X2 DOVBS over long distance

X2 DOVBS over long distance

Post by Jeffrey Rhode » Sat, 17 Apr 1999 04:00:00




> There's more to it than 'fake modem tone', Jeffrey.  In addition to ANS,
> the 2100 Hz answer tone (1500 milliseconds?), the modems are required to
> send continuous speech energy (in some band at some amplitude which I
> forget).  Speech is more than 50 % pauses, so if an echo cancellor were
> mistakenly turned off due to speech that looked like ANS, it would come
> right back on.

> Not all data patterns (even scrambled) will have the right characteristic.

If the data is truely random, like it will be when packets are
compressed (and/or encrypted), then I disagree. Again, why have I had so
much success with Bitsurfer to Bitsurfer V.120 56K DOVBS long distance?
As I recall, either end can set %A96=E to Enable echo cancelling
disabling (how's that for department of redundancy department and even
when the other end is %A96=D to Disable echo cancelling disabling), then
the DOVBS call stays up and transfers data flawlessly, albeit at
56K-only. If I could just FORCE the Bitsurfers to TRY 64K DOVBS, I
wonder how flawless that connection, via AT&T long distance, would be? I
kind of think that idle HDLC flags of 01111110 at 64K will not present
enough enery to keep echo cancellers disabled, but idle flags of single
zero delimited repetitions of 1111110 WILL present the same energy as
rate adapted idle flags of 01111110.

Quote:> Take a look at the V.25 recommendation.  While you are at it, have a look
> at V.8, for the really interesting way they extended V.25 for V.34 modems.

> Laurence V. Marks
> IBM Corp. - Research Triangle Park, NC

I would look at those documents, Laurence, but IMO, they are written by
analog modem vendors who (like you) see digital impairments that do not
exist in a correctly operating ISDN!!! I believe that persuing an
auto-adjusting algorithm for end-to-end ISDN digital-only connections is
a better approach, since I believe that I can resolve whether, or not,
digital impairments exist in the LSBs of transmitted octets in about
20000 received bits (or so). If impairments do not exist in the LSBs of
an end-to-end ISDN digital-only connection, then the connection can be
assumed to be on clear channels that are 64k-worthy. So I believe that
V.25 and V.8 need to first consider my auto-adjusting algorithm after
answer indication is given, before "negotiating" anything else that is
that is needed for low grade and misconfigured voice circuits.

I notice that the Courier I-modem is able to "negotiate" X2 symmetric
proprietary DOVBS using somekind of V.8 answer modem tone, yet this
algorithm seems to give up too easily and settle for V.34 data rates on
long distance, when I know that DOVBS data rates like 56000 bps are
possible on the same connection. Also, I am getting CONNECT
64000/ARQ/x2/LAPM/V42BIS connections within the city of Seattle (like
from a US WEST BRI to an AT&T POP for Worldnet) yet calls to other
cities like Tacoma or Olympia only resolve to 56K, and calls beyond that
ALWAYS resolve to 33600 bps or less V.34 analog speeds.

X2 Symmetric is not WORTHY of the ITU's so-called "V.91", in my opinion.
I'd like to help USR get this fixed but it seems the almost year old
external I-modem firmware is "fixed" as good as it will get???

regards, jcr