Crosstalk Elimination Tech Bulletin
Crosstalk is being able to
hear a conversation on
telephone line from another line,
or a station on a phone system from another station.
In this Tech Bulletin we're talking about crosstalk on real wired analog phone lines, analog lines from a cable or VoIP provider (from a box), or analog station ports on phone systems or ATAs.
If the crosstalk is coming in on a line from the phone company (two conversations heard), there's nothing you can do about it except report it accurately to the phone company.
If crosstalk is on real phone lines, even if it's not heard by the telco repairman, their automated tests should show a problem with the line you're reporting it on. It will have a short to ground or a short to foreign voltage, probably caused by a defect in the jacket of the cable. It could be a cross with one of the other lines in the premise, or one or more lines going to another premise.
While you may have crosstalk on the RF section of a
cordless phone or on a digital T1/PRI, that would be very unusual and
would be harder to fix - way beyond the scope of this tech bulletin.
The first thing to do is figure out where the crosstalk is coming from:
- Between two outside lines
- Between two analog station ports
- Between a POTS line and an analog station port
- On the phone line (POTS or digital) that's coming in from the phone company, well before it ever gets to the premise.
Figuring out where the crosstalk is coming from is often the hardest part of the service call. It takes time. An accurate log kept by the customer noting the lines/stations it was heard on and the times really helps. Especially since crosstalk is almost always intermittent.
You may not be able to fix a crosstalk problem if you can't hear it when you're on the service call, but there are some easy tests to perform that will let you check pretty accurately.
There's one of two reasons you're going to hear crosstalk:
1. A physical short between conductors in telephone cable... caused by:
Bare wires physically touching each other, either as a direct short (all the time) or intermittently. An intermittent short on a phone line is referred to as a "swinger" - as in when the wires strung between two telephone poles swing into each other when it's windy, causing an intermittent short.
Telephone lines originally were bare conductors strung between insulators on telephone poles, spaced out horizontally so they couldn't touch (there was no insulation on the wires).
When you have an intermittent short on a phone line, you'll also hear static.
If you short a speaker wire from your stereo, you'll hear the volume drop either a little or a lot. There is only AC audio on the pair. No DC at all.
The DC on a telephone pair makes crackling static a common trouble.
If you short a wire on a phone line the volume will drop either a little or a lot, but you'll also hear a click when the short is placed or removed from the conductor, or more commonly a crackling type static as an intermittent short changes resistance between the conductors - or between a conductor and a "foreign voltage" or ground.
A foreign voltage could be from a conductor from another phone line, some other source of AC or DC voltage, or even from a wire from a phone line or station port touching an AC power cable.
The short can be from tip to ring (on the same line - but then you wouldn't hear crosstalk), from tip or ring to ground, or any combination caused by water invading the cable (water conducts electricity) through a defect in the insulation.
Not all cable is made to be waterproof, but most will resist water for at least a short time. Animals biting into the cable definitely causes water to get into it.
It takes a while for wet cable to dry out by itself. Some cable can be dried using air, which is how the phone company protected many of their cables for years. Modern cable for use outside (aerial or direct burial) has a waterproofing gel inside and a heavy jacket to try to prevent problems.
In the old days before electronic telephone equipment (when there were just switches and relays), a repairman would use a battery box (burn box) to put a very high current down a pair that was wet. The idea is that the conductors would get hot and dry out the water in the cable.
That worked until subscribers started putting in electronic phone systems, when the high current would burn up the trunk cards on these new systems. The phone company didn't want to stop drying cables that way but they were getting sued by the subscribers whose expensive new phone system was destroyed. It's seldom done today.
The phone company's last choice, and yours, should be to replace a wet buried or aerial cable. The first thing you try is finding a spare pair that isn't wet/bad. If you can't find a good pair you may be able to detect where the fault is (where the water is getting into the cable) with a TDR (Time Domain Reflectometer) which will tell you how many feet away you are from the problem in the cable. You walk that far out and dig the cable up, or otherwise look for the cable and try to repair the fault in the cable.
If it's a really easy cable run replacing the cable is probably
better and faster than fixing it.
