JPL's Wireless Communication Reference Website Chapter: Capita Selecta

Layered Organisation of Telecommunication Nets

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






 I    II



------------------------------------------------------------------------------------------------



(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.)

• A new protocol layer should be created where a different level of abstraction of the tasks to be performed is needed.
• Each layer should perform a well-defined function.
• The function of each layer is chosen to ensure compatibility with internationally standardised protocols
• Each layer only has interfaces with the layer immediately above or below. Layer boundaries should be chosen to minimize the information flow from one layer to its lower or upper neighbouring layers.
• The number of layers should be chosen large enough that essentially distinct functions should not be performed in the same layer, and small enough to ensure that the complexity of the architecture remains within limits.

Audio

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