The protocols contributing to moving data from one computer to another are represented as the layers of a stack. This is a teaching aid. Networking involves a large number of functions and a vast quantity of standards and protocols. Dividing up this mass of knowledge aids the student to get a grip on the field. Once universities started teaching networking by this method, the industry began to segment along the same lines. Many different skills contribute to the networking industry and those that work in the field define the scope of their career by the layers in which they specialize.
Networking technology follows the layering concepts of two main models. The first is TCP/IP, the Transmission Control Protocol/Internet Protocol stack. This was originally one large protocol, but eventually split out into separate layers. The TCP/IP model does not have as many layers as the Open Systems Interconnection (OSI) model produced by the International Standards Organization (ISO).
The OSI model has seven layers. TCP has only three, but the bottom layer is divided into two sublayers. An important principle of layering networking protocols is that of "abstraction." A protocol at one layer does not need to know how a protocol at a lower layer works in order to use its services. This enables specialization. An application asks for a connection without going into the detail of how that task is achieved. The layer immediately below offers a service to that above. The lower layer itself uses services of the layer below it in order to provide its function.
In both stacks, layers are numbered from one at the lowest layer, and each higher layer is numbered one higher. Traditionally, TCP/P only had three layers. However, it is usually represented with an additional application layer on top, making four layers as compared to the OSI stack's seven layers.
The Application Layer deals with the user. The Presentation Layer covers data formatting and issues such as compression and encryption. The Session Layer maintains a connection between two computers, establishing parameters for the session and breaking the session when all transactions have been completed. The Transport Layer breaks data up into packets. In the TCP/IP model, the functions of the Presentation Layer and the Session Layer are bundled into the Transport Layer.
The Internet Layer is the heart of the TCP/IP stack. This is where the IP address structure is defined. IP addresses are a key element of Internet technology and have made the TCP/IP model more widely used than the OSI model. The equivalent layer in OSI is called the Network Layer. This far down the stack, protocols deal with data in packets. The next layer down, the Data Link Layer, converts data packets into frames of bits ready for transfer across the network. The bits are converted into an electronic pulse to journey across the cable. The Physical Layer defines the properties and configuration of the cable. In TCP/IP, the Data Link Layer and the Physical Layer are joined together as the Network Access Layer.
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