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What is the difference between E&M and junction lines, and do you have any advice on how to check fa

Learn about E&M lines and how to check faults in them.

What is the difference between E&M and junction lines, and do you have any advice on how to check faults in these?
E&M means ear & mouth or listen and talk or receive and transmit. E&M circuits (analog) are either 4 wire or 6 wire E&M. In the 6-wire version, one pair of wires is used for signaling (E&M), one pair for transmit and one pair for receive. E&M is also used generically as a signaling means when setting up T1's between the Central Office (CO) and customer equipment. I have never heard of "junction lines" used in the same context. See Junction lines here.

How do you check for faults on E&M? It pays off to understand the user complaints because that will point you in the right direction. You will need an analog voltmeter (some prefer a digital voltmeter) and with an analog meter you can then visualize when a seizure is made on the E&M pair -- the meter will spike then drop. With a butt set, you can listen to the transmit pair for voice and do the same on the receive pair -- you may find noise on one pair or you may find the E&M pair permanently seized, which indicates trouble in the signaling equipment onsite, blown fuse or shorted pairs to the CO. E&M requires a lot more work because you may need to test between two points. Assuming audio is an issue, you will need to swap tones @ 1000 Hz to establish some base lines. Many non-circuit issues with E&M are associated with settings-- the signaling methods: Wink Start, Wink Delay, Delay Dial, Wink-Wink-- there are others. It's all about seizure, timing and sending/receiving digits and then voice.

Many issues are improper settings for signaling and this is due to confusion of terms or capabilities such as between the CO and a PBX. Normally, the CO must slow down to the PBX or customer premise hardware speed. The signaling may mean using a Wink-Delay, meaning after the seizure of the channel, a short delay ensues before sending digits to the PBX. Another typical issue with E&M is glare -- that is when two devices attempt to access the channel at the same time. In the PBW world there are two preventive measures to counter glare. First, set one PBX as the Master and the other as Slave if the settings are available. Then, in PBX A, set the hunt group to seize calls from the last channel first. Speaking of digits, this is when it's best to have a digit grabber because hot audio levels will distort DTMF digits and that's another line of troubleshooting.

This was last published in November 2009

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