THE OSI MODEL

Communication Protocols

Define the rules and data formats for exchanging information in a computer network, and provide the basis for network programming Well-known communications protocols are Ethernet a hardware and Link Layer standard that is ubiquitous in Local Area Networks and the Internet Protocols suite which defines a set of protocols for inter networking, i.e. for data communication between multiple networks, as well as host-to-host data transfer, and application-specific data transmission formats.

The Internet Protocol Suite

The Internet protocol suite is the set of communications protocol used for the Internet and other similar networks. It is commonly known as TCP/IP from its most important protocols: Transmission Control Protocol (TCP) and Internet Protocol (IP), which were the first networking protocols defined in this standard.

 Open Systems Interconnection (OSI) model is a reference model developed by ISO (International Organization for Standardization) in 1984, as a conceptual framework of standards for communication in the network across different equipment and applications.

Most of the network communication protocols used today have a structure based on the OSI model. The OSI model defines the communications process into 7 layers, which divides the tasks involved with moving information between networked computers into seven smaller, more manageable task groups.

The OSI 7 layers model has clear characteristics. Layers 7 through 4 deal with end to end communications between data source and destinations. Layers 3 to 1 deal with communications between network devices. 
On the other hand, the seven layers of the OSI model can be divided into two groups: upper layers (layers 7, 6 & 5) and lower layers (layers 4, 3, 2, 1). The upper layers of the OSI model deal with application issues and generally are implemented only in software. The highest layer, the application layer, is closest to the end user. The lower layers of the OSI model handle data transport issues. The physical layer and the data link layer are implemented in hardware and software. The lowest layer, the physical layer, is closest to the physical network medium (the wires, for example) and is responsible for placing data on the medium.

The specific description for each layer is as follows:

Layer 7:Application Layer
Defines interface to user processes for communication and data transfer in network
Provides standardized services such as virtual terminal, file and job transfer and operations

Layer 6:Presentation Layer
Masks the differences of data formats between dissimilar systems
Specifies architecture-independent data transfer format
Encodes and decodes data; Encrypts and decrypts data; Compresses and decompresses data

Layer 5:Session Layer
Manages user sessions and dialogues
Controls establishment and termination of logic links between users
Reports upper layer errors

Layer 4:Transport Layer
Manages end-to-end message delivery in network
Provides reliable and sequential packet delivery through error recovery and flow control mechanisms
Provides connectionless oriented packet delivery

Layer 3:Network Layer
Determines how data are transferred between network devices
Routes packets according to unique network device addresses
Provides flow and congestion control to prevent network resource depletion

Layer 2:Data Link Layer
Defines procedures for operating the communication links
Frames packets
Detects and corrects packets transmit errors

Layer 1:Physical Layer
Defines physical means of sending data over network devices
Interfaces between network medium and devices
Defines optical, electrical and mechanical characteristics