WiFi Standards

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Del curso dictado por Princeton University

Networks Illustrated: Principles without Calculus

39 calificaciones

Princeton University

Networks Illustrated: Principles without Calculus

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

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 WiFi is officially called IEEE 802.11 and that's a technical attribute

standard. And if you look there, you will see the

specifications for WiFi. So the 802 part is for Local Area

Networks or LANs and LANs can either be wireless, like in this case or they can

also be wire line. Like in the case of an Ethernet local

area network. You may have heard of Ethernet before if

you have an Ethernet can able /g, if you're not connecting via WiFi to your

internet right now, then you're, might be connecting directly with an Ethernet

cable. And the .11 part is for wireless.

So 802, as we said, is for local area networks, and then .11 specifies that

it's a wireless Local area network. But nobody really wants to look around

say IEEE 802.11 so people just coined the better term WiFi which stands for

Wireless Fidelity. So, for a while up until around 1999

there were no real standards. Even on WiFi, or nobody was, people would

walk around and say that their devices were WiFi that didn't necessarily conform

to the WiFi standards. So in 1999, the WiFi alliance was set up

to ensure interoperability, and we looked at the word in the last section when we

talked about Europe and adopting GSM as the global standard for over in Europe

for cellular networks. in this case, we mean interoperability

among devices for WiFi. And so if they stamp this symbol on your

device, it means that it conforms the WiFi standard, or a specific WiFi

standard. And what do we mean by specific WiFi

standard? Well, there's more than just 802.11 we

actually cook some of an alphabet soup if you want to look at it that way.They're a

lot of different standards and you may have heard of some of these before:

there's 802.11B, 802.11A, 802.11G, 802.11N.

And these are just four of them, right so WiFi started maybe around 1997 when it

was officially called WiFi. and it came around in a frequency band of

2.4 GHz at first and supported a maximum speed of 2 Mbps.

You can see over the years, the maximum speed has improved, and so let's stop

there for a second and let's say, what do you mean by speed, well We measure speed

in what's called a megabit per second.So, what does that exactly mean?

Well, when we talk about speed, when you're driving your car, you say, miles

per hour, kilometers per hour, meters per second, or however you say it.

What you're saying there, you're saying that you go a certain distance in a

certain amount of time or on average, that's kind of how you're driving right

now. And when we measure speed in a network,

we measure the number of bits that we're sending per second.

So your device will transmit at a certain number of bits per second, and we talked

about how a bit, in the last lecture, is either a zero or a one, just a logical

zero or one. Like in computer code and so when we talk

about bits per second or bps for short, we're saying the number of bits that are

going over the network in one second and that's on average.

So you're sending a number of bits over the network and then in one second you're

just basically probing and saying, well, how many bits are going to pass through

here in one second on average? And we measure in mega bits per second,

because its many, many and many its per second.

Because bit is small little piece and we transmit millions of them in each second

and gigabits per second and now what we can handle, and so we set mega bits per

second with in millions and just like when we say megahertz we meant millions

of hertz, and, and so on. And so, then in 1999 we went to 802.11b

and that was the first official [UNKNOWN] standard still at 2.4 gigahertz and went

up to 11 megabits per second. And so, these are mostly in alphabetical

order, there's a little variation there, for the most part it is, but here's one

example. b came out and then actually a came out

with an even higher data rate of 54 Mbps at a different frequency range, in the

same year, so they found out they could transmit much, much higher in that

frequency range, and then g came out in 2003.

And that was still at 54, but it was down then at the 2.4 GHz range and then as of

2009, we have 802.11n, which is the most commonly used right now, and that's in

both 2.4 and around 5 GHz range, and that supports up to a maximum speed of 100

Mbps. And, clearly that will vary, as we will

see, so you're not always going to get 100 Mbps, it'll depend on who else is on

the network, and so on. And so, as the data rates improved over

the years, the number of devices that wanted to access WiFi went way up.

So back in 2009, when IEEE 802.11n came out so 802.11n came out, we had somewhere

around 600 million devices, and then by 2015, it's projected that that's going to

go up to 2.2 billion. So as the data rates go up, there's more

and more of a demand. We saw a very similar trend with cellular

networks.

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