2. Crosstalk induced onto one pair from another because the wires are too close together, and/or they aren't paired:
In the old days some telephone station wire wasn't twisted. The old red/green/yellow/black (JAKE or JKT) inside wiring cable had basically no or very random twists. It was CAT nothing cable and it really picked up crosstalk if there were two lines on the four wires in the single cable. It also picked up RFI (Radio Frequency Interference) from radio stations like crazy.
That cable was first used when essentially nobody had two lines in their house. The third and fourth wires were used for the princess phone dial transformer in houses, and for A-Lead control on single line phones attached to 1A key systems in businesses (to light the light on key phones when someone was using the single line phone).
Regular twists in a cable resist crosstalk and interference. Even some twists, like CAT1 which would be very lightly twisted, does a pretty good job of reducing voice crosstalk and RFI. CAT3 does a great job.
Essentially all twisted pair voice cable is now CAT3. Either 4 pair or 25/32/50/100 pair.
CAT5 cable has more twists per foot, but seldom does a better job for voice than CAT3 except in high RF situations (like at a radio station).
Speaking of radio stations, some of the engineers decide to run their own telephone cable and they use shielded audio cable. That usually causes crosstalk and RF. If you see that stuff or the red/green/yellow/black cable on a case of trouble like crosstalk or RF, just abandon the cable and run twisted pair.
If someone has done something really goofy either on-purpose or mistake, and used one wire from each of two pairs, like the white/blue wire and the orange/white wire, that's not a pair at all and could easily get crosstalk or RFI.
Likewise if you double up pairs the wire is no longer twisted pair. Don't do that!
You can also get crosstalk if speaker wires or something with loud audio is on the same cable as a telephone line, or tie-wrapped to a telephone cable. The telephone network was carefully engineered to set the maximum volume of a phone line to prevent crosstalk. The level of audio on a pair going to a 70V paging speaker, with the speaker wire tie wrapped to a telephone cable, is definitely going to bleed over to the phone pairs.
Generally speaking you want to have a speaker wire cross a phone wire at a 90 degree angle, never run parallel to the phone wire.
You may have noticed that if you put a toner used to trace wires on a
phone pair it's usually heard faintly on lots of other
pairs. That's because the toner is much louder than what
the phone network was designed for - which is OK because it's just a
tool used to repair stuff and not left in-place for very long.
Checking analog lines for crosstalk
If you determine that you hear two conversations on a single incoming line at the demarc, the rest is easy.
Well, kind of easy because if it's intermittent it will be hard to get the phone company to fix it if they can't hear it. On the good side if the cable was wet but is drying out when the phoneman checks it and he can't hear crosstalk (maybe because the other line isn't being used at the time), the phone company also runs an automated test on the line - which will most likely show that there's a problem on the pair.
This checking procedure will work for two lines or 100 lines. I'm going to use three lines at the RJ-21 as an example.
You must know the phone number for a 1KC (1000 cycle) tone (milliwatt) line. You can't use a toner for tracing wires for this procedure since the tone it puts out is way too loud. You aren't testing for how loud the tone is, you just need to listen for it, so you can use any 1KC tone you have the number for. The local 1KC tone here in Roselle, IL is 630-980-9940, which doesn't seem to time-out. That will work OK for this application.
I've used 312-856-9996 on service calls here in Chicago for years but AT&T changed it from a 1KC tone that would stay on the line (with a slight pause every 10 seconds) to 8 seconds of tone followed by silence (silent termination). That's just not useful for checking crosstalk. You need a tone that will stay there for a while, like the 630-980-9940 line. Don't hog the line because there are others that need to use it!
If you need test numbers for your local CO we may have them in our CO Lookup (enter the area and exchange code to see if we have any test numbers for that CO).
You'll also need two butt-sets. The second butt-set doesn't have to be fancy, but you need two.
It's best to do this before or after hours so you're not fighting the users trying to use the phone!
The simplest test you can do is simply use one butt set to go on each line and listen in monitor mode, and then go off-hook to see if you hear anything that doesn't sound right. If there is a serious imbalance you'll hear that the line sounds different from the others right away. You can skip that if you're going to perform a real crosstalk test because you'll be doing it anyway.
