OSI REFFERENCE MODEL AND TCP/IP PROTOCOL SUITE
OSI Model The international Standards Organization (ISO) developed the OSI model in 1983. This model connects systems that are open to communicate with each other regardless of the protocols that each system uses. This model allows computers from different manufacturers to communicate with each other without requiring any logical changes to the hardware and software. The OSI model is represented in seven layers that define the entire process of communication between two computers on a network. The common functions of each layer are -
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Data unit
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Layer
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Function
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Host
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Data
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7. Application
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Network process to application
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6. Presentation
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Data representation, encryption and decryption, convert machine dependent data to machine independent data
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5. Session
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Interhost communication, managing sessions between applications
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Segments
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4. Transport
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End-to-end connections, reliability and flow control
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Media
layers |
Packet/Datagram
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3. Network
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Path determination and logical addressing
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Frame
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2. Data link
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Physical addressing
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Bit
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1. Physical
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Media, signal and binary transmission
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1. Physical layer The physical layer defines electrical and physical specifications for devices. In particular, it defines the relationship between a device and a transmission medium, such as a copper or optical cable. This includes the layout of pins, voltages, cable specifications, hubs, repeaters, network adapters, host bus adapters (HBA used in storage area networks) and more.
2. Data link layer The data link layer provides the functional and procedural means to transfer data between network entities and to detect and possibly correct errors that may occur in the physical layer. Originally, this layer was intended for point-to-point and point-to-multipoint media, characteristic of wide area media in the telephone system. The IEEE 802.2 LLC layer is not used for most protocols on the Ethernet, and on other local area networks, its flow control and acknowledgment mechanisms are rarely used.
3. Network layer The network layer provides the functional and procedural means of transferring variable length data sequences from a source host on one network to a destination host on a different network, while maintaining the quality of service requested by the transport layer .The network layer performs network routing functions, and might also perform fragmentation and reassembly, and report delivery errors. Routers operate at this layer.
4. Transport layer The transport layer provides transparent transfer of data between end users, providing reliable data transfer services to the upper layers. The transport layer controls the reliability of a given link through flow control, segmentation/desegmentation, and error control. Some protocols are state- and connection-oriented.
5. Session layer The session layer controls the dialogues (connections) between computers. It establishes, manages and terminates the connections between the local and remote application. It provides for full-duplex, half-duplex, or simplex operation, and establishes check pointing, termination, and restart procedures.
6. Presentation layer The presentation layer establishes context between application-layer entities, in which the higher-layer entities may use different syntax and semantics if the presentation service provides a mapping between them. If a mapping is available, presentation service data units are encapsulated into session protocol data units, and passed down the stack.
This layer provides independence from data representation by translating between application and network formats. The presentation layer transforms data into the form that the application accepts. This layer formats and encrypts data to be sent across a network. It is sometimes called the syntax layer.
7. Application layer The Application Layer refers to the higher-level protocols used by most applications for network communication. In application layer protocols include the File Transfer Protocol (FTP) and the Simple Mail Transfer Protocol (SMTP). Data coded according to application layer protocols are then encapsulated into one or (occasionally) more transport layer protocols (such as the Transmission Control Protocol (TCP) or User Datagram Protocol (UDP)), which in turn use lower layer protocols to effect actual data transfer.
Some protocols are used in this layer are as under
* Hypertext Transfer Protocol (HTTP),
* File Transfer Protocol (FTP),
* Simple Mail Transfer Protocol (SMTP)
* Simple Network Management Protocol (SNMP).
TCP/IP Protocol Suite
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The TCP/IP layer model is based on a four-layered network. Protocols came first and model was a description of the existing protocols. Supports only connection- less communication in the network layer, but both connectionless & connection-oriented communication in transport layer.
8. Network Access Layer (Link Layer) Network Access Layer is the networking scope of the local network connection to which a host is attached. This is the lowest component layer of the Internet protocols.
The Link Layer is used to move packets between the Internet Layer interfaces of two different hosts on the same link. The processes of transmitting and receiving packets on a given link can be controlled both in the software device driver for the network card, as well as on firmware or specialized chipsets.
9. Internet Layer The Internet Layer solves the problem of sending packets across one or more networks. Internetworking requires sending data from the source network to the destination network. This process is called routing.
In the Internet Protocol Suite, the Internet Protocol performs two basic functions:
* Host Addressing and identification: This is accomplished with a hierarchical addressing system.
* Packet routing: This is the basic task of getting packets of data (datagrams) from source to destination by sending them to the next network node (router) closer to the final destination.
10. Transport Layer The Transport Layer's responsibilities include end-to-end message transfer capabilities independent of the underlying network, along with error control, segmentation, flow control, congestion control, and application addressing. End to end message transmission or connecting applications at the transport layer can be categorized as either connection-oriented, implemented in Transmission Control Protocol (TCP), or connectionless, implemented in User Datagram Protocol (UDP).
11. Application Layer The Application Layer refers to the higher-level protocols used by most applications for network communication. In application layer protocols include the File Transfer Protocol (FTP) and the Simple Mail Transfer Protocol (SMTP). Data coded according to application layer protocols are then encapsulated into one or (occasionally) more transport layer protocols (such as the Transmission Control Protocol (TCP) or User Datagram Protocol (UDP)), which in turn use lower layer protocols to effect actual data transfer.
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