Telephone tracing an endurance sport?

September 20, 2009 1:02 PM

How did old-fashioned mechanical-relay telephone tracing work?

I've just watched Black Christmas for the first time, and a central component of the plot is the race through the telephone switching station to trace a mysterious call. I was intrigued that someone from the phone company had to come put a physical tap on the line and then follow that line somehow at the station. Can anyone explain how this words?

Bonus points if you can explain to me why old phone numbers had a word in front of the numbers. :-) (Fellatio-451, for example)

posted by jefficator to technology (4 comments total)

Dunno about tapping, but the words were Telephone Exchange Names.
posted by jquinby at 1:05 PM on September 20, 2009

For an electromechanical exchange, where there's no central computer controlling the whole thing, the process is simple but tedious:

The tracing process starts with the technician finding where the answering line comes into the telephone exchange--a pair of wires which are hopefully labeled in some convenient manner so that they're easy to find based on looking up a circuit number in the records.

This line will connect to some type of electromechanical switch that allows the line to be connected to one of several other sets of wires. The technician checks to see what position the switch is in, and therefore which set of wires has been connected to the line.

Then he follows those wires to wherever they connect, likely another electromechanical switch, and repeats the process of examining the switch to see what position it's in, then following the connected wires to wherever they come from.

This process continues until either the technician finds the physical phone line that the call originates from (in which case hopefully good records exist as to whose line that is), or until a trunk line to another exchange is encountered. If the latter happens, he has to get in touch with a technician at the exchange on the far end of that trunk line, to continue the same process at that exchange.

A telephone exchange is basically just a big collection of wires and switches that allow any line to be connected to any other line, via a path that usually involves several switches and sets of wires. Tracing a call is simply a matter of checking switches and following wires to trace the connection path back to its source.
posted by FishBike at 1:27 PM on September 20, 2009 [1 favorite]

years ago a friend worked for gte in what was called the "cross bar" I believe. This is the switching equipment (it took up the entire floor) & this was where phone taps by law enforcement took place. All of this mech/elect. equipment has been replaced by computers taking up a fraction of the space.
posted by patnok at 1:43 PM on September 20, 2009

And that's why it's called tracing!

As for the letters, it was just an easier way of remembering phone numbers in the day. PEnnsylvania 6-5000 was 736-5000 (you'd use the first two letters), just like 1-800-CALL-UPS works out today. The words would often be related to the region they were in, but not necessarily.
posted by mendel at 9:17 PM on September 20, 2009