[Asterisk-Users] Analog FXO Woes Continue

[Jim Van Meggelen]

Rich Adamson wrote:
>>> Don't have any real answers, but might check the following... at
>>> least to rule them out. 
>>> 
>>> Telco folks _always_ check lines from their demarc (which in some
>>> cases is the protector box on the outside of the building). Most
>>> will not come inside to measure anything from the customer equipment
>>> jack. If that's true in your case, then you have to question the
>>> cabling inside the building (to asterisk). That cabling is most
>>> often simple inside wire that can easily pick up noise (eg,
>>> induction from florescent lights, motors, wall-wart transformers,
>>> some desk lamps). If you don't know where the inside wire is run,
>>> might try to find out or bypass it with cabling laying on the floor
>>> for at least an elementary test.
>> 
>> Testing from the demarcation point is essential, and poor inside
>> cabling can contribute to the problem, but if the cable is Cat 3 or
>> better, it is unlikely that it will be succeptible to induced noise;
>> that's why twisted pair is twisted - to protect it from induced
>> noise. 
> 
> "Inside wire" in the US is NOT twisted pair. That _was_ the
> point. Doubtful it is in canada either. Check any of the
> cable specs for the 4-wire el-cheapo inside wire that's been
> in use for years. Inside wire has been known to create issues
> for well over twenty years _if_ the cable is located anywhere
> near noise-generating electrical devices.

LOL!

Inside wiring used to be untwisted, many years ago. But UTP has been in
use for so long now it'd be very unusual to find a building without it.
In fact, I have never seen anything less than CAT2, and I have been
involved in a lot of PBX installations; many of them in very old
buildings indeed.

You are correct that untwisted cabling will be susceptible to the
aforementioned problems. You are incorrect in asserting that such cable
is common.

Perhaps you are talking about the grey cords with RJ11s on the ends that
run from the wall to your phone. Those are not typically twisted, but
they are also NOT referred to as inside wiring. Those should not be used
to terminate telco circuits, and, if so used, should not be more than
six feet in length. Also, you would never run those through walls or
plenum, not only for technical reasons, but also because they would
generally violate fire code.

>>> If you did not _see_ a telco person on site doing the transmission
>>> checks, you have to assume that someone did them from the central
>>> office (most common approach). That's okay in many cases, but its
>>> not okay in other more serious cases. The majority of the telco
>>> people that would be dispatched for testing only know enough to
>>> follow printed procedures using whatever testset they've been given;
>>> they don't have the skills to actually interpret the readings
>>> for cases they've never seen or been trained to recognize.
>>> 
>>> Its not hard to plug an ordinary phone into the same rj11 jack used
>>> by asterisk. Do it and listen close. Given the problems that you've
>>> stated, it should not be difficult to hear noise, hum, low volume,
>>> etc, if it is in fact bad lines. Also, compare lines; it is not very
>>> often four of four lines go bad in exactly the same way. Can you
>>> hear any difference between lines?
>> 
>> This is not a bad idea, but is not always conclusive. I've done
>> numerous tests on circuits where it sounded great on a butt set, but
>> was nevertheless out of spec. Also, if the problem is due to loss, it
>> is quite reasonable to expect all the lines to have the exact same
>> problem, because they will all be exactly the same distance from the
>> C.O.
> 
> The point was the poster is suggesting some very serious line
> deficiencies, and if those deficiencies are truly the result
> of bad lines, he should be able to detect at least _some_
> issues by using at least some of his five senses.

Do you say this from experience? Because I _have_ seen lines that
sounded perfect with a butt set, and nevertheless measured out of spec.
My _experiences_ do not support your _theory_. Sure, he can test the
line as you suggest, and if he detects noise he can report it as such.
If, however, nothing can be detected, it does NOT indicate that the
circuit is nominal.

>>> Bridge an ordinary phone on the same pstn line as asterisk. Place
>>> some calls from asterisk and listen to what's going on via the
>>> analog phone. (Example: some central offices don't like dtmf tones
>>> within xxx milliseconds after going off-hook. You'll get wrong
>>> numbers, etc. Insert the 'w' option in your Dial statement to delay
>>> those dtmf tones a little bit.) To be a little sneaky, unscrew and
>>> remove the mouthpiece from the analog phone and you can monitor
>>> calls all day long without impacting asterisk's ability to handle
>>> calls.
>> 
>> Say WHAT?!?!
>> 
>> OK look, I'm sorry, but this is just plain wrong. Disconnecting the
>> transmitter in your handset will not alter the fact that you have
>> introduced a device in the loop that is in an off-hook condition.
> 
> Better try it before you knock it (but use a real analog set,
> not the el-cheapo electronic ones). Disconnecting the mic is
> exactly the same thing as the old multi-party phones with the
> little switch on its side. (In fact, playing with the mic use
> to be one way to bypass coin operated requirements. :)

I promise you this: I will try it.

