Wednesday 23 July 2014

The Open Systems Interconnect (OSI) Model has Seven Layers

The Open Systems Interconnect (OSI) model has seven layers 

This text describes and explains them, beginning with the 'lowest' among the hierarchy (the physical) and continued to the 'highest' (the application). The layers unit stacked this way:
Application
Presentation
Session
Transport
Network
Data Link
Physical

PHYSICAL LAYER

The physical layer, the lowest layer of the OSI model, cares with the transmission and reception of the unstructured raw bit stream over a physical medium. It describes the electrical/optical, mechanical, and sensible interfaces to the physical medium, and carries the signals for all of the higher layers. It provides:
Data encoding: modifies the simple digital signal pattern (1s and 0s) utilized by the laptop computer to raised accommodate the characteristics of the physical medium, and to help in bit and frame synchronization. It determines:

What signal state represents a binary one
How the receiving station is attentive to once a "bit-time" starts
How the receiving station delimits a frame
Physical medium attachment, accommodating varied prospects among the medium:

Will Associate in Nursing external transceiver (MAU) be accustomed connect with the medium?
How many pins do the connectors have and what is each pin used for?
Transmission technique: determines whether or not or not the encoded bits ar transmitted by base band (digital) or broadband (analog) sign.
Physical medium transmission: transmits bits as electrical or optical signals applicable for the physical medium, and determines:

What physical medium decisions are used
How many volts/db got to be accustomed represent a given signal state, using a given physical medium

DATA LINK LAYER

The data link layer provides error-free transfer of data frames from one node to a unique over the physical layer, allowing layers on prime of it to assume near to error-free transmission over the link. To do this, the knowledge link layer provides:

Link establishment and termination: establishes and terminates the logical link between a pair of nodes.
Frame traffic control: tells the transmitting node to "back-off" once no frame buffers unit out there.
Frame sequencing: transmits/receives frames consecutive.
Frame acknowledgment: provides/expects frame acknowledgments. Detects and recovers from errors that occur among the physical layer by re transmitting non-acknowledged frames and handling duplicate frame receipt.
Frame delimiting: creates and acknowledges frame boundaries.
Frame error checking: checks received frames for integrity.
Media access management: determines once the node "has the right" to use the physical medium.

NETWORK LAYER

The network layer controls the operation of the sub net, deciding that physical path the knowledge got to take supported network conditions, priority of service, and various factors. It provides:

Routing: routes frames among networks.
Sub net traffic control: routers (network layer intermediate systems) can instruct a inflicting station to "throttle back" its frame transmission once the router's buffer fills up.
Frame fragmentation: if it determines that a downstream router's most transmission unit (MTU) size may be a smaller quantity than the frame size, a router can fragment a frame for transmission and re-assembly at the destination station.
Logical-physical address mapping: interprets logical addresses, or names, into physical addresses.
Subnet usage accounting: has accounting functions to remain track of frames forwarded by subnet intermediate systems, to produce request information.
Communications Subnet

The network layer computer code ought to build headers therefore the network layer computer code residing among the sub net intermediate systems can acknowledge them and use them to route information to the destination address.

This layer relieves the upper layers of the need to know one thing concerning the knowledge transmission and intermediate shift technologies accustomed connect systems. It establishes, maintains and terminates connections across the intervening communications facility (one or several intermediate systems among the communication sub net).

In the network layer and so the layers below, peer protocols exist between a node and its immediate neighbor, but the neighbor is {additionally} a node through that information is routed, not the destination station. the availability and destination stations is additionally separated by many intermediate systems.

TRANSPORT LAYER

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

The size and quality of a transport protocol depends on the kind of service it'll get from the network layer. For a reliable network layer with virtual circuit capability, a bottom transport layer is required. If the network layer is unreliable and/or alone supports datagrams, the transport protocol got to embrace intensive error detection and recovery.

The transport layer provides:
Message segmentation: accepts a message from the (session) layer on prime of it, splits the message into tinyer units (if not already little enough), and passes the smaller units right all the way down to the network layer. The transport layer at the destination station reassembles the message.
Message acknowledgment: provides reliable end-to-end message delivery with acknowledgments.
Message traffic control: tells the transmitting station to "back-off" once no message buffers unit out there.
Session multiplexing: multiplexes several message streams, or sessions onto one logical link and keeps track of that messages belong thereto sessions (see session layer).
Typically, the transport layer can accept relatively large messages, but there unit strict message size limits obligatory by the network (or lower) layer. Consequently, the transport layer ought to break up the messages into smaller units, or frames, pretending a header to each frame.

The transport layer header information ought to then embrace management information, like message begin and message end flags, to change the transport layer on the other end to acknowledge message boundaries. additionally, if the lower layers do not maintain sequence, the transport header ought to contain sequence information to change the transport layer on the receiving end to induce the things back on among the proper order before handing the received message up to the layer on prime of.
End-to-end layers

Unlike the lower "sub net" layers whose protocol is between in real time adjacent nodes, the transport layer and so the layers on prime of unit true "source to destination" or end-to-end layers, and do not appear to be involved the most points of the underlying communications facility. Transport layer computer code (and computer code on prime of it) on the availability station carries on a language with similar computer code on the destination station by victimization message headers and management messages.

SESSION LAYER

The session layer permits session establishment between processes running on fully totally different stations. It provides:

Session establishment, maintenance and termination: permits a pair of application processes on fully totally different machines to see, use and terminate a association, cited as a session.
Session support: performs the functions that modify these processes to talk over the network, activity security, name recognition, logging, and so on.

PRESENTATION LAYER

The presentation layer formats the knowledge to lean to the applying layer. it'll be viewed as a result of the translator for the network. This layer may translate information from a format utilised by the applying layer into a typical format at the inflicting station, then translate the common format to a format known to the applying layer at the receiving station.

The presentation layer provides: 

Character code translation: as associate example, computer code to EBCDIC.
Data conversion: bit order, CR-CR/LF, integer-floating purpose, and so on.
Data compression: reduces the quantity of bits that need to be transmitted on the network.
Data writeion: cipher information for security functions. as associate example, information cryptography.

APPLICATION LAYER

The application layer is that the window for users and application processes to access network services. This layer contains a variety of commonly needed functions:

Resource sharing and device redirection
Remote file access
Remote printer access
Inter-process communication
Network management
Directory services
Electronic messaging (such as mail)
Network virtual terminals

No comments:

Post a Comment