CS101 - Introduction to Computing - Lecture Handout 30

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Internet Services

During the last lecture …

(Introduction to the Internet)

We looked at the role Internet plays in today’s computing
We reviewed some of the history and evolution of the Internet

Internet: The Enabler

Enables attractively-priced workers located in Pakistan to provide services to overseas clients
Enables users to easily share information with others located all over the world
Enables users to easily, inexpensively communicate with others remote users
Enables the users to operate and run programs on computers located all over the world

The Internet is unlike any previous human invention. It is a world-wide resource, accessible to all of the humankind.

Key Characteristics

Geographic Distribution Global - reaches around the world
Robust Architecture Adapts to damage and error
Speed Data can travels at near ‘c’ on copper, fiber, airwaves
Universal Access
Same functionality to everyone
Growth Rate
The fastest growing technology ever
Freedom of Speech
Promotes freedom of speech
The Digital Advantage
Is digital: can correct errors

Internet: Network of Networks

A large number of networks, interconnected physically
Capable of communicating and sharing data with each other
From the user’s point view, Internet – a collection of interconnected networks – looks like a single, unified network

TCP/IP Transmission Control Protocol/Internet Protocol

TCP breaks down the message to be sent over the Internet into packets
IP routes these packets through the Internet to get them to their destination
When the packets reach the destination computer, TCP reassembles them into the original message
1960's
1969 - DoD-ARPA creates an experimental network – ARPANET – as a test-bed for emerging networking technologies
ARPANET originally connected 4 universities & enabled scientists to share info & resources across long distances
1980's
1983 - The TCP/IP protocols becomes the only set of protocols used on the ARPANET
This sets a standard for all networks, and generates the use of the term Internet as the net of nets
1990's1993 - CERN releases WWW, developed by Tim Berners-Lee It uses HTTP and hypertext, revolutionizing the way info is presented & accessed on Internet
1990's1993-1994 - Web browsers Mosaic & Netscape Navigator are introduced
Their GUI makes WWW & Internet more appealing to the general public

Today’s Goal: Internet Services

To look at several services provided by the Internet

  • FTP
  • Telnet
  • Web
  • eMail
  • Instant messaging
  • VoIP

But first, we need to find out about the addressing scheme used on the Internet

Internet Addressing

Regular post cannot be delivered unless we write a destination address on the envelope Same is true for the Internet
Regular post can be delivered at the intended address even if the given address is not precise. That is not the case for Internet addressing

Internet Addressing

www.vu.edu.pk
203.215.177.33

IP Address

A unique identifier for a computer on a TCP/IP network
Format: four 8-bit numbers separated by periods. Each 8-bit number can be 0 to 255
Example:
–203.215.177.33 (IP address of the VU Web server)
Networks using TCP/IP route messages based on the IP address of the destination
Any IP addresses (as long as they are unique) can be assigned within a PN
However, connecting a PN to the Internet requires using unique, registered IP addresses

Domain Names

IP addresses are fine for computers, but difficult to recognize and remember for humans
A domain name is a meaningful, easy-to-remember ‘label’ for an IP address
Examples:
www.vu.edu.pk
216.239.33.101 www.google.com

DNS: Domain Name System

DNS is the way that Internet domain names are located & translated into IP addresses
Maintaining a single, central table of domain name/IP address relationships is impractical

  • Billions of DNS-IP translations take place every day
  • The DNS-IP tables get updated continuously

Tables of DNs & IP addresses are distributed throughout the Internet on numerous servers
There is a DNS server at most ISPs. It converts the domain names in our Internet requests to actual IP addresses
In case it does not have a particular domain name in its table, it makes a request to another DNS server on the Internet

Internet Services

There are many, but we will look at only the following:
FTP
Telnet
Web
eMail
Instant messaging
VoIP

FTP: File Transfer Protocol

Used to transfer files between computers on a TCP/IP network (e.g Internet) Simple commands allow the user to:

List, change, create folders on a remote computer
Upload and download files
Typical use: Transferring Web content from the developer’s PC to the Web server

Telnet Protocol

Using Telnet, a user can remotely log on to a computer (connected to the user’s through a TCP/IP network, e.g. Internet) & have control over it like a local user, including control over running various programs
In contrast, FTP allows file operations only
Typical use: Configuring and testing of a remote Web server

The Web

The greatest, shared resource of information created by humankind
A user may access any item on the Web through a URL, e.g.
http://www.vu.edu.pk/cs/index.html
Before, going any further, let us dissect this URL

The Web

How does the web works

User launches the browser on his/her computer

web works

User types in the URL into the browser
The browser breaks down the URL into 3 parts :
Protocol Identifier
Server Address
Directory & File Name
Browser sends server’s name to the DNS server

Domain Name

Browser sends a ‘GET’ request for cs/index.html

Browser sends a ‘GET’ request for cs-index.html

Server sends the requested file to the browser

Server sends the requested file to the browser

Browser displays index.html

email

Computer-to-computer messaging
Inexpensive, and quite quick, but not instant!
The most popular service on the Internet, even more than surfing, but soon to be overtaken by instant messaging
Billions are sent every day

How does an eMail system work?

