You pick up your iPhone while waiting in line at a coffee shop. You google a not-so-famous actor, get linked to a Wikipedia entry listing his recent movies and popular YouTube clips of several of them. You check out user reviews on Amazon and pick one, download that movie on BitTorrent or stream that in Netflix. But suddenly the WiFi logo on your phone is gone and you're on 3G. Video quality starts to degrade, but you don't know if it's the server getting crowded or the Internet is congested somewhere. In any case, it costs you $10 per Gigabyte, and you decide to stop watching the movie, and instead multitask between sending tweets and calling your friend on Skype, while songs stream from iCloud to your phone. You're happy with the call quality, but get a little irritated when you see there're no new followers on Twitter. You may wonder how they all kind of work, and why sometimes they don't. Take a look at the list of 20 questions below. Each question is selected not just for its relevance to our daily lives, but also for the core concepts in the field of networking illustrated by its answers. This course is about formulating and answering these 20 questions. All the features of this course are available for free. It does not offer a certificate upon completion.
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来自 Princeton University 的课程
Networks: Friends, Money, and Bytes
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从本节课中
Why is WiFi Faster at Home Than at Hotspot?
WiFi hotspots have become an essential feature of our wireless lifestyle. In this lecture, we will study WiFi, and focus specifically on common link layer protocols that are used to manage interference. In doing so, we will see why WiFi does not scale well beyond several devices sharing one access point.
与讲师见面
Mung ChiangProfessor
Electrical Engineering
Christopher BrintonLecturer
Electrical Engineering
The first major type is called IPTV, now IP stands for internet protocol in the
network layer of the protocol stack. Okay.
And TV is just TV okay, television. So, while we say this is an IPTV service,
we really mean that it is delivered over a private and managed network that runs
IP protocol. But nonetheless, is not the public
internet in its entirety. And they used part of the public internet
but it consists of a private and managed network that is paid for, deployed, by an
internet service provider, okay? An ISP.
Could be a cable company, could be a phone company, so on.
And often there is a set-top box, STB, on the consumer premises.
And in the old days, okay, TV was delivered by broadcast over the air, by
satellites, or through cables. So why is this switching now to IP
technology? And this is a, a powerful example of the
phenomena of IP convergence that happened in the early part of the the century.
One reason is cost reduction, okay as you consolidate uniform platform based on IP,
then it can reduce cost. Another is flexibility, as we have seen
IP's greatest strength is that it's ability to support diverse applications
and the needs arriving in the future. And the third is that the compression
technology has gotten better and better over the last twenty years, for video,
such that at this point access network capacity can tolerate video, TV
distribution. So that is the switch from the
traditional satellite or over the air into, IP-based TV.
And this is a, typical picture, showing the, main component.
This is a home, another home, another home.
This one might be using a cable, modem, okay,
through a, customer premise, equipment CPE.
And that's connected into a set-top box on, your TV, and then, a WiFi gateway
for, the rest of your data traffic, in, your home.
And this one's connected through a passive optical network, okay.
So the physical medium is not a cable line but a fiber optic line.
And this goes into, an optical network unit which again is connected to, TV and
to WiFi gateway. And this one, goes through DSL, which is
a copper-based broadband access, into, a DSL modem, which in turn goes to set-top
box and WiFi gateway. Now next lecture we'll talk a lot about
these WiFi, including in-home and public area WiFi access points.
Then. This is the access network part okay, it
may cover in each distribution area somewhere between you know, around 32 to
128 kind of houses. And this, then, goes into, metro and then
backbone network. Skipping quite a few hops, we reach, a
media serving head end, and this head end may be connected to a local antenna that
can receive signals from local TV stations.
And these are then connected into a super head end.
usually there are just a few in a country like US, which has a big satellite dish
that can get the, content signal distributed via satellites.
And you, as you can see that, there are lot of equipments, both on access part
and also on the, backbone content on the very prior to this, deployed by a company
such as cable or phone company. In contrast, another way of using the
Internet for video, is VOI, Video over the Internet.
In this case all you need is a big data pipe, leading into, the user.
And then they can view application overlays to deliver the video you want.
So this is delivered over a public network, okay.
Or, in, perhaps some strict sense of the word, over, the Internet, okay.
It is not delivered, over, a managed private network.
And there are few variances, variations, one is a client server based
architecture. For example, YouTube or say ABC, BBC and
so on. Okay.
And they don't charge you a fee, they have other business model to support
their operation. Another par, possibility is CLI server
base architecture. But they will charge you a fee.
For example, Amazon Prime or Hulu Plus, or HBO Go, or Netflix.
So they will may charge you a monthly or annual fee.
And the third type is a P2P. And often they are free.
For example, Bit Torrent or PPLives. So there are different business revenue
model for them, and they have to also pay.
Even though they don't have to build their own network, they do have to pay
for the content they are delivering. Okay? Often, these companies have to
strike some special deal with the owner of the content.
