Transcript circuit switching

Arab Open University - AOU
T171
You, Your Computer and the Net:
Learning and living in the information age
Module 2 – Section 2 & 3
Session 6
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Module 2: ARPANET
Main topics covered in this section are:
1.
2.
3.
The origins of the ARPANET
The development of Packet Switching
Design for the network
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Who though of it first?
1.
When we talk about the Internet Origins, we must consider
the computer beginnings:
It is the main part of any network
2.
Computers were invented in the midway of the 20th century
3.
However, this wasn’t the actual start!
4.
Go back to the analytical engine of Charles Babbage (1820s):
precursor to modern computers
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Who though of it first?
5.
Used punch cards to input data
Borrowed from the Jacquard loom (1804)
6.
7.
And still, we need to go back further to search for the origins of
Internet  endless!
Instead of searching for a single start point, which is too
difficult, we can visualize the origins of any brilliant
achievement as a mixture of many ideas, for many great
thinkers, in many different times!:
- The origins are diverse
- They are from different times.
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Who though of it first?
8.
The key institution of the Internet was the MIT:
(Massachusetts Institute of Technology)
9.
The following are three of the most important figures in the
Internet story:
Vannevar Bush.
2. Norbert Weiner.
3. Claude Shannon.
1.
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Who though of it first?
Vannevar Bush:
1.
•
Forgotten from 1939-1945 (as a result of the second world war).
Influenced the creation of WWW
•
Norbert Weiner:
2.
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Worked on control systems for anti-aircraft guns in the second
world war.
Noticed similarities between human-beings and machines:

Recognized the important of feedback.
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Published ‘Cybernetics : Control and Communication in the
Animal and the Machine’.
Talked about ‘the boundary regions’ of science:

Where different disciplines overlapped.

New ideas and researches would best be generated.
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Who though of it first?
Claude Shannon:
3.
Created a model for the process of communication:
1.

The scientific basis for the communications theory
His model contained 5 elements:
2.
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An information source
A transmitter
A transmission channel
A receiver
A destination
It also raised the concept of redundancy in communication:
3.

Vital in the design of the Internet
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Origins of ARPANET:
ARPA – Advanced Research Projects Agency

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Was a special agency within the United States Department of
Defense (US DOD)
To fund and foster researches in many important areas, including
computing
At that time:


Computers were thought of as vastly expensive and cumbersome
machines
It was abnormal that an individual uses a computer for himself:


It was only used for institutions and organizations.
Computers operated only in batch mode:

No direct interaction with the user, during the processing.
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Origins of ARPANET:

Licklider:
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A director in ARPA (1962- 1964)
Appreciated the idea that computers were machines with which
humans could fruitfully interact
Interactive computing had replaced batch computing
Had promoted time-sharing concept as a basis for interactive
computing
In a time-sharing platform:

The computer passes control from one program to the other:
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In a ‘circular’ manner
Very quickly
So every user thinks the computer is serving him only
The CPU is shared by many programs.
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Origins of ARPANET:
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The first time-sharing computer was set up, by John McCarthy, at
MIT in 1957
ARPA’s networking project Managed by Lawrence Robert from
MIT
Robert’s view of the network topology was, simply:
Every node is connected to all other nodes over dial up telephone lines

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Nodes with large time-sharing computers at the connected sites
are called “ hosts”.
A revised network topology was discussed in GatlinburgTennessee:
On which the term ARPANET was heard for the first time

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The ARPANET’s basic topology had included 4 nodes (4
universities).
After the success of ARPANET, security and reliability issues were
raised
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A survivable network:

The initial network topology was centralized.

In such a way that the network would be completely disabled by
a single node failure:
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Avoid centralization!
Hence, decentralization and redundancy have been added to
the ARPANET.
Decentralization means that nodes communicate with their
neighbors with no central control.
Adding redundant interconnection between nodes will enable
decentralized network to function under fault situations.
Results in a distributed network topology
A distributed network uses digital signals rather than analog
signals:

Because otherwise, the signal quality would deteriorate too
quickly
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Circuit-switching vs. packet-switching

In analog communication:
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A physical connection has to be established between the
communicating points (sender and receiver):
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Telephone calls
This process is called circuit switching
The switched connection should remain active until
communication is done
Circuit-switching advantages:
1.
2.
Reliable
Rarely breaks down.
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Circuit-switching vs. packet-switching
Circuit-switching disadvantages

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Very inefficient and expensive.
The entire path has to be dedicated regardless of how much data
is being transmitted

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50% of the telephone call is ‘silent’ -> no data
The connection has to be made prior to transmission and is
maintained until the connection is terminated
In digital communication

There is no direct physical connection between the communicating
points
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Circuit-switching vs. packet-switching

The sender’s message

Converted into sequences of Zeroes and Ones.