But first, the components:

eMail client
SMTP server
POP3 server

eMail Clients

Programs used for writing, sending, receiving, and displaying eMail messages
Examples: Outlook, Communicator, Hotmail, YahooMail

SMTP: Simple Mail Transfer Protocol

A protocol used to send and receive eMail messages over a TCP/IP network

POP3: Post Office Protocol

A protocol used for receiving eMail messages
A POP3 server maintains text files (one file per user account) containing all messages received by a user
eMail client interacts with the POP3 server for discovering and downloading new eMail messages
The message is prepared using the eMail client

Post Office Protocol

The eMail client sends it to the SMTP server

The eMail client sends it to the SMTP server

If the receiver is local, it goes to the POP3 server

If the receiver is local, it goes to the POP3 server

The receiver picks it at his/her convenience

The receiver picks it at his-her convenience

Otherwise, it is sent to receiver's SMTP server

Otherwise, it is sent to receiver's SMTP server

Which forwards it to the local POP3 server

Which forwards it to the local POP3 server

The receiver picks it at his/her convenience

The receiver picks it at his-her convenience 1

The Trouble with email

Slow response times
No way of knowing if the person we are sending eMail to is there to read it
The process of having a conversation through eMail by exchanging several short messages is too cumbersome
Instant messaging (IM) solves these problems

Instant Messaging

  • The IM services available on the Internet (e.g. ICQ, AIM, MSN Messenger, Yahoo! Messenger) allow us to maintain a list of people (contacts) that we interact with regularly
  • We can send an instant messages to any of the contacts in our list as long as that contact is online

Using Instant Messaging

Whenever a contact in our list comes online, the IM client informs us through an alert message and by playing a sound
To send an instant message to a contact, just click on the contact in the IM client, and start typing the message
The selected contact will receive that message almost immediately after you press ‘Enter’
When the contact’s IM client receives the message, it alerts the contact with a blinking message and by playing a sound
That contact then can type a response to the received message, and send it instantly
Several such conversations can be carried out simultaneously, each occupying a separate IM windows

How instant messaging works?

User launches the IM client

User launches the IM client

IM client finds the IM server & logs in

IM client finds the IM server & logs in

It sends communication info (IP address, etc) to the IM server

It sends communication info (IP address, etc) to the IM server

IM server finds user’s contacts & sends him/her the communication info for the ones online

IM server finds user’s contacts

IM server also tells the contacts that the user is online; sends his/her communication info to them

IM server also tells the contacts that the user is online

Now the user’s & the contact’s IM clients are ready to communicate directly (P2P)

Now the user’s & the contact’s IM clients are ready to communicate

As new contact’s come online, IM server informs them about the user being online & vice versa

As new contact’s come online

Multiple, simultaneous conversations are possible

Multiple, simultaneous conversations are possible

When the user logs-off, his/her IM client informs the IM server

When the user logs-off, his-her IM client informs the IM server

IM server erases the temporary file and informs the user’s contact’s about his/her‘offline’ status

IM server erases the temporary file

Key Point

Once the IM server provides the communication info to the user and his/her contact’s IM client, the two are able to communicate with each other without the IM server’s assistance
This server-less connection is termed as a P2P connection

Question

Why do we require the server in the first place?
Why doesn’t my IM client look for the user’s contact’s IM client without the IM server’s help?

Answer

Many users (including almost all home users) do not have permanent IP addresses. They are assigned temporary IP addresses by their ISP each time they connect to the Internet
The server-based IM scheme removes the need of having permanent IP numbers It also gives IM users true mobility, allowing them the use of IM from any Internetconnected computer

VoIP: Voice over IP

Voice delivered from one device to another using the Internet Protocol
Voice is first converted into a digital form, is broken down into packets, and then transmitted over a TCP/IP network (e.g. Internet)
Four modes:
C2C
C2T
T2C
T2T (with a TCP/IP net somewhere in between)

Pro

Much cheaper than traditional phone service

Con

Noticeably poor quality of voice as compared with land-line phone service, but not much worse than cell phone service

Today’s Goal: Internet Services

We looked at several services provided by the Internet
FTP
Telnet
Web
eMail
Instant messaging
VoIP
We also found out about the addressing scheme used on the Internet

Next Lecture:

Next lecture (Lecture 31) - the third one in the four-lecture productivity SW sequence - will be on developing presentations
However, during lecture 33, we will become familiar with the role that graphics and animations play in computing