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 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.
The main idea in OSI is that the process of communication between two end points in a telecommunication network can be divided into layers, with each layer adding its own set of special, related functions. Each communicating user or program 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 or program. 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 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.)
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.
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.
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.
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.
Layer 2: The data-link layer-This layer provides 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.
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.
AND
TCP/IP
The Internet Protocol Suite (commonly known as TCP/IP) is the set of communications protocols used for the Internet and other similar networks. It is named from two of the most important protocols in it: the Transmission Control Protocol (TCP) and the Internet Protocol (IP), which were the first two networking protocols defined in this standard.
The Internet Protocol Suite, like many protocol suites, may be viewed as a set of layers. Each layer solves a set of problems involving the transmission of data, and provides a well-defined service to the upper layer protocols based on using services from some lower layers. Upper layers are logically closer to the user and deal with more abstract data, relying on lower layer protocols to translate data into forms that can eventually be physically transmitted.
The TCP/IP model consists of four layers (RFC 1122). From lowest to highest, these are the Link Layer, the Internet Layer, the Transport Layer, and the Application Layer.
Application layer
-Refers to standard network services like http, ftp, telnet as well as communication methods used by various application programs
-Also defines compatible representation of all data
Transport layer
-Manages the transfer of data by using connection oriented (TCP) and connectionless (UDP) transport protocols
-Manages the connections between networked applications
Internet layer
-Manages addressing of packets and delivery of packets between networks
-Fragments packets so that they can be dealt with by lower level layer (Network Interface layer Network)
Network Interface layer
-Delivers data via physical link (Ethernet is the most common link level protocol )
-Provides error detection and packet framing
Different authors have interpreted the RFCs differently regarding whether the Link Layer (and the four-layer TCP/IP model) covers physical layer issues or a "hardware layer" is assumed below the link layer. Some authors have tried to use other names for the link layer, such as Network interface layer, in effort to avoid confusion with the Data link layer of the seven-layer OSI model. Others have attempted to map the Internet Protocol model onto the seven-layer OSI Model. The mapping often results in a five-layer TCP/IP model, wherein the Link Layer is split into a Data Link Layer on top of a Physical Layer. Especially in literature with a bottom-up approach to computer networking, where physical layer issues are emphasized, an evolution towards a five-layer Internet model can be observed out of pedagogical reasons.
The Internet Layer is usually directly mapped to the OSI's Network Layer. At the top of the hierarchy, the Transport Layer is always mapped directly into OSI Layer 4 of the same name. OSIs Application Layer, Presentation Layer, and Session Layer are collapsed into TCP/IP's Application Layer. As a result, these efforts result in either a four- or five-layer scheme with a variety of layer names. This has caused considerable confusion in the application of these models. Other authors dispense with rigid pedagogy focusing instead on functionality and behavior.
