OSI Reference Model

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DUE 0903239

OSI Reference Model

Chapter 05

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OSI Reference Model


The Open System Interconnection reference model
(OSI) was created in the early 1970’s by the International
Standards Organization (ISO).



The OSI is the standard that provides communication
between end nodes in a network.



It is a 7 layered model that allows independent
development of layered equipment.



Allows communication from one network system to
another: UNIX to Windows


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Application layer



The action: you type in a

URL and press Enter. The

Browser begins its
journey


Application
-

Layer 7

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Application layer


Presentation layer



The action: you type in a

URL and press Enter. The

Browser begins its
journey

Presentation
-

Layer 6

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Session
-

Layer 5


Application layer


Presentation layer


Session layer



The action: you type in a

URL and press Enter. The

Browser begins its
journey

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Transport
-

Layer 4


Application layer


Presentation layer


Session layer


Transport layer

The action: you type in a

URL and press Enter. The

Browser begins its
journey

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Network
-

Layer 3


Application layer


Presentation layer


Session layer


Transport layer


Network layer

The action: you type in a

URL and press Enter. The

Browser begins its
journey

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Data Link
-

Layer 2


Application layer


Presentation layer


Session layer


Transport layer


Network layer


Data
-
link layer



The action: you type in a

URL and press Enter. The

Browser begins its
journey

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Physical
-

Layer 1


Application layer


Presentation layer


Session layer


Transport layer


Network layer


Data
-
link layer


Physical layer

The action: you type in a

URL and press Enter. The

Browser request begins

its digital
journey

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The 7 Amazing OSI
Layers as
seen by your
browser


Application layer


Presentation layer


Session layer


Transport layer


Network layer


Data
-
link layer


Physical layer

The action: you type in a

URL and press Enter. The

Browser request begins

its digital
journey

Packet Request
goes out on the media

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OSI
Reference Model Open
Systems
Interconnect

Computers communicate guided by
a series of layers


Developed
by ISO


International Standards Organization


7 Layers


Layer 7


Application


Closest to end user


access information on network via application


e
-
mail, databases


Layer 6


Presentation


Translates data and encrypts or compresses it


Layer 5


Session


Controls sessions between computers


Layer 4


Transport


Transfers data between end users and checks for reliability


Layer 3


Network


Performs network routing


determines paths


Layer 2


Data link


Provides physical addressing


MAC address


packages bits of data


Layer 1


Physical


Transmits bits (0 and 1)


regulates transmissions over medium such as
wire, optical fiber, or air

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Remembering The Layers


P
lease



A
ll


D
o




P
eople


N
ot




S
eem


T
ake




T
o


S
ales




N
eed


P
ersons



D
ata


A
dvice




P
rocessing


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APPLICATION
-

Layer 7


Layer 7 is the layer that allow the human interface with
the network using tools such as TFTP, FTP POP and
Telnet just to name a few.

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PRESENTATION
-

Layer 6


Layer 6 provides coding, encryption, decryption
compression and decoding of the data between two
nodes.



This is the layer that allows two completely different
systems to communicate.



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SESSION
-

Layer 5


Layer

5

is

responsible

for

the

session

setup

and

ending

of

communication

between

the

two

end

devices
.

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TRANSPORT
-

Layer 4


Layer 4 is the layer that takes the user data and breaks
or chops it into smaller more manageable sizes called
segment.



Layer 4 supplies flow control and also error
retransmission of segments it does not get
acknowledgments.



Layer 4 uses UDP and TCP as its transport protocol.

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NETWORK
-

Layer 3


Layer 3 is considered the traffic cop of the network. Its
job is to read the destination address and decide which
port to send it out so it can reach its destination.



Routers operate at this layer and use an address or
route table to forward packet to their destination.



Logical IP address are applied at this layer
.

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DATA LINK
-

Layer 2


Layer 2 is for reliable transit across the physical network by creating the it to
be placed on the physical link. Some error control and frame placement is
done here.



Data Link layer is divided into two separate sub
-
layers. The MAC layer and
the Logical Link Control.


MAC is responsible for access to the physical media and contains the
physical (MAC) address
.
Communicates with layer 1


Example 00
-
16
-
3f
-
51
-
54
-
99


LLC manages the communication between the two end devices. It uses
flow control, error control and frame synchronization.



Error
-
checking checksums

are added to the end of the packet (also
known as a trailer).



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Layer 2 Components


Bridge
-

A simple device that has the ability to look at
incoming traffic and examine it to determine if it can be
sent out the other side. Often referred to as a filter.



Switch
-

Multiport bridge that determines which switch
port traffic goes out by comparing the destination MAC
address to a MAC table stored in its memory



Network Interface Card
(NIC)
-

the interface between
the layer one media and the layer 2 device.

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PHYSICAL
-

Layer 1


In order for date to move from node the data is
converted to a data bit stream.



That bit stream can be electrical pulses, light pulses or
radio waves.



The Physical layer is the copper, fiber or air waves the
data signal travel on.



Physical layer devices/ components are Hubs, wires,
connectors and pins.

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Encapsulation


Each layer of the OSI model creates its own Protocol
Data Unit (PDU) these PDU’s include the header

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OSI Layers of Encapsulation


Application layer


Presentation layer


Session layer


Transport layer


Network layer


Data Link layer


Physical layer

DATA


DATA


DATA


SEGMENTS


PACKETS


FRAMES


BITS

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TCP Model VERSES OSI Model


Application layer


Presentation layer


Session layer


Transport layer


Network layer


Data Link layer


Physical layer



Application layer


Transport layer


Internet layer

Network Interface
layer

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Why are Layers 1
-

4 important to
Convergence?


As data filters down through the OSI Model layers, it is molded by
layers 7


5



At layer 4 it receives a TCP header



At layer 3 it receives an IP header



At layer 2 it completes the “header” process by receiving Layer 2
headers and a trailer called a frame check sequence (FCS),
encapsulating all the previous digital information.



At layer 1 the formatted bits exit enter the cable

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Credits

Icons: permission from Cisco Systems

Chart and artwork: Mike Harsh


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This material is based upon work supported by the National Science Foundation under

Grant No. 0402356. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the
author(s) and do not necessarily reflect the views of the National Science Foundation



Questions?


Feel free to contact the creator of this material

Mike Harsh, Professor, Collin College


mharsh@collin.edu