OSI 7 Layers

OSI 7 Layers Reference Model For Network Communication

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 by different vendors. It is now considered the primary architectural model for inter-computing and internetworking communications. 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. A task or group of tasks is then assigned to each of the seven OSI layers. Each layer is reasonably self-contained so that the tasks assigned to each layer can be implemented independently. This enables the solutions offered by one layer to be updated without adversely affecting the other layers.

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 characteristic

The OSI 7 layer model is defined by ISO in document 7498, The protocols defined by ISO based on the OSI 7 layer mode are as follows:

Application	ACSE: Association Control Service Element
	CMIP: Common Management Information Protocol
	CMIS: Common Management Information Service
	CMOT: CMIP over TCP/IP
	FTAM: File Transfer Access and Management
	ROSE: Remote Operation Service Element
	RTSE: Reliable Transfer Service Element Protocol
	VTP: ISO Virtual Terminal Protocol
	X.400: Message Handling Service (ISO email transmission service) Protocols
	X.500: Directory Access Service Protocol (DAP)
Presentation Layer	ISO-PP: OSI Presentation Layer Protocol
	ASN.1: Abstract Syntax Notation One
Session Layer	ISO-SP: OSI Session Layer Protocol
Transport Layer	ISO-TP: OSI Transport Protocols: TP0, TP1, TP2, TP3, TP4
Network Layer	ISO-IP: CLNP: Connectionless Network Protocol
	CONP: Connection-Oriented Network Protocol
	ES-IS: End System to Intermediate System Routing Exchange protocol
	IDRP: Inter-Domain Routing Protocol
	IS-IS: Intermediate System to Intermediate System
Data Link	HDLC: High Level Data Link Control protocol
	LAPB: Link Access Procedure Balanced for X.25