gordon - Saturday, 11 October 2014, 5:30 PM
 
The midterm exam results and answers are available via:

http://ict.siit.tu.ac.th/moodle/mod/assign/view.php?id=645

You can find your grade as well as the scanned PDF of your exam. Please check that I have correctly summed the points for each question, and that the grade on Moodle matches the points on the exam. I do not give any written feedback on your exam, but you can check your answer with my answer sheet to see where you went wrong. Please discuss with me if you have any queries.

With the scanned exams, sometimes the scanner misses some pages. In those cases I am too lazy to find the missing pages and re-scan, so you may see a blank page with a short message "Missing page". See me if you want to see the hard copy of your exam. Also, if I incorrectly uploaded the wrong exam for you, please let me know asap and I will fix it.

See you next week in the lecture.

Steve

PS Power will be off on Sunday (tomorrow) in the afternoon at Bangkadi, which probably means ICT server (including your exam) will be unavailable until Monday morning.

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Make-up Lecture for CS on Friday
by Steven Gordon - Monday, 20 October 2014, 4:16 PM
 
Due to Thursday 23 October being a public holiday, and to keep lectures consistent between IT and CS sections, a make-up lecture will be held for CS section on Friday 24 October:

Location: Rangsit R316

Time: 13:00 to 14:20

If you have an problems with this, please let me know.

Steve

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Assignment Released
by Steven Gordon - Sunday, 26 October 2014, 1:24 PM
 
I have released the Assignment instructions on the course website:

http://ict.siit.tu.ac.th/moodle/mod/assign/view.php?id=647

In short, form a group of 3 and tell me of who is in that group by 4pm this Wednesday. The assignment involves using some software to run experiments. You should at least have the software installed and tested by the end of this week (Fri 31st). Instructions are on the website.

I will give further help and hints on the assignment each week, including adding further info to the assignment web page when necessary.

Steve

PS Lecture notes for next 4 topics - Multiplexing, Networks, Switching and Routing - are on the course website. They will be in the copy centre on Monday. I will print one-sided only, so you have space on the back of each page for notes. We will probably start these topics either in the 2nd lecture this week, or next week. 

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Makeup Lecture for CS Section
by Steven Gordon - Thursday, 30 October 2014, 3:31 PM
 
As all classes are cancelled on Wednesday 12 and Thursday 13 November (due to Thammasat Expo), for CS Section I will hold a make-up lecture on:

Monday 10 November

12:20 to 14:20

Room BKD 2605

Note that Dr Boontawee has cancelled CSS321 on that day, so I will take over his time slot, but we will start at 12:20. There will be no ITS323 lecture on 12th or 13th.

The schedule for IT section, Monday 14:40 and Tuesday 10:20, remains the same.

Steve

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Quiz 6 Answers and Assignment Tasks
by Steven Gordon - Friday, 31 October 2014, 3:43 PM
 
For the previous quiz, some students had a question about PCM asking them to calculate the size of a WAV file. If they gave a correct answer in "Mb" (megabits) then it was mistakenly marked incorrect. For example, "1 MB" was accepted, but "8 Mb" was not. I manually remarked those questions so that their megabits answer was also marked correct.

If you are confident that your answer is correct in a quiz, but marked wrong, please contact me (talk to me or send an email) or "flag" the question in Moodle. I will manually check your answer.

Also on the quiz (and lecture), there were many questions about sliding window. For the quiz question that asked you to calculate the new window size or frame ACKed/transmitted, I have created an 8 minute video that gives a sample answer:

https://www.youtube.com/watch?v=JWREZ5HwIxw

Please view if you are still unsure about sliding window.

Finally, for the assignment. A reminder to setup the virtual networking software according to the instructions at:

http://sandilands.info/sgordon/automatic-creation-of-virtual-network-with-vboxmanage

I've included a video that shows the steps (on Ubuntu - check the instructions for the additional steps needed for OSX or Windows). Next week I will give some hints on how to proceed with running the flow control and ARQ experiments (i.e. using arq_client and arq_server). So it is important you have the virtual network setup before then.

