1. CYBERLAW 2002 Professor Fischer Class Four: Introduction to Internet Technology September 9, 2002

2. ACLU v. Reno (E.D. Pa. 1996) Significant opinion in that it is the first federal district court pronouncement on the Internet Para.81: The Internet is a unique and wholly new medium of worldwide human communication. Para. 74: The Internet is as diverse as human thought

3. 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”

4. 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

5. Signals Explained A signal is any physical characteristic of the universe to which we attach meaning The 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

6. 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?

7. 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?

8. Bandwidth What is bandwidth? Why do we care about bandwidth? What is bps? What is a bit? What is a byte?

9. The Rise of Computers A 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 speeds Although computers have very old historical roots (abacus, Babbage engine), first general purpose electronic digital computer introduced not so long ago, in 1945 Computers have gotten much faster and smaller!

10. The Rise of Computer Networks Note a cyclical pattern over the years: Dumb terminals to Personal Computers Personal computers back to network computers which act more like dumb terminals serviced by a powerful server Server Client What reasons are there for the use of networks serviced by powerful central computers?

11. 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

12. Network Elements Computers Some kind of transmission channel (or line) Adapter See Figure 1-5 p. 21

13. 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

14. Internet Architecture What’s a backbone? What are routers? What’s an ISP? An ISP POP?

15. 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

16. Applying Layer Concepts to the Internet What constitute the Internet’s physical, protocol and application layers?

17. Applying Layer Concepts to the Internet Application Protocols Transmission Control Protocol (TCP) layer Internet Protocol Layer (IP) Hardware Layer

18. IP Address Each computer that is connected to the Internet must have an IP address What does IP stand for? What is the IP address for www.yahoo.com?www.yahoo.com Ping program Note 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

19. Protocols What’s a protocol?

20. 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

21. Applications Layer What applications protocols do internet users use?

22. 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 work Voice-over IP, video-over-IP, instant messaging

23. 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 address Web browser connects to Web server and sends HTTP request (via protocol stack) for web page Web server checks for page. If can’t find it, sends HTTP 404 error message. If can, sends it Web browser receives page and connection closed Browser 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.