Sharing revisited

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来自 Princeton University 的课程

Networks Illustrated: Principles without Calculus

43 个评分

Princeton University

Networks Illustrated: Principles without Calculus

43 个评分

What makes WiFi faster at home than at a coffee shop? How does Google order its search results from the trillions of webpages on the Internet? Why does Verizon charge $15 for every GB of data we use? Is it really true that we are connected in six social steps or less? These are just a few of the many intriguing questions we can ask about the social and technical networks that form integral parts of our daily lives. This course is about exploring the answers, using a language that anyone can understand. We will focus on fundamental principles like “sharing is hard”, “crowds are wise”, and “network of networks” that have guided the design and sustainability of today’s networks, and summarize the theories behind everything from the social connections we make on platforms like Facebook to the technology upon which these websites run. Unlike other networking courses, the mathematics included here are no more complicated than adding and multiplying numbers. While mathematical details are necessary to fully specify the algorithms and systems we investigate, they are not required to understand the main ideas. We use illustrations, analogies, and anecdotes about networks as pedagogical tools in lieu of detailed equations. All the features of this course are available for free. It does not offer a certificate upon completion.

从本节课中

Routing Traffic through the Internet

It is hard to overstate the impact that the Internet has had on society. In this lesson, we will overview the fundamental concepts behind the way the Internet is designed. We will also take a look at routing, which is the process of determining how packets of information are transported.

与讲师见面

Christopher Brinton
Lecturer
Electrical Engineering
Mung Chiang
Professor
Electrical Engineering

Hello and welcome back once again.

Today we're going to look at how your

Internet gets messages where they need to go.

So we're going to switch actually now and

talk about technical networks for this lecture as

well as the last one for the course which will be also on the Internet.

and the recurring theme here, as you will see,

is that the Internet is really a network of networks.

So it's composed of a bunch of sub-networks.

So, to start off,

we, we're going to revisit the idea of sharing, which takes us all the

way back to when we were discussing cellular networks, in the first lecture.

So, we talked that FDMA, TDMA, and CDMA right?

They're divisions in either frequency, time, or codes in

terms of allocating users' sum of the network resources.

So, those are dedicating resources, so the way that

we're sharing the network medium is by dedicating resources.

Right?

Whether those resources are frequency, time, or code, it's the same thing.

That's how two sessions are differentiated from one another or two links

are differentiated from one another, in

terms of the resources that they're assigned.

And this is just a diagram to show that here, we have two senders, Senders 1 and

2, and then Receiver 1 and 2, which are

the recipients, again, of whatever information that we're sending.

We're showing this here as as a wire.

Right, this is a wire.

It could also be the air, it doesn't have to be a wire.

but basically the idea is that over time, right?

So time is actually coming down here. Um,

[INAUDIBLE]

first Sender 1 has the time slot, then Sender 2 has the time slot,

then Sender 1 would have it again and Sender 2 would have it again.

Right.

So during that time, the entire link is allocated

just to Sender 1, this is a TDMA scheme.

We could also make this frequency or we can make it codes, as well.

But TDMA sometimes just makes the most sense.

So that's the idea here.

But there's one

negative to this, is that we're not using all the resources constantly, right?

So, for instance, if you look at this right here.

If Sender 1, this is Sender 1's term in time.

Sender one doesn't have anything to send right now, okay?

So this is just going to be an empty or a wasted link.

So, Sender 1 doesn't need to send anything during this time slot.

And Sender 2 has something to send then.

Then why wouldn't you just have let center 2 send it?

Well, we can't do that because the

way we're distinguishing users is by dedicating resources.

Okay.

So the way that we distinguish them is by knowing that during

that time period, the entire session is just allocated to that user.

Right. This is what we call circuit switching.

Okay?

This is a circuit switch network, and each of these

are circuits, so each, each of these time slots are circuits.

There is a paradigm shift, around the time when people

start to think about the Internet, to what's called packet switching.

So packet

switching is different from circuit switching, it's the way

that we share the network is by sharing the resources.

Under circuit switching, we share the network by dedicating the resources.

Under packet switching, we let everyone share the resources all the time.

This is how it looks, okay?

We divide the messages into smaller pieces called packets, and we'll

talk more about what packets are in the next lecture, actually.

But just suffice it to say, it's just a really, it's just a small

piece of a message.

Under packet switching, we don't dedicate any circuits or anything like that.

We allow everybody to use the network medium, right?

And they can send, right?

And they can use the time slots accordingly.

So you see, we don't have any wasted space here, because any of the time

that Sender 1 or Sender 2 isn't sending, they can be filled in by Sender 2.

[INAUDIBLE]

anything or Sender 1 could fill in the time slot for Sender 2.

So we don't dedicate anything.

We just allow everyone to share all the resources.

And we call a session here because I've, I've been using the term session.

Session is really an application level.

So user and session, we'll use interchangeably

but they're not exactly the same thing, right?

So the session would be, you'd open up a

session between, for instance, your computer and my computer.

Like if we were on Facebook messaging so those

should be a session between my Facebook application and

your Facebook application.

We could also have another session opened if

we were talking on instant messenger somewhere else.

then those would be two different sessions.

Right.

But we're not, we're going to say that would be one user with two sessions

but we're just going to call users and sessions are the same thing here.

And we're also going to assume the sessions are

unicast, meaning there's one source and one destination.

So I'm not going to send to

multiple people at once which would be multicasting.

we're just going to assume that everything is unicasting.

So with packet switching

here you can see that we're cutting

everything up into finer grains and we're not

dedicating the network at any given time to

anyone or frequency or code or anything else.

We're letting everybody share all of the network resources by allowing

them to pour their packets, so to speak, into the network.

And then, allowing the network really to deal with things

like congestion, right or deal with how to get this packet.

So how do I know this packet is meant for this

receiver if there's a whole bunch of other packets in the network.

Well, we'll look at all that.

It requires us to do different ways in terms of

addressing, right, to be able to actually address this receiver.

And so that's the main idea behind packet switching.

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