I still stand by what I said: telling people to take their telephone
apart to troubleshoot line impairments is simply not sound advice. Also,
when you say a "real analog set" do I assume you mean an
electromechanical set? Those are rather rare these days, don't you
think?

As for "el cheapo" electronic sets, you are certainly quite comfortable
lumping a rather diverse collection of technology under that moniker.
Can you clarify, in technical terms please, what exactly differentiates
an "el cheapo" electronic phone from one which is not?

>> To do what you are suggesting, one needs a butt set; which is
>> equipped to passively monitor the line without affecting it.
> 
> Better take your butt set apart, draw the schematic, and do
> the same for what is stated above.

You have not yet explained how removing the transmitter prevents your
invention from causing an off-hook condition on the line. This is a
rather important point for you to overlook in your rebuttal. Perhaps I
don't understand telephone networks as well as you, so I'm hoping you
can clarify this for me.

>>> If asterisk is having an
>>> echo issue (as an example) and you don't hear it with the bridged
>>> phone, you at least know where to look.
>> 
>> That isn't really true. Since the analogue phone will not have a
>> transcoding delay, the echo might still be there, just occurring at
>> the same time as the side tone.
> 
> If you actually think about what you just said, you'll
> probably want to take that statement back. Think real hard
> though! (Oh well, let me give you a clue: near-end verses far-end.)

Alright, I'm thinking real hard. 

Let's picture a hypothetical circuit between two points. The sending
device transmits audio down the circuit, which arrives at the far end
and, unfortunately, is echoed back. If the total end-to-end circuit is
all TDM, there will be negligible propagation delay. Echo will arrive
back to the sender in such a short time that it will be very nearly in
phase with the side tone, and thus indistinguishable from it.

On the other hand, if you have an IP phone at the near end (or a cell
phone, or anything that introduces a transcoding delay), the side tone
will still be perceived with no delay, but the signal echoed back from
the far end will be noticeably delayed, and will thus arrive out of
phase from the side tone, which will be perceived as echo.

I do appreciate the clue, and I thank you for it. Unfortunately, I have
no idea what you meant by it.

>>> The telco's have a telephone number for a "quiet termination" and
>>> another one for their "milliwatt generator". Get those numbers and
>>> use the test set to measure noise (quiet termination) and loss
>>> (milliwatt generator). If those results are reaonable, then you've
>>> got an asterisk configuration problem (and/or digium card problem).
>> 
>> Yes. And test the lines from the demarcation point, with all Customer
>> Premise Equipment (CPE) REMOVED from the circuit (if you leave your
>> inside wiring connected, you are not isolating the telco circuit from
>> CPE). After testing from the demarc, test from the final termination
>> (i.e. the cord you've plugged into your FXO). Any difference is your
>> responsibility.
> 
> The above statement certainly tells us which part of the telco you
> work(ed) in. :) 

It does? How so? This has nothing to do with my career history. This is
a regulated reality (actually, a de-regulated reality).

>> The spec for loss on a circuit is -8.5dB. Nominal is -3dB to -6db.
> 
> Actually, the spec is rated in terms of xx db loss per 1,000
> feet from the CO per gauge of copper actually used, but your training
> department translated that into something that doesn't require a
> calculator.

Interesting theory. 

So what you are saying is that the telephone company "specifies" the
laws of physics, and decides what the loss will be per unit of distance
from the CO? 

What you are describing is a characteristic of all electrical circuits,
not a specification for telephone lines. For a given length of cable
(with a given diameter or gauge, as well as some other factors), the
attenuation can be determined.

The specification, on the other hand, defines the parameters within
which a line must fall in order to satisfy the engineering requirements
of the telephone network. If a line does not meet those specifications,
it is "out of spec", and can thus be expected to have problems, all the
laws of physics notwithstanding.

Have you been doing all of your research on howstuffworks.com? Much of
your logic follows the types of articles they publish (there's an
article on wiretapping that sounds suspiciously similar to your butt-set
design). Also, your assertions lack technical depth.

You certainly have a fascinating view of things. I am very much enjoying
our little tête-à-tête, but I suspect our topic of conversation is no
longer relevant to this discussion list.

Shall we agree to disagree? I must admit much concern with your
application of logic, and I find some of your advice displays a complete
lack of experience, but if you are content, I see no point in
belabouring the point.

Warmest, heartfelt regards,

Jim.