Welcome to lecture eighteen of Networks, Friends, Money, and Bytes.

And today's question is, why is WiFi faster at my home then at hot spots?

And this might be a question that many of us are wondering about.

And the root cause, in two bullets, is that.

The interference managing system used in WiFi does not scale well as the number of

stations or user devices increases even from just several to just say ten or a

dozen. In other words, the trajedy of the

commons which we observed in many different social and tech networks is not

efficiently litigated in the case of WiFi by what they use as the random access

method as soon as the crowd is reasonably big.

For example, in a hotspot. And then increasingly perhaps at home as

the number of WiFi devices active at the same time increases at home too.

In order to get into the more complete answer, we have to look at a little bit

of the history of WiFi. Okay?

Now WiFi started in the mid to late 90's in the standardization body by IEEE.

Now, you have seen quite a few different standardization bodies names such as

IETF, mostly for internet, type of standards.

You've seen ITU, and you've seen, in cellular world, so called third

generation, standardization body, 3GPP and 3GPP2.

And, indeed, there are quite a few other standardization bodies such as MPAC Such

as DSL forum, Wimax forum. Some of them are less restrictive than

others in terms of the standards that they issue.

Now IEEE is the one that's responsible for a lot of local area networks.

In a Homer problem, back in lecture thirteen, we looked at wire lined local

area network, the Ethernet. And, the switching method over therefore,

detecting the spanning tree distributedly.

Now, WiFi is really a wireless version of local area network.

So, sometimes it's also called wireless LAN, okay.

And then as the first and the second generation of cell owner standards took

off, people in the community wondered, maybe we can invent another,

complimentary, type of wireless assistance.

Now today there are billions of WiFi devices, and hundreds of millions are

added every year around the world. And they are used at home for WiFi

connectivity. Imagine the number of wires you have to

put together in your home if you didn't have WiFi.

It's used in the office extensively, and it's also used in public hot-spots.

Which could be hotel, airport, coffee shop,

and so on. Now, similar to a cellular network, WiFi

also used a fixed architecture. We'll see in a picture soon that there is

something like a base station called access points, okay.

And they are connected among themselves by some kind of wires.

But there are also crucial differences between WiFi and cellular.

For example, in WiFi, the range of communication is much shorter, and the

allowed transmission power is also smaller, okay.

So, this short range is something around, say, a 100 meters.

There's little mobility support. If you move between, say, two access

points controlled by same controller, may be that's okay.

But, in general, as soon as you move away from one WiFi-AP access pointer then,

then you lose connectivity. You have to re-associate again and the

session may drop. But, the biggest the difference might be

the underlying physical media in cellular what they use is a licensed spectrum,

okay? You have to pay billions of dollars to

the government to acquire the right to transmit in that frequency range.

But the WiFi they use so called unlicensed spectrum, okay?

There are quite a few different unlicensed spectrum, one is the so called

ISM band, industrial, science and medical frequency.

And there is one, in the so called S band.

And that is around 2.4 gigahertz. And the other in the so called c band

around 5.8 gigahertz. All these are unlicensed meaning you do

not have to pay the government or anybody any money.

You can just go ahead and use it. Now in fact in the 2.4 gig which is where

WiFi started in the 90's the dominate usage before WiFi was actually microwave.

Good thing that you don't have to pay government money in order to microwave

popcorns. And, then WiFi community said that, you

know what? Let's use this, two bands of unlicensed

spectrum, starting with 2.4 and getting. So here is a typical topology of a wifi

deployment. In your home, hotspot, and so on.

As I mentioned there is something called the access point, AP.

This is similar to a base station, BS, back in lecture one.

So, this is lecture eighteen and we've come a long way. Now we're back to the

wireless world. And then there are a lot of in use

devices. Almost all devices have a WiFi connection

today. Laptop, desktop, TV, games console, of

course your phone, your tablet. And they communicate wirelessly denoted

by these dotted lines, to the access point.

Now how do they find the right access point to talk to?

That is one question we'll answer later. And this is called the basic service net.

Bss. Okay.

And a bunch of BSS can form what's called extended service net.

Ess. These AP's are then connected among

themselves through some wires. For example Ethernet.

And then tapped into the access network, which then further gets into the cloud

into the rest of the internet. So, this is similar to a base station and

a mobile station. And, a base station is sometimes called

Node B or evolvable E-node B in different generations of cellular.

