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

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. Data Encryption

It encodes data in a specific format so as it can not be understood by each and every user or application.

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.

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.

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.