Unlicensed Spectrum

Loading...

Do curso por Princeton University

Networks Illustrated: Principles without Calculus

40 classificações

Princeton University

Networks Illustrated: Principles without Calculus

40 classificações

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. Please note that per Princeton University policy, no certificates, credentials or reports are awarded in connection with this course.

Na lição

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

Conheça os instrutores

Christopher Brinton
Lecturer
Electrical Engineering
Mung Chiang
Professor
Electrical Engineering

Hello and welcome back. Today we're going to continue with the

principle that sharing is hard, which we looked at last time with cellular

networks. Except today, we're going to focus on

WiFi networks. And we're going to answer the question

why WiFi is faster at home than it is at a hotspot, which you probably noticed for

one time or another. And if you stay at home, sometimes you

get much better internet connection on WiFi than if you go to Starbucks,

especially if it's crowded at wherever, whichever hotspot you're at, like for

instance, a coffee shop. So we'll start off today by talking about

unlicensed spectrum. And so if revert to what we were talking

about in the last lecture with cellular we saw that all the cellular spectrum

operated on licensed bands. So all of your 3 or 4G networks all

require licenses to operate. And your cellular provider needs to have

a license to operate in those bands and to allocate you those frequencies.

So in 1985, the FCC started to release some unlicensed bands.

And a large part of reason for this is as, you know, the process [UNKNOWN] FCC

in trying to terms of additional inspection took lots and lots of money,

millions and millions of dollars, and a very, very long time.

So this is especially inconvenient for any researchers who wanted to operate in

these bands, especially in non-closed proximities.

So if you're even a researcher, just as a side note, you can, as long as it's in a

contained area and you're not actually projecting it out into the atmosphere,

which we all have to share. But if you're trying to do any type of

research, and you want some real world scenarios, it was very inconvenient.

And so the FCC, we saw this logo last time, initially released this as the ISM,

the Industrial Science and Medical bands. So it was for use mainly for the ISM, for

these fields, like for in hospitals, for medical or any science or an industry.

and they were centered around 2.4 and 5.8 gigahertz.

And engineers quickly seized this opportunity, so even though it was, it

was pushed out with the idea being used for people on industries of science and

medicine. And engineers were very quick because

they wanted to do research in these bands and try to use it for these different

purposes. [LAUGH] So, in fact, the most, the most

commonly encountered device that operates in these bands is the microwave oven, and

that's where it gets the name microwave from.

Because these are at very high frequencies, at 2.4 and 5.8 GHz, and we

won't get into detail on why those are called microwave or what the microwave

band is, for instance, but these are microwave range frequencies and you can

look at that, for instance, more in your Wikis if you want.

But we're talking low low gigahertz range like 2.4 or 5.8.

Remember a gigahertz is 10 to the 9th or a billion Hertz, so this is 5.8 billion

hertz. And so these bands as engineers started

to seize this opportunity became increasingly used for WiFi devices.

So you're probably wondering, well why didn't they just use them as additional

spectrum for cellular? Well the reason is that we came to a

fundamentally different paradigm and a fundamentally different way of thinking

for specific devices in close proximity. Okay.

And this is WiFi, as we know. So it's not the same setup as cellular

anymore. These WiFi devices are used for different

setup and the setup meaning that they're closer in proximity to one another.

So for instance if you have laptop or a smartphone or a tablet or any other

devices that can. Operate on WiFi.

As you notice you generally have to be pretty close to the access point.

So WiFi is typically used for a stationary devices or it's ideally used

for a devices that aren't moving around a lot as well as devices that are close to

the access point. So the reason we didn't allocate these

for cellular is because engineers started to think, well, for devices that are

stationary and close to the access point, why do they have to go, do power control

with all the other cell phones that are out and about anywhere else?

Why can't we just have different WiFi or small different sections where they can

access the Internet differently? And that was the fundamental idea behind

WiFi which is, still permeates to the present day.

And as you probably know, if you've ever gone to a Starbucks or anywhere before,

that as you have more and more people around it, it gets harder and harder to

get a good data rate. And the data rate on WiFi fluctuates

quite a bit. And we'll talk about why that is exactly,

but as more and more people come, the data rate tends to go down a lot.

And so its not scalable, in the sense that it can't go up to a high number of

people like we can do in cellular. We can have many, many more devices in a

cellular region but with WiFi, you know, if you go above.

Even ten devices is getting a lot, in one specific access point, the data rates can

be pretty slow.

Explore nosso catálogo

Registre-se gratuitamente e obtenha recomendações, atualizações e ofertas personalizadas.

O Coursera proporciona acesso universal à melhor educação do mundo fazendo parcerias com as melhores universidades e organizações para oferecer cursos on-line.

© 2018 Coursera Inc. Todos os direitos reservados.

Baixar na App Store

Baixar no Google Play