By John Larmouth, University of Salford - A Greater Manchester University, 11 Nov 97. OSI ( Open Systems Interconnection ) represents the totality of protocol definitions and associated additional texts which provide international standardization of many aspects of computer-to-computer communication. In theory it extends from the lowest level of signalling techniques to high-level interactions in support of specific applications. Perhaps the best-known aspect of OSI is the 7-layer model ( formally The Basic Reference Model for Open Systems Interconnection ). Most readers will have at least heard of it, and can probably recite the names of the seven layers. What is the purpose of the layer concept? Why do we call it an architecture? Why seven layers? In this introductory discussion we will take a very broad look, and develop more detail as we proceed. The form of combination of separate specifications in OSI ( and in many other architectures ) is as follows: 1. the bottom-level set of messages is defined, usually involving some header information and some tail information, but with a hole in the middle which is left undefined by this level or layer. 2. The next layer ( which could comprise a set of different independent specifications, any one of which could be used for an actual application ) defines the contents of the hole, usually in terms of some header information, another hole (to be completed by a still higher layer, and so on ), and some trailing information. 3. Eventually we reach a specification that fills in the last hole ( the top layer ), and have our messages ( given an appropriate selection of the specifications ) completely defined. This approach is called layered protocol definition or just layering. Each of the layered specifications adds some value to the functionality of the layer below or, in OSI terms, uses the services of the layer below to build an enriched service. In the earliest days of computer communication, typically when a single link of copper wire or a radio-link connected the two communicating computers, a single monolithic specification completely defined all aspects of communication in an unstructured manner. Aspects which were very application-specific (such as signalling the end of a deck of cards, or pressing some specific button on one of the communicating machines) were signalled using some specific voltage or current on the wire. In other words, the low-level signalling that was used did not merely indicate zeros and ones that higher layers of specification used to carry information. Rather, the signalling system and the application were inextricably intertwined. Some of these so-called "link" protocols lasted into the 1990s, mainly in the military communications area, but they are very inflexible because of the intermingling of application matters with details of the signalling system and hence the particular medium of communication. If we are trying to produce International Standards for a large and ever-increasing number of applications ( m say ) to run over a large and ever-increasing number of different types of media and signalling systems ( n say ), then if we were to produce monolithic specifications, we would need m times n standards, an unmanageable task. The first and most important step must be to try to separate the application-dependent aspects from the signalling and routing technology dependent aspects by defining the functionality to be provided by the latter, and assumed by the former. We talk about "defining the OSI Network Service", that is, specifying the functionality for end-to-end communication to be provided by networks and to be used by application specifications. In principle we have now reduced the problem to an m plus n plus 1 problem - one end-to-end Network Service Definition, m application specifications assuming this functionality, and n network technologies providing this functionality to end-systems wishing to communicate. This would produce a two-layer model - one network layer, and one application layer. There have indeed been ( non-OSI ) protocol suites that have adopted this architecture. It is the minimum layering that makes any sense today.

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