Answer»
- OSI layers 3 to 6 are MISSING in most fieldbuses (and in HART). These are the Network, Transport, Session and Presentation Layers. The services provided by these layers in a full protocol stack are concerned with features of a full wide area network, where (logical and physical) CONNECTIONS are made and broken, addressing and routing functions must cover multiple networks, data packets may be corrupted or lost, and data may be carried in multiple FORMATS. In general, these services are not needed, or can be greatly simplified, in a fieldbus where a fixed set of devices is permanently connected to a single network.
- Fieldbuses usually provide only a bare minimum of these features, and include them within those layers which are fully implemented. Different fieldbus protocols vary in the extent to which such functions are provided (particularly, support for multi-segment network addressing and segmented FILE transfer). Features which are not provided in the protocol stack may be implemented within the device or host application itself, but this is somewhat less satisfactory, since different implementations may not inter-operate properly.
- Leaving out unwanted functionality results in simpler embedded software or hardware. Since each layer has to be specified both by its function and its interfaces with the layers above and below it, this also SIMPLIFIES the protocol specification.
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