Node type codelist: Consider adding a 'junction' code

[duncandewhurst]

Background

Fibre network maps often include nodes without a specified type. For example, in the following screenshot, the node on Yaa Asantewaa Avenue is explicitly declared as a PoP, but the type of the node at which the span running along Accra / Konongo - Elisu Road branches is not explicitly declared:

The nodeType codelist lacks a code for these generic junction/branch/fork nodes. A 'junction' code was proposed in #46, but not added for want of further clarification of its semantics.

Use cases

Tracing, digitising and conversion

Users who are tracing, digitising or converting network maps to OFDS format are likely to come across this scenario. They need to know what (if anything) to put in Node.type.

Mapping and filtering

Users who are producing network maps based on OFDS data are likely to want to filter on Node.type to only show specific node types, e.g. points of presence, or to apply differing symbology for different node types. They might want to exclude junctions from their maps.

Modelling options

Omit Node.type

Node.type is an optional field so it could simply be omitted for junction nodes. Explicitly declaring that a node is a junction code doesn't provide users with any extra information, since that fact can be deduced from more than two spans referencing the node in Span.start or Span.end. If we pursue this option, we can consider providing guidance to publishers and users, either in the description of Node.type or the documentation.

Add a 'junction' code

A 'junction' code (or similar) could be added to the node type codelist so that junctions can explicitly be declared. That would mean slightly more work for data publishers and slightly less work for data users (in identifying junctions). However, this option comes at the risk of inconsistent data. For example, a Node.type could be set to 'junction', but only referenced by one or two spans, in which case users would need to make a judgement call on how to interpret the data.

We could add a normative rule that 'junction' must only appear in Node.type if the node is referenced by >2 spans. However, a check for that rule would need to be implemented in code in CoVE and any other tool that produces or validates OFDS data.

@stevesong, I think that I captured everything from our discussion, along with a few extra thoughts, but please add anything that I missed. In particular, I'm interested to know if you have any other use cases in mind for a 'junction' code. Thanks!