OSI Model

OSI (Open Systems Interconnection) is a standard description or "reference model" for how messages should be transmitted between any two points in a telecommunication network. Its purpose is to guide product implementors so that their products will consistently work with other products. The reference model defines seven layers of functions that take place at each end of a communication. Although OSI is not always strictly adhered to in terms of keeping related functions together in a well-defined layer, many if not most products involved in telecommunication make an attempt to place describe themselves in relation to the OSI model. It is also valuable as a single reference view of communication that furnishes everyone a common ground for education and discussion.


Developed by representatives of major computer and telecommunication companies beginning in 1983, OSI was originally intended to be a detailed specification of interfaces. Instead, the committee decided to establish a common reference model for which others could develop detailed interfaces, that in turn could become standards. OSI was officially adopted as an international standard by the International Organization of Standards (ISO). Currently, it is Recommendation X.200 of the International Telecommunication Union.


The main idea in OSI is that the process of communication between two end users in a telecommunication network can be divided into layers, with each layer adding its own set of special, related functions. Each communicating user is at a computer equipped with these seven layers of function. So, in a given message between users, there will be a flow of data through each layer at one end down through the layers in that computer and, at the other end, when the message arrives, another flow of data up through the layers in the receiving computer and ultimately to the end user. The actual programming and hardware that furnishes these seven layers of function is usually a combination of the computer operating system, applications (such as your Web browser), TCP/IP or alternative transport and network protocols, and the software and hardware that enable you to put a signal on one of the lines attached to your computer.

OSI divides telecommunication into seven layers. 


The layers are in two groups. The upper four layers are used whenever a message passes from or to a user. The lower three layers (up to the network layer) are used when any message passes through the host computer. Messages intended for this computer pass to the upper layers. Messages destined for some other host are not passed up to the upper layers but are forwarded to another host. The seven layers are:


Layer 1: The physical layer...This layer conveys the bit stream through the network at the electrical and mechanical level. It provides the hardware means of sending and receiving data on a carrier. In the Open Systems Interconnection (OSI) communications model, the physical layer supports the electrical or mechanical interface to the physical medium. For example, this layer determines how to put a stream of bits from the upper (data link) layer on to the pins for a parallel printer interface, an optical fiber transmitter, or a radio carrier.

The physical layer is usually a combination of software and hardware programming and may include electromechanical devices. It does not include the physical media as such.


Layer 2: The data link layer...This layer provides error control and synchronization for the physical level and does bit-stuffing for strings of 1's in excess of 5. It furnishes transmission protocol knowledge and management. The Data-Link Layer is the protocol layer in a program that handles the moving of data in and out across a physical link in a network. The Data-Link Layer is layer 2 in the Open Systems Interconnect (OSI) model for a set of telecommunication protocols.


The Data-Link Layer contains two sublayers that are described in the IEEE-802 LAN standards:

1) Media Access Control (MAC)

2) Logical Link Control (LLC)

The Data-Link Layer assures that an initial connection has been set up, divides output data into data frames, and handles the acknowledgements from a receiver that the data arrived successfully. It also ensures that incoming data has been received successfully by analyzing bit patterns at special places in the frames.


Layer 3: The network layer...This layer handles the routing of the data (sending it in the right direction to the right destination on outgoing transmissions and receiving incoming transmissions at the packet level). The network layer does routing and forwarding. In the Open Systems Interconnection (OSI) communications model, the Network layer knows the address of the neighboring nodes in the network, packages output with the correct network address information, selects routes and quality of service, and recognizes and forwards to the Transport layer incoming messages for local host domains. Among existing protocols that generally map to the OSI network layer are the Internet Protocol (IP) part of TCP/IP and Netware's IPX/SPX. Both IP Version 4 and IP Version 6 (IPv6) map to the OSI network layer.


Layer 4: The transport layer...This layer manages the end-to-end control (for example, determining whether all packets have arrived) and error-checking. It ensures complete data transfer. In the Open Systems Interconnection (OSI) communications model, the Transport layer ensures the reliable arrival of messages and provides error checking mechanisms and data flow controls. The Transport layer provides services for both "connection-mode" transmissions and for "connectionless-mode" transmissions. For connection-mode transmissions, a transmission may be sent or arrive in the form of packets that need to be reconstructed into a complete message at the other end. The TCP portion of TCP/IP is a program that ceaan be mapped to the Transport layer.


Layer 5: The session layer...This layer sets up, coordinates, and terminates conversations, exchanges, and dialogs between the applications at each end. It deals with session and connection coordination.

In the Open Systems Interconnection (OSI) communications model, the Session layer (sometimes called the "port layer") manages the setting up and taking down of the association between two communicating end points that is called a connection. A connection is maintained while the two end points are communicating back and forth in a conversation or session of some duration. Some connections and sessions last only long enough to send a message in one direction. However, other sessions may last longer, usually with one or both of the communicating parties able to terminate it.


For Internet applications, each session is related to a particular port, a number that is associated with a particular upper layer application. For example, the HTTP program or daemon always has port number 80. The port numbers associated with the main Internet applications are referred to as well-known port numbers. Most port numbers, however, are available for dynamic assignment to other applications.


Layer 6: The presentation layer...This is a layer, usually part of an operating system, that converts incoming and outgoing data from one presentation format to another (for example, from a text stream into a popup window with the newly arrived text). Sometimes called the syntax layer.

In the Open Systems Interconnection (OSI) communications model, the presentation layer ensures that the communications passing through are in the appropriate form for the recipient. For example, a presentation layer program may format a file transfer request in binary code to ensure a successful file transfer. Programs in the presentation layer address three aspects of presentation:

Data formats - for example, PostScript, ASCII, or binary formats

Compatibility with the host operating system


Encapsulation of data into message "envelopes" for transmission through the network

An example of a program that generally adheres to the presentation layer of OSI is the program that manages the Web's Hypertext Transfer Protocol (HTTP). This program, sometimes called the HTTP daemon, usually comes included as part of an operating system. It forwards user requests passed to the Web browser on to a Web server elsewhere in the network. It receives a message back from the Web server that includes a Multi-Purpose Internet Mail Extensions (MIME) header. The MIME header indicates the kind of file (text, video, audio, and so forth) that has been received so that an appropriate player utility can be used to present the file to the use


Layer 7: The application layer...This is the layer at which communication partners are identified, quality of service is identified, user authentication and privacy are considered, and any constraints on data syntax are identified. (This layer is not the application itself, although some applications may perform application layer functions.)  In the Open Systems Interconnection (OSI) communications model, the Application layer provides services for application programs that ensure that communication is possible. The Application layer is NOT the application itself that is doing the communication. It is a service layer that provides these services:


Makes sure that the other party is identified and can be reached

If appropriate, authenticates either the message sender or receiver or both

Makes sure that necessary communication resources exist (for example, is there a modem in the sender's computer?)

Ensures agreement at both ends about error recovery procedures, data integrity, and privacy

Determines protocol and data syntax rules at the application level

It may be convenient to think of the Application layer as the high-level set-up services for the application program or an interactive user.