If you don't have access to a computer that runs VirtualBox, then in the IT Mac lab on 3rd floor, the following computers already have VirtualBox and virtnet installed under OSX: 3, 4, 29, 30, 31, 32, 33, 34, 36, 37, 38.

At least one group has already successfully setup and run the software. If anyone has problems, let me know.

Steve

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Quiz 7 and Practice
by Steven Gordon - Wednesday, 5 November 2014, 7:43 PM

Quiz 7 is now available. I've also added a practice lesson on "packets" which explains the structure and terminology of packets (including frames) and gives some real examples. Some of if you have seen before, some is new. The lesson includes about 45 minutes of videos. I suggest you use your free time on Wednesday and Thursday next week to complete the lesson and quiz.

Also you should have already installed and have virtnet working, and tried the Python code for the stop-and-wait and sliding-window protocols. Now you should start running the Python code with different parameters and investigate how those parameters impact on throughput. If you haven't met your group members yet, check the final list at:

http://ict.siit.tu.ac.th/moodle/mod/assign/view.php?id=647

Finally, just a reminder the CS section that there will be a make-up lecture on Monday 10 Nov in BKD2605, starting 12:20pm. There are no lectures on 12 or 13 Nov.

Steve

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Talking to Martians ... and Quiz 8
by Steven Gordon - Wednesday, 12 November 2014, 4:21 PM
 
A reminder that Quiz 7 is due 10am tomorrow. And Quiz 8 is now available with several questions on switching and routing:

http://ict.siit.tu.ac.th/moodle/mod/quiz/view.php?id=653

Due to the shorter make-up lecture for CS students this week, there were some differences in what I covered between the CS and IT section. Some points that will be useful in Quiz 8, but may not have yet been covered include:

- "flooding" is the process of sending copies of a packet to everyone in the network

- a "routing table" stores the least cost paths from a node to each potential destination. However the entire path is not instead; instead the routing table stores only the destination and the "next node" in the path. Videos and notes of the routing lecture is at:

http://ict.siit.tu.ac.th/moodle/mod/page/view.php?id=607

Finally, over the last month there has been lots of interesting space news: India deploying their Mars orbiter, the NASA Curiosity Mars rover taking photos of a comet, the MAVEN spacecraft forwarding 550 Mbits of data back to Earth, and unfortunately the failures of the Orbital Sciences rocket and Virgin Space Ship Two.

Space systems, including space communications, often lead the development of new technologies and also face difficult challenges. For example one of the challenges of the Mars projects is communications between the rover on Mars and NASA headquarters on Earth, e.g. downloading the photos and uploading commands. For example, the path loss is such that the received signal is about 1,000,000,000,000,000,000,000,000,000 times weaker than the transmitted signal. Big antennas and powerful transmitters are needed.

Also the round-trip-time is about 28 minutes! Imagine browsing the web when it takes 28 minutes from when you click on a link until when the page is displayed.

When the Curiosity Mars rover landed I wrote about some of these communication challenges from the perspective of ITS323. I discussed path loss, antenna gain, frequencies, bandwidth and spectrum, data rates and transmission and propagation delays. Feel free to read at:

http://sandilands.info/sgordon/communications-with-mars-curiosity

Steve

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New Lectures Notes and Assignment Extension
by Steven Gordon - Monday, 17 November 2014, 5:08 PM

The final set of lecture notes are now available in the copy centre and online. They will be needed in tomorrow's lecture (Tuesday) for IT section, and in Thursdays lecture for CS section.

I've added some brief notes in the "Update" section of the Assignment webpage and also extended the deadline of the assignment submission until Friday 28 November 5pm.

I think most students have the virtnet software running and I've seen selected results, including plots and equations, from some students which look good. Feel free to use the extra week for the assignment to get some feedback from me by asking questions or showing me your results/plots.

Steve

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Course Evaluations
by Steven Gordon - Monday, 24 November 2014, 8:48 AM
 
Course evaluation will be held at the start of the lecture today for IT section (and Wednesday for CS section). Please make an extra effort to attend today or Wednesday to provide feedback on the course.

Steve

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Questions about assignment
by Steven Gordon - Wednesday, 12 November 2014, 7:51 AM
 
Several students have asked me some questions about the assignment. Here are my answers:

> is Down link  the same as client?? when client downing file??

