OSI Model

OSI MODEL:-

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.

Physical Layer

The Physical layer comprises the physical components that make up the networking hardware of the network, including the network adapter, connectors, network media (copper wires or optical cables), and so on. To sum it up in a simple sentence: The Physical layer gets the data from here to there. This layer covers both electrical and mechanical aspects of the network. For example, the method used to encode data into electrical or light signals on the network media is decided at this layer.

Data Link Layer

The Data Link layer serves several functions, which the IEEE has divided into two sublayers. The first is the Logical Link Control (LLC) and the second is the Media Access Control (MAC). As a whole, the Data Link layer is responsible for transmitting data from one place to another and doing some minimal error correction. The purpose of the LLC is to provide Service Access Points (SAPs) that devices can use to send information. The MAC component takes care of transmitting the data and correcting errors.

The Data Link layer is responsible for putting together the Ethernet frame, for example. This includes formatting the header information into the correct fields and placing the data in the right place. Functions operating at this layer also determine the order in which bits are interpreted (that is, big- or little-endian), and add checksum information used to ensure that the frame arrives intact at its destination.

Bridges are network devices that operate at this level in the model. Bridges examine the MAC addresses of packets and use that information to decide whether to forward a packet to another port.

Network Layer

The network layer controls the operation of the subnet, deciding which physical path the data should take based on network conditions, priority of service, and other factors. It provides:

·         Routing: routes frames among networks.

·         Subnet traffic control: routers (network layer intermediate systems) can instruct a sending station to "throttle back" its frame transmission when the router's buffer fills up.

·         Frame fragmentation: if it determines that a downstream router's maximum transmission unit (MTU) size is less than the frame size, a router can fragment a frame for transmission and re-assembly at the destination station.

·         Logical-physical address mapping: translates logical addresses, or names, into physical addresses.

·         Subnet usage accounting: has accounting functions to keep track of frames forwarded by subnet intermediate systems, to produce billing information.

Transport Layer

The transport layer ensures that messages are delivered error-free, in sequence, and with no losses or duplications. It relieves the higher layer protocols from any concern with the transfer of data between them and their peers.

The size and complexity of a transport protocol depends on the type of service it can get from the network layer. For a reliable network layer with virtual circuit capability, a minimal transport layer is required. If the network layer is unreliable and/or only supports datagrams, the transport protocol should include extensive error detection and recovery.

Session Layer

The Session layer is responsible for deciding the format of the data transmitted. Session protocol examples are the remote procedure call (used by NFS and other applications). Another way to think of the Session layer is that it functions to allow processes on networked computers to talk to each other. TCP and NetBIOS are both protocols that reside at the Session layer.

Presentation Layer

The Presentation layer interprets the actual data that is being exchanged. For example, different systems can use different methods to represent floating-point numbers or other data. The order of bits in a byte is translated at this level. The necessary conversions take place at this layer. It is at this layer that translations from different character-encoding methods take place. For example, when one computer uses ASCII characters and another computer uses IBM's EBCDIC encoding, translations between these two methods of representing characters are made at the Presentation layer.

Application Layer

The user comes into the picture in the Application layer. Without applications that need to use the network, we network administrators would be out of a job. Examples of network components that reside at this layer include firewalls or networked file systems (such as NFS). End users can recognize Application layer components as programs that they use every day, such as email and FTP.

August 01, 2012 Tags : Technology