They have to still buy servers. We'll see both content servers and
control servers. They may have to lease or buy network
capacity. They may also roll out some customer
premise equipments, although they may also just leverage existing mobile
devices, like your phone or tablet. And then, of course, they have to pay for
the software that monitors the services and configures the video delivery.
Now whether you're talking about IPTV or, video over the internet as we see here in
this picture okay with a source of the video could be your own camera, okay, or
could be your phone and then uploading through the internet to a media server.
Okay? For example, Facebook server, okay?
Then through the internet it is cached in different places and ready to get play
out. Then through the internet people want to
view the video will grab those content. Okay?
Either way, they have to face the question of quality of video.
Unlike just other kind of data traffic video is something that people can
visually experience. So, they care about, the quality which
consists of quite a few components. Part of quality is determined by bit
rate, slash distortion, because usually, everything else, at constant higher bit
rate would give you, less distortion. But the relationship is not linear.
Okay, it's not like I double my bit rate and I will always have the distortion
metric, whatever that might be. We'll see an example later in this
lecture. Okay. It is not a linear relationship.
Okay, the kind of relationship it shows up is called a ray distortion curve.
And the ray distortion curve, R as a function of D, we will see that in a
minute. And the kind of bit rate that's required
depends on quite a few things. For example, the amount of motion in the
picture. If there's, too much, motion in a media,
if it's an action film, then, you'd demand higher quality.
If it's a talk show, then, a lower bit rate will still give you a similar visual
impact. It depends on the screen resolution.
Okay? if you're upgrading to retina display, then, your eyes will be spoiled
and say, hey, I can't go back to look at the original screen.
Once you're used to HD on your big TV screen, then you can't go back.
Another key element is the ratio between viewing distance and the screen size.
If you view it too closely, A it's not good for us and B it's actually going to
detect some kind of defect. You'll see the pixelation and so on, no
uniformity of the cutters but most people don't view it that way.
It's not pleasant in any case. So there is a reasonable ratio between
these two numbers you'll will ping ting. Another factor is the efficiency of
compression okay. If you have a more advanced a smarter
more efficient compression as you will see in a minute than you can get a lower
bit rate for the same amount of distortion.
Or alternatively, use the same bit rate to get much lower distortion.
It also depends on, the kind of content we're talking about, okay.
For standard definition, we usually need something like one megabit per second,
one to two megabits per second for a, reasonable combination of these factors.
And a typical movie would take something like one to two gigabyte.
Now for HD, however. This is SD.
HD. We need usually at least six to eight
megabit per second. but if you want a true HD quality then
usually you need something more like 20 to 25 megabit per second.
And a typical movie at this bit rate, it would take something like 5 to 8
gigabyte, for HD. And then now people are talking about 3D.
Holographic and ultra HD so ultra HD you pretty much can't see the pixelation and
usually you need a factor 4 above a true HD so something like 100 megabit per
second now you may think this is a very big number a 100 times of SD but of
course you viewing experience is much better.
And plus, this actually incorporates a huge factor
of compression capability without which you do need two or three orders magnitude
higher through but which is really not deliverable in any access network today.
Now, is 100 mega per second or let's say 25 mega per second deliverable in today's
technology of the access network? Well, it depends.
Okay well certain kind of media cell certain fiber base delivery and for
certain kind of WiFi say 802.11 and can, you can reach over 100 megabit per
second. And for your wire line access, if you
want to get to 25, mega per second, most of the, latest broadband technology over,
different media can manage that. But if you want to go much higher than
this, it's going to be, quite a challenge.
Now, on your cellular network, and we're going to talk about that in lecture
nineteen, the actual application layer speed that you can expect, even with 4G
LTE. Okay.
It will be hard to achieve this. Achieving this actually is possible.
Okay. It depends on other factors, but it is at
least feasible in the coming month and couple of years that you can expect to be
able to watch reasonable quality HD-like kind of video on RTE if you are at a good
spot and definitely be able to watch SD video on RTE.
Now all this is about a bit rate. How many bits per second can you play it
back? There're also other factors for example
delay, such as how long do I have to buffer this?
We'll see an example of how buffer smooths, the playback experience, as well
as the variance of delay. Okay.
And this is important because well, video is a motion picture if the relative
timing across the frames, becomes an issue then you will see, say, frozen
screens and so on. So now the questions are, A, how can the
pipe take on so many bits per second? In fact without compression, the raw data
rate of even standard television would not be available today, on today's
technology. So how can, the pipe take on so many bits
per second? A second question is, how do I keep track
of different states of the video? Unlike, a normal, like, a standard file,
now I have to keep track of, play, and pause, fast forward, and reverse, so on.
The third question is, how do I support, different qualities of service, over
best-effort network? Remember, we talked about, in lecture 13
that Internet is over best effort, or no effort to guarantee.
So can it ever support something like video, isn't that amazing.
In advanced material part of the lecture we will talk about this but in the next
two modules we will talk about the other two equations.
