Cool to see you here on HN. I hadn't heard the name PairGain since I worked for a small corporate ISP in the early 2000s. We'd recommend PairGain modems for clients who needed seriously high-speed links of 2Mbps! This was just before ADSL and SHDSL were rolled out en masse, or at least well before they were reliable enough for corporate use. We had to organise a special installation of a direct copper line from their premises to ours. I guess they just patched them together at the exchange? It was a pretty small catchment area. Fun times!
The tariff in my locality allowed for dry copper pairs to be installed ("burglar alarm circuits") and some of my Customers took advantage of that along w/ PairGain devices to get high speed links between sites serviced out of the same central office.
I can't tell exactly from a quick scan of Cringeley's commentary... but I get the impression that a "dry copper pair" the single-pair POTS equivalent of "dark fiber"... with the critical caveat that a dry copper pair can link two points only if they are serviced by the same central telephone company switch.
A leased loop is any arrangement where a customer pays the telco for a dedicated loop for continuous use. A dry loop is a type of leased loop that isn't connected to any telco equipment, just spliced together to create a continuous circuit (a typical or 'wet' leased loop has at least battery and sometimes dial tone from the telephone switch). It's perfectly possible to get a leased line that spans telco offices, in which case your loop has to be spliced to an interoffice or toll lead (long distance line). In the days before heavy use of multiplexing, toll leads were scarce and so interoffice leased lines were extremely expensive.
Dry loops were often sold as burglar alarm circuits because one of the most common uses was for burglar alarm communicators that often operated on polarity reversal - meaning that they applied a potential to the pair and just swapped its polarity when an alarm condition occurred. Of course there were more sophisticated burglar alarm communicators at the time that used telegraphy techniques, but these usually ran on private networks since they could share a bus in a way that was not typical of telco infrastructure (most of these were basically Gamewell systems even if not made by Gamewell proper). In the early days of burglar alarm monitoring, if the monitoring service didn't have a private network (typical in urban areas) they usually installed their monitoring equipment or a multiplexing system at each telephone exchange in the covered area, allowing for more economical dry loops within a single office. Actually this pattern continued well into the '90s with some burglar alarm services using DSL-like high-frequency digital communicators that interacted with a monitoring system that had to be connected to the line card at the telephone exchange.
1990s AppleTalk networks could work over a single pair. I ran a network with hundreds of Macs for a tech company. Ten buildings in a small campus.
Still remember the day the fire-alarm techs punched down over one of the Macs in the executive office. Not much of a network signal when there's 24 Volts on the same line.
