[walrus01]

It should be noted that this is a map at what we would call layer 3 in the OSI model. ipv4/ipv6 space and BGP relationships between ASes.

Giant transit ISPs like Cogent own little if any underlying fiber infrastructure, which is much harder to map and have 100% topology discovery of by researchers sitting at desks somewhere. Lots of transit ISPs like HE, Telia or Cogent buy inter-city transport from "carrier of carrier" specialists who own the underlying assets, run DWDM systems on them, and so forth.

Without proprietary information from carriers' GIS systems (at Level3/Lumen/Centurylink, Zayo or similar) it's much harder to do that.

[astrange]

What are they providing if they don't own the cables? Are they shared with other non-ISP issues, or is it more like when a company builds an office building and then leases it from themselves?

[walrus01]

what I described above would be at layers 1 and 2 in the OSI model (the cable itself and underlying physical assets)

a company like Cogent, HE, they're providing at layer 3 the routers, network engineering, IP space (in many cases, not all, mostly for enterprise customers), and what is known as "IP transit" - transiting a customer's traffic through their network to all of their peers. And maintaining the cost of buying all those inter-city transport circuits to link all of their routers within their AS together.

https://www.teliacarrier.com/knowledge-hub/what-is-guides/wh...

https://www.thousandeyes.com/learning/techtorials/transit-pr...

https://blog.telegeography.com/what-is-ip-transit-definition