Transcript internet_basics

Internet Basics
Dr. Norm Friesen
June 22, 2007
Questions
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What is the Internet?
What is the Web?
How are they different?
How do they work?
How do they differ technically from other
communications media (e.g. telephone)?
The Web for Absolute
Beginners (1 of 2)
• The Web is a series of documents, joined
together by links. These links (which can take
the form underlined text, text labels or images)
can be followed by simply being selected or
clicked. These documents can originate from
almost anywhere in the world.
• Hypertext - a term that significantly predates the
Web, referring to a collection of documents (or
"nodes") containing cross-references or "links"
which, with the aid of an interactive browser
program, allow the reader to move easily from
one document to another.
The Web for Absolute Beginners (1
of 2)
• Hypermedia - The extension of hypertext to
include other media - sound, graphics, and video
- has been termed "hypermedia."
• Web documents - Unlike text documents, Web
pages can be dynamic and interactive: their
contents can change in response to the reader
or user's actions. Most dynamic Websites
(www.amazon.com, www.blogger.com,
www.cbc.ca) use information stored in
databases to provide textual, numeric and other
types of content for these pages.
• A Browser - A browser (Internet Explorer,
Firefox or another system) is the software used
to view and interact with Web documents.
• Client and Server - There are two basic
categories of computers associated with the
Web and the Internet.
• The client computer is used to view Web pages
with a browser and an Internet connection of
some kind. When you (or your students)
access WebCT from home, office, a computer
lab, or a classroom, you will be using a client
computer.
• The server computer stores Web documents
and "serves" their contents across the
Web. Server computers can be located
anywhere in the world, but must be connected to
the Web using a permanent, high-speed link.
• Software that makes Websites interactive and
dynamic is generally located on the server. It
generally involves a database. This means that
for most types of Web-based interactivity, a
standard Web browser is sufficient.
• Protocol: specific rules for accessing and
transmitting data. Different protocols (e.g.
"http" or "shttp") are used for different
kinds of information. (often includes errors)
• URL or Web Address: a combination of
characters (letters, numbers, punctuation)
that is unique on the Web, and that
indicates the specific location of a file or
resource on a particular server.
Addresses
• Flash & Java Applets are examples of technologies that
provide interactivity by being downloaded to and
operating on the client computer.
Document Locations or Web Addresses
• Each page or document on the Web has a unique
address. This address identifies every important
element involved in accessing the document, including:
– the protocol or set of technical rules used to transmit the
document
– the organization providing the server that hosts the document on
the Web
– the nature or nationality of the organization
– the part of the server accessed (port number)
– the computer directories or folders in which the Web page is
stored (optional)
– the name of the Web page file
"Layers"
• Computer and network systems are
designed in terms of clearly separated but
independent components.
• In a network, these are referred to as
"layers" (can be divided in different ways):
– Physical: the actual wire or fibre on which
information is transmitted; can be phone line
– Application:
– Content:
Websites
• sub-site structure often contains the real "content" of
the web site; often uses a nonhierarchical organizational
logic
• linear: most common format; useful for documents
• listing or index (usually alphabetical): useful for links or
alternative organization of larger sites
• grid: useful for large, uniform information sets, like an
expanded table. Can be combined with a hierarchy.
• database: for searches on large information sets.
Requires special server software.
Databases on the Web: An important example of
Internet/PC Architecture
To make the contents of a database accessible over the
Web, it is necessary to make use of special software
that resides on a server. Special software forms the
middle component or tier in what is commonly known
as a three-tier system. This simple "stack" architecture
is comprised of:
1. Database Tier: the data or database file residing on
the server. Also known as the database server.
2. Middle Tier: database connectivity software making
the database securely available on the web.
3. Client Tier: the client or browser used to search and
modify database records.
• data is passed back and forth between these tiers or
components, basically in the form of search requests
and results.
• The middle tier acts as an interpreter, translating
information into the different languages or protocols
understood by the browser and database.
– It receives a query from a web page form, coded in a web
protocol.
– It translates this query into a simple but powerful Structured
Query Language (SQL) statement, which can be understood by
almost any type of database.
– the connectivity software receives the results from the database
as pieces of text that it must then format as a web document
using HTML (HyperText Markup Language).
TCP/IP Protocols
• The internet as a postal system, not a
telephone system.
• Why?
Cold War and the Internet
• One central "hub" makes
the entire network
vulnerable
• Many "hubs" means that
the network can operate
even if one or more ceases
to exist.
• DARPA: Defense
Advanced Research
Projects Agency