THE OSI MODEL

• Established in 1947, the International Standards Organization (ISO) is a multinational body dedicated to worldwide agreement on international standards.
• An ISO standard that covers all aspects of network communications is the Open Systems Interconnection (OSI) model. It was first introduced in the late 1970's.
• The open system is the set of protocols that allows any two different systems to communicate without regarding of their underlying architecture (h/w and s/w).

Note

ISO is the organization.
OSI is the model.

Seven layers of the OSI model

The interaction between layers in the OSI model

An exchange of data using the OSI model

LAYERS IN THE OSI MODEL

In this section we briefly describe the functions of each layer in the OSI model.

Topics discussed in this section:

• Physical Layer
• Data Link Layer
• Network Layer
• Transport Layer
• Session Layer
• Presentation Layer
• Application Layer

PHYSICAL LAYER

The physical layer is responsible for movements of individual bits from one hop (node) to the next.

The major functionalities of physical layer are:

• Physical characteristics of interfaces and medium
• Representation of bits
• Data rate
• Synchronization of bits
• Line configuration
• Physical Topology
• Transmission mode

DATA LINK LAYER

The data link layer is responsible for moving/sending frames from one hop (node) to the next.

The major functionalities of data link layer are:

• Framming
• Physical addressing
• Flow control
• Error control
• Access control

Physical Addressing

Most local-area networks use a 48-bit (6-byte) physical address written as 12 hexadecimal digits; every byte (2 hexadecimal digits) is separated by a colon, as shown below:

07:01:02:01:2C:4B

A 6-byte (12 hexadecimal digits) physical address.

NETWORK LAYER

The network layer is responsible for the delivery of individual packets from the source host to the destination host.

The major functionalities of Network layer are:

• Routing
• Logical Addressing
• Packet Switching

Source-to-destination delivery

TRANSPORT LAYER

The transport layer is responsible for the delivery of a message from one process to another.

Reliable process-to-process delivery of a message

The major functionalities of Transport layer are:

• Segmentation
• Port addressing
• Flow control
• Error control

SESSION LAYER

The session layer is responsible for dialog control and synchronization.

The major functionalities of Session layer are:

• Synchornization
• Dialog Control

PRESENTATION LAYER

The presentation layer is responsible for translation, compression, and encryption.

APPLICATION LAYER

The application layer is responsible for providing services to the user.