The International Standards Organization (ISO) developed the Open Systems Interconnection (OSI) Reference Model to define functional communications standards. This reference model is widely used by equipment manufacturers to assure their products will interoperate with products from other vendors. The OSI model describes the functions that are performed in data communications. The model architecture is seven layers those are listed below with each layer defining specific functions.
Layer 7 Application
The Application layer provides the interface to the user. Any user application that requires network communication accesses the communication resources through this layer. This layer also is responsible for finding and determining the availability of communication partners. Typical applications in the TCP/IP protocols are Simple Mail Transfer Protocol (SMTP), Telnet, and File Transfer Protocol (FTP).
Layer 6 Presentation
The Presentation layer is responsible for encoding and decoding data that is passed from the application layer to another station on the internetwork. This layer is responsible for encoding data in a format that the receiving station can interpret and for decoding data received from other stations. Data compression and encryption are accomplished at this layer. Typical coding schemes include ASCII, EBCDIC, MPEG, GIF, and JPEG.
Layer 5 Session
The session layer is responsible for creating, managing and termination sessions that are used by entities at the presentation layer. The session layer is responsible for coordinating the service requests and responses generated and received by a station when it is communication with other entities on the internetwork.
Layer 4 Transport
The Transport layer implements reliable internetwork data transport services that are transparent to upper-layer protocols. The services include flow control, multiplexing, and error checking and recovery. If virtual circuits are needed for the communication to be accomplished, they are built and maintained at this layer. Flow control is responsible for making sure that a sending station does not transmit data faster than the receiving station can process it. Multiplexing allows multiple applications to share a common network interface. Error checking is implemented to discover errors on transmission and to provide a recovery mechanism when errors are found. Typical error recovery includes retransmission of the data.
Layer 3 Network
The Network layer defines routing services that allow multiple data links to be combined into an internetwork. The Network layer defines network-addressing schemes that logically identify network devices. The logical network addresses are different from the physical addresses defined at the MAC layer, and are used by routing protocols running at this level to transfer packets from one network to another. The most common network addressing protocols are IP, IPX, and AppleTalk. Typical routing protocols that run at this level are RIP, OSPF, IGRP, and NLSP.
Layer 2 Data Link
The Data Link layer provides reliable transit of data across a physical network link. The Data Link layer also defines the physical network-addressing scheme, such as the MAC Address on network interface cards in a workstation connected to a LAN. The Data Link layer also defines the topology of the network (bus, star, dual ring, and so on). Flow control at the Data Link layer is defined to ensure receiving stations are not overrun with data before they can process data already received. The Institute of Electrical and Electronics Engineers (IEEE) has redefined the Data Link layer into two sublayers. The sublayers are the Logical Link Control (LLC) layer and the Media Access Control (MAC) layer. The LLC and MAC sublayers are defined in the IEEE 802.2 standards. The LLC manages communications between devices over a single link of a network. The MAC sublayer manages access to the physical medium from multiple upper-level protocols. The MAC layer also defines the MAC address, which uniquely identifies devices at the Data Link layer.
Layer 1 Physical
The Physical layer defines the parameters necessary to build, maintain, and break the physical link connections. It defines the characteristics of the connectors, data transmission rates and distances, and the interface voltages.