Transcript CYBERLAW 2002 Professor Fischer Introduction to September 9, 2002

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 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
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 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
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 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!
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?
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 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?
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
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 work
Voice-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 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.