Transferred by first breaking it into equal size pieces 
Packets
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Each packet includes the destination IP address.
And a sequence number
Packets are then transmitted to the receiver one after another
Via different routes.
A packet will be redirected by a router, after checking its IP
address
A router is a device that reads the IP address of the packet and
passes it on (to another router) towards the destination
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Circuit-switching vs. packet-switching
These packets are finally collected at the receiving end to
assemble the original message
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If any part of the message is missing:
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The destination sends a message back to the source, requesting
the missing part.
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That portion only is resent
This process is called Packet Switching.
This concept was developed by Paul Baran and Donald Watts
Davies.
Packet-switching advantages
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No entire circuit dedicated for one single message.
Many messages may be sent on the network at any time.
No wastage when data is not being sent.
Very efficient, when network is being flooded with packets
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Shifting paradigms
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The development of:
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Time-sharing instead of batch systems.
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Digital instead of analog systems.
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Distributed instead of centralized networks.
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And packet-switching instead of circuit-switching
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 led to the Internet as known today
This is what we call paradigm shift:
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Moving from one technology to the other.
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It was the idea of Thomas Kuhn
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WWW – World Wide Web
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World Wide Web (WWW) was invented in Geneva in 1989.
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The technical infrastructure for the web existed in 1991

But not used commonly till 1993 when the first browser
program was launched
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Summary
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The ideas of Bush, Wiener and Shannon
The importance of MIT
The setting up of ARPA
J. C. R. Licklider and his vision
Circuit-switching vs. Packet-switching
Analog and digital communications
Paradigm shifts of Thomas Kuhn .
WWW – World Wide Web
Read Chapters: 4,5,6 and 7 from the book:
“A brief history of the future”.
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Section 3: Making the Network:
Main topics covered in this section are:
1.
2.
3.
4.
Interface Message Processors (IMP’s)
Request for Comments (RFC)
Network Protocols
E-mail
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Interface Message Processors (IMP’s)

In a packet switching network, a small computer is inserted
between each host and the network of transmission lines.
These are called IMP’s.

These IMP’s are linked together to form a sub-network of
IMP’s.

The IMP’s acts as the host’s interface to the network.
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Interface Message Processors (IMP’s)
The advantages of this structure are:
1.
Freeing the host computer from the routing load such as
dividing the messages into packets, routing the packets and
gathering packets.
2.
The routing programs at each site are the same since we
have identical routing devices (IMP’s).
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Networking Group (NWG)
Request for Comments (RFC)
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The first packet switching network using 2 IMP’s was built in
October 1969.
That network was a packet switching IMP’s, sending bits
between each other.
In 1968, a group of graduate students from two universities
(University of California at L.A and MIT) set together to
discuss what applications would be built on the ARPANET.
This group was called Network Working Group (NWG)
They organized a series of meetings and documented their
notes and findings on what they called Request for
Comments (RFC), so RFC’s are technical notes written by
the NWG to exchange ideas on the design of host software
for the ARPANET.
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Networking Group (NWG)
Request for Comments (RFC)
RFC documents are very important because:
1.
They promoted cooperative and open work methods.
2.
They are timely documents containing information that
comprise the consensus of the network developers.
3.
Today, the RFC’s present and accurate trace on how
Internet software evolved.
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Networking Group (NWG)
Request for Comments (RFC)

The network working group used RFCs to evolve a whole
set of protocols for the new network.

Vint Cerf and Jon Postel were key persons for protocol
development.
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Network Protocol
Main functions of Network Protocol are:
1.
Provide rules that govern how the computers communicate.
2.
Give exact meaning to the bits flowing between the IMPs.
3.
Concerned with passing messages.
4.
Specify the format that a message must take, and the way
in which computers must exchange a message within the
context of a particular activities such as exchanging e-mail,
establishing remote connections or transferring files.
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Network Protocol
Some of the protocols used on the Internet are:
1.
Simple Mail Transfer Protocol (SMTP)
Used to send and receive electronic mail.
2.
File Transfer Protocol (FTP)
Used to transfer files between computers.
3.
Hyper Text Transfer Protocol (HTTP)
Used to transmit information on the world wide web.
4.
Network News Transfer Protocol (NNTP)
Used to transmit network news
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Network Protocol

Internet protocols are much more complex than its
predecessor, the ARPANET.

Network Working Group (NWG) created a Network Control
Protocol (NCP), which enabled different hosts on the
network to communicate.

Communication via computers involves many processes at
different levels, from passing signals down cables or over
wireless or fiber optic links, to the formatting messages.
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Network Protocol
Two protocols developed in early days:
1.
Telnet
Used for logging into remote hosts.
2.
FTP (File Transfer Protocol)
Used for secure transfer of files between hosts.
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Layers & Protocols
Protocols that are used to deal with all these different levels are defined in
the table below:
Layers Name
What goes on this layer
Typical Protocols
Application
E-mail formatting, file transfer remote login, etc,
FTP, Telnet, SMTP
Transport
Breaking messages into packet, routing messages
TCP
Internet
Assigning and resolving Internet addresses
IP
Sub Network
Passing packets round a local area network on their
way to an Internet router
Ethernet
Link
Setting up a connection between the transmitting
Computer and the Internet (e.g. by dial up Modem)
PPP, SLIP
Physical
The medium which carries the signals
RS232 (Serial cable),
10BasedT (Ethernet),
Fiber Optic, etc,
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E-mail
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One of the most important application that was developed
on the ARPANET was e-mail In 1970.
Email was a method of machine-to-machine message
exchange.
Ray Tomlinson (a hacker) had written the e-mail program,
he was the first who used the symbol “@” to separate the
sender’s name from the network – ID.
In 1973, ARPANET community formalized and email
protocol.
A revised email protocol has been introduced in RFC 680
“Message Transmission Protocol” IN 1975.
In 1977, the email protocol was revised and remained
unchanged since then.
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E-mail
Since its production in 1973, three quarters of the network traffic was
email because:
1.
It is a fast way of communication informal message.
2.
Message are easily composed with the help of email clients
and delivered instantly to the addresses.
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