Applications of Information and
Communication Technologies
Lecture 17
Presented By
Dr. Muhammad Umar Javed
Assistant Professor
Department of Computer Science
OSI Model
OSI stands Open System Interconnection.
An open system is a model that allows any two systems to communicate even
if their architectures are different
Developed by the International Standard Organization (ISO) in 1983.
Provides a logical framework for data communication through computer
networks.
The standards were created for computer industry to allow different networks
to work together efficiently.
OSI Model
OSI model consists of seven layers and each layer perform its specific function
in network communication
The layers are divided into two groups
Upper Layer:
The upper layer focus on user applications and how files are represented on
computer before transfer.
Lower Layer:
The lower layers are concerned with how the communication across a network
actually occurs
Layers of the OSI Model
Physical Layer
Data Link Layer
Network Layer
Transport Layer
Session Layer
Presentation Layer
Application Layer
OSI Model
Network Layers Interaction
Application Layer
Topmost layer of the OSI model.
Provides services directly to the user applications to access the network.
Provides a user interface for exchanging information between users.
Give support for services such as email, remote file access and transfer,
shared database management, and other types of distributed information
services.
Example: users exchange information on Internet (application program).
Presentation Layer
Performs data transformations to provide a common interface for user
applications.
Data Conversion
Sender side converts data into standard format that network requires.
Receiver side changes data from standard format into the format, the receiver can
understand it.
Encryption & Decryption
Encryption Sender, transforms original data/information into another form.
Encrypted information is sent over the network.
Decryption Receiver, re transforms (decrypts) the information back to its original
form.
Compression
Sender side compresses data into a small size (to reduce data transmission
duration)
Receiver side de compresses the data.
Session Layer
Establishes, maintains, and manages connections between end user
applications.
Session: The connection between applications running on separate computers
and an exchange of messages between them.
Synchronization involves the use of checkpoints in data stream. If a failure
occurs only the data from the last checkpoint is retransmitted.
Suppose, we want to send 1000 pages of data. Checkpoint can be used after
each 100 page. If there is an error at page 320, the pages from 301 will be
retransmitted. Page from 1 to 300 will not be retransmitted.
Transport Layer
Provides a mechanism for the exchange of data between source and
destination.
Controls the flow of data and ensures complete data transfer.
Sender Side
Breaks the large message into segments (small data packets).
Destination computer
reassembles the segments or data packets.
Also sends an acknowledgment of the receiving of data to the sender.
Network Layer
Makes routing decisions
Means it is responsible for establishing, maintaining, and terminating network
connections.
It manages the delivery of data from source to destination.
Determines a logical path between sender and receiver.
There may be many networks between two computers.
This layer manages to send data from source computer to the destination
computer.
Data Link Layer
Deals with the flow of data
Provides error free transfer of data
Checks whether the data has arrived properly and safely at the destination.
Also responsible for the reliability of the physical link established at the
Physical layer.
Data Link Layer
Data link layer must decide the following
Framing
The data link layer divides the stream of bits into manageable data units called frames
Flow control
The rate of sending data can be more than the rate of receiving data.
Data link layer imposes flow control mechanism to prevent it.
Error control
The data link layer detects and retransmits damaged or lost frames. It also prevents the
duplication of frames.
Access control
If two or more devices are connected to the same link, data link layer determines which
device has control over the link at a given time.
Data Link Layer –Sub Layers
Logical Link Control (LLC)
Establishes and maintains links between the communication devices.
Function is to ensure the reliability of the physical connection.
Media Access Control (MAC)
Controls the way in which multiple devices share the same communication
medium.
Specifies how different terminals or workstations share the same
communication media.
Physical Layer
Bottom most layer of the OSI model.
It transmits stream of bits and defines how the data is transmitted over the
network and what control signals are used.
Its main function is to control how a stream of bits is sent and received over the
physical medium
Physical layer decides the following
Characteristics of Media
The physical layer defines the characteristics and type of transmission
medium.
Representation of Bits
The bits are encoded into electrical signals for transmission. The physical
layer defines the type of encoding.
Lecture End