I usually do not remove the bridging clips on the RJ-21X when doing this test since the test will catch problems looking out to the phone company and looking in to the telephone equipment. Once I identify a suspect lien, then I do testing with the bridging clips removed.
• For three phone lines I put one butt-set on the last line and dial the 1KC test number (and make sure I hear it).
• I lay that butt-set down (depending on how quiet the phone room is I may cover the receiver with my hand so I don't hear the tone coming from the butt-set itself), and take the other butt-set and clip it on line two. I dial a Silent Termination phone number (like 312-856-9997 which times out after a while)
• I listen for the tone I've got dialed up on Line 3, and for other unusual noise. You must dial a silent termination number (or your office to talk to someone). Just listening in monitor mode going off-hook with the butt-set and not dialing a number is not a valid test . An unterminated line is likely to pick up noise and crosstalk, so you must dial a phone number to perform a valid test.
• If I hear noise or the tone I make a note of which line it's on. You really need to make a chart while you're doing these tests.
• If I don't hear the tone I move the butt-set to Line 1 and dial the Silent Termination number and listen for the tone or noise.
• Then I remove the butt-set from Line 3, put it on Line 2 and dial the 1KC number and lay that butt-set down.
• I put the other butt-set on Line 1, dial the Silent Termination number and listen for the one or nose.
This procedure tests all of the lines with the least amount of work.
If you have ten lines you'd start by dialing the 1KC number on Line 10, then checking with silent termination from Line 9 through Line 1 for the tone or noise. Then I'd dial the tone on Line 9 and check Line 8 through Line 1 for the tone or noise, repeating the process until I finally dialed the Tone on Line 2 and checked for the tone or noise on Line 1.
If the crosstalk is between two of the lines at that premise you will find the problem with that test. On any lines I heard the tone or noise on I then repeat the test with the bridging clips open so I can be sure whether the crosstalk is coming from the phone company or the telephone equipment / inside wiring.
If I hear the crosstalk between two or more lines with the bridging clips open I report the lines to phone company repair. There's nothing more I can do at that point.
If I don't hear the tone checking for crosstalk my only choice is to try to get the customer to note which CO line they are hearing crosstalk on, and if it's always the same line(s) I just report what the customer told me to the phone company and hope for the best.
Just like a bad card in a phone system could cause crosstalk, a bad card in the CO or a SLC in the neighborhood can be causing the crosstalk - not related to cable at all.
Damage from Crosstalk
So how much damage can crosstalk cause? If a customer or competitor hears something they shouldn't it could cost a lot of money.
The biggest loss of money due to crosstalk was the divorce settlement of former GE CEO Jack Welch.
It turns out he had GE's KSU-less multiline phones in his house. Those phones are known to be poorly designed (as are many of the two or more line consumer phones), and have crosstalk built-in.
Well, one day he was talking to his girlfriend on one line and his wife went to use the other line (the phones have busy lights). Because of the built-in crosstalk (defect from the factory) his wife was able to hear the conversation.
So how much could that possibly cost? The divorce settlement was reportedly $180 million.
Crosstalk Caused by Modular Non-Twisted Line Cords
The GE and RCA KSU-less phones have poorly designed circuit boards and they just have crosstalk no matter what anybody does (except throw the phones away).
2 Line, 3 Line and 4 Line KSU-less phones normally come with a 7 foot long modular mounting cord (or sometimes two cords for a 4 Line phone). Some come with a special twisted pair mounting cord, which helps prevent crosstalk. Even if it's not twisted, a 7 foot flat mounting cord (not twisted) may not cause crosstalk?
When the 7 foot cord won't put the phone where the user needs it, they just get a regular flat 14' or 25' modular cord. The phone works with that flat cord, but they end up having crosstalk between the lines on the flat cord (because the pairs aren't twisted in the flat cord - they're laying next to each other).
Click HERE to see our 14' and 25' 2 Pair Twisted Pair Modular Cords for those situations. The wire is round, not flat, and contains two twisted pairs. It has regular modular plugs on it (not the bigger RJ-45 8 pin type). This line cord is more expensive but it will prevent crosstalk compared to a flat cord, and also reject RFI.