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23 Οκτ 2013 (πριν από 3 χρόνια και 7 μήνες)

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OSI LAYERS
-
Networks

Tony Caleb D

14
-
July
-
2010


OSI: Open System Interconnection .



Developed by the International Organization for
Standardization in 1974.



There are seven layers in OSI reference Model .



It is a layered approach.







OSI Reference
Model

An Exchange using the OSI Layers

OSI Reference Model

Application

Presentation

Session

Transport

Network

Data Link

Physical

Layer
-

7

Layer
-

6

Layer
-

5

Layer
-

4

Layer
-

3

Layer
-

2

Layer
-

1

Upper Layer

or

Software Layer

Lower Layer

or

Hardware Layer

Heart of OSI

Application Layer


Application Layer

is responsible for
providing Networking Services to the user.
It is also known as Desktop Layer.
Identification of Services is done using Port
Numbers.



Ports are Entry and Exit Points to the Layer



Total

No
.

Ports



0



65535



Reserved

Ports



0



1023


Open

Client

Ports

1024



65535

Application

Presentation

Session

Transport

Network

Data Link

Physical

Application

Example of HTTP request

Client

Web Server

http://www.zoomgroup.com

Client

Web Server

Example of HTTP request

HTTP Request


http:// www.zoomgroup.com

Webpage

HTTP Request

Listen on

Port 80

Sending HTTP Reply

Webpage

Received HTTP Reply


http://
www.symantec.com

Webpage

Client

Web Server

Examples of Networking Services

Service



Port No.

HTTP




80


FTP




21

SMTP



25

TELNET



23

TFTP



69

Application

21

80

25

67

53

69

Data flow from Application Layer

Data

Presentation

Session

Transport

Network

Data Link

Physical

Presentation Layer


Presentation Layer

is responsible for
converting data into standard format.



Examples :
JPEG
,
MPEG,
MIDI, WAV, MP3



Following tasks are perform at Presentation
layer :



Encoding


Decoding


Encryption


Decryption


Compression


Decompression

Application

Presentation

Session

Transport

Network

Data Link

Physical

Presentation

Data flow from Presentation Layer

Data

Data

Application

Presentation

Session

Transport

Network

Data Link

Physical

Session Layer


Session Layer

is responsible for establishing,
maintaining and terminating session.


Session ID works at Session Layer.




Examples :



RPC


Remote Procedure Call



SQL


Structured Query Language


NFS


Network File System

Application

Presentation

Session

Transport

Network

Data Link

Physical

Session

Data flow from Session Layer

Data

Data

Data

Application

Presentation

Session

Transport

Network

Data Link

Physical

Transport Layer


Transport Layer is
responsible
for end
-
to
-
end
connectivity. It is also known as the heart of
OSI Layers. Following tasks are performed at
the Transport Layer :
-




Identifying Service


Multiplexing & De
-
multiplexing


Segmentation


Sequencing & Reassembling


Error Correction


Flow Control

Application

Presentation

Session

Transport

Network

Data Link

Physical

Transport

Identifying Service


Transmission Control Protocol


Connection Oriented


Acknowledgement


Reliable


Slower


Port No. 6


e.g. HTTP, FTP, SMTP


User Datagram Protocol


Connection Less


No Acknowledgement


Unreliable


Faster


Port No. 17


e.g. DNS, DHCP, TFTP

TCP

UDP

Multiplexing & De
-
multiplexing

Application

Presentation

Session

Multiplexing & De
-
multiplexing

21

80

25

67

53

69

Transport

TCP
-

6

UDP
-

17

Network

Data Link

Physical

Segmentation

Hello! How are you ?

Hello!

How

Are

You

?



Hello! How
are you ?


A

B

Sequencing & Reassembling

Hello! How are you ?

Hello!

How

Are

You

?



Hello! How
are you ?


A

B

You Hello! How ? Are

Hello!

How

Are

You

?



you Hello!
How ? are


Sequencing & Reassembling

Hello! How are you ?

Hello!

1/5

How

2/5

Are

3/5

You

4/5

?

5/5



Hello! How
are you ?


A

B

Hello! How are you ?

Hello!

1/5

How

2/5

Are

3/5

You

4/5

?

5/5

Hello!

1/5

How

2/5

Are

3/5

You

4/5

?

5/5



Hello! How
are you ?


Error

Correction

Hello! How are you ?

Hello!

1/5

How

2/5

Are

3/5

You

4/5

?

5/5



Hello! How
are you ?


Hello!

1/5

Are

3/5

You

4/5

?

5/5

Hello!

1/5

Are

3/5

You

4/5

?

5/5

Segment Missing

Hello!

1/5

Are

3/5

You

4/5

?

5/5

How

2/5

How

2/5

Hello! How are you ?



Hello! How
are you ?


