The Open Systems Interconnection Reference Model (OSI Reference Model or OSI Model) is an abstract description for layered communications and computer
network protocol design. It was developed as part of the
Open Systems Interconnection (OSI) initiative.
[1] In its most basic form, it divides network architecture into seven layers which, from top to bottom, are the Application, Presentation, Session, Transport, Network, Data-Link, and Physical Layers. It is therefore often referred to as the OSI Seven Layer Model.A layer is a collection of conceptually similar functions that provide services to the layer above it and receives service from 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 the path.
Contents
1 History2 Description of OSI layers2.1 Layer 7: Application Layer2.2 Layer 6: Presentation Layer2.3 Layer 5: Session Layer2.4 Layer 4: Transport Layer2.5 Layer 3: Network Layer2.6 Layer 2: Data Link Layer2.6.1 WAN Protocol architecture2.6.2 IEEE 802 LAN architecture2.7 Layer 1: Physical LayerHistoryIn 1977, 2work on a layered model of network architecture was started, and the
International Organization for Standardization (ISO) began to develop its OSI framework architecture. OSI has two major components: an abstract model of networking, called the Basic Reference Model or seven-layer model, and a set of specific protocols.Note: The standard documents that describe the OSI model can be freely downloaded from the ITU-T as the X.200-series of recommendations.
[2] A number of the protocol specifications are also available as part of the ITU-T X series. The equivalent ISO and ISO/IEC standards for the OSI model are available from the ISO, but only some of the ISO/IEC standards are available as cost-free downloads.
[3]All aspects of OSI design evolved from experiences with the
CYCLADES network, which also influenced Internet design. The new design was documented in ISO 7498 and its various addenda. In this model, a networking system is divided into layers. Within each layer, one or more entities implement its functionality. Each entity interacts directly only with the layer immediately beneath it, and provides facilities for use by the layer above it.Protocols enable an entity in one host to interact with a corresponding entity at the same layer in another host. Service definitions abstractly describe the functionality provided to an (N)-layer by an (N-1) layer, where N is one of the seven layers of protocols operating in the local host.Description of OSI layersLayer 7: Application LayerMain article:
Application LayerThe application layer is the OSI layer closest to the end user, which means that both the OSI application layer and the user interact directly with the software application. This layer interacts with software applications that implement a communicating component. Such application programs fall outside the scope of the OSI model. Application layer functions typically include identifying communication partners, determining resource availability, and synchronizing communication. When identifying communication partners, the application layer determines the identity and availability of communication partners for an application with data to transmit. When determining resource availability, the application layer must decide whether sufficient network resources for the requested communication exist. In synchronizing communication, all communication between applications requires cooperation that is managed by the application layer. Some examples of application layer implementations include
Telnet, Hypertext Transfer Protocol (HTTP), File Transfer Protocol (FTP) , and Simple Mail Transfer Protocol (SMTP).Layer 6: Presentation LayerMain article:
Presentation LayerThe
Presentation Layer establishes a context between Application Layer entities, in which the higher-layer entities can use different syntax and semantics, as long as the Presentation Service understands both and the mapping between them. The presentation service data units are then encapsulated into Session Protocol Data Units, and moved down the stack.This layer provides independence from differences in data representation (e.g., encryption) by translating from application to network format, and vice versa. The presentation layer works to transform data into the form that the application layer can accept. This layer formats and encrypts data to be sent across a network, providing freedom from compatibility problems. It is sometimes called the syntax layer.The original presentation structure used the Basic Encoding Rules of
Abstract Syntax Notation One (ASN.1), with capabilities such as converting an
EBCDIC-coded text
file to an
ASCII-coded file, or
serializing objects and other
data structures into and out of
XML. ASN.1 has a set of cryptographic encoding rules that allows end-to-end encryption between application entities.[
edit] Layer 5: Session LayerMain article:
Session LayerThe
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 checkpointing, adjournment, termination, and restart procedures. The OSI model made this layer responsible for "graceful close" of sessions, which is a property of
TCP, and also for session checkpointing and recovery, which is not usually used in the Internet Protocol Suite. The Session Layer is commonly implemented explicitly in application environments that use
remote procedure calls (RPCs).Layer 4: Transport LayerMain article:
Transport LayerThe
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. This means that the Transport Layer can keep track of the segments and retransmit those that fail.Although not developed under the OSI Reference Model and not strictly conforming to the OSI definition of the Transport Layer, the best known examples of a Layer 4 protocol are the
Transmission Control Protocol (TCP) and
User Datagram Protocol (UDP).[
citation needed]Of the actual OSI protocols, there are five classes of transport protocols ranging from class 0 (which is also known as TP0 and provides the least error recovery) to class 4 (which is also known as TP4 and is designed for less reliable networks, similar to the Internet). Class 0 contains no error recovery, and was designed for use on network layers that provide error-free connections. Class 4 is closest to TCP, although TCP contains functions, such as the graceful close, which OSI assigns to the Session Layer. Detailed characteristics of TP0-4 classes are shown in the following table.Perhaps an easy way to visualize the Transport Layer is to compare it with a Post Office, which deals with the dispatch and classification of mail and parcels sent. Do remember, however, that a post office manages the outer envelope of mail. Higher layers may have the equivalent of double envelopes, such as cryptographic presentation services that can be read by the addressee only. Roughly speaking,
tunneling protocols operate at the Transport Layer, such as carrying non-IP protocols such as
IBM's
SNA or
Novell's
IPX over an IP network, or end-to-end encryption with
IPsec. While
Generic Routing Encapsulation (GRE) might seem to be a Network Layer protocol, if the encapsulation of the payload takes place only at endpoint, GRE becomes closer to a transport protocol that uses IP headers but contains complete frames or packets to deliver to an endpoint.
