JPL's Wireless Communication Reference Website

Chapter: Capita Selecta

Layered Organisation of Telecommunication Nets

The 'Open Systems Interconnection' (OSI) reference model plays an important role in standardising protocols for exchange of data between complex information systems.

The OSI model consists of seven layers (1 - 7), each of which is specified to support reliable interconnection between two systems, I and II, irrespective of the technical implementation of lower layers. Each layer adds specific functions to lower layers to enrich the value of the services offered. There is a division between the lower three and the upper four layers. The lower layers (1, 2 and 3) address 'transport' or 'bearer' services. The higher layers (4 - 7) add user-oriented functions necessary for services such as electronic mail, EDI or videotex. The combination of layers 1 to 7 offer 'value added' services. During the 1980's, the distinction between the three lower and the upper four layers was clearly drawn in legal and regulatory issues, e.g. in considering the extent of the PTT monopoly for transport services.

At present, the applications of information and computer systems become increasingly interwoven with telecommunication networks; this interaction is known as 'telematics'. For a more complete view of telematic services, one may therefore also have to consider the specific user applications which are located above layer 7. A corresponding extension of the OSI layers has been proposed in research papers: Social and juridical aspects of information transactions are addressed by the information structure layer (8) and the information delivery and access layer (9).


System	Layer 			Tasks / Aspects
(9) (9) Information delivery Legitimate delivery to parties
in economic and social processes
(8) (8) Structuring and organization of information
7 7 Application layer Interface to application software and user services
6 6 Presentation layer Handling and translation of structured data
5 5 Session layer Coordination and synchronization of the session
4 4 Transport layer "End-to-end" transfer of data
3 3 Network layer Routing and switching
2 2 Data link layer Error control (coding)
1 1 Physical layer Control of physical circuits
Physical media for interconnection (cables, radio links, optical fibre etc.)
The principles behind the structuring into the above layers were


User mobility requires precautions at all layers of the network protocol stack. Hear Randy Katz discuss the potential problem of strictly layering protocols in wireless systems. He researches mobile computing.

JPL's Wireless Communication Reference Website (c) 1993, 1995.