Osi Model

The Open Systems Interconnection (OSI) model (ISO/IEC 7498-1) is a conceptual model that characterizes and standardizes the internal functions of a communications system by partitioning it into abstraction layers. The model is a product of the Open Systems Interconnection project at theInternational Organization for Standardization (ISO). The model groups similar communication functions into one of seven logical layers. A layer serves the layer above it and is served by the layer below it. For example, a layer that provides error-free communications across a network provides the path needed by applications above it, while it calls the next lower layer to send and receive packets that make up the contents of that path. Two instances at one layer are connected by a horizontal connection on that layer.

History
Work on a layered model of network architecture was started and the International Organization for Standardization (ISO) began to develop its OSI framework architecture. OSI had two major components: an abstract model of networking, called the Basic Reference Model or seven-layer model, and a set of specific protocols. The concept of a seven-layer model was provided by the work of Charles Bachman, Honeywell Information Services. Various aspects of OSI design evolved from experiences with the ARPANET, the fledgling Internet, NPLNET, EIN,CYCLADES network and the work in IFIP WG6.1. The new design was documented in ISO 7498 and its various addenda. In this model, a networking system was divided into layers. Within each layer, one or more entities implement its functionality. Each entity interacted directly only with the layer immediately beneath it, and provided facilities for use by the layer above it. Protocols enabled an entity in one host to interact with a corresponding entity at the same layer in another host. Service definitions abstractly described the functionality provided to an (N)-layer by an (N-1) layer, where N was one of the seven layers of protocols operating in the local host. The OSI standards documents are available from the ITU-T as the X.200-series of recommendations. Some of the protocol specifications were also available as part of the ITU-T X series. The equivalent ISO and ISO/IEC standards for the OSI model were available from ISO, but only some of them without fees.

Description of OSI layers
According to recommendation X.200, there are seven layers, labelled 1 to 7, with layer 1 at the bottom. Each layer is generically known as an N layer. An "N+1 entity" (at layer N+1) requests services from an "N entity" (at layer N). At each level, two entities (N-entity peers) interact by means of the N protocol by transmitting protocol data units (PDU). A service data unit (SDU) is a specific unit of data that has been passed down from an OSI layer to a lower layer, and which the lower layer has not yet encapsulated into a protocol data unit (PDU). An SDU is a set of data that is sent by a user of the services of a given layer, and is transmitted semantically unchanged to a peer service user. The PDU at a layer N is the SDU of layer N-1. In effect the SDU is the 'payload' of a given PDU. That is, the process of changing an SDU to a PDU, consists of an encapsulation process, performed by the lower

layer. All the data contained in the SDU becomes encapsulated within the PDU. The layer N-1 adds headers or footers, or both, to the SDU, transforming it into a PDU of layer N-1. The added headers or footers are part of the process used to make it possible to get data from a source to a destination.

OSI Model

Data unit

Layer

Function

7. Application

Network process to application

Data Host layers

6. Presentation

Data representation, encryption and decryption, convert machine dependent data to machine independent data

5. Session

Interhost communication, managing sessions between applications

Segments

4. Transport

End-to-end connections, reliability and flow control

Packet/Datagram 3. Network

Path determination and logical addressing

Media Frame layers

2. Data link

Physical addressing

Bit

1. Physical

Media, signal and binary transmission

Application layer
In computer network programming, the application layer is an abstraction layer reserved for communications protocols and methods designed for process-to-process communications across an Internet

Protocol (IP) computer network. Application layer protocols use the underlying transport layer protocols to establish host-to-host connections.

In the OSI model, the definition of its application layer is narrower in scope. The OSI model defines the application layer as being the user interface. The OSI application layer is responsible for displaying data and images to the user in a human-recognizable format and to interface with the presentation layer below it.[1] It separates functionality above the transport layer at two additional levels, the session layer and the presentation layer. OSI specifies strict modular separation of functionality at these layers and provides protocol implementations for each layer.

TCP/IP protocols
The following protocols are explicitly mentioned in RFC 1123 (1989), describing the application layer of the Internet protocol suite.  Remote login category   Telnet is a network protocol used on the Internet or local area networks to provide a bidirectional interactive text-oriented communication facility using a virtual terminal connection. File transfer category  FTP File Transfer Protocol is a standard network protocol used to transfer files from one host to another host over a TCP-based network, such as the Internet  TFTP Trivial File Transfer Protocol is a file transfer protocol notable for its simplicity. It is generally used for automated transfer of configuration or boot files between machines in a local environment.  Electronic mail category  SMTP Simple Mail Transfer Protocol is an Internet standard for electronic mail (e-mail) transmission across Internet Protocol (IP) networks.  IMAP Internet Message Access Protocol is an Application Layer Internet protocol that allows an e-mail client to access e-mail on a remote mail server.   POP Post Office Protocol is an application-layer Internet standard protocol used by local e-mail clients to retrieve e-mail from a remote server over a TCP/IP connection. Support services category  DNS Domain Name System is a hierarchical distributed naming system for computers, services, or any resource connected to the Internet or a private network.  RARP Reverse Address Resolution Protocol is an obsolete computer networking protocol used by a host computer to request its Internet Protocol (IPv4) address from an administrative host, when it has available its Link Layer or hardware address, such as a MAC address.  BOOTP Bootstrap Protocol is a network protocol used by a network client to obtain an IP address from a configuration server.

