OSI MODEL By : Alok Pareek, Satish Solanki & Harsh Vardhan Sikhwal
WHAT IS OSI? The Open Systems Interconnection model (OSI model) is a product of the Open Systems Interconnection effort at the International Organization for Standardization. It is a prescription of characterizing and standardizing the functions of a communications system in terms of abstraction layers. Similar communication functions are grouped into logical layers. An instance of a layer provides services to its upper layer instances while receiving services from the layer below. 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
DESCRIPTION OF OSI LAYERSAccording to recommendation X.200, there are seven layers. These Seven layers are Layer 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. 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 resourcesare effectively shared among multiple users. Forexample, contention resolution and flow control. Modulation, or conversion between the representation of digitaldata in user equipment and the corresponding signals transmitted over acommunications channel. These are signals operating over the physicalcabling (such as copper and optical fiber) or over a radio link. Parallel SCSI buses operate in this layer, although it must beremembered that the logical SCSI protocol is a transport layer protocolthat runs over this bus. Various physical-layer Ethernet standards arealso in this layer; Ethernet incorporates both this layer and the data linklayer. The same applies to other local-area networks, such as tokenring, FDDI, ITU-TG.hn and IEEE 802.11, as well as personal areanetworks such as Bluetooth and IEEE 802.15.4.
2) The Data Link Layer: The main task of the data link layer is to take a raw transmission facility and transform it into a line that appears free of undetected transmission errors to the network layer. To accomplish this, the sender breaks the input data into data frames (typically a few hundred or a few thousand bytes), transmits the frames sequentially, and processes the acknowledgment frames sent back by the receiver. The issues that the layer has to solve: To create and to recognize frame boundaries - typically by attaching special bit patterns to the beginning and end of the frame, To solve the problem caused by damaged, lost or duplicate frames (the data link layer may offer several different service classes to the network layer, each with different quality and price),to keep a fast transmitter from drowning a slow receiver in data, If the line is bi-directional, the acknowledgment frames compete for the use of the line with data frames.
3) The Network Layer: The main task of the network layer is to determine how data can be delivered from source to destination. That is, the network layer isconcerned with controlling the operation of the subnet. The issues that the layer has to solve: To implement the routing mechanism, To control congestions, To do accounting, To allow interconnection of heterogeneous networks.
4) The Transport Layer: The basic function of the transport layer is to accept data from the session layer, split it up into smaller units if need be, pass them to the network layer, and ensure that the pieces all arrive correctly at the other end. All this must be done in a way that isolates the upper layers from the inevitable changes in the hardware technology. The issues that the transport layer has to solve: To realize a transport connection by several network connections if the session layer requires a high throughput or multiplex several transport connections onto the same network connection if network connections are expensive, To provide different type of services for the session layer, To implement a kind of flow control
5) The Session Layer: The session layer allows users on different machines to establish sessions between them. A session allows ordinary data transport, as does the transport layer, but it also provides enhanced services useful in some applications. Some of these services are: Dialog control - session can allow traffic to go in both directions at the same time, or in only one direction at a time. If traffic can go only in one way at a time, the session layer can help to keep track of whose turn it is. Token management - for some protocols it is essential that both sides do not attempt the same operation at the same time. The session layer provides tokens that can be exchanged. Only the side holding the token may perform the critical action. Synchronization - by inserting checkpoints into the data stream the layer eliminates problems with potential crashes at long operations. After a crash, only the data transferred after the last checkpoint have to be repeated
6) The Presentation Layer: The presentation layer performs certain functions that are requested sufficiently often to warrant finding a general solution for them, rather than letting each user solve the problem. This layer is, unlike all the lower layers, concerned with the syntax and semantics of the information transmitted. A typical example of a presentation service is encoding data in a standard agreed upon way. Different computers may use different ways of internal coding of characters or numbers. In order to make it possible for computers with different representations to communicate, the data structures to be exchanged can be defined in an abstract way, along with a standard encoding to be used "on the wire". The presentation layer manages these abstract data structures and converts from the representation used inside the computer to the network standard representation and back
7) The Application Layer: The application layer contains a variety of protocols that are commonly needed. For example, there are hundreds of incompatible terminal types in the world. If they have to be used for a work with a full screen editor, many problems arise from their incompatibility. One way to solve this problem is to define network virtual terminal and write editor for this terminal. To handle each terminal type, a piece of software must be written to map the functions of the network virtual terminal onto the real terminal. All the virtual terminal software is in the application layer. Another application layer function is file transfer. It must handle different incompatibilities between file systems on different computers. Further facilities of the application layer are electronic mail, remote job entry, directory lookup ant others
Problem’s in OSI Model Seven layers not widely accepted Standardized before implemented Top three layers fuzzy Internet or TCP/IP layering widespread
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