Public cellular network accessibility issue

[mike1813]

At present, it is assumed that a public cellular network must be accessible from the World. The idea is that such a network should be considered ubiquitous in any physical location related to the modelled system (unless this is prevented by security measures).

However, the World represents the global public space, i.e., everywhere outside the Spaces defined explicitly in the system model where there are no restrictions on access. This space may be too big to be compatible with the assumption that a public cellular network can be accessed from it.

There are two scenarios:

1. A public cellular network represents the ability for cellular network User Equipment to connect from anywhere, using roaming access if necessary. In this case, having one public cellular network accessible from the World (and any other Space containing connected UE devices) is appropriate.
2. The system may extend over a larger geographical region than is covered by a single public cellular network. In this case, having all UE devices on a single public cellular network is an oversimplification. One should use more than one public cellular network, each restricted to a set of Spaces.

Up to now, system models have used public cellular networks to represent scenario (1). At present, the only way to model scenario (2) is to make each cellular network private, so its coverage must be asserted explicitly. However, a private cellular network is assumed to have fewer routing restrictions in place, so those would also need to be added (modelling them by selecting the corresponding Controls).

If we want to enable the use of public cellular networks to handle scenario (2), some further refinement would be needed. The simplest extension is to create two distinct classes of public cellular network, one of which represents a set of networks covering the World via roaming agreements, while the other represents a geographically constrained public cellular network covering a specific area such as a single country.

[mike1813]

One other reason why public cellular networks are assumed to be accessible from the World is because it is then possible to infer that they are connected to the Internet without using a disconnected matching pattern. In the past, system-modeller did not support such matching patterns, although this limitation may now have been addressed.