Now, let's take a look at each of these principles.

As you will see, they're in no particular order.

But we've structured them in a way to give a logical flow to the course.

If you see a star next to a principle, it means we'll be covering it in this class.

Otherwise, we'll be referring you to the associated textbook.

So our first principle is that sharing is hard.

So what do we mean when we're talking about sharing?

Well, specifically, we're talking about sharing the network medium.

That medium could be the air or a cable. So let's take the idea first of sharing

the air. Suppose each of you and your friends are

on a cell phone and you're in relatively close proximity to one another.

Now if each of you were on a call, and you're transmitting the signal out to

some base station that might be pretty far away.

And we'll look at that in more detail in the first chapter.

But, now the problem is that each of you is on the same air.

So, your signals are naturally going to collide with one another.

And caused interference. Now you're probably thinking, well this

is really bad, right. Because our signals are hitting each

other which is naturally going to cause them to get jumbled up?

But at the same time, shouldn't your cellphone, your talk, be coming into my

phone. Shouldn't this conversation be coming

into my phone? This one, too?

And at the same time, shouldn't all of my conversation be going into the output of

your phone and so on? Shouldn't we all be able to hear each

other's conversations then? Well, clearly you know that that's not

the case, because you've been next to someone, I'm sure.

While you been, both been on the phone before and that doesn't happen.

But why doesn't it happen? I mean, clearly it looks like it should.

Well, that boils us down to whole idea behind sharing.

So network operators have had to deal with different ways of getting us to be

able to share the network medium, and it's kind of similar to if you've ever

shared an ice cream with someone else. So each of you will split the ice cream

cone maybe in half, and You can each eat half of it and then you're both happy.

Now, the problem is that what happens if one or more people want the same ice

cream cone? Then each of you gets a smaller and

smaller share and that's the same idea with sharing the air is that as we have

more and more users, each of us gets a smaller share of that resource.

And so we have to do the same with less. So a similar thing would happen to you if

you're ever on your laptop and you go to a local coffee shop.

If you try to connect to the internet you're trying to go to this point over

here with your transmission that's going to connect you to the rest of

everyone on the outside world. But you've probably noticed that lots of

times your signal when you're sitting in a coffee shop is a lot worse than it is

when you're at home. But why should that be if you're on wifi

in both places? Well, the reason is that a lot of other

people have the same idea. They all want their coffee too.

So they come to the coffee shop and each of you is trying to talk to this point.

[SOUND] And at the same time this point is sending its transmissions back to each

of you. And so, those are naturally going to

collide and cause a lot of lost messages and require devices to have to retransmit

as we say the messages and will look at ways the engineers help make it that.

So let's move on to our second principle now, the idea that consensus is hard.

And again, this will be covered in the material in the course.

So, what I want to start off by doing is considering three different examples of

cases where we have to find consensus, and how it's hard in each case.

First one is ranking set of items. So if we items A, B, C and D, say, we

have to come up with a consensus as to which one is the most important and which

one is the least important in the ranks accordingly.

So maybe B is most important, A is the 2nd, C is the 4th, D is the 3rd.

So while we need some information to be able to come up with this consensus, it

might come in the form of something like this, where we have relationships between

the items. Here those relationships are the forms of

referrals. So if I'm on this smiley face, which is

one of these items, then I could get referred to either the yellow item, the

red item, or the green item. And likewise if I'm on this red item, I

could get referred to this item or this item.

And we could use those referral relationships to establish an idea of

what's called importance. And then we use that importance to come

up with this consensus idea. But clearly, there's no right answer

because there's no saying what we should and shouldn't do with that information.

And also, as we get more and more people involved from more and more items the

answer gets much more more complicated as to what the right consensus is.

The second example I want to consider is the idea of an auction.

And with an auction, we have a number of bidders, maybe we have a few, one, two,

three bidders here, and you can have a single or multiple item.

So I'll draw three items on this side and we want to determine what the proper

matching is between bidder and item and that will depend upon what they bid.

But again there's no, necessarily right answer.

Maybe the matching has a set answer. The highest bidder will get the most

valuable item. Second highest will get second most

valuable, and so on. But then, what do we charge them?

And that's something again, that doesn't have a right answer.

And it gets more complicated as we have more and more bidders.

The third thing I want to consider is something similar.

So, in an option is when we have voting. So, when in a voting system, we have each

person submit a vote which is like a ballot where they rank each of the

candidates. Maybe we have candidates A, B, C and D

and then we have to use these votes here to determine which person wins, which

person loses. So, in this case maybe we say that C

wins. B gets second place.

D gets third place, and A gets fourth. I'm just making this up.

And so again, as we have more and more people voting, and we have more and more

candidates and so on, this is going to get much more complicated.

So this is all great, and now you can see why we're finding consensus in these

situations and why it's hard, but how's this relate to networking?

Well, turns out that companies on the web every single day, like Google and

Wikipedia, actually use these systems to come up with a consensus on things that

we deal with. So, if we consider a standard Google

query. We get a standard results of ten items on

the first page. And let's not look at the colored items

right now, or the items on the side. We'll look at those in a minute.

And these are pages. And Google has to, uses this process

called page rank. Which is very similar to some of the

ideas we were talking about up here conceptually to determine the rank order

of those pages. And so each of the pages has a referral

which is in the form of a hyperlink in which they connect to each other.

And it uses that to establish a notion of importance and rank the pages

accordingly. At the same time, Google uses the idea of

an auction. To determine who gets allocated to which

ad space. And the way that that's done is each

website here, submits a bid to Google, and Google uses those bids to determine

which person gets which ad space. So, someone gets ad space one of over

here, someone gets ad space two and so on.

And again, they have to determine what to price, accordingly.

Now, if we look at Wikipedia on the other hand.

And we look a typically Wikipedia article.

We have a few tabs up here read us the main that we look we're reading fittingly

computer networks. And we also have this Edit tab here were

anyone can go and edit the article as they please.

So as we have more and more people that want to edit this article, we have more

and more opinions, which is good in one way because we have access to more

information but it's bad because then we have more conflicts.

And more people will want to make their edits appear and they will keep rewriting

over each other and then someone has to determine which edits are correct and

which edits aren't. So Wikipedia follows the idea of rough

consensus in determining this. Which is somewhat similar to some of the

ideas that we have in voting theory, so we'll look at voting theory.

Sharing Is Hard & Ranking is Hard

Networks Illustrated: Principles without Calculus

Rencontrer les enseignants