TELECOM Digest OnLine - Sorted: Re: Linux Asterisk embedded PBX, HELP!!

Re: Linux Asterisk embedded PBX, HELP!!

Jay Hennigan (
Thu, 28 Oct 2004 22:58:00 -0700

On Thu, 28 Oct 2004 07:58:42 -0700, Vish wrote:

> Hi again folks,

> Is there a PBX which is Linux/Asterisk based that can work independent
> of a PC? A PBX with embedded Linux OS and Asterisk that can be
> configured locally initialy with a PC, (later works independent of the
> PC). The device must also be subsequently configurable remotely via
> the internet (VPN if required). Maybe I am just dreaming but am new to
> this. Any help appreciated.

Well, yes. But it's really a PC under the hood. PBXs have essentially
been computers since the crossbar days.

There are a number of single-board dedicated x86 boxes that can run
asterisk without the "look and feel" of a PC. You can boot from a
solid-state "disk" and have no hard drive, you don't need a monitor,
keyboard or mouse, etc. Some even run on -48VDC, just like your
grandfather's PBX.

But, if you want the features and reliability, a server-class rack
mount "PC" is your best choice. You'll want a large amount of storage
for voice mail, standard PCI slots for the Digium boards, etc. And
you can configure it locally with a PC (connected via console) or
remotely via ssh or telnet, or even with a real green-screen VT-100.

If you wanted to, you could probably run asterisk on a Tivo, a Cisco
2500, or any of a number of other esoteric non-PC platforms. It's
open source and there are a number of cross-compilers available. With
a little effort, most any platform with a C compiler can likely handle
the basic engine.

Date: Thu, 28 Oct 2004 14:54:58 -0500
From: Gordon S. Hlavenka <>
Newsgroups: comp.dcom.telecom
Subject: Re: Clicking in Phone Line From Electric Fence
Message-ID: <>
Organization: Crash Electronics
X-Telecom-Digest: Volume 23, Issue 519, Message 4 of 14
Lines: 122

Matt wrote:

> Now ... about 500 feet - 1000 feet from the field where the
> electric fence is a youth camp. On the camp is the office telephones,
> caretaker, and a program director.

> The phone line for the caretaker and ONE of the two office
> lines experience a continual hum as well as a click, click, click,
> click, click, every time the electric fence fires off.

Hum in particular, and also noise to some extent, are symptoms of an
unbalanced pair.

> The program director does not experience any known issue on his
> line, and the other line in the office is fine. I find this very
> odd, since both of the office lines come in (presumably) on the same
> cable?

These are three separate phone lines, each with their own phone number,
right? Not extensions from a single number?

Check the lines where they enter the building; hopefully there's a NID
there. You can disconnect an RJ-11 type plug inside the NID and
connect a phone directly to the line there. If there's still hum and
clicking, then it is absolutely Telco's problem. If the hum and
clicks disappear when tested at the NID then the problem is with your
inside wiring, which Telco will happily repair for you at a
substantial fee ...

(If you're talking about one phone number with several extensions, of
which some are clean and some are noisy -- then it's definitely a
problem with your phones or inside wiring. Possibly both.)

If there's not a NID it shouldn't be too difficult to have Telco put
one in for you.

The fact that one line is OK shows that it is possible to provide
service to your location without interference. So all your lines
should be clean.

Yes, they all come in on one cable but that cable is probably not
continuous back to the CO. It's likely opened up at one or more pints
to tap off pairs for other subscribers. Any of these points provides
an opportunity for corrosion or foreign material to affect one or more
(but not necessarily all) pairs and cause an unbalanced condition.

> Any thoughts? Verizon is kinda stumped on this issue, so I'm trying
> to see if I can figure anything out to help them out.

This shouldn't be a stumper at all. It can be labor-intensive to
track down where the pair is going out of balance, so maybe they're
just dragging their feet hoping you'll give up and accept the service
you've got.

A telephone pair is two wires twisted together. The wires are as near
to identical electrically as they can be made. So, if one lead has a
600V pulse induced in it by a nearby electric fence, the other will
have an identical 600V pulse induced in it. Since the wires'
electrical characteristics are identical, these pulses propagate at
identical speeds and arrive at your telephone simultaneously. But
since one lead attaches to one side of the phone's circuits and the
other lead attaches to the other side, the two pulses cancel each
other perfectly and you hear nothing. Only the audio placed onto the
pair at the CO should be heard.

The hybrid in your telephone is in essence an analog computer; it
calculates the difference of the voltages on the two wires and outputs
that difference as audio. The CO places different signals on each
wire (representing audio) but noise sources place the SAME signal on
each wire.

Telephone pairs generally carry a LOT of noise -- but since they carry
the IDENTICAL noise on both wires, it all cancels perfectly at the
ends and nobody hears any of it. Even the twist plays a role; if the
wires were parallel they would electromagnetically couple slightly
different audio signals from adjacent pairs in the cable. Twisting
the pairs ensures that each wire picks up an identical signal.

(If your house is wired with old Red-Green-Black-Yellow "cloverleaf"
cable and you use the B-Y pair for a second line, you will probably
notice crosstalk. This happens because the cloverleaf cable is not
twisted-pair and so it acts like a coupling transformer. It can cause
crosstalk even if you don't have anything connected to the far end of
the cloverleaf cable.)

When the electrical characteristics of one wire are changed, the pair
becomes unbalanced. This can happen because a spiderweb is built in a
junction box, and provides a high-resistance leakage path to ground,
or to other pairs (which leads to crosstalk). It can happen because
rain leaks into a splice and the resulting corrosion inserts a few
dozen ohms of DC resistance into one side of the circuit. It can
happen when a mouse chews up insulation on a bunch of wires and
randomly alters their capacitance.

In an unbalanced condition, signals do NOT propagate identically down
the two wires. They may propagate at different speeds, and/or they
may have different frequency response curves. When noise signals
reach your telephone instrument, the difference between them (which
would be zero on a perfectly balanced line) becomes audible. Power
lines produce a 60Hz hum, electric fencers produce regular clicks,
other types of problems cause hissing and popping.

Gordon S. Hlavenka
"If we imagined he could _find_ the car,
we could pretend it might be fixed." - Calvin

{TELECOM Digest Editor's Note: I had telephone service once where the
line went bad, and telco decided it was necessary to swap the pairs.
Trouble was, they had a hard time finding *any* spare pairs in the
cable, but about a half-dozen pairs which were out of order. So what
telco wound up doing (I found out later) was to take *one* good wire
from one pair and a second good wire from another pair, in effect
making one good pair out of four wires (two of which were out of
order.) When I started listening to my phone calls later on, it just
did not seem right, it sounded sort of out of balance somehow. I then
complained about that also, and a few days later they came out and
were working in the hole in the street changing things around again,
and that time it sounded better. Is that some sort of problem, when
you 'mix and match' wires from different pairs to get a good pair? PAT]

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