OSI Model

The Open Systems Interconnection (OSI) model is a reference tool for understanding data communications between any two networked systems. It divides the communications processes into seven layers. Each layer performs specific functions to support the layers above it and offers services to the layers below it. Such a model of layers is also called a “protocol stack” or “protocol suite”.

Application Layer:

This is the layer with which the user interacts. It provides services, such as

·         file transfers

·         electronic messaging

·         e-mail

·         network management

·         web browsing

Examples of application layer implementations include File Transfer Protocol (FTP), Simple Mail Transfer Protocol (SMTP), Hyper text transfer protocol (HTTP).

 

Presentation Layer:

Translates data from the Application Layer into a network format. It provides a variety of coding and conversion functions. Examples of presentation layer functionality include:

·         Encryption and decryption of a message for security

·         Compression and expansion of a message so that it travels efficiently

·         Graphics formatting

·         Content translation

·         System-specific translation

Session Layer:

The session layer establishes, manages (regulating which side transmits, and when and how long it transmits) and terminates communication sessions. Communication sessions consist of requests and responses between applications on different devices. These requests and responses coordinate by protocols at the session layer. Examples of session-layer implementations include Session Control Protocol (SCP), Zone Information Protocol (ZIP).

 

Transport Layer:

The Transport layer handles error recognition and recovery. It also repackages long messages when necessary into small packets for transmission and at the receiving end, rebuilds packets into the original message. The receiving Transport layer also sends receipt acknowledgments. The most common transport layer protocols are Transmission Control Protocol (TCP).

Network Layer:

This layer is concerned with the transmission of packets. It translates logical addresses and names into physical addresses and chooses the best path to send a packet. The routing may be complex in a large network (e.g. Internet).Routers operate and take routing decision at the Network Layer. The most common network layer protocols are Internet Protocol (IP).

Data Link Layer:

It is responsible for communications between adjacent network nodes. Hubs and switches operate at the Data Link Layer. It handles errors that occur at the lower levels of the network stack. It has two sub layers:

Logical Link Control (LLC): defines how data is transferred over the cable.

Medium Access Control (MAC): defines who can use the network when multiple computers are trying to access it.

The most common data link layer protocols are Ethernet protocols.

 

Physical Layer:

The Physical Layer of the OSI model is responsible for bit-level transmission between network nodes. The Physical Layer defines items such as:

·         connector types

·          pin assignments

·         cable types

·         voltages

·         Network Interface Cards