The document provides an overview of the 7-layer OSI model that is used to connect computers over a network. It describes each layer in order from the application layer down to the physical layer. The application layer allows user communication programs like web browsers, email clients, and file transfer protocols to operate. Lower layers deal with networking functions like packetizing data, routing, error checking, and physical connections between devices. The physical layer transmits the raw bit data over various transmission mediums like wires or fiber optics.

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OSI Model
Computer Networks
& Internet Computing
Nifras Ismail

2. 2
OSI / ISO MODEL
Contents
Introduction ....................................................................................................................................3
Layer 7 – Application Layer ................................................................................................................5
Layer 6 – Presentation Layer ..............................................................................................................6
Layer 5 – Session Layer ......................................................................................................................7
Layer 4 – Transport Layer ..................................................................................................................8
Layer 3 – Network Layer ....................................................................................................................8
Layer 2 – Data Link Layer ....................................................................................................................9
Layer 1 - Physical Layer .................................................................................................................... 10
References
1. TechExams.net Tech Notes by. John Hiemstra
2. Computer Networks by. Andrew S. Tanenbaum
3. Computer Networks and Internet computing Lecture notes by. Dr. K. Sarveswaran
4. http://www.9tut.com/
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3. 3
OSI / ISO MODEL
Introduction
SI Model network devices data transmit
O .
postal system
1. Write your letter
2. Insert it into an envelope
3. Write information about sender and receiver on that envelope
4. Stamp it
5. Go to the post office and drop it into a mail inbox
we go through some steps to achieve the target,
2 computers connect steps follow .
OSI Model . There is 7 steps in this model to connect to
pc’s
This OSI Table is must be memorized in your heart by the correct order. You can easily
All People Seem To Need Data Processing
memorized by the sentence below.

4. 4
table step layer .
device want to send a data to another one that data 1st send to top of the table to bottom of
the table , but when it receive that data from another it goes in another way ( device
Data send data
data receive .)
Note: The OSI model layers are often referred to by number than by name (for example, we refer
saying “layer 3″ to “network layer”) so you should learn the number of each layer as well.
data processing layer header
, header information ( where the data
goes down , how its going , network devices MAC address , network IP) header
Layer
Receiving part understand .

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Now we see the deeper look of each layer on this OSI Model
Layer 7 – Application Layer
It provides the interface between the applications ( application user
communication .) But notice that the programs you are using (like a web
browser – IE, Firefox or Opera…) do not belong to Application layer,
Telnet, FTP, email client (SMTP), Hyper Text Transfer Protocol (HTTP) , DNS are
examples of Application layer.
HTTP :
FTP : file transfer .
SMTP : email .

6. 6
OPTIONAL :
DNS (Domain Name System)
It provides domain name to the IPs
All hosts and IPs are saved in a file, that is name as host.txt in ARPANET, So thousands of host
name providing is not a simple problem its very complex, so they invented a method to provide the
Domain Name System that is simply say DNS – This is an hierarchically domain based system.
This domain names are provide like as postal system that is country , state, town, road and home.
Domain names provide in 2 main top levels they are
1. Generic
2. Countries
Example diagram for the levels
The top-level domains come in two flavors: generic and countries. The original generic
domains were com (commercial), edu (educational institutions), gov (the U.S. Federal
Government), int (certain international organizations), mil (the U.S. armed forces), net
(network providers), and org (nonprofit organizations). The country domains include one entry
for every country, as defined in ISO 3166.
Layer 6 – Presentation Layer
This layer ensure the presentation of the data. 2 computers communicate
format bit format (1,0) data
transmit . data format .
Presentation layer concentrate syntax and semantics of the information transmit. In
order to make it possible for computers with different data representations to communicate, the
data structures to be exchanged can be defined in an abstract way, along with a standard
encoding to be used ''on the wire.'' The presentation layer manages these abstract data
structures and allows higher-level data structures

7. 7
In general, it acts as a translator of the network. For example, you want to send an
email and the Presentation will format your data into email format. Or you want to send photos
to your friend, the Presentation layer will format your data into GIF, JPG or PNG… format.
Dictionary
1. Concentrate –
2. Syntax –
3. Semantics –
4. Transmit –
5. Representation –
6. Exchanged –
7. Abstract way –
8. Encoding –
9. Manage –
10. High-level structure – Human structure
11. General –
Layer 5 – Session Layer
The session layer allows users on different machines to establish sessions, maintained
communication between them.
- communication .
Sessions offer various services including
1. Dialog control -
2. Token Management
3. Synchronization. – communication
. 120MB Download file 80MB
crash 80MB .
Dictionary Dig
1. Crash -
2. Token -
3. Establish -

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Layer 4 – Transport Layer
Accept data from above layer and split it up into small segments – layer
segments Network layer pass
duty .
Ensure all segments arrive correctly and upper layers isolated by inevitable changes in
hardware devices ( send order all segment .
hardware devices ( Routers , hubs , etc…….)
layers .
This layer maintain
1. Flow control of data – data .
2. Provides for error checking and recovery of data - error checking ( error- free – point –
to – point channels) point point errors check .
protocol
TCP ( Transmission Control Protocol) , UDP ( User Diagram Protocol ) , NETBEUI , SPX
Dictionary Dig
1. Split –
2. Segments –
3. Arrive –
4. Isolate –
5. Inevitable-
6. Flow –
7. Provide –
8. Recovery –
9. Diagram –
10. Destination -
Layer 3 – Network Layer
Transport Layer segments packets . packets
. packets original data
destination . address routing table
packets routing .
Network layer :
1. Path determination – source to destination path .
2. Routing – putting address
3. Delivery of packets across the inter-networks. – networks packets
delivery .
packets connection , .
4. Responsible for logical addressing (Example :IP)

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protocols :
IP, IPX, ICMP, RIP, OSPX, BGF
Network layor devices :
Layer 3 Switches , Routers , WAP ( Wireless access point – wireless routers)
Dictionary Dig
1. Determination –
2. Across –
3. Path –
4. Delivery –
5. Responsible –
6. Put -
Layer 2 – Data Link Layer
Packets Layer3 message Data frames header
add .
This header contains :
1. Adding hardware destination (MAC address) – doing by switches
2. Source address
And this header is also responsible to finding the next destination device on the local network,
Note. Layer-3 ( Network layer ) finding the last destination path ( network ) not care who is the
receiving device
Example of devices use in layer 2 are : switches , bridges ,WAPs ,
It divides into mainly 2 parts :
1. LLC – Logical link control
- Managing frames
- Flow control
- Error control
2. MAC – Media access control
- Carries physical addresses (MAC addresses)
Dictionary Dig
1. Contains-
2. Find-
3. Local network –

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4. Care –
5. Divides-
6. Manage
7. Carries-
8. Access -
Layer -1 Physical Layer
The Physical Layer defines the physical characteristics of the network such as connections,
voltage levels and timing. – .
layer bit (1,0) data .
- Computer wire data transfer 1,0 format 1
voltage 0 voltage layer .-( how many volts
should be used to represent a 1 and how many for a 0)
- bit nano seconds
- Initial connection established
- How many pins networks connectors pluged. physical layer
.
layer devices : Repeaters and hubs Transmission medium
(copper wire, UTP, STP ,fiber …) .
i
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