While plugging my phone cord into the modem jack of my computer, I noticed that the cord has 4 seperate wires bundled inside the cord, while my telephone jack only has 2 contacts. Is there some use for the extra two wires? How many wires do you need for a phone or for a modem?
I asked this same question to a telephone repair guy a few years back. He says its primarily to power the back-lit buttons on touch-tone phones. Also, one can get a cable signal splitter to have two phone lines…but you need tho call up the phone company to activate the additional line and get assigned a new number.
In a typical 4 wire cord the red and green pair is typically for line one and the black yellow pair is for line two (if available).
I’m not sure the backlighting answer is correct as I have used touch tone phones where the light for the dailpad was taken directly from the line current of the red green pair. Possibly this was true for some very old style phones but from the 70’s onward I never saw a lighted dial need more than 2 wires.
While we’re at it, why does ethernet cable have eight wires, and only use four of them? Why is there a correct way to arrange the unused wires? Why are the two pairs used 1 & 2 and 3 & 6?
The four pair arrangement was used to allow future enhancements to take advantage of the extra two pairs. That day is actually here, as Gigabit ethernet uses all four pairs.
As did VGAnyLan. Anyone remember that ? Excellent 100 Mb technology, but everyone knew CSMA/CD better and so the market won.
Generally speaking, it’s a Good Idea to connect all the pairs in a twisted-pair cable - because a load of interfaces come with RJ-45/twisted-pair these days.
Looking around my office, I see E1 (1,2,4,5 - good to know if you ever need to make a physical loopback), ISDN BRI, POTS and, heaven have mercy, Token Ring on TP. To say nothing of Cisco’s console and auxillary ports.
It’s a given that if you only connect the pairs that you need, someone else will attempt to reuse the cable in another setting, leading to end- and needless grief. I’ve done my share of $150/hour troubleshooting visits that revolved about the wrong type of cable. (Don’t connect the ISDN with the console cable. I know the connectors fit, and I know it’s right there in the box with the router, and I know it’s a pretty baby blue colour. Just - don’t use it for ISDN. Please.)
A lot of times when you run bundles of wire through walker ducts, some of the lines of the cables will short. Having the extra lines in the cable allow you wire the connection using alternate wires.