A

B

Flow Control
-

Windowing

Sending 3
-
window

Received 3
-
window

Ack
-
Rec
-
3

Sending

5
-
window

Received 4
-
window

A

B

Ack.
Received
-
4

PC
-
A


can send 4 Segments at a
time to

PC
-
B

Data flow from Transport Layer

Data

Data

Data

Data

TH

Segment

Application

Presentation

Session

Transport

Network

Data Link

Physical

Network Layer


Network Layer is

responsible
for providing best
path for data to reach the destination. Logical
Addressing works on this layer. Router is a
Network Layer device.



It is divided into two parts


Routed Protocols


e.g. IP, IPX, Apple Talk.


Routing Protocols


e.g. RIP, IGRP, OSPF, EIGRP

Application

Presentation

Session

Transport

Network

Data Link

Physical

Network

Routed Protocols



Hello! How
are you ?


192.168.1.1



192.168.1.2

A

B

Segment

Segment

Source IP


192.168.1.1


Destination IP

192.168.1.2

Source IP


192.168.1.1


Destination IP

192.168.1.2

Segment



Hello! How
are you ?


Routing Protocols

A


www.symantec.com

Hop to Hop Delivery

IP Addresses

Data flow from Network Layer

Data

Data

Data

Segment

Segment

NH

e.g. Router

Packet

Application

Presentation

Session

Transport

Network

Data Link

Physical

Data link Layer


Datalink

Layer

is divided into two Sub



LLC


Logical Link Control


It talks about Wan protocols e.g. PPP, HDLC,
Frame
-
relay



MAC


Media Access Control


It talks about Physical Address.
It is a 48 bit
address i.e. 12 digit Hexadecimal Number.


It is also responsible for Error Detection


Devices working on Data Link Layer are
Switch, Bridge, NIC.


Application

Presentation

Session

Transport

Network

Data Link

Physical

Data Link

Error Detection


CRC Check



Hello! How
are you ?


Packet

Packet

Source MAC

00
-
20
-
18
-
C0
-
07
-
71


Destination MAC

00
-
20
-
18
-
C0
-
07
-
72

00000

32434

22222

89696

55434

99323

43434

99434

46323

11114

33333

00000

32434

22222

89696

55434

99323

43434

99434

46323

11114

Error Detected

192.168.1.1

00
-
20
-
18
-
C0
-
07
-
71

192.168.1.2

00
-
20
-
18
-
C0
-
07
-
72

A

B

PC
-
A

CRC No. 33333

PC
-
B

CRC No. 11114

Error Detected


For Error Correction

Contact Source Transport layer

Error Detection


CRC Check

Packet

Packet

Source MAC

00
-
20
-
18
-
C0
-
07
-
71


Destination MAC

00
-
20
-
18
-
C0
-
07
-
72

00000

32434

22222

89696

55434

99323

43434

99434

46323

11114

20202

00000

32434

22222

89696

55434

99323

43434

99434

46323

11114

20202

Source MAC

00
-
20
-
18
-
C0
-
07
-
71


Destination MAC

00
-
20
-
18
-
C0
-
07
-
72

Packet



Hello! How
are you ?


192.168.1.1

00
-
20
-
18
-
C0
-
07
-
71

192.168.1.2

00
-
20
-
18
-
C0
-
07
-
72

A

B

e.g. Switch

Data flow from Data Link Layer

Data

Data

Data

Segment

DH

Packet

DT

Application

Presentation

Session

Transport

Network

Data Link

Physical

Packet

Packet

Frame

Physical Layer


Physical Layer is

responsible for electrical,
mechanical and procedural checks. Data will be
converted into Binary (
i.e
) 0’s & 1’s. Data will be in
the form of electrical pulses if it is Coaxial or
Twisted Pair cable and in the form of Light if it is
Fiber Optic Cable.



Devices working at Physical Layer are Hubs,
Repeaters, Cables, Modems etc.

Application

Presentation

Session

Transport

Network

Data Link

Physical

Physical

Physical Layer Example

Frame

Frame

1010101010101010

1010101010101010

Frame



Hello! How
are you ?


A

B

0101011010101010101010101

1010101101010101010101010

0101011010101010101010101

1010101101010101010101010

0101011010101010101010101

Data flow from Physical Layer

Data

Data

Data

Segment

Packet

Frame

Bits

e.g. Hub

Application

Presentation

Session

Transport

Network

Data Link

Physical

Segment

Packet

Frame

A

B

Packet

Data

Data

Data

Bits

Data

Segment

Data Encapsulation & De
-
capsulation

Packet

Data

Data

Data

Application

Presentation

Session

Transport

Network

Data Link

Physical

Application

Presentation

Session

Transport

Network

Data Link

Physical

Bits

DH

DT

NH

TH

Segment

Packet

DT

Packet

DH

DT

DH

NH

Segment

NH

TH

Data

TH

Frame

Packet

Application

Transport

Internet

Network

Access

Comparing OSI with TCP/IP Layers

OSI Layers

TCP/IP Layers

Application

Presentation

Session

Transport

Network

Data Link

Physical

Relationship Between Layers and Address in TCP/IP

Summary of layers