Transcript DOK 422: Bilgi Ağları - Hacettepe Üniversitesi

WWW and Internet Access
Yaşar Tonta
Hacettepe Üniversitesi
Bilgi ve Belge Yönetimi Bölümü
DOK 422: Bilgi Ağları
DOK 422 – Bahar 2005
Kaynak: Bogdan Ghita – http://ted.see.plymouth.ac.uk/bogdan
2005.03.01 - SLAYT 1
Session Content
• The World Wide Web
– HTTP (HyperText Transfer Protocol), HTML
(HyperText Markup Language), URL (Uniform
Resource locator)
• Dial-up Internet access
– PPP (Point-to-Point Protocol) and SLIP (Serial
Line Internet Protocol)
• (Why) is my / the Internet slow?
• ISPs (Internet Service Providers)
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The history of Internet
1969
From: historyoftheinternet.com
1999
From: bell-labs.com
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The World Wide Web
• “The World Wide Web is the universe of networkaccessible information, the embodiment of
human knowledge” (WWW Consortium - W3C)
• The most popular Internet application
• Also known as "WWW', "Web" or "W3"
• Began as a networked information project at
CERN (European Laboratory for Particle
Physics)
• Developed by Tim Berners-Lee
– now Director of the W3C
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WWW - Background
• Early ‘vision statement’ from Tim Berners-Lee
– Merges the techniques of networked information and
hypertext to make a global information system.
– Represents any information accessible over the
network as part of a seamless hypertext information
space
– Allows information sharing within internationally
dispersed teams and dissemination of information by
support groups.
– Originally aimed at the High Energy Physics
community, it has spread to other areas and attracted
much interest in user support, resource discovery and
collaborative work
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What is Hypertext?
CNET209/left_border.htm
CNET209/CNET_209_handouts.htm
CNET209/CNET_209_links.htm
CNET209/left_border.htm
Default.htm
CNET209/Contact.htm
http://t.extreme-dm.com
• Hypertext is text with links
• Links can be made within or between documents
• Allows reader to escape sequential presentation and
pursue threads
• Not unique to the web and did not originate with it
• Documents can be text, graphics, movies, sound, etc
• The term "hypermedia", meaning "multimedia hypertext"
is often applied to WWW
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HyperText Transfer Protocol
(HTTP)
• The data transmission protocol used to
transfer web resources across the Internet
• Format negotiation
– A feature of HTTP is that the client sends a list
of the representations it understands along
with its request (e.g. graphics formats such as
GIF and JPEG)
– server can then ensure that it replies in a
suitable way
– allows the web to distance itself from the
technical and political battles of the data
formats
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HTTP (cont.)
• HTTP is stateless
– Each request is an independent entity that
does not depend upon the state left over from
any prior request
• HTTP is transaction-based
– Non-persistent (version 1.0) requires a new,
independent connection to the server for every
element to be downloaded
• inefficient, particularly for low bandwidth scenarios
– Persistent (version 1.1) - the requests are
pipelined
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HyperText Markup Language
(HTML)
• The basic language used to create / present web
documents
• Written and stored as plain ASCII text
• Based on SGML - Standard Generalised Markup
Language
• Describes the logical structure of the document
instead of its formatting
– Allows it to be displayed optimally on different
platforms using different fonts and conventions
• Uses special ‘tags’ to encode styles and effects
– Interpreted by the client / browser to render
document in correct format
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HTML (cont.)
• Has evolved in several versions - current is
HTML 4.0
• Also other related WWW markup languages
– e.g. VML (Vector ML), WML (Wireless ML),
DHTML (Dynamic HTML), ...
– The future seems to belong to XML (eXtensible
Markup Language) – a better HTML (more tags,
more flexible)
• WWW also has standards for ‘active’ content
– e.g. Java, JavaScript, Active-X
– can be embedded within HTML documents
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HTML Example - The Markup
Q: How to obtain the above?
A: Right-click on a web page then select View source
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HTML Example - The result
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Uniform Resource Locator (URL)
• Each resource on the web has an address
e.g. http://ted.see.plym.ac.uk/bogdan/CNET209/default.htm
1
1
2
3
4
2
3
4
Transfer protocol used to access site (e.g. HTTP, FTP, TELN
Address of the host (DNS name or IP address)
Directory path of resource
Resource name
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WWW Client-Server Model
• To allow the web to scale, it was designed
without any centralised facility
• Anyone can publish information, and anyone
(authorised) can read it
• To publish data you run a server, and to read
data you run a client
• All the clients and all the servers are connected
to each other by the Internet
• The WWW protocols and other standard
protocols allow all clients to communicate with
all servers
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Web Browsers
• Web browser - client program used to visit web pages
• First ever - written by Tim Berners-Lee, CERN (1990)
• The first used heavily by public – Mosaic (1993)
– Marc Andreessen (undergraduate at the time) and
colleagues at the National Center for Supercomputing
Applications (NCSA)
– Most of that group went on to form Netscape Corporation
• The current main competitors: Netscape (Navigator)
and Microsoft (Internet Explorer)
• Web browser has increasingly become the only
network application client you need to use
– Migration of applications to web interface
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Internet Access methods
• It’s all about money - price dependent on
–
–
–
–
Connectivity method: leased line (always-on) or dial-up)
Speed
Amount of traffic exchanged
Contention rate (number of users vs. connectivity speed to
Internet ratio)
• Alternatives
– Dial-up
– ADSL
– LAN, ...
