Refinement needed in the model of Network provision

[mike1813]

The relationship type providedBy from an AbstractSubnet refers to the entity that provides the means of communication over that AbstractSubnet. Any type of network can have this relationship:

• an IP subnet would normally be provided by a Router
• a Bluetooth or USB pairing relationship (which is modelled as a subnet) would be provided by one of the paired devices
• internal communication within a Host using sockets (possibly below an IP layer) would be provided by that Host
• a wide area Cellular Network would be provided by the device providing its backbone network

There are two aspects with the current model that allow anomalous system models to be created, and/or create ambiguity or unwanted dependencies affecting some of the domain model inference rules.

• the providedBy relationship can also refer to a DataCentre, but really this only makes sense for an IP subnet connecting more than one Host (i.e., not an internal communication subnet within one Host)
• a Cellular Network is strictly also provided by the Base Stations that provide RAN (Radio Access) subnets in each of its cells

These should be addressed as far as is practical given the potential backward compatibility issues it may cause for existing system models. We can't do anything about the fact that the relationship can be from any AbstractSubnet, so the restrictions on DataCentre provision of networks will need to be addressed by adding modelling error threats. The issue related to a Cellular Network causes fewer problems but could be addressed by allowing a RAN to be inferred if a Base Station provides a Cellular Network.

This leads to two sets of changes

• Add modelling errors in package#Network to detect where a Data Centre provides a type of network that should only be provided by a Host
• Alter the construction sequences in package#5G so the host providing any part of a cellular network is regarded as providing the cellular network, and this can be used to specify other inferred elements such as RAN