L2TP carries
PPP frames inside transport packet.Layer 3: Network LayerMain article:
Network LayerThe
Network Layer provides the functional and procedural means of transferring variable length
data sequences from a source to a destination via one or more networks, 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—sending data throughout the extended network and making the Internet possible. This is a logical addressing scheme – values are chosen by the network engineer. The addressing scheme is hierarchical.The best-known example of a Layer 3 protocol is the
Internet Protocol (IP). It manages the
connectionless transfer of data one hop at a time, from end system to ingress router, router to router, and from egress router to destination end system. It is not responsible for reliable delivery to a next hop, but only for the detection of errored packets so they may be discarded. When the medium of the next hop cannot accept a packet in its current length, IP is responsible for fragmenting the packet into sufficiently small packets that the medium can accept.A number of layer management protocols, a function defined in the Management Annex, ISO 7498/4, belong to the Network Layer. These include routing protocols, multicast group management, Network Layer information and error, and Network Layer address assignment. It is the function of the payload that makes these belong to the Network Layer, not the protocol that carries them.Layer 2: Data Link LayerMain article:
Data Link LayerThe
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. Local area network architecture, which included broadcast-capable multiaccess media, was developed independently of the ISO work, in
IEEE Project 802. IEEE work assumed sublayering and management functions not required for WAN use. In modern practice, only error detection, not flow control using sliding window, is present in modern data link protocols such as
Point-to-Point Protocol (PPP), and, on local area networks, the IEEE 802.2 LLC layer is not used for most protocols on Ethernet, and, on other local area networks, its flow control and acknowledgment mechanisms are rarely used. Sliding window flow control and acknowledgment is used at the Transport Layer by protocols such as
TCP, but is still used in niches where
X.25 offers performance advantages.Both WAN and LAN services arrange bits, from the Physical Layer, into logical sequences called frames. Not all Physical Layer bits necessarily go into frames, as some of these bits are purely intended for Physical Layer functions. For example, every fifth bit of the
FDDI bit stream is not used by the Layer.WAN Protocol architecture
Connection-oriented WAN data link protocols, in addition to framing, detect and may correct errors. They also are capable of controlling the rate of transmission. A WAN Data Link Layer might implement a
sliding window flow control and acknowledgment mechanism to provide reliable delivery of frames; that is the case for
SDLC and
HDLC, and derivatives of HDLC such as
LAPB and
LAPD.
IEEE 802 LAN architecturePractical,
connectionless LANs began with the pre-IEEE
Ethernet specification, which is the ancestor of
IEEE 802.3. This layer manages the interaction of devices with a shared medium, which is the function of a
Media Access Control sublayer. Above this MAC sublayer is the media-independent
IEEE 802.2 Logical Link Control (LLC) sublayer, which deals with addressing and multiplexing on multiaccess media.While IEEE 802.3 is the dominant wired LAN protocol and
IEEE 802.11 the wireless LAN protocol, obsolescent MAC layers include
Token Ring and
FDDI. The MAC sublayer detects but does not correct errors.
Layer 1: Physical LayerMain article:
Physical LayerThe
Physical Layer defines the electrical and physical specifications for devices. In particular, it defines the relationship between a device and a physical medium. This includes the layout of
pins,
voltages,
cable specifications,
Hubs,
repeaters,
network adapters,
Host Bus Adapters (HBAs used in
Storage Area Networks) and more.To understand the function of the Physical Layer in contrast to the functions of the Data Link Layer, think of the Physical Layer as concerned primarily with the interaction of a single device with a medium, where the Data Link Layer is concerned more with the interactions of multiple devices (i.e., at least two) with a shared medium. The Physical Layer will tell one device how to transmit to the medium, and another device how to receive from it (in most cases it does not tell the device how to connect to the medium). Standards such as
RS-232 do use physical wires to control access to the medium.The major functions and services performed by the Physical Layer are:--Establishment and termination of a
connection to a
communications medium.Participation in the process whereby the communication resources are effectively shared among multiple users. For example,
contention resolution and
flow control.
Modulation, or conversion between the representation of
digital data in user equipment and the corresponding signals transmitted over a communications
channel. These are signals operating over the physical cabling (such as copper and
optical fiber) or over a
radio link.
Parallel SCSI buses operate in this layer, although it must be remembered that the logical
SCSI protocol is a Transport Layer protocol that runs over this bus. Various Physical Layer Ethernet standards are also in this layer; Ethernet incorporates both this layer and the Data Link Layer. The same applies to other local-area networks, such as
Token ring,
FDDI, and
IEEE 802.11, as well as personal area networks such as
Bluetooth and
IEEE 802.15.4.
Posted by Rahmat Ullah at
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