The Osi Model

The OSI Model

NTC/360 Network & Telecommunications Concepts

The OSI Model

The Open Systems Interconnection Reference Model (OSI) is the accepted model that describes the how the communications for a computer network should be designed. OSI was developed as part of the Open Systems Interconnect Initiative.

Basically, OSI divides the functions of a protocol in to seven layers. This makes the reasoning easy to follow and the system much more reliable. The implementation of several OSI layers is often referred to as a Stack or a TCP/IP Stack.

The layers are often referenced in descending order because layer 7 is the first layer that an end user sees. A detail of each layer in descending order follows:

* Layer Seven of the OSI Model - The Application Layer of the OSI model is responsible for providing end-user services, such as file transfers, electronic messaging, e-mail, virtual terminal access, and network management. This is the layer with which the user interacts.

* Layer Six of the OSI Model - The Presentation Layer of the OSI model is responsible for defining the language that enables two network hosts to communicate. Encryption and compression should be Presentation Layer functions.

* Layer Five of the OSI Model - The Session Layer of the OSI model is responsible for establishing process-to-process communications between networked hosts.

* Layer Four of the OSI Model - The Transport Layer of the OSI model is responsible for delivering messages between networked hosts. The Transport Layer should be responsible for fragmentation and reassembly.

* Layer Three of the OSI Model - The Network Layer of the OSI model is responsible for establishing paths for data transfer through the network. Routers operate at the Network Layer.

* Layer Two of the OSI Model - The Data Link Layer of the OSI model is responsible for communications between adjacent network nodes. Hubs and switches operate at the Data Link Layer.

* Layer One of the OSI Model - The Physical Layer of the OSI model is responsible for bit-level transmission between network nodes. The Physical Layer defines items such as: connector types, cable types, voltages, and pin-outs.

The layers have the several characteristics in common. Eeach layer is separate