COMPATIBILITY AND RELIABILITY ISSUES WHEN
CONNECTING IN VFC MODE BETWEEN ROCKWELL BASED MODEMS
AND US ROBOTICS COURIER VFC/V.34 MODEM

by Intertex Data AB 94-09-27


Users have experienced severe unreliable problems between Rockwell
based VFC modems and USR VFC modems. (Whether these problems also
exist between the USR modems in VFC mode is unknown.)

Intertex has made some extensive tests and analyses to detect the
cause of the problems and has also introduced some fixes to
improve reliability towards the USR modems. These fixes are
included in the firmware version 1.27A for the Intertex (PowerBit)
VFC modems.

The USR modems tested (probably) used the Swedish rev A or US rev
B code for USR V.34. Tests have been done towards several BBSs
from both good and bad lines.


Some of the issues found:


1) In the current version, USR does not seem to implement the
   highest symbol rate, 3429 baud, but falls back to 3200 baud
   instead. This means that the bandwidth available on the
   telephone network is not fully utilized. To obtain the same
   quality and speed, the line then has to be less noisy.

   This is the reason why Intertex modems often select 26.4kbps
   towards an USR modem, while Intertex select 28.8kbps towards a
   Rockwell modem on the same line.

   (Other Rockwell based modems often select 28.8 even when they
   shouldn't, thus giving unreliable connections. We prefer to
   play safe! If a user wants to force higher speed, he can set
   aggressive line speed selection by the AT%S2 command in the
   Intertex modems.)


2) In the current version, USR does not seem to implement
   secondary channel rate change. (A rate change is a fast
   negotiation between the modems for switching speed up or down.
   All other modem settings remain, so the procedure only takes a
   second.)

   Normal ("in band") rate changes work poorly with VFC modems at
   high speeds. Therefore Rockwell has introduced "secondary
   channel rate change" that is used between 19.2 and 28.8 kbps.
   This mode uses the available secondary channel to negotiate
   about rate changes. That works much more reliable and faster
   since the negotiation can be done without interrupting the
   character flow.

   Unfortunately USR only uses the normal "in band" rate change
   method. If the line is not perfect, the rate change then often
   fails and leads to a retrain. (A retrain is a slow process
   - 17 seconds between a USR and Rockwell modem when successful -
   where the complete modem training (equalizer setting etc.) is
   redone.)


3) Rate changes between Rockwell and USR modems on non perfect
   lines often fails!

   Testing from our good line (AXE subscriber line) most
   rate changes were successful. Testing from our bad line (old
   country side subscriber line) ALL rate changes failed!

   A failing rate change is converted to a retrain.

   (Please note that Rockwell modems using Rockwell's standard
   code have another problem with rate changes. If a higher speed is
   requested by the remote modem, the standard Rockwell code agrees
   to shift up without checking the current line quality on the
   local side. Since the own line quality may be much worse that on the
   remote side, the higher speed may cause the modem to loose sync,
   leading to a retrain that forces the speed down again! The
   Intertex modems properly checks the own line quality and
   rejects a shift up in speed if the quality is not sufficient.
   We could not see how the USR implements this since all rate
   changes from the bad line failed.)


4) Retrains between Rockwell and USR modems on non perfect
   lines often failed and led to hangup!

   Testing from our good line (AXE subscriber line) most
   retrains were successful. Testing from our bad line (old
   country side subscriber line) ALL retrains led to a hangup!
   (Rockwell to Rockwell works.)

   Independent of whose fault it is, we have found an effective
   fix for this problem. By detecting the fail-situation early, a
   corrective measure could be taken. The fix is included in the
   1.27A firmware release.


Issue 4) is probably the cause of the reported sudden hangups.
Since V.fast modems use the lines close to their theoretical
limit, rate changes and retrains are likely to happen, so it is
important that they really work!

Because of the limited symbol rate and poor performance of rate
changes in the current version of the USR firmware, we recommend
users to run safe towards the USR modem. The new dialling string
parameter "%" allows Intertex users to select safety when dialling
a certain modem! It has the same effect as AT%S0 (safe speed
selection) and AT%E2 (no initiation of shift up in speeds), but
only applies to the current call.

The Intertex firmware 1.27A includes the fixes for the failing
retrains as well as the percent dialling parameter.