In this assignment you set the downlink (server to client) to 100Mb/s with 0ms delay. Then you can change the uplink (client to server) to different data rates and delays. The reason is that the uplink is for DATA from client to server, while downlink is for ACK from server to client. In flow control and error control, the performance mainly depends on the DATA (not the ACK), because the ACK is usually very small. In summary, do not change the downlink values (100 Mb/s and 0ms), but change the uplink values.

> I tried changing the window size and recorded the result from the
program.I changed the window size to 1,4,8,12,64 and 128 and found out that the through put has ...

First, the window size should be 2^k - 1. E.g. 1, 3, 7, 15, 63, ... Also make sure when you start the server and client that they both use the exact same window size. As you change the window size you may observe different throughput. Why? That is what this assignment is about. Think about how sliding-window works and then think about transmission and propagation delay and try to understand how the window size impacts on throughput.

> which parameter should i change for each test?

There are some suggestions on the assignment website. You should think: what values of the parameters will likely cause a change in throughput? Then try them. Because you have limited time you cannot try all values. So you may try 2 or 3 values, look at the results, then from those values, predict what the 'good' values of the parameter should be.

> When I start arq_server it reports an error about could not connect to address. What should I do?

Make sure when you create the topology that you create topology 2. E.g.

bash vn-createtopology 2

In the assignment you must use topology 2. But in the instructions for virtnet it gives an example of using topology 5. arq_client and server will not work in topology 5 (without some code changes). Note that topology 5 has 3 nodes. If you have 3 nodes, then delete them and create topology 2

> I run the server and the client but they never finish.

Make sure the first three parameters for client and server are identical. E.g.

arq_client.py 100 10000 3
arq_server.py 100 10000 3

If any of the first 3 parameters (number of frames, frame size, window size) are different between client and server, then it won't work correctly.

If you still have a problem (eg arq_client and server are not running) then my 1st suggestion is to delete the nodes from VirtualBox (make sure to "Remove all files") and then create topology 2 again. If that still doesn't help, then take a screenshot of the error and email me.

Steve

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Re: Questions about assignment
by Steven Gordon - Wednesday, 12 November 2014, 2:05 PM
 
I have added the video from the lecture that demonstrates using the Python client and server:

https://www.youtube.com/watch?v=bdDua3t7ets

A question from a student:

> I have started arq_server on node2 but nothing happens.

It looks like you are running "arq_server" on node2, which is correct. But now you need to run "arq_client" on node1.

Remember with stop-and-wait, A sends DATA to B, and B sends an ACK back to A (look in the lecture notes). In this assignment "A" is "arq_client" on node1 and "B" is "arq_server" on node2.
So you must start/login to node2 (as you did before using bash vn-ssh 2) and start arq_server, e.g.

python arq_server.py 100 10000 1 9 0.0

And then open a new Cygwin terminal and start/login to node1 (bash vn-ssh 1) and start arq_client.

python arq_client.py 100 10000 1 9 1.0

Note the "9" above means when they run they will print a lot of information on the screen. If you want to print just the results, then change "9" to "1".

If everything is working correctly, then after you start arq_client, then you should see many messages about TxData etc. Then eventually it should finish, showing summary statistics.
If it doesn't work, then take screenshot of both client (node1) and server (node2) and send to me.

Steve

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Re: Questions about assignment
by Steven Gordon - Friday, 14 November 2014, 9:53 AM
 
Some answers to questions from students:

> I was confused about the unit of throughput in ARQ (both client and server) whether it's in bits or bytes.

All data units are Bytes. So data length, amount of transmitted/received data is in Bytes. And throughput is in Bytes per second. Throughput at the server is calculated as:

TotalDataReceived / Duration

(You can see that if you open arq_server.py in a text editor).

> I wasn't sure how to calculate the expected throughput.

In the assignment you must "measure" the throughput using the Python code, as well as "calculate" the expected throughput.

