OSI Model - Department of Electronic and Information Engineering

warmersafternoonNetworking and Communications

Oct 23, 2013 (3 years and 5 months ago)

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1



6.
Network Model




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6. Network Model

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Why Network Protocol Models?



Network communication is an extremely
complex task


Need cooperative efforts from all nodes involved


A standard model helps to
describe the task of a
networking product or service


Also help in
troubleshooting

by providing a
frame of reference

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Who define Network Model?



Need non
-
profit making organizations


ISO

-

International Standards Organization


e.g. OSI
, MPEG
-
1, 2, 4, etc. (
http://www.iso.ch/
)


IEEE

-

Institute of Electrical & Electronic
Engineers


e.g
IEEE 802,

IEEE 754, etc. (
http://www.ieee.org
)


ITU

-

International Telecommunication Union


e.g. V.34,

H.323, H.324, etc. (
http://www.itu.int
)

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The OSI 7
-
layer Model


A
ll


P
eople

S
eem

T
o

N
eed

D
ata

P
rocessing

A
way


P
izza

S
ausage

T
hrow

N
ot

D
o

P
lease


OSI

-

Open Systems Interconnection


Defined in 1984 and become an international standard


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Layered Architecture



Layering specifies different level
of functions and services


Each layer works with the layer
below and above it


Each layer provides services to
next layer


Shield the upper layer from the
details of actual implementation

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Relationship of OSI layers


Virtual

Communication

Physical

Communication

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Protocols in a layered architecture



Network communication is possible only if
machines speaking the same languages (
protocols
)


Since each layer work independently, each layer
speaks different languages (
protocols
)


Lead to the concept of a
Protocol Stack


Network communication is possible only if the
Protocol Stacks on two machines are the same

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To identify the language (protocol) of each layer,
identifier (header and trailer) are added to data


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1. Physical Layer


Define how cable is attached to
the Network Adapter Card


How many pins in the connector?


The impedance?


Max/min electrical voltage?


Responsible for transmitting
bits from one computer to
another

Example protocols: Ethernet, Token Ring (Physical part)

Appl.

Pres.

Sess.

Tran.

Netw.

Data.

Phys.

Appl.

Pres.

Sess.

Tran.

Netw.

Data.

Phys.

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2. Data Link Layer



Make data frames





Provide error
-
free frame
transfer by
acknowledgment

and
retransmission

Example protocols: Ethernet, Token Ring

Frame

Appl.

Pres.

Sess.

Tran.

Netw.

Data.

Phys.

Appl.

Pres.

Sess.

Tran.

Netw.

Data.

Phys.

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3. Network Layer



Logical address to physical
address translation



For TCP/IP running on Ethernet


Logical address: IP address


158.132.148.99


Physical address: Ethernet address


00 00 E2 15 1A CA


Determine the
route

from
source to destination computer

Example protocols: IP, IPX

Appl.

Pres.

Sess.

Tran.

Netw.

Data.

Phys.

Appl.

Pres.

Sess.

Tran.

Netw.

Data.

Phys.

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4. Transport Layer



Ensure packets are delivered
error free
,
in

sequence



Translate between packets and
message

1

3

2

Receiving

1

2

3

Sending

Example protocols: TCP, SPX, NWLink

Appl.

Pres.

Sess.

Tran.

Netw.

Data.

Phys.

Appl.

Pres.

Sess.

Tran.

Netw.

Data.

Phys.

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5. Session Layer


Example application program: Winsock, UNIX Socket

Appl.

Pres.

Sess.

Tran.

Netw.

Data.

Phys.

Appl.

Pres.

Sess.

Tran.

Netw.

Data.

Phys.


Establishment, maintaining and
release of session


Provide
dialog management


Regulate which side transmit,
when, for how long (Sync.)


Provide
synchronization

between user tasks

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A. Consecutive sessions on a
transport layer connection

1

2

3

B. Consecutive sessions on
a multiple transport layer
connections

1

2

3

What is a Session?


C. Both

1

2

3

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6. Presentation Layer


Example application program: redirector (NT), SSL

Appl.

Pres.

Sess.

Tran.

Netw.

Data.

Phys.

Appl.

Pres.

Sess.

Tran.

Netw.

Data.

Phys.


Translate

data from
Application Layer to the
format suitable for session
layer (the network)


Provide data
encryption
,
compression


Changing

or
converting

character set, graphic, and file
format

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7. Application Layer


Example protocols: FTP, Telnet, HTTP


Entry point

for application to
access network


Directly support user
applications


E.g. File transfer, email


General capabilities:


Network access, flow control,
Error recovery

Appl.

Pres.

Sess.

Tran.

Netw.

Data.

Phys.

Appl.

Pres.

Sess.

Tran.

Netw.

Data.

Phys.

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Example: File Transfer


PictureTel

FTP Server

FTP Client

Network


The most traditional network task


Implemented by a simple Application Layer
protocol called
FTP

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Utility: WS_FTP


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FTP

TCP

IP

Ethernet

FTP

TCP

IP

Ethernet

PictureTel

FTP Server

FTP Client

WS_FTP

Layers 1,2

Layer 3

Layer 4

Layers 5
-
7

Layers 1,2

Layer 3

Layer 4

Layers 5
-
7

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The 802 Project Model



IEEE launched the
Project 802

in 1980 February


Predate the ISO standard, but the development
was performed in roughly the same time


Objective
: To clearly define the network standards
for different kind of physical components of a
network
-

the interface card and the cabling


Enhance the
Data Link

and
Physical

layers of the
OSI model

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IEEE 802 Categories


802.1

Internetworking

802.2

Logical Link Control (LLC)

802.3

Carrier
-
Sense Multiple Access with Collision



Detection (CSMA/CD) LAN (Ethernet)

802.4

Token Bus LAN

802.5

Token Ring LAN

802.6

Metropolitan Area Network (MAN)

802.7

Broadband Technical Advisory Group


802.8

Fiber
-
Optic Technical Advisory Group

802.9

Integrated Voice/Data Networks

802.10


Network Security

802.11

Wireless Networks

802.12

Demand Priority Access LAN, 100 BaseVG
-
AnyLAN

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Enhancements to the OSI Model



Divide the
Data Link layer

into
2 sub
-
layers


Logical Link Control Sub
-
layer


Define the use of
service access points

(SAPs) as
an interface for higher layer to Data Link layer


Media Access Control Sub
-
layer


Directly interface

with the network card


For
delivering error
-
free

data communications

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Media Access

Control (MAC)

Logical Link
Control
(LLC)

802.3

802.4

802.5

802.12

802.2

802.1

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6. Network Model