January 1, 2008

TELEPHONY:
Avoiding Telephony Gremlins

By Justin J. Junkus

Square 1

The first rule of telephony troubleshooting is trouble avoidance. While it’s tempting to cut time per installation by doing a quick disconnect-reconnect from the other telephony service to yours without a check of all phone outlets, don’t take this route.

With the prevalence of do-it-yourself outlet additions and contractors without any formal telephony training, the odds are good that at least one of the phone outlets in the home was wired incorrectly or equipped with sub-quality materials.

My favorite is the cheap wallplate where the RJ-11 jack only accepts the connector when the connector is jammed past the retaining tab. Visually, it passes inspection, but the forced fitting may have bent or shorted the internal prongs.

The best procedure is to test each outlet before disconnecting the incumbent phone service provider. Go to each outlet, unplug any phone or wire, and use a butt set or a phone you know is working to check for dialtone and the ability to dial out. Document your findings on a room-by-room basis, indicating pass or type of failure mode.

If the incumbent phone service is already disconnected, or if this is new construction, you have two choices. You can use a toner at the demarc point to inject a tone into the home wiring and check for continuity and reversals, or you can install your service and then do the room-by-room check. I’d go with the latter option, given the time to set up the tone and the fact that a toner test is not a complete phone service equivalent. Realize, however, that when you do this, you have effectively accepted the responsibility for ensuring a good connection at each outlet, usually free of charge to the subscriber.

Once your service is installed and you’ve checked for dialtone at the embedded multimedia terminal adapter (EMTA) before it is connected to the customer’s wiring, you can verify service at each outlet. Begin by disconnecting all phones from outlets to isolate any potential problem in customer premises equipment (CPE - phones, fax machines, etc.) that might feed back into the customer’s wiring. This is particularly pertinent if noise on the line is a problem for the installation site.

Checking each outlet

At each outlet, check for availability of dialtone, the ability to dial a number, and the clarity of a telephone connection. Once again, your best test tool for doing this is the butt set, but a known working phone will do most of the job in a pinch.

Lack of dialtone at an outlet usually means an open in the path back to the EMTA. If this condition occurs, check the other outlets for dialtone. If it is absent at all outlets, you should check the connection from the EMTA to the subscriber’s wiring. If it is absent at only a couple of outlets, chances are these outlets are connected as a loop back to the demarc. You will need to temporarily disconnect the EMTA from the subscriber’s wiring and use a toner to isolate where tone is lost as you check the outlets that do not have dialtone.

If there’s no dialtone at just one outlet, unscrew the wallplate and visually inspect the wiring connections to it for opens. If all is well, disconnect the wires from the wallplate and use the alligator clip connections on your butt set to directly check for dialtone on the line. If dialtone is not present, check the path from the outlet back to the EMTA using a toner.

Similar to the case where several outlets do not have dialtone, this will require temporarily disconnecting the EMTA phone side from the subscriber’s wiring. Because wiring is typically buried in the wall, if there is no tone, you will most likely need to rewire the outlet.

On outlets where dialtone is present, verify that dial tone can be removed (broken) as soon as a number is dialed. This is a special testing scenario where just dialing on your butt set or a working phone will not reveal a problem. Failure to break dialtone is caused by the wire pair being reversed at a connection point. Newer phones and test sets automatically detect and compensate for this condition, but it may cause problems later with older or low cost phones and other equipment, such as fax machines.

Most butt sets have a polarity detector built in, which will indicate reversals via a visual indicator on the set. Low cost, stand-alone reversal detectors that plug into a wall outlet are also available.

A reversal may stem from the use of a standard phone cord, rather than a specially built “straight through” cord, which keeps the connector pin assignments on the same color wire on both ends, for the connection between the EMTA and the wall in the typical backfeed configuration. If this is the case, all outlets will have the problem. If only one outlet shows a reversal, it has most likely been wired incorrectly and never detected.

Last tests

If dialtone and dialing work, the last tests are for electrical noise on the line and voice quality. Hum will be discernable on a test set or phone and is usually caused by electrical interference. The source may be an AC outlet that is too close to the phone outlet or feedback though an AC powered device with a phone output. If hum appears when nothing but the test equipment and the EMTA are connected to the wiring, the source is probably inductive coupling from proximity to an AC line or a short between electrical and phone wiring.

Voice quality testing is emerging as a way to quantitatively measure the quality of a telephone connection. Although not yet widely used, test sets are available that indicate a numerical mean opinion score (MOS) from 1 on the low side to 5 on the high side to indicate how well the transmission meets the criteria for carrier grade telephony. MOS scoring is also built into some EMTAs and can be accessed in the field via a browser interface on a laptop connected to the EMTA.

Finally, there’s the simple case of phones that won’t ring. This tends to happen in homes with many outlets, loaded with telephone sets. The problem may be that the maximum number of ringer equivalents (the REN number) has been exceeded. REN is a number that indicates the current draw of the phone ringer compared to the vintage electromechanical gong and clapper, which has a REN of 1.

Five REN is the maximum per phone number. Typical solid-state ringers have a REN around 0.1, so you’d need 50 of them to cause a problem; however, homes with older or unusual phones may come close to the maximum. Inability to give a ringing indication may also stem from the ringer being shut off or lack of AC power for a multifunctioning phone or fax machine.

And for those of you who don’t watch old “Twilight Zone” episodes with Bill Shatner as the phobic flyer, a gremlin is a creature of World War II airline assembly plant workers, who blamed it for loose rivets and other stuff that unexpectedly fell off planes.

Justin J. Junkus is president of KnowledgeLink and telephony editor for Communications Technology. Reach him at jjunkus@knowledgelinkinc.com.