VoIP (Voice Over IP) is a
general term that methodizes
making conventional telephone
calls over the Internet
Protocol... a.k.a. IP)
So you have your new VoIP
equipment installed and your
gloating over how you've shafted
old Ma Bell. It was simple
enough; you pay a skimpy $17 to
$25(+) per month for
unlimited calls and maybe paid
that one time charge for their
VoIP Converter Box. This box
has two types of jacks:
RJ45 Ethernet that plugs into
your broadband (DSL or Cable)
and one or more RJ-11(s) that
your Telephone(s) plugs into.
But wait, you have a fax machine
and sometimes use your dial-up
modem (why you want to use a
dial-up modem if you have
broadband escapes me, but hey,
stay with me here).
You plug in your telephone and
make a voice call. You get
dial tone, you dial out,
terrific sound... works just
like your land line... wow, this
works great!
Then you plug in your modem or
fax machine to make some test
calls and you get mixed results
<darn!>. Those baby's
don't <quite> work on your new
VoIP connection.
Why not?... well, I will be very
basic here... fundamentally your
clashing two technologies...
Contiguous Analogue and Digital
Packetizing:
Modems on VoIP:
Analogue dial-up modems require
a bi-directional continuous
(uninterrupted) analogue
signals. Their design
incorporates several types of
signal coding including PCM,
PSK, DPSK, QAM and others.
These special modulations
require careful
consideration of amplitude,
phase jitter, signal delay, echo
cancellation and other signaling
attributes to function properly.
VoIP on the other hand,
immediately digitizes and
packetizes the analogue signals
and puts these packets through
the internet digital network.
Since the telephone network is
designed for voice, their
network allows some data loss
and packet delays and are of
little concern because voice
signaling is very forgiving.
Not so with Modems, especially
the higher speed 33.6kbps and
faster.
Bottom line with modems, to
operate over VoIP the modem must
change, or the VoIP Converter
Box (aka 'gizmos')
must change to adapt to the
analogue modems signaling
requirements.... not an easy
task considering the enormous
installed base of legacy
equipment.
"Dial-up
modems over VoIP are unreliable
and
<at this
time>
should be
avoided"
However, if you
must, then limit your speeds to
0-300bps<Bell 103/V21> or
V.22-<1200bps> and
V.22bis<2400bps> which will have
be best reliability.
Higher speed attempts will
proportionally and dramatically
degrade.
~
Floyd "The Modem Guy" ~
Faxes over VoIP:
Faxes have a better chance
operating over VoIP but it is
not guaranteed. Why?,
The older fax modulation
standards are 'half duplex'
This means the analogue signals
transmit in one direction at a
time. This operation is
more tolerant to VoIP packet
delays and most of the time you
may be fine.
There are a few types of FAX
modulations out there.
Originally, faxes started slow
with 100bps to 300bps, then
settled for many years with V.29
(half duplex 9600bps)... then
was enhanced to 14.4kbps.
The two <friendly> standards
that evolved are T.38 and T.711
which are universally supported
and could
be fine (but not necessarily)
with your VoIP. Almost all
fax machines support these two
schemes/speeds.
What will my VoIP provider
say? They will be
hesitant to give a definitive
answer on FAX reliability.
When your having FAX problems on
VoIP, your provider will give
some suggestions such as
lowering your FAX baud rate,
this may help a little. (If
asked about modems,... they'll
probably put their hands in
their pockets, shrug their
shoulders and make circles in
the sand with their toes - and
will definitely not commit.) You
should note that many VoIP
providers are now offering a fax
service that basically uses your
email to send/receive faxes. A
bit more inconvenient, but to
eliminate your POTS you should
look into a fax service.
Look out, there is an
enhanced fax standard called
"Super" G3 modulation that
boosts the fax speeds to
33.6kbps. Used over VoIP
could cause your Faxes to pass a
gall stone and be even more
unreliable or simply not
function.
What is in the future?
There is work in ITU-T
(International
Telecommunications
Union-Telecommunications) to
address this issue. It has
a few preliminary names...
MoIP (Modem over IP or
also called Media over IP), and
AoD
(Analogue over Digital).
Other names will evolve as
proposals are introduced and
settle into a formal standard.
T.38 Fax Relay Standard, aka
FoIP (Fax over IP), has
actually been around since 1998,
and proponents argue its
reliability. Is it
reliable???... In theory yes,
but practically, you should test
with your application to be sure
before you commit to it.
The problem is, you need both
fax machines (sending and
receiving) to be supportive of
the same standards, and there
are literally millions of
<older> machines out there so
each fax session will most
likely vary.
Any new standard must be
defined, argued, ratified and
then incorporated... this takes
years, not months.
