Transcript Computer Networks - University of Northampton

OSI and TCP Reference Models
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

To understand
Basic definitions
 Protocol
 Application

Understand communication process using
Reference models
◦ OSI Reference Model
◦ TCP/IP - Internet Model
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A
network protocol defines the
◦ format and the order of messages
◦ exchanged between two or more
communicating entities, as well as the
◦ actions taken on the transmission
and/or receipt of a message or other
events.
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mail
server
user
agent
user
agent
mail
server
mail
server
user
agent
user
agent
user
agent
user
agent
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Messages format and order
 From a client to a mail server
◦
◦
◦
◦
HELO
MAIL FROM: <address>
RCPT TO: <address>
DATA
<This is the text end with a
line with a single .>
◦ QUIT

user
agent
mail
server
From a mail server to a client
◦ status code
 1xx - Informative message
 2xx - Command ok
 3xx - Command ok so far, send the rest of
it.
 4xx - Command was correct, but couldn't
be performed for some reason.
 5xx - Command unimplemented, or
incorrect, or a serious program error
occurred.
◦ content
SMTP
SMTP
mail
server
user
agent
SMTP
POP3,
IMAP
SMTP
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outgoing
message queue
user mailbox
user
agent
mail
server
user
agent
user
agent
user
agent
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Network Applications
are Programs that
run on different end
systems and
applicati
on
transpor
t
network
data link
physical
communicate over
a network.
applicati
on
transpor
t
network
data link
physical
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applicati
on
transpor
t
network
data link
physical
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Number of Hosts on the
Internet:
Aug. 1981
213
Oct. 1984
1,024
Dec. 1987
28,174
Oct. 1990
313,000
Jul. 1993 1,776,000
Jul. 1996 19,540,000
Jul. 1999 56,218,000
Jul. 2004 285,139,000
Jul. 2005 353,284,000
Jul. 2007 489,774,000
Jul. 2008 570,937,000
Jan. 2009 625,226,000
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
All standards of the Internet are published as
RFC (Request for Comments)

e.g., the SMTP protocol is specified in RFC821
A typical (but not the only) way of standardization:

Internet draft
 RFC
 proposed standard
 draft standard (requires 2 working implementations)
 Internet standard (declared by Internet Architecture Board)
We reject: kings, presidents, and voting. We believe in: rough
consensus and running code.

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


Why need standards?
The use of standards in protocols ensures that
products from different manufacturers can work
together
Do you know of name of a standards body for :
◦ food industry
 FSA
◦ Computer Science and Engineering
 IEEE
 And many others
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


The problem of moving information
between computers over a network
medium is divided into smaller and
more manageable problems
This separation into smaller more
manageable functions is known as
layering.
Two Reference Models:
◦ OSI -7 Layer Model
◦ TCP/IP Model
 Also known as the Internet model
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Ambassador
in Korea
Ambassador
in France
여보세요
Bonjour!
Translator
Translator
She is
saying
hello
The Korean
ambassador said
“Hello”
Secretary
Secretary
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


A standard reference model
called OSI (Open Systems
Interconnect ) is now considered
the primary Architectural model
for inter-computer
communications
The OSI model divides the
networking process into seven
layers.
Each layer in the model has a
unique function in the
communication process and
communicates with the layer
directly above and below it.
OSI Model
Application
Presentation
Session
Transport
Network
Data Link
Physical
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
Information passed from
source to destination has to
pass through each layer on
each side.

Which layer does the user
interact with:
 Application or Physical ?

Which layers are connected
by a cable?
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Phrase to remember 7 layers is:
All People Seem To Need Data Processing
 What is a protocol?
 Examples of some network applications?
 Need for standardisation?
 Example of a standard?
 How many layers in OSI model?

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
Physical Layer
◦ Media
◦ Topology
◦ Making Cables
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