aerial vs. underground plant
expressionISP & telecom
The two ways outside plant reaches customers: on poles (cheap, fast, storm-exposed) or buried (costly, slow, backhoe-exposed).
Every ISP's build map is a negotiation between these two economics.
Aerial plant is cable strung on poles; underground plant is cable in ducts or buried. The choice is made once and determines the economics and failure modes of that route for decades.
Aerial is dramatically cheaper to install and to repair, because the cable is visible and reachable, and a crew can be at the fault in an hour. It is also exposed to everything: storms, ice loading, vehicle strikes on poles, vegetation, and the shared-pole disputes that arise when power, telephone and cable operators all attach to the same structure. Underground survives weather almost entirely and costs several times as much per metre, more in a city, and when it does fail the repair means locating the fault precisely and then excavating.
The operational consequence is that outage patterns are predictable from the plant type. A region with aerial distribution has weather-correlated outages and short repair times; underground has rare failures and long ones. Anyone reading availability figures for an access network is reading a fact about civil engineering choices made long before the service existed.