If you remember back in our lectures on Data Link Control Protocols we showed an example of stop-and-wait flow control and calculated the throughput. There was 1000 Bytes of real data received every 8341 us, giving a throughput of 959118 b/s. Since the data rate from A to B was 1 Mb/s, the efficiency was 95.9%. The 1000 Bytes was the amount of data in each frame (e.g. in your assignment the default value is 10,000 Bytes). The time 8341 us was calculated from DATA transmission, propagation and ACK transmission. In your assignment you can calculate them because you know data size (including header), data rate and propagation delay from client to server and from server to client (i.e. the Round Trip Time is the sum of the delay on uplink and downlink).

Note that the ACK size is quite small (several Bytes, compared to 10,000 Bytes of data) and the downlink data rate is 100 Mb/s, so the ACK transmission time will be very small. You may ignore it if your calculation if you want. So for stop-and-wait flow control the total time is:

total time = DATA transmission + uplink delay + downlink delay

expected throughput = original data / total time

expected efficiency = throughput / uplink data rate

So you now have equations for calculating efficiency with stop-and-wait flow control (no errors). When you plot your "measured" efficiency you must also plot the "expected" efficiency. They should be similar (but may not be identical).

Now you need to find similar equations for:

1. sliding-window flow control (hint: look at the red and green coloured examples of sliding window from the lectures where in one case the efficiency is 52.94% and the other 90%. You may want to find different equations for each case).

2. stop-and-wait error control when there are errors/retransmissions. (hint: consider the time it takes if there is no error - same as stop-and-wait flow control - and the time it takes if there is 1 error. Then the average time depends on the error rate. You may ignore the cases of being two errors in a row, i.e. two retransmissions of the same packet, because it is very very unlikely to happen).

> could you please explain what is the meaning of expected efficiency?

When you run the arq_client and server code for an experiment you get the measured throughput at the server. You should convert that to measured efficiency as:

measured efficiency = measured throughput / uplink data rate

Similar when you calculate the expected throughput from your equations, you should convert that to expected efficiency as:

expected efficiency = expected throughput / uplink data rate

For example, if the data rate from client to server (uplink) is 10 Mb/s and you calculate expected throughput to be 7 Mb/s, then the expected efficiency is 70%.

In your plots you should use measured/expected efficiency (not throughput) as we are interested in comparing the protocols under different conditions, independent of the link data rate.

> Do we need to edit the code in arq_client.py and arq_server.py?

No. There is no reason to change the code in this assignment.

However you may open the files with a text editor (e.g. "nano arq_client.py") to view the source code if you like. The code was written by me, and as I have never programmed in Python before it is probably very bad code with many bugs. If anyone fixes bugs and provides significant improvements of the code for the assignment before the final exam, then I may give you a bonus. But don't attempt this until you have completed the assignment.

> Can we work with other groups?

You may discuss with others groups when using the software and running the experiments. You should select your own parameter values though. For example, one group may run experiments for data rate of 1, 2, 3 and 4 Mb/s, another group for 2, 4, 6, 8 and 10 Mb/s. You must run your own set of experiments - you must NOT use other groups data. When writing your report (e.g. creating plots, writing the conclusions) you must NOT work with other groups - this part should be the ideas and work of your group only. Bonus points are available to groups that demonstrate independent thinking, intelligent selection of parameter values, and excellent presentation of data in plots.

> How many experiments should be performed? How many different parameter values should we use?

Enough so that your plots illustrate the main trends, i.e. they show how the parameters impact on efficiency. There is no exact number of experiments that is correct. Some groups may perform different numbers of experiments but still get full marks.

Once you have the software working, then it takes up to about 2 minutes to run one experiment. So within 1 hour you could run about 30 experiments. Since you have 3 people in your group, then you could collect data for 90 experiments in just 1 hour. So with several hours of work, I think a group could perform 100's of experiments quite easily. I don't require you to run 1000's of experiments, but if you submit results for just 10 or 20 experiments, you may get a very low score for the assignment. 

Steve

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Re: Questions about assignment
by Steven Gordon - Wednesday, 19 November 2014, 9:02 AM 

> What is the header size for DATA frames?

4700 Bytes.

So if you set the data to 10,000 Bytes, the DATA frame is 14,700 Bytes. If you set the data to 20,000 Bytes, then the DATA Frame is 24,700 Bytes.