But, Idea is that there is one hub wireless followed by wire line access.

However, as we mentioned that, spectrum is un-liaison, range is short and

mobility support is limited. And this is the underlying spectrum

around the 2.4 gig. There is slight difference in the way the

channels are decided in chopping up the frequency bands across different

countries. In the US, here is the typical picture.

Okay, you've got eleven channels. One, two, six, up to eleven.

Okay? The middle channel six is centered around

2.437 [COUGH] gigahertz. And the width of this channel is 22

megahertz, with a little guard band of five megahertz on the,

on this side, okay? So, I should say, this is not joined to

scale, okay? You can imagine, three, four, five, and

so on, so forth in between, okay. So as you can see that a lot of these

channels actually are overlapped. Okay?

With the signal processing's help you can take care of some of that overlap.

But if you want truly orthogonal frequency bands then you actually need to

say I can only use channels one, six, and eleven.

These are the three channels that are truly non-overlapping in the frequency

band. Okay.

So there are eleven channels but three that are truly orthogonal.

Now the question, We're going to try to address first are

the following, okay. Terminology wise, is wi fi the same as 82

dot eleven? Yes.

So 82 dot eleven is the IEEE standardization body, okay, for local

area network, and. Wifi is just a sharper name.

Wireless fidelity was the original name, now where a few people remember that, and

they just call it WiFi now. So imagine if you called your home

connectivity I triple E, A two, two dot eleven.

You know, that's not as, attractive, as to say, Hey, we got a WiFi here.

But what about this business of 11. a and b, and g and n, and e and h and r.

Sounds like a very lazy way to do alphabet soup.

What these are just different subclasses of the.11 family.

It started actually with b. Oh, of course they tried to start it with

a first but the standardization around family b gelled before that of a.

So, .11b back in 1998 for example, that was the only family available if you want

to buy a wifi product. And this used two point four gig.

And it can give you a reasonable data rate upto one, eleven megabit per second,

upto okay? We will see that if when the interference

is strong then you can go all the way down to one megabit per second but in

1998, eleven in the best case was already pretty impressive and then came along G

which still uses 2.4 gig but with more advance the signal processing can give

you up to 54. [INAUDIBLE] per second.

Again, up to 54. And then A finally came along.

It also can achieve 54 mega bit per second, but it uses a different spectrum.

The 5.8 gigahertz range. Okay?

So the radio analog front end requires a different kind of chips.

So A, B, G were sort of the dominant three variance in mid 2000's and so.

In the last several years there is the superleferation of dot 11N, and now we

use even more fancy signal processing. Including something called multiple

input, multiple output. It's really, multiple entenours at both

the axis point say two, as well as your device.

Say two. In next lecture, eight, nineteen, we'll

talk a little about the throughput implication of using the mymode devices.

Together with earlier advances such as so called OFDM Orthogonal frequency-division

multiplexing. This is not a wireless signal processing

class so we wont go in to that detail but together with all these techniques people

are able to take this speed from a max of 54 all the way to say around 300 megabit

per second. Okay, again, ought to.

But typically you can observe something around 100 megabit per second, maybe half

of that, but still that's very impressive.

In fact, if you look at, in many cases, the wired backbone of the.

Wifi network. They don't have more than say 50 megabit

per second. So the air interface is already not the

weakest link any more as soon as thyroin N is used.

So now people are talking about using the 11N as a way to do very high speed

transmission within a home even for transmitting HD TV signals.

And then there are other kinds of letters associated with eleven family.

Okay. And these are not different through put

classes anymore. But are I mean, different kinds of

enhancement. of the 11N. family.

For example, e and h and r, they give you better quality of service, they give you

better security, and better roaming and mobility support.

Now there are many interesting stories in recent days.

For example, once Steve Jobs announced iPhone four,

Couple of years ago, he couldn't get on the WiFi network inside the auditorium so

he had to ask everybody else please get off the WiFi and then he was able to get

on the WiFi. In 2011 of summer, the Korean government

announced that the City of Seoul will be blanketed by WiFi around year 2015, okay?

So every street corner will be provided with a WiFi capability.

And then now, as I mentioned a lot of manufacturers and service providers are

talking about getting rid of the wire. So from the residential gateway, all the

way to the HD quality setter box on your TV, using WiFi.