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Dial-Up (Internet) Access
• The ‘Ideal’ access - dedicated connection
– Leased line between a subscriber and a provider
• Dial-up - the affordable choice
– “Refers to connecting a device to a network via a
modem and a public telephone network”
(webopedia.com)
– Aside from contention rate, access depends on
the number of modems used - modems vs. users
ratio
• Most used protocols – SLIP and PPP
– SLIP (Serial Line Internet Protocol)
– PPP (Point to Point Protocol)
– Common core, but with a few differences
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SLIP / PPP
• SLIP/PPP provides the ability to transport TCP/IP
traffic over serial lines (such as dial-up telephone
lines) between two computers
• Both computers run TCP/IP based network software
• Equipment: modem + phone line
• It is a form of direct Internet connection as your computer
has
– A communications link to the internet, even if it is via a service
provider
– Networking software that can communicate using TCP/IP
protocols with other computers on the Internet
– An identifiable address (IP address) at which it can be contacted
by other computers on the Internet
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SLIP / PPP Differences
• PPP is a newer, better designed protocol
• Multi-Protocols
– SLIP can only transport TCP/IP traffic
– PPP is a multi-protocol transport mechanism
• Configuration Negotiation
– SLIP requires to know your IP address (assigned by the
ISP) and the IP address of the system to be connect to
• Difficult if IP addresses are dynamically assigned
– PPP addresses this problem by negotiating
configuration parameters at the start of the connection
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SLIP / PPP Differences (cont.)
• Automatic Login
– Most SLIP software can dial-up and automatically
login
– …but it depends on the ISP’s system standard
prompts
• e.g. "login:" to get userid and "password:" to get
password
• If these are non-standard, you need to write a script to
automate the login process or login manually
• PPP provides PAP (Password Authentication
Protocol) and CHAP (Challenge-Handshake
Authentication Protocol)
– Both provide the means to automatically send your
login userid/password information to the remote
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ADSL
• ADSL - Asymmetric Digital Subscriber Line
– Why asymmetric?
• Uses the telephone lines as transport medium
– improvement in the “local loop” (the cable between the phone
exchange and the home socket) usage
• Speeds (depend also on physical factors) asymmetric
– Downstream - up to 9 Mb/s
– Upstream – up to 1.5Mb/s
• Requires specific modem, but works on a normal
telephone line
– The phone can work in parallel with the ADSL connection
• It is the dial-up of tomorrow…but much faster and
always-on!
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Is it over for dialup?
• U.S. 2001 statistics – home connectivity
– Dialup – 80%
– Cable modem – 12.9%
– DSL – 6.6%
– Other – 0.5%
• Not everybody needs 10x bandwidth! – U.S. 2001 application
usage
–
–
–
–
–
–
E-mail – 45.2%
Information search – 36.2%
News, weather, sports – 33.3%
Product / service purchase – 21.0%
Employment search – 7.5%
Phone calls – 2.8%
(Source: U.S. Dept. of Commerce and U.S. Census Bureau - 2001)
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Internet Service Providers (ISPs)
• A company that provides access to the Internet
– Has infrastructure for: connectivity (computers, routers),
access (modems, phone lines), and technical support
• Selection issues : cost, PoP(s), contention
• Cost - taxes are moving towards the cost of the phone
call
• Point of Presence (PoP) - A local computer/network
owned and operated by an ISP – used to dial into to get
onto the Internet
• Good user-to-modem ratio in each POP (minimise
occasions when users get an engaged tone) –
minimum approx. 12:1
• Good contention rate – between 1:20 and 1:50
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Other ISP selection issues
• Personal Homepage
– Does the ISP allow you to set up your own WWW page on their
server? How much space do you have allocated?
• Shell Access
– Does the ISP give you access to a Unix-type login shell account
on one / more of their servers? (good for nerds!)
• POP3 Mail Server
– Lets you pick up mail from the ISP's server so that you can
read (and write) them on your own PC
– Your mails are first received by the ISP and stored on their
servers - how much server space is allocated?
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The World Wide Wait?
• Factors affecting Internet / WWW performance
•
•
•
•
•
•
•
Modem speed
Choice of ISP (and their facilities / capacity)
Response times of remote sites
Power outages / server downtime
Time of day and site location
General Internet characteristic of 'best effort' transfer
Hardware of the computer - less of an issue lately
• The Internet will NOT be faster or enhanced if you have a faster
machine (unless you have a very fast / demanding connection)
• 486s or an old Pentiums are VERY slow machines
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“My Internet is slow” - Why?
• Is it?
• Congestion - the main factor to affect
performance
– Queuing - increases the delay
• The output rates and router speed are not limitless
– Loss - requires retransmission
• The queues have limited sizes
• ‘The Internet’ is faster!
– But the information content changes
text
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image
audio
video
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Summary
• WWW - the universe of information
– HTTP - the protocol that makes it work
– HTML - the language used to produce it
– URL - the addressing system behind it
• WWW - is it slow or are we demanding?
• Dial-up - the current solution to connect
– PPP vs. SLIP - PPP is the winner
• ADSL – the future (?)
• ISP - the bridge between user and the Internet
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