Open System
Interconnect (OSI) Model:
In 1978
International standard organization introduced a model called open system
interconnection. OSI is not a protocol,
it is a model of understanding and network architecture that is flexible robust
and interoperable.
OSI model is built of seven layers.
Seven Layers of
OSI
1) Physical layer
2) Data link layer
3) Network layer
4) Transport layer
5) Session layer
6) Presentation layer
7) Application layer
When a message is to be sent from device A to device
B this message must pass through these layers . Figure1 (On the next slide)
shows the layer involved when message is sent from device A to device B
These layers can be thought of as belonging to three subgroups.
Layer 1,2 and 3 i.e. physical data link and network layer deals with physical
aspect of moving data, layer 4 is responsible for end-to-end data transmission
for entire message. Layer 5 6 and 7 that is session presentation and
application can be thought of as user support layers. They allow
interoperability among on related software systems.
Functions of Layers
1. Physical Layer
Physical layer is
the lowest layer of OSI model. It deals with mechanical and electrical
specification of interface and transmission medium. The physical layer
concerned with the following:
1) Physical Characteristics of interface and transmission media .
2) Type of encoding that is which signal pattern represents a binary 1 and which represents a binary zero
3) The transmission rate that is the number of bits sent/received per second
4) Synchronization of bits. the sender and the
receiver must be synchronized at bit level
2. Data Link Layer
The data link
layer provides error free transfer of data frames from the one computer to
another over the physical layer. Data link layer performs the following
functions-
a. Framing:
The data link
layer divides the stream of BITS received from the upper layer into the manageable
data units called frames.
b. Physical addressing
Frames are to be
distributed to different systems on the same network, the data link layer adds
a header to the frame to define physical address of sender and receiver of the
frame.
c. Flow control:
if the rate at
which the data absorbed by the receiver is not equal to the rate at which the
data is produced at the centre side the data link layer impose is a flow
control mechanism to prevent overwhelming of receiver.
The data link adds
reliability to physical layer by hiding a mechanism to detect and retransmit
damaged or lost frames. It is also
useful mechanism to prevent duplication of frames.
When two or more
devices are connected to the same link, data link layer protocols are necessary
to determine which device has control over the link at any given time.
3. Network Layer
v
The
network layer is responsible for source to destination delivery of data packet
possibly across multiple networks.
v
If two systems
are connected to the same network, there is usually no need for network layer,
if two systems are attached to different networks with the connecting device
between them often there is need for network layer to accomplish source to
destination delivery.
The physical addressing implemented by Data link layer
handles addressing problem locally. Is packet crosses the network boundary we
need another addressing to help distinguish the source and destination systems.
The network layer ads are headed to the packet coming from the upper layer that
include logical address of sender and receiver.
if independent
networks are connected together to create an internetwork or large network, the
connecting device called routers or gateways route the packet to the final
destination.
4.Transport Layer
Responsibilities of
Transport Layer
A message is
divided into transmittable segments. Each segment contains a sequence number.
These numbers enables the transport
layer to reassemble the entire message correctly upon arriving at the
destination and to identify and replace packets that we are lost in the
transmission.
The transport
layer can be either connectionless are connection oriented. Connectionless
transport layer treats each segment as independent packet and delivers it to
the destination machine. A Connection oriented transport layer makes a
connection with the transport layer at the destination machine before
delivering the packet. After all the data is transferred the connection is
terminated.
Like data link
layer the transport layer is responsible for error control. However error
control at this level is performed end to end rather than across a single link
Like the data link layer, the transport layer is responsible for flow control. However flow control at this level is performed end to end rather than across a single link
The services
provided by the first three layers (physical, data link and network) are not
sufficient for some processes. Session layer is network dialogue controller. It
is establishes maintains and synchronizes the interaction between the
communication system.
Specific
responsibilities of session layer include the following:
a. Dialog Control:
The session layer
allows two systems to enter into dialogue. it allowed the communication between
two processes to take place either in half duplex or full duplex mode.
The session layer
allows a process to add checkpoints (synchronization points) into the stream of
data for example if the system is sending a file of two thousand pages it is
advisable to insert checkpoints after every hundred pages to ensure that
hundred page unit received and acknowledged independently.
6.Presentation Layer
Presentation
Layer is concerned with the syntax and
the semantics of information exchange between two systems.
Specific
responsibilities of presentation layer include the following
a. Translation
The processes in
the two systems are usually exchanging information in the form of character
strings numbers and so on. The information should be changed to bit stream
before being transmitted. because different computers use different encoding
systems, the presentation layer is responsible for interoperability between these
different encoding methods the presentation layer at the sender's side changes
the information from sender dependent format to a common format
To carry sensitive
information a system must be able to achieve privacy. the sender transforms the
original information to another form and sender the resulting information out
over the network.
Data compression
reduces the number of bits to be transmitted data compression becomes
particularly important in transmission of multimedia such as text audio and
videos.
a. Network virtual
terminal
Network virtual
terminal is a software version of physical terminal and allows a user to login
to a remote host the user computer talks to the software terminal which in turn
talks to the host and vice versa
Application layer
enables the user to access the network. It provides user interface and support
for services such as electronic mail, remote file access transfer and
management remote database management and another type of distributed
information source services.
Specific services provided
by the application layer include the following
b. File Transfer access
and Management
This application
allows a user to access a file in a remote computer, to retrieve file from
remote computer and to manage file in remote computer
This application
provides the basis for email forwarding and storage
This application
provides distributed database source and access for global Information about
various objects and services.