Re: [hylafax-users] Question about a modem

["Duncan Thomas" <duncan@goldtel.com.au>]

Two wire cables are typically used for PSTN (analogue) lines or the new
breed of analogue/digital PABX systems. Digital PABX systems multiplex the
Voice and data paths over the same pair.

>However, I've another modem that does not work because it wants only 2
wires
>phone cables, while the VoIP cable is 4 wires; this modem always "hears"
RINGs
>on the line while there's none.

Sometimes problems can occur with 4 wire cables as the extra pairs act as an
antenna and induce noise into the device or line (unlikely on analogue
because it sits somewhere around 48 V dc and is analogue as oposed to
digital).
Communication vendors tend to encourage the use of two wire cords on a lot
of newer systems.

Four wire cables are used on older telephone systems that use separate data
and speech paths, Security systems that need a forward and return path, some
Fax machines and four wire modems that use a separate transmit and receive
path.

Four wire cables are also used for Basic rate ISDN devices on the subscriber
side (although they usually supply an eight wire cord).

Purely for interests sake some devices require 3 wires such as pay phones
(old ones) which uses the third wire for earthing their meter pulses that
they use for charging the call. Really old telephones used the third wire
for generating rings (you know, the phones with the alarm clock style bells)

> (is there any modem that works with VoIP
> lines??)

Pretty well any modem (fax only) or fax machine will operate thru an ip
gateway, so long as the gateway has a fax module installed. Data won`t work
thru them (i`m not entirely sure but i guess it is simply too much bandwidth
in terms of compression), but it doesn`t need to since the ip gateway would
primarily be used to pass data in the form of ethernet via routing anyway.

Regards Duncan




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