1. Presentation Layer
This layer converts data received from application layer
into machine's native internal numeric format and
encoded transmitted data into displayable form for
output.
It performs code conversions, text compressions, security
encryption etc. on the message.
It represents data according to the software/hardware
environment of the node. For example, presenting Unix
formatted data in windows.
The relationship between the presentation layer and the
application and session layer is shown in fig
2.
3. Functions of Presentation layer are:
1. Data Compression
It reduces the number of bits to be transmitted by compressing the data.
2. Data Conversion
It formats the data on different nodes as per their Software/Hardware
environment.
3. Code conversion
The form and syntax (language) of the two communicating systems can be
different. For example one system is using the ASCII code for file transfer and the
other one uses IBM's EBCDIC. Under such conditions the presentation layer
provides the "translation" from ASCII to EBCDIC and vice versa.
4. 4. Data Encryption
It encodes data in a specific format so as it can not be understood by each
and every user or application.
5. Application Layer
This is the top most layer of the OSI architecture.
It is the layer seen by application program, and hence is user oriented layer, which
provide services to support end user of network directly.
A message to be sent across network enters the OSI model at this point and then
travels downward into physical layer then through transmission line upto physical
layer of destination & then upwards upto application layer of receiver end system.
It provides services that directly support user applications such as database access,
e-mail and file transfer. Some popular application layer protocols are X400(Email
protocol) X500(Directory Server Protocol), FTP (File Transfer Protocol), and rlogin
(Remote Login Protocol). Fig. shows the relationship of the application layer to the
user and presentation layer.
6.
7. Functions of Application layer are:
1. User Interface
It defines user interface to low level layers and various application
processes.
2. Security
It is responsible for implementation of security checks at user entry
points.
3. File Transfer
The application layer provides file transfer access and management
(FTAM) which allows a user to access files in a remote computer to retrieve files
from a remote computer and to manage or control files in a remote computer.
8. 4. E-mail
It provides a basis for e-mail forwarding and string.
5. Database Access
It provides distributed database sources and access for global
information about various objects and services.
