Controlled vs. Random Access

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Del curso dictado por Princeton University

Networks Illustrated: Principles without Calculus

40 calificaciones

Princeton University

Networks Illustrated: Principles without Calculus

40 calificaciones

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.

De la lección

Random Access in Wifi Networks

In this lesson, we will investigate WiFi, another type of wireless network. Rather than having stringent power control algorithms as we saw for cellular, WiFi relies on "random access" methods to manage interference among users in the same location.

Unlicensed Spectrum5:01

Traffic Analogy4:43

WiFi Standards5:34

WiFi Deployment6:24

Accessing WiFi10:55

Interference7:49

Controlled vs. Random Access6:49

Random Access Protocols & ALOHA7:00

ALOHA Successful Transmission5:15

ALOHA Throughput7:12

ALOHA Inscalability8:19

CSMA Carrier Sensing5:54

CSMA Backoff7:49

CSMA vs. ALOHA5:41

Conoce a los instructores

Christopher Brinton
Lecturer
Electrical Engineering
Mung Chiang
Professor
Electrical Engineering

So this brings us now to the term called Medium Access Control or MAC.

And we'll see that in the internet, MAC is a layer in what's called the Protocol

Stack. And we won't concern ourselves with that

yet. But just know that that is something that

the internet does. And it's a task managed by the internet.

And MAC defines how people share the network medium, right, so it's defining

how people can access the network. There's two different types of access out

there. There's Controlled Access, which is what

we looked at with [UNKNOWN], FDMA, TDMA, CDMA.

That's controlling, assigning, regulating, any of those synonyms, it's

doing that, it's controlling how people are getting on the medium.

Then there's what's called Random Access which is the way the Wi-Fi works.

So under control, we have our cellular and our regulated ways and under Random

Access, we, it's a different set of protocols.

They are all unified under MAC, so they are both types of MAC but it's just

better controlled or Random Access. So we can come back to our cocktail party

analogy to see the differences here again.

And with CDMA, as we said last time in the last lecture.

when you're at a cocktail party. If the host was having everyone speak

according to a CDMA standard. Which is kind of weird because when you

go to a cocktail party hopefully you're not talking about CDMA necessarily.

But they would ask people to adjust their volumes basically.

Right? Which is really the way that a cocktail

party would probably work is that everyone would be talking at the same

times. You'd just be, oh, adjusting your volume

so that no one was overpowering another person's conversation.

TDMA would be a really strange way to run a party.

It's when you assign people different time slots on which they would speak.

So, this person would be able to talk in his conversation then you'd get this next

group of people. It's hard to tell who's talking to who in

this picture but you get the idea. And then under WiFi.

WiFi itself isn't, you know, a multiple access like, we'll see more of those in a

minute. But with WiFi, you're allowed to speak in

this cocktail party. You'd be allowed to speak, as long as

nobody else wants to. There's no set rule.

Like there's no time slot that you're being assigned like in TDMA.

No one's telling you how you adjust your volume.

You just listen to say, okay, is there another conversation going on?

If not, then you'd say, well okay, then I can start talking.

That's the idea. So you just listened, you have some

sensing protocol, you listen. And then you can talk as long as nobody

as else is. And we allude back again to our stop

light versus our stop sign analogy. when you're sitting at the stop light

that's a way of controlling access to the intersection through the stop light.

Where as the stop sign is more so along the lines of random access where a

person's just looking and then going. And so they make their own decision in

the end. It's ultimately up to them whether

they're going to tran... Whether they're going to go or not.

So let's think a little deeper into Random Access, let's look at this example

over here. So, in these diagrams that we draw again

these circles are denoting the transmission ranges.

And here we have two access points D and E.

But note that the access points themselves also have to talk back to the

devices. So, they're really both transmitters and

both receivers. Just like we talked about in the last

chapter, the idea of a link. And then the other thing we should

mention is that there is a difference between the transmission range, the

sensing range and the interfering range. So, those are three different things that

you can there's a lot of research that goes into defining what they are and how

large they are. But for our purposes here, we're not

going to really make any distinction between them.

Just intuitively, if someone's in your transmission range; like when you're

speaking out loud, if somebody can hear you, it means they're near a transmission

range. So, you would know that B for instance is

in D's transmission range as we can see right now.

And C is in E's transmission range as well and A is also in there too.

And in terms of interference is if their transmission range collide at all.

So, at all. So, if when you transmit, the other

persons transmission can intersect with you whether that's happening at the

receiver itself or before you get to the receiver.

That will cause a collision and we're assuming that collisions in the worst

case scenario always going to cause both of the frames to be lost.

Again, these are assumptions and they're not always true in practice.

But it's going to simplify our analysis a lot in what we're going to be looking at.

And so, in this diagram right here, as we can see, there's always different

transmitters. Right?

And they're all, all their transmission ranges would intersect, if we drew the

circles out. We could draw the circles around C.

We could draw it around B, D, A and E. And, at most one of them can be

transmitting it at any given time. Because they're all going to interfere

with each other. Even though D and A are on a different

access point than C and E, they're still going to collide because they're all

within each others transmission ranges, alright.

So, at the beginning of every time slot, a device has to ask itself, should I

transmit or not? That's the question that they have to

answer. Do I transmit in this time slot?

Or do I not transmit in this time slot? And different Random Access protocols

will dictate how they answer that question.

So, if we look at this diagram right here.

At the bottom right here we have different time slots 1, 2, 3, and 4.

And for purposes of our discussion we will make another assumption which is

that each of these time slots. If someone decides to transmit, the frame

will be present for the entire time slot. So we can look at these different time

slots as being our standards of time here, which will make our analysis more

easy to lay out. So, in any one of these time slots, I

mean, let's suppose in the first half of it just A transmits.

So A decides that it wants to send a frame to D.

That's it. B and C don't decide, don't want to

transmit at all. So that's fine and A's frame will get

there. Everything will be okay.

Now, in B, in time slot 2, both B and C decide that they want to transmit at the

same time. But if B and C are transmitting, they're

going to collide with one another. And therefore, you're going to have a

lost frame. Now, in time slot 3, nobody decides to

transmit. So, we can call that, what we say, a

wasted opportunity. So someone could of transmitted if they

wanted but nobody decided to by virtue of the medium access protocol we're using.

And in the fourth time slot then, A, B and C all transmit, and again if any more

than one person's transmitting, we're going to have a lost frame.

So in that case the frame would be lost.

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