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Del curso dictado por Princeton University

Networks Illustrated: Principles without Calculus

43 calificaciones

Princeton University

Networks Illustrated: Principles without Calculus

43 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

Alright. So, we covered a lot of material in this

lecture and just to give a brief summary, we talked all about WiFi.

We talked about its topology, how it's typically laid out and basic service

sets, devices within service sets and access points as well within service

sets. We talked about the differences between

WiFi and cellular, you know a WiFi device is un, unregulated frequency band.

It's, it's typically run over a much smaller range and has a much smaller

number of devices within each basic server set, or on the other hand, within

a cell, in a cellular network. And it has much varying data rate and

that data rate's going to vary a lot depending upon how much we've backed off.

And depending upon the current interference conditions.

And that's something that doesn't necessarily happen as much with cellular,

because it's much more regulated. And then, we talked about random access

protocols and what exactly those are. We focused mainly on ALOHA, a protocol

that was invented in Hawaii, a very simple and very non scalable as we saw.

And we talked about CSMA, which is still largely in use today, and we talked about

some of the concepts that underlies CSMA. And we just saw overall, how random

access protocols work very well for a small number of devices.

well ALOHA really doesn't, but CSMA works very well for a small number of devices.

But it's not very scalable to a large number of devices.

Which is why you can't get a very good data rate sometimes when you're near a

hot spot, if there is a lot of other people trying to access the same access

point. Because you're going to cause a lot of

interference and collisions. And that's the price that we pay for

simple protocols. So, some major themes we've saw again the

idea of feedback here, in terms of sending acknowledgement packets back.

In CSMA, the idea of feedback is really that you use the acknowledgement packets

to know that you can keep sending without having to back off.

Unless you don't see an acknowledgement, that really indicates you if, there's

congestion, then you have to back off. So, this is a form of negative feedback.

And we saw distributed implementation again, that CSMA can be done completely

distributedly just like we've talked about with distributed power control in

the last section. So, overall over the past two lectures,

we've looked at all different methods of sharing.

In the firsthand, we looked at cellular and how we have things like FDMA, TDMA,

CDMA, and we looked at power control. And we looked at how your cellphone uses

power control to determine what power to transmit.

And then, in this section, we looked at random access methods for WiFi and why

your WiFi device is going to have a better data rate at your house than it

would at a hot spot. But overall we've really looked at the

theme of how do we deal with lots of people in networks at the same time.

And how do we share the air most efficiently.

Whether the air is being shared over cellular network or over a WiFi network.

And so, the, really the big theme is that sharing is hard to do.

It's not easy. And to this day, researchers are still

trying to develop methods to squeeze more capacity improvement out of what we have

in the network and all the resources that we have.

And so, that's the major theme that sharing is hard.

And I hope you've enjoyed it.

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