CYBERLAW 2002

Professor Fischer

Class Four: Introduction to Internet Technology

September 9, 2002

ACLU v. Reno (E.D. Pa. 1996)

Significant opinion in that it is the first federal district court pronouncement on the InternetPara.81: The Internet is a unique and wholly new medium of worldwide human communication.Para. 74: The Internet is as diverse as human thought

C.P. Snow,The Two Cultures: And a Second Look (1964)

“It is dangerous to have two cultures which can’t or don’t communicate. In a time when science is determining much of our destiny, that is, whether we live or die, it is dangerous in the most practical terms. Scientists can give bad advice and decisionmakers can’t know whether it is good or bad. On the other hand, scientists in a divided culture provide a knowledge of some potentialities which is theirs alone”

SIGNALS

By clapping my hands, I (the sender) am encoding a message (attention!) onto a signal of pressure waves which propagate through the channel of the atmosphere which is decoded by your eardrums back into the message which you (receiver) understand– or at least pay attention to!

See Figure 1-2 on p. 8

Signals Explained

A signal is any physical characteristic of the universe to which we attach meaningThe electro-magnetic (e-m) wave is the signal of choice for modern computing communications. Figure 1-3 p. 9 (not a very good depiction of a sine wave!)What is amplitude? What is frequency?What is hertz? Mhz? Wavelength

Signal Types

What’s the difference between analog and digital?Contrast Figures 1-3 (p. 9) and 1-4 (p. 11)Can an analog signal be converted to a digital signal?What are the benefits of digital signals?Is digital always better?

Channels of Communication

Signals have to be sent across channels to receivers

Wireless: frequency bands – Table 1-1 (p. 13) and Table 1-2 (p. 15)

Wireline: passband

Is there any reason to prefer wireline channels over wireless?

Bandwidth

What is bandwidth?

Why do we care about bandwidth?

What is bps?

What is a bit?

What is a byte?

The Rise of ComputersA computer is essentially just a collection of binary logic circuits which accepts electrical inputs (1s or 0s) and generates electrical outputs (1s or Os)Computers can manipulate symbols at very high speedsAlthough computers have very old historical roots (abacus, Babbage engine), first general purpose electronic digital computer introduced not so long ago, in 1945Computers have gotten much faster and smaller!

The Rise of Computer Networks

Note a cyclical pattern over the years: Dumb terminals to Personal ComputersPersonal computers back to network computers which act more like dumb terminals serviced by a powerful serverServerClientWhat reasons are there for the use of networks serviced by powerful central computers?

Advantages of Networks

Lower cost maintenance

Don’t have to install software on individual user machines

Reduce storage needs on client computers

Client computers can communicate with other network users

Network Elements

Computers

Some kind of transmission channel (or line)

Adapter

See Figure 1-5 p. 21

What is the Internet?

Often described as a network of networks

Is there really an internet – does the phrase have any real technological meaning?

The Internet is like a foreign country – it’s everywhere you’re not

No real technological difference between Internet and LAN (local area network)

NIC – Network Interface Card

Internet Architecture

What’s a backbone?

What are routers?

What’s an ISP? An ISP POP?

Network Layers

Networks can be described in terms of abstract layers

See Figure 1-6 (p. 22) – shows a simple model of such conceptual layers:

1. Physical Layer

2. Protocol Layer

3. Application Layer

Applying Layer Concepts to the Internet

What constitute the Internet’s physical, protocol and application layers?

Applying Layer Concepts to the Internet

Application Protocols

Transmission Control Protocol (TCP) layer

Internet Protocol Layer (IP)

Hardware Layer

IP AddressEach computer that is connected to the Internet must have an IP addressWhat does IP stand for?What is the IP address for www.yahoo.com?Ping programNote computers on other networks (eg corporate networks)have IP addresses too. If that network is not on the Internet, these IP addresses can be the same as Internet IP addresses IPv4 (32 bit addresses), IPv6 (128 bit addresses)Domain name; URL

Protocols

What’s a protocol?

Protocols

What’s a protocol? A set of rules that governs communications. Communication protocols are required for Internet to function

Example – every web server on the Internet conforms to the HTTP protocol

Applications Layer

What applications protocols do internet users use?

Applications Layer

What applications protocols do internet users use? Hypertext Transfer Protocol – makes the World Wide Web work. It is the protocol used by specific applications, web browsers and web servers, to communicate over the Internet Simple Mail Transfer Protocol – makes your e-mail workVoice-over IP, video-over-IP, instant messaging

HTTP: What happens when you type a URL into web browser

If you type a domain name, browser connects to domain name server and retrieves IP addressWeb browser connects to Web server and sends HTTP request (via protocol stack) for web pageWeb server checks for page. If can’t find it, sends HTTP 404 error message. If can, sends itWeb browser receives page and connection closedBrowser reads HTML tags and formats the page for screen. Browser then looks for all other elements it needs like images or applets to complete the page. For each, makes additional connections and HTTP requests to web server.