The OSI Reference Model

For many years the OSI reference model was the core reference framework used by all communications courses. It remains a theoretical reference model that can be used to compare protocols and stacks.

The OSI reference model specifies standards for describing "Open Systems Interconnection" with the term 'open' chosen to emphasise the fact that by using these international standards, a system may be defined which is open to all other systems obeying the same standards throughout the world. The definition of a common technical language has been a major catalyst to the standardisation of communications protocols and the functions of a protocol layer.

The seven layers of the OSI reference model showing a connection between two end systems communicating using one intermediate system.

The structure of the OSI architecture is given in the figure above, which indicates the protocols used to exchange data between two users A and B. The figure shows bidirectional (duplex) information flow; information in either direction passes through all seven layers at the end points. When the communication is via a network of intermediate systems, only the lower three layers of the OSI protocols are used in the intermediate systems.

Services provided by each Protocol Layer

The OSI layers may be summarised by:

1. Physical layer: Provides electrical, functional, and procedural characteristics to activate, maintain, and deactivate physical links that transparently send the bit stream; only recognises individual bits, not characters or multicharacter frames.
2. Data link layer: Provides functional and procedural means to transfer data between network entities and (possibly) correct transmission errors; provides for activation, maintenance, and deactivation of data link connections, grouping of bits into characters and message frames, character and frame synchronisation, error control, media access control, and flow control (examples include HDLC and Ethernet)
3. Network layer: Provides independence from data transfer technology and relaying and routing considerations; masks peculiarities of data transfer medium from higher layers and provides switching and routing functions to establish, maintain, and terminate network layer connections and transfer data between users.
4. Transport layer: Provides transparent transfer of data between systems, relieving upper layers from concern with providing reliable and cost effective data transfer; provides end-to-end control and information interchange with quality of service needed by the application program; first true end-to-end layer.
5. Session layer: Provides mechanisms for organising and structuring dialogues between application processes; mechanisms allow for two-way simultaneous or two-way alternate operation, establishment of major and minor synchronisation points, and techniques for structuring data exchanges.
6. Presentation layer: Provides independence to application processes from differences in data representation, that is, in syntax; syntax selection and conversion provided by allowing the user to select a "presentation context" with conversion between alternative contexts.
7. Application layer: Concerned with the requirements of application. All application processes use the service elements provided by the application layer. The elements include library routines which perform interprocess communication, provide common procedures for constructing application protocols and for accessing the services provided by servers which reside on the network.

The communications engineer is concerned mainly with the protocols operating at the bottom four layers (physical, data link, network, and transport) in the OSI reference model. These layers provide the basic communications service. The layers above are primarily the concern of computer scientists who wish to build distributed applications programs using the services provided by the network.

"Hop-by-Hop" "Network-wide" and "End-to-End" Communication

The two lowest layers operate between adjacent systems connected via the physical link and are said to work "hop by hop". The protocol control information is removed after each "hop" across a link (i.e. by each System) and a suitable new header added each time the information is sent on a subsequent hop.

The network layer (layer 3) operates "network-wide" and is present in all systems and responsible for overall co-ordination of all systems along the communications path.

The layers above layer 3 operate "end to end" and are only used in the End Systems (ES) which are communicating. The Layer 4 - 7 protocol control information is therefore unchanged by the IS in the network and is delivered to the corresponding ES in its original form. Layers 4-7 (if present) in Intermediate Systems (IS) play no part in the end-to-end communication.

A detailed example of the interaction between OSI layers is provided in an example.