Hi All,

I have a system where a modem is connected to a residential voip
gateway using a two wire connection. This residential gateway is
talking to an H323 - ISDN gateway over a satellite connection. In
normal voice calls I use G.729. But whenever there is a modem call,
both the gateways upspeed to G.711 and I m using G711 A-Law right now.
Disabling of Echo Cancellation in both the gateways is configurable
(by default echo cancellation is disabled whenever there is a modem
call). I am using modems from a spanish manufacturer called SATELSA
and need to make just the low data rate modems work (V.21, V.22,
V.22bis, V.23)

The problems is that when I try to establish a call between two
modems, calls go through using some models and at some specific data
rates/modulation schemes but not other combinations of models and data
rates. I tried to experiment with disabling echo cancellation just at
one end and keeping it enabled at the other. Sometimes it helps to an
extent that I see the circuit 109 (line signal detected) on at one end
(e.g. at the PSTN gateway end when it is the called end and the echo
cancellation is kept enabled at this end) but not at the other end.
The call still fails.

Now searching for ideas on how to improve this performance.
Any help in this regard will be greatly appreciated.

Warm Regards
Manish

Manish Bhasin wrote:
Why ??

The average analog 2-wire dial modem has enough trouble working
over the channels for which they were designed to work. 3khz
band-limited and all that. To try to operate them over a "lossy"
medium like IP is going to be marginal and unreliable at best. A
few dropped or late packets and your done.

Even with analog voice compression schemes such as ADPCM, most
modems cannot maintain connections. Now you want to try a
re-digitized, lossy, variable delay path ??

Imagine cutting a garden hose in half, then setting up a team of
monkeys with buckets to try to maintain the same water flow at the
far end of the hose.

Good luck...

--
(remove N0SPAM from address to reply)

Reed <N0SPAMreedh@rmi.net> wrote:
I agree with your over all point. However, ADPCM, and any
number of other methods that are used to provide quasi-analog
voice channels using 32 Kbps for the digital transport do in
fact work with modems. I once had the opportunity to test Ayden
ADPCM, Hughes DAMA, Scientific Atlanta DAMA, an AT&T packet
system, and a couple of others, plus some older analog satellite
equipment. (Basically everything available in Alaska in 2000.)

The common characteristics of all these were that they did not
have variable path delay (although some of them have significant
phase jitter compared to standard 64Kbps circuits) and all
provided the equivalent of a 32Kbps bandwidth on the digital
side (except for the really old SA and California Microwave
analog SCPC satellite systems).

Basically they all function at 14.4 Kbps with a quality v.32bis
or v.34 modem. Most would handle 16.8 Kbps, and some could hit
19.2 Kbps (for example, the DAMA systems). In fact the old
analog systems could be, in some cases, configured for wide
enough bandwidth to even get 21.6 Kbps now and then, on an
otherwise good channel.

I found that a USR Courier and a SupraFAX modem were the best
that I had. I tried several PCMCIA cards on my laptop, and
never did find one that would work well over these limited
bandwidth systems.

But one thing came through loud and clear: you *cannot* tandem
two of *any* of that type of reduced bandwidth channel and get a
decent modem connection! I think 7200 bps was the highest
connect that I ever got, and it wouldn't stay up for long. 1200
bps wasn't even stable on many connections. And on many there
was simply nothing that the modems could actually connect at.
(The effect of this was that from villages I could make modem
calls to an ISP near the major switching centers like Anchorage
and Fairbanks, but could not get a connect calling my own
computer here in Barrow, because it always involved a tandem
route using at least 2 of those systems.)

VoIP was just starting to appear when I retired, so I've never
worked with it. I cannot imagine that a modem connection would
be possible over a real world VoIP channel. Every dropped
packet causes a phase hit larger than a modem can tolerate.
The phase jitter during normal operation, even when packets are
not being dropped, will almost certainly be higher than can be
tolerated by any type of PSK modem.

--
FloydL. Davidson <http://web.newsguy.com/floyd_davidson>
Ukpeagvik (Barrow, Alaska) floyd@barrow.com

Floyd L. Davidson a écrit:
There's a solution for service providers from an Israeli company :
http://www.surf-com.com/c6x.asp?section=3

Stegozor <stegozor@removethis.altern.org> wrote:
...


I'm certain that for for OEM's there are several similar solutions
available. However, it is unlikely that bulk providers will be
opting to include all of that functionality until the technology
is significantly improved (smaller footprint and lower price).

Any individual company that is building their own network, where
the requirement will see high visibility for the functionality
needed, however, might well want to include that into a design.
(I would argue that it would be poor engineering to do so, and
that a fully digital solution, absent the modems, would be by
far preferred.)

For now, I'd guess that the above list of systems that I worked
with (most of which were fairly old at that time) represents
what most customers will get from most bulk providers (ATT, for
example). Which is to say, virtually nothing! Both of the DAMA
(Demand Assigned Multiple Access) systems mentioned above
include optional plug in modules to provide the same
functionality for FAX services only. Presumably modules are
also available for v.32 and v.34 modem protocols, but they were
not installed in the units I saw. Basically, the unit can
provide a 32 Kbps digital data channel, which for voice calls
uses a form of ADPCM. When a FAX call is detected, it is
demodulated with the internal "modem" (the optional dsp module)
and each end of the DAMA channel acts as a modem between the
callers modem and the destination modem, thus using the 32Kbps
channel as a digital channel rather than as a quasi-analog
channel (with its inherent SNR limitations that affect the
analog output of the modem).

--
FloydL. Davidson <http://web.newsguy.com/floyd_davidson>
Ukpeagvik (Barrow, Alaska) floyd@barrow.com