The International organization for standardisation (ISO) developed the Open System Interconnection (OSI)reference model to describe how information is transferred from one machine to another, from the point when a user enters information using a keyboard and mouse to when that information is converted to electrical or light signals transferred along a piece of wire or radio waves transferred through the air. By understanding the basics of the OSI reference model, one can apply these to real protocols to gain better understanding of them as well as to more easily troubleshoot the problems.
Layer 7 Application Layer (Application network services): It provides network services to any network aware application requiring access to the network. Examples include FTP, HTTP and SMTP.
Layer 6 Presentation Layer (Data Representation): It handles data representation. It ensures data is readable, and formats and structures data. It also negotiates transfer syntax for the application layer. This layer defines how various forms of text, graphics, video and/or audio information are presented to the user. For example text is represented in two data forms: ASCII and EBCDIC. The presentation layer takes care of translating the one data format to the other such that data is represented legibly to the user. Examples of presentation layer protocols and standards include ASCII, BMP, GIF, JPEG, WAV, AVI and MPEG.
Layer 5 Session Layer (Inter-host communication): It facilitates inter host communication. In doing so, it establishes, manages and terminates the sessions between applications. Ex maples include RPC (Remote Procedure Call) and NFG (Network File System).
Layer 4 Transport Layer (end-to-end connections): The transport Layer faciliates end to end communications. It handles transport issues between hosts and ensures data transport reliably. It also establishes, maintains and terminates virtual circuits, and provides reliability through fault detection and recovery information flow control. The transport layer is responsible for the actual mechanics of a connection, where it can provide both reliable and unreliable delivery of the data. For the reliable connections, the transport layer is responsible for error detection and correction: when an error is detected, the transport layer will resend the data, thus providing the correction. For unreliable connection, the transport layer provides only error detection. In this case error correction is left to one of the higher layers ( typically the application layer).
Layer 3 Network Layer (data delivery): The network layer ensures data delivery. It provides connectivity and path selection between two host systems, routes data packets and selects the best path to deliver the data.
Layer 2 Data Link Layer (media access): The data link layer provides access to the network media. It defines how data is formatted and how access to the network is controlled.
Layer 1 Physical Layer (Binary Transmission): The physical layer handles binary transmission. It defines the electrical, mechanical, procedural and functional specifications for activating , maintaining, and deactivating the physical link. It is responsible for transmitting the data onto the physical media.