(Although not relevant for your assignment, the data size, e.g. 10,000, and header size, 4,700, are very unrealistic. In reality they will be much smaller. The reason for such large values for this assignment is to allow the virtual network software to work correctly).

> What is the ACK size?

44 Bytes.

So if the downlink data rate is 100 Mb/s, the ACK transmission time is about 0.0035 ms. Compared to the DATA frame transmission time (usually at least 10's of ms) the ACK is insignificant. In calculations of expected efficiency, you may assume the ACK transmission time is 0.

> How do I calculate expected efficiency for sliding window?

There are 2 cases to consider with sliding window, which depend on the window size. Example of the two different cases are at:

http://ict.siit.tu.ac.th/~sgordon/its323y14s1/reports/its323y14s1h13-sliding-window-pictures.pdf

In the first example you see the window is small such that the client must wait for the first ACK to come back. In the second example you see the window is large enough such that the first ACK comes back while the window of DATA frames is still being transmitted (giving maximum efficiency). Try to write equations for each case.

Finally, I have an online calculator for stop-and-wait and sliding-window flow control efficiency (it doesn't consider errors) that you can use to check your equations/results:

http://sandilands.info/sgordon/flow-control-efficiency-calculator

Steve

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Re: Questions about assignment
by Steven Gordon - Sunday, 23 November 2014, 8:56 AM

> I am using Windows/Cygwin and I cannot login to node 2

If you want to start and login to a node there are two ways:

1. In VirtualBox GUI, click on node2 and press "Start". Then it will boot, open a terminal and after a minute it will show a prompt "login:". Enter the username and password (network/network). You don't need to use Cygwin to start a node and login; Cygwin is only required for creating the topology.

2. Follow instructions as on the website under the section "Microsoft Windows (Cygwin) Only". Each time you start cygwin you must run the 3 commands starting with "cd", "PATH" and "ln". Then run the "bash vn-ssh 2" command.

> In sliding window experiments, I set the number of frames to 100, but when it sends some more frames (106, 114). Does this affect my results?

No. If you measure the throughput at the server it automatically calculates based on the duration and number of data bytes received. So if it receives 100 frames or 106 frames or 114 frames, it will still calculate correct throughput. And generally, you should expect the throughput to be about the same in those cases.

> How accurate do our results need to be? 83573 b/s or is 83 kb/s ok?

To the nearest kb/s is ok. There will be small variations in each experiment. So one experiment you may get 83573 b/s and then if you run the exact same experiment again you may get 84204 b/s. They are both 84 kb/s. Even a result of 85 kb/s would be ok. You can expect variations of about 1000 or 2000 b/s (maybe more in some cases).

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Re: Questions about assignment
by Steven Gordon - Monday, 24 November 2014, 5:58 PM

I've added a few notes on experiments, sliding window and plots under the subsection "More on Experiments, Plots and Sliding Window" on the assignment page:

http://ict.siit.tu.ac.th/moodle/mod/assign/view.php?id=647

This week I will be available in my office to help with the assignment:

Tuesday 8-10am

Wednesday: after 4pm

Thursday: after 1pm

Friday: most of the day

If you can't find me in my office during these time, then I may be in the AI3 research lab on 3rd floor of the other building.

Steve

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Re: Questions about assignment
by Steven Gordon - Thursday, 27 November 2014, 1:10 PM

With the expected throughput (and efficiency) of stop-and-wait with errors, you need to consider the average time it takes to transfer a frame given two cases:

Case 1: no error, time to transfer = X

Case 2: 1 error, time to transfer = Y

For example, you need to write equations for X and Y (the equation for X is in fact the same as for stop-and-wait for no errors). Then if the error rate is 5% (or 0.05) then the average time will be:

AveTime = 0.95 * X + 0.05 * Y

Then you can use that average time to calculate throughput (e.g. DataSize/AveTime) and efficiency.

