Web Science/Part1: Foundations of the web/Internet Architecture/Ethernet/Communication over a shared Medium

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Video of the lesson: Communication over a shared medium

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Script for: Communication over a shared medium

Why Ethernet protocol was developed?

Let’s assume that we have one or more computers that want to communicate. The key is a shared medium. They want to exchange information. As we all know the smallest node of information is 1 bit. So what we can do is to put this 1 bit to the voltage cable in order to transfer.

But if you do this, some problems will arise:

- VOLTAGE TIME

How long do you have to put voltage on the cable?

If you just start putting the voltage on the cable, there will be electromagnetic waves starting to transfer proper data through the cable. The question is how long do you have to do this that others can measure this?

- SENDER

Who is actually a sender of the data?

Let’s imagine 3 computers: A, B and C. We assume that A sends a bit of data. At some point of time B and C receive the data. But the question is who send the data? You can’t figure this out.

- RECIPIENT

Who is the recipient of the data?

If A transfers data then B and C might not know if this data is actually for them.

- TRANSERING SIMULTANIOUSLY

Who is supposed to use medium?

A starts transferring some data then at some point of time B also starts transferring some data. What we have is that C receives corrupted data.

- IS DATA CORRUPT?

The data might get corrupted while being in the cable due to some external events. For example, if cell phone is close to the cable the electromagnetic waves might cause a change. Participants should figure this out.

In order to solve this problem the Ethernet protocol was developed.

So what Ethernet can do is:

• SYNCHRONIZE TIME INTERVALS

Let’s assume computer A sends data at 10 Mbits/s. And computer C has interface that allows 100 Mbits/s. Ethernet as the protocol will be able to synchronize the clocks between these 2 devices.

• SPECIFY RECIPIENT AND SENDER

If A wants to send data for C then B will know that this data is not for him (while C will know that it’s for him).

• DETECT CORRUPTED DATA

If data gets corrupted for some reason then computer C will first see that the data is corrupted.

• DETECT COLLISIONS If A and B starts to transfer data at the same point of time they know that something went wrong and C will receive corrupted data(because it’s overload by 2 packages).

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Further information, readings and exercises about Communication over a shared medium


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Check your understanding of: Communication over a shared medium

Web Science/Part1: Foundations of the web/Internet Architecture/Ethernet/Communication over a shared Medium/Quiz


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"Check your understanding" clarification

Rene, I think there are some controversies in the questions. For example, in the first group of questions called:
What general problems exist while making digital communication over a shared medium?

  • one has to ensure that everyone gets the same amount of time to put data to the shared medium. (in your opinion should be unmarked)

You explain that as: This is how a Protocol like token ring is designed. Ok, it means that we should NOT care about that fact (same amount of time), because we assume that we DO use one of the protocols for communication.

VS

  • Participants have to have a way not to talk at the same time. (in your opinion should be marked).

In my opinion, that problem can also be solved by using one of the protocols ("token ring" for example). Moreover, in previous case we have already ASSUMED that we DO use one of the protocols for communication. Hense, the 2nd case should be unmarked, if we follow the same logic as in previous case.

All in all, if we assume that we do NOT use protocols (but want to design a good protocol), correct answers should be:
1, 2, 4, 6, 7.


To the second group of questions I have only one remark:
Collisions will always occur if using a shared medium. (if we want to make this question wrong). When I marked this question I was thinking of "sometimes they will occur, sometimes will not, depending on the protocol we use or do not use". --oleamm (discusscontribs) 10:43, 25 October 2013 (UTC)

smalles amount of information

I don't get it: The text says that a bit is the smallest node of information.... and when I click the answer in the second question, the answer is wrong. I might be wrong, but I don't understand: If 1 bit is the smallest amount of data, then it must be the smallest amount that can be transfered, right?

--Stefanbazan (discusscontribs) 12:29, 29 October 2013 (UTC)

you are totally right. Of course I can transfer a bit on a shared medium but this woul result in a broadcast. i could not determin where the bit comes from or who is supposed to be the recipient. So in order to make any meaningfull communication on a shared medium I cannot just start sending data but need a protocol and some meta information. This is what the question leads up to but there might be a chance of better formulating it. --Renepick (discusscontribs) 13:21, 29 October 2013 (UTC)

Information from the video

I thought that text version of the video might be helpful while refreshing the topic. So here is it.

Why Ethernet protocol was developed?

Let’s assume that we have one or more computers that want to communicate. The key is a shared medium. They want to exchange information. As we all know the smallest node of information is 1 bit. So what we can do is to put this 1 bit to the voltage cable in order to transfer.

But if you do this, some problems will arise:

- VOLTAGE TIME

How long do you have to put voltage on the cable?

If you just start putting the voltage on the cable, there will be electromagnetic waves starting to transfer proper data through the cable. The question is how long do you have to do this that others can measure this?

- SENDER

Who is actually a sender of the data?

Let’s imagine 3 computers: A, B and C. We assume that A sends a bit of data. At some point of time B and C receive the data. But the question is who send the data? You can’t figure this out.

- RECIPIENT

Who is the recipient of the data?

If A transfers data then B and C might not know if this data is actually for them.

- TRANSERING SIMULTANIOUSLY

Who is supposed to use medium?

A starts transferring some data then at some point of time B also starts transferring some data. What we have is that C receives corrupted data.

- IS DATA CORRUPT?

The data might get corrupted while being in the cable due to some external events. For example, if cell phone is close to the cable the electromagnetic waves might cause a change. Participants should figure this out.

In order to solve this problem the Ethernet protocol was developed.

So what Ethernet can do is:

• SYNCHRONIZE TIME INTERVALS

Let’s assume computer A sends data at 10 Mbits/s. And computer C has interface that allows 100 Mbits/s. Ethernet as the protocol will be able to synchronize the clocks between these 2 devices.

• SPECIFY RECIPIENT AND SENDER

If A wants to send data for C then B will know that this data is not for him (while C will know that it’s for him).

• DETECT CORRUPTED DATA

If data gets corrupted for some reason then computer C will first see that the data is corrupted.

• DETECT COLLISIONS
If A and B starts to transfer data at the same point of time they know that something went wrong and C will receive corrupted data(because it’s overload by 2 packages).

--Jane Kruch (discusscontribs) 17:51, 26 October 2013 (UTC)

Added your text to the script for this lesson. Just enter "script=" then text and check for the pipe "|" delimiter Methodood (discusscontribs) 14:18, 25 January 2014 (UTC)

test

test

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navigational context


Video
and script
Associated Lesson
Communication over a shared Medium
Ethernet Header
Minimum Package length vs Maximum cable length
Collision Detection
no video Summary, Further readings & Homework

The following video of the flipped classroom associated with this topic are available:

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