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SNMP Simple Network Management Protocol is an "Internet-standard protocol for managing devices on IP networks". Devices that typically support SNMP include routers, switches, servers, workstations, printers, modem racks, and more.



CMOT

Common

Management

Information

Protocol

is

the

OSI

specified network

management protocol.

Presentation layer
In the seven-layer OSI model of computer networking, the presentation layer is layer 6 and serves as the data translator for the network. It is sometimes called the syntax layer.

Description
The presentation layer is responsible for the delivery and formatting of information to the application layer for further processing or display. It relieves the application layer of concern regarding syntactical differences in data representation within the end-user systems. An example of a presentation service would be the conversion of an EBCDIC-coded text computer file to an ASCII-coded file. The presentation layer is the lowest layer at which application programmers consider data structure and presentation, instead of simply sending data in form of datagrams or packets between hosts. This layer deals with issues of string representation - whether they use the Pascal method (an integer length field followed by the specified amount of bytes) or the C/C++ method (null-terminated strings, e.g. "thisisastring\0"). The idea is that the application layer should be able to point at the data to be moved, and the presentation layer will deal with the rest. Serialization of complex data structures into flat byte-strings (using mechanisms such as TLV or XML) can be thought of as the key functionality of the presentation layer. Encryption is typically done at this level too, although it can be done on the application, session, transport, or network layers, each having its own advantages and [1] disadvantages. Decryption is also handled at the presentation layer. For example, when logging off [1] bank account sites the presentation layer will decrypt the data as it is received. Another example is representing structure, which is normally standardized at this level, often by using XML. As well as simple pieces of data, like strings, more complicated things are standardized in this layer. Two common examples are 'objects' in object-oriented programming, and the exact way that streaming video is transmitted. In many widely used applications and protocols, no distinction is made between the presentation and application layers. For example, HyperText Transfer Protocol (HTTP), generally regarded as an application-layer protocol, has presentation-layer aspects such as the ability to identify character encoding for proper conversion, which is then done in the application layer. Within the service layering semantics of the OSI network architecture, the presentation layer responds to service requests from the application layer and issues service requests to the session layer.

In the OSI model: the presentation layer ensures the information that the application layer of one system sends out is readable by the application layer of another system. For example, a PC program communicates with another computer, one using extended binary coded decimal interchange code (EBCDIC) and the other using ASCII to represent the same characters. If necessary, the presentation layer might be able to translate between multiple data formats by using a common format.

Services
    Data conversion is the conversion of computer data from one format to another. Character code translation Compression involves encoding information using fewer bits than the original representation. Encryption and Decryption   Encryption is the process of encoding messages (or information) in such a way that eavesdroppers or hackers cannot read it, but that authorized parties can. is able to decode the ciphertext using a decryption algorithm, that usually requires a secret decryption key, that adversaries do not have access to.

Sublayers
The presentation layer can be composed of two sublayers: common application service element (CASE) and specific application service element (SASE).

CASE
The common application service element sublayer provides services for the application layer and request services from the session layer. It provides support for common application services, such as:     ACSE (Association Control Service Element) ROSE (Remote Operation Service Element) CCR (Commitment Concurrency and Recovery) RTSE (Reliable Transfer Service Element)

SASE
The specific application service element sublayer provides application specific services (protocols), such as       FTAM (File Transfer, Access and Manager) VT (Virtual Terminal) MOTIS (Message Oriented Text Interchange Standard) CMIP (Common Management Information Protocol) JTM (Job Transfer and Manipulation) a former OSI standard MMS (Manufacturing Messaging Service)

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RDA (Remote Database Access) DTP (Distributed Transaction Processing)

Protocols
Other protocols sometimes considered at this level (though perhaps not strictly adhering to the OSI model) include:         Apple Filing Protocol (AFP) Independent Computing Architecture (ICA), the Citrix system core protocol Lightweight Presentation Protocol (LPP) NetWare Core Protocol (NCP) Network Data Representation (NDR) Telnet (a remote terminal access protocol) eXternal Data Representation (XDR) X.25 Packet Assembler/Disassembler Protocol (PAD)