A reminder that the deadline is 5pm tomorrow (Friday 28th). You must submit on Moodle. It is a hard deadline - no changes/submissions are allowed after 5pm. I suggest you upload your files today, so you know how to submit, and then you can update the files up until 5pm tomorrow. If you leave uploading until the last 10 minutes tomorrow, then you may encounter problems of slow server or network, and miss the deadline.

Steve

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Final lectures this week
by Steven Gordon - Monday, 1 December 2014, 6:14 PM

Some (potential) changes to lectures this week:

For IT Section, at the request of the students I held a special lecture today 4pm-5:30pm on the final topic. I will repeat that lecture during the normal time slot (Tuesday 10:40am). Students that attended today do not need to attend tomorrow. However if you missed today you may attend tomorrow. (Note that if you missed the normal lecture today, tomorrows lecture will not make much sense - you should view the video beforehand).

For CS Section, there is a request to hold a makeup lecture on Wednesday 1pm - 2:20pm (and then the normal lecture 2:40pm to 4pm) and cancel Thursdays lecture (or was it cancel 2:40pm Wednesday??). Either way I am happy to do this, however I haven't heard the preferences of many students on this. Unless I hear from other students (whether they prefer the makeup or normal lecture) I will continue with the normal schedule: Wednesday 2:40pm and Thursday 10:40am.

Videos from todays lectures, covering the final topics for all students, are now available via:

http://ict.siit.tu.ac.th/moodle/mod/page/view.php?id=610

I will provide exam hints by early next week, but in summary it will cover the topics after the midterm, will be similar questions to past exams and quizzes. Although I will try, you may not receive assignment marks before the final exam.

Steve

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Re: Final lectures this week
by Steven Gordon - Tuesday, 2 December 2014, 1:59 PM
 
For CS Section, there have been further requests for the makeup lecture. Lets do it:

Wednesday (tomorrow): 1pm - 4pm, BKD2605

No lecture on Thursday, however you can find me in my office all day if you have questions.

A reminder that Quiz 9 is still open, and the final Quiz 10 is also now open.

Steve

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Final Exam Hints
by Steven Gordon - Monday, 8 December 2014, 2:52 PM

Hints on the final exam are available on Moodle.

On Quiz 10, there was an error in 1 question about the features of IP. It asked to select three features of IP, but unfortunately required you to select them in a specific order otherwise would be marked wrong. I have manually marked the question for those that selected the correct features and will check again at the end of the quiz.

Steve

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Assignment Results
by Steven Gordon - Thursday, 11 December 2014, 6:05 PM
 
You can find scores and some feedback on the assignment on the course website. Specifically, the assignment page links to some feedback for all groups:

http://ict.siit.tu.ac.th/moodle/mod/assign/view.php?id=647

You can see your score on that page, or on your "Grades" page for the course (which also includes comments for your group if there is a penalty):

http://ict.siit.tu.ac.th/moodle/grade/report/index.php?id=18

Finally, a reminder to complete Quiz 10 before the exam. I will be in my office most of tomorrow or contact me via email for any last questions. Otherwise see you in the exam.

Steve

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Final Exam and Course Results
by Steven Gordon - Friday, 19 December 2014, 9:29 AM

Final exam has been marked. It seems the exam was a little bit harder than I expected, but still there were some excellent scores from students. You can view the exam answers online. Please view your "Grades" on Moodle to see all your scores:

http://ict.siit.tu.ac.th/moodle/grade/report/user/index.php?id=18

On Moodle, the relevant scores are "All quizzes" (out of 15), "Assignment" (out of 15), and both exams (Midterm 30, Final 40). Students that received a bonus for the assignment will also see "Assignment above 15". These scores are summed to obtain your Course Total (out of 100).

I have no time to scan your exams. You can see the scores you received for each question on the Grades page; you have to see me if you want to view your exam. Please check all your scores and make sure there are no mistakes. You must inform me by Wed 24 December if you have an problems, after which the grades will be finalised.

I cannot reveal any information about letter grades, including cutoffs; you have to wait until letter grades are released by SIIT. The exception is for students planning to re-enter - they need to see me if they need their grade. I will not reply to emails about grades, nor will I consider requests for "extra tasks" to improve your score.

I hope to see you next semester in CSS322, ITS335 and the Network lab (ITS332).

Steve

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