Now you may have also seen quite a few annoying things about WiFi.

One is that you cannot get on. Sometimes you know why you can't because

there's a lock and you don't have the pass code.

Sometimes there's no law, but sometimes you can't get on.

Okay. Part of that is because of the backbone

issues. For example, by backbone might be

congested or there usually is a some good DHCP or there usuall is some good DHCP

server that can rent you an IP address for a period of time,

and without IP address you can't do much. So we talk briefly about this in lecture

eleven in lecture thirteen and more in the next lecture.

Sometimes you say that,"Hey the performance is very poor.

Poor performance. Very low fruper.

For example, on the train.

Okay? Like Amtrak in the U.S.

They announced a couple of years ago they will provide onboard WiFi, I tried many

times, either I can't get on or get on and then get a 50 kilobits per second.

So where is my whatever, 100 megabit per second, where is my ten where is my one,

where is 0.1 megabit per second. While I was getting such a low speed I

really cannot use it. Sometimes you will see that hey just at

certain time of the days for example 8:00 p.m.

on every weekday, suddenly it doesn't work very well.

Now, again, part of that could be backbone congestion.

But part of that could be your neighbors, especially living in the high-rise

building, turn up their WiFi as they get back home.

Now a lot of homes have something like ten or so WiFi devices, that are all

active. Then you see a lot of interference.

So, our question is, how can we understand and tackle lot of these

performance issues. Now, part of the problems stand from, the

fact that WiFi has very limited radio planning.

By planning, I mean radio planning, okay. First of all, it's unlicensed band, so

it's just hard to plan, okay. Anybody can stick an AP over there.

It could be your service provider, it could be your neighbor You can't really

control that. Okay.

So some people say, I'm just give up control, trying to plan.

Instead of doing that, I'll just say, you know, lets just open up a box and, and do

it. Now compare that to cellular world.

The spectrum is very expensive. You need to have the legal right to

transmit over there and the equipment is also much more expensive.

It's not 50 bucks box of AEP. So it is tightly controlled by the

cellular providers and they do very careful rating and planning, where and

how to put their antenna together, okay. Second is that there's very light

management. Unlike the Internet or the telephone

cable were where there's a lot of management going on traditionally within

the network. there's very light management going on

for Wi-Fi okay. Very little stable tools for root cause

analysis. And very few signal, signaling channel on

the control plane to control and adapt the Wi-Fi's.

But the situation might be changing over the next few years.

The backlog might be the bottle neck, either because the DHCP server doesn't

give you an IP address for example, certain campus networks banned on iPhone

because their DHCP server has a bug relating to iPhone wants to give you an

IP address, it won't be able to get it back.

So they say, alright, no more iPhone on our WiFi.

Sometimes it's because of the air interface.

Because of interference, for example. And that is the heart of this lecture.

So what are the degrees of freedom that you can use to tackle these, these

problems? Well, starting with the first, order one,

which is find the right access point to associate this WiFi device.

What. Part of that is carried out by sending

beaconing signal and comparing the kind of signal strength that you can get from

diff, from different AP's. Then you have to select the right

channel. Okay we mentioned for example in.11B okay

there areonly three truly orthogonal channels and that's one, six, and eleven.

Now unfortunately most enterprise or residential customers, they just open up

a box and the default channel by the manufacturer say you know,

Linksys would say that it's all channel six for this batch of, APs.

Then, right there, you got many APs transmitting in the same channel, even

though they co-spread themselves around the different channels.

But, many residential and quite a few, percentage of the enterprise customers

simply do not adjust that. And then, there's the rate selection.

'Kay, for example, should I transmit at one megabit per second?

Or eleven megabit per second? And somewhere in between?

So the question this This question depends on what kind of

interference condition are you experiencing.

Sometimes you try to be to aggressive sending a to high rate even though the

medium is bad. For example in a noisy cocktail if you

talk to much like what I'm doing right now then the other end may not hear you

clearly. You say well bad channel let me talk very

slow. And maybe louder as well.

Right? Let's say you get the right AP.

You do some smart channel array selection.

Then what? Then is the problem of collision.

Okay, collision of frames. And this will be picked up in the next

module of the video.

Numerical Example and Summary

网络：朋友、金钱和比特

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