waltje@uwalt.nl.mugnet.org (Fred 'The Rebel' van Kempen) (04/05/91)
Dear All,
Me mentioning the existence of a document describing some of
the possible PEP protocol optimalization registers resulted
in a true HEAP of "I want it- can you mail it?" messages....
So, here it is. Any followups to this posting to me please,
do not abuse the net for it...
Enjoy,
--
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Telebit Corporation Revision 1.00 01 SEP 1990
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TELEBIT PEP MODEM CONSIDERATIONS
FOR
ADVERSE COMMUNICATIONS LINKS
By TELEBIT CORPORATION Technical Support
Edited by C. E. Castillo, UNIX Communications Specialist, TELEBIT Corp.
The Telebit modem's advanced multi-carrier design, utilizing the
patented Packetized Ensemble Protocol (PEP), is ideally suited to
operate over international circuits which can often cause insur-
mountable obstacles to standard "slow" or other high speed modems.
There are several things to consider when attempting to establish
a link over such a line that should help insure reliable connections
with the maximum performance possible under the conditions.
INTERNATIONAL CALLS
When originating a call and dialing internationally, you should
increase the time to wait for carrier from the default of 40
seconds, to about 80 seconds, to allow more time to dial, get a
connection to the other end and then negotiate carrier. This is
controlled in the modem by the S7 register, which can be set any-
where up to 255 seconds as needed. Your software package may also
have a parameter that controls the time to wait for carrier so be
sure to set that accordingly.
Leave the answering modem S7=40, because if 40 seconds has elapsed
without establishing a connection, it is likely that it will fail
to do so and it would be better to redial. If possible, set S50=
255 to force the modem to only send PEP connect tones. This will
not allow the answering modem to connect at any slow speed.
It is usually good to set S110=1, (only on a TrailBlazer Plus,
T2000, or T2500) so the modems can negotiate to do data compression.
Do not use the data compression feature if the files have been com-
pressed prior to transmission. If you are going to use Kermit,
Xmodem or UUCP to do a file transfer, set the S111 to the approp-
riate value to allow the modems to negotiate for protocol support.
It is recommended that you use one of the supported protocols to
optimize throughput to the maximum level the telephone line will
allow. Zmodem and variations might also attain good results
without any protocol support, but there is no need for complex
sliding window protocols with the protocol support feature in
TELEBIT modems.
Echo suppression equipment on long distance circuits can cause
timing delays that disrupt our packet transmission. The modem
can be configured to compensate for this problem if S121=1.
This is a negotiated feature, which means that it is not re-
quired to set S121 on both modems, only one of the two need be
set to allow us to compensate for echo suppression. Experimen-
tation is about the only way to determine if echo supression
equipment is present on the circuits. If you seem to be able to
operate without setting S121, don't change it. But if you are
experiencing problems with frequent long delays in data trans-
mission, try setting this and see if there is an improvement.
CELLULAR, MICROWAVE, AND SATELLITE CALLS
In satellite and microwave channels, Compansion may be an obstacle
to obtaining good data throughput. Compansion is the Compression
and Expansion (amplification) of an analog signal over a voice or
data communications link. The use of Compansion to ensure the best
voice quality possible over a satellite or microwave link may cause
PEP packets to be "clipped" during a data connection. Uncompanded
communications channels may be available from the satellite or
microwave service provider. Experimentation is the best way to
determine which type of service will best suit your application.
The S121, S120, and J6 S36 registers will allow for a reliable and
effective connection over either of these services.
In cellular communications channels Compansion may also be a
factor, but the leading obstacle is usually signal strength.
Due to the use of passive or active RJ-11 adapters, the modem
signal strength is crucial to maintaining a reliable and effective
connection. The CellBlazer product has software registers (S175)
which set signal strength on the modem. Other TELEBIT modems have
the ability to set signal strength via an internal gain resistor or
an external (RJ-45 connector) gain resistor. The S121, S120, and
J6 S36 registers, in combination with the proper signal strength
adjustments, will provide a reliable and effective connection over
cellular channels.
The communications equipment providing the RJ-11/RJ-45 interface
to the modem may also be an important factor in cellular, microwave,
or satellite connections. Active RJ-11/RJ-45 adapters are highly
recommended for cellular use via cellular telephones. The satellite
tracking mechanisms of a ship/earth station terminal may be just
as important as its signal processing and transmission capabilities.
The ability to adjust signal strength for any modem interfacing unit,
along with the S121, S120, and J6 S36 registers, will provide a
reliable and effective communications link.
DIAGNOSING AND SOLVING MODEM COMMUNICATIONS PROBLEMS
Some indications that your TELEBIT modem is experiencing difficulty
in communicating are:
1) Inability to connect after several attempts at "Handshaking"
(the PEP connect tones) resulting in a NO CARRIER status.
2) After connection, frequent pauses during which there is
no response nor data received for 10 to 30 seconds at a time.
3) Disconnection after a delay of 60 seconds with no data trans-
mission.
In many cases, depending on the types of communications link, it
is possible to "fine-tune" PEP to allow for more reliable
connections and maximum performance. TELEBIT Technical Support
prefers to troubleshoot these situations on a case by case basis.
It is therefore recommended that they be consulted prior to
modifying your TELEBIT modem modulation characteristics. TELEBIT
Technical Support should be able correct most, if not all, of these
problems. Contact TELEBIT Technical Support at 408-734-5200 (U.S.)
or 1-800-TELEBIT (U.S.) or one of the TELEBIT international offices
listed in this document.
Many international calls will connect without any problems, but
if difficulties in connecting or maintaining connections occur, the
following settings are recommended.
The three levels are in order of severity of line difficulties,
from minor to major.
Level 1.
AT S120=12 J6 S36=1
Generally, Level 1 will have a very minor impact on performance,
both in interactive use and file transfer. It is useful, however,
for achieving a degree of reliability, even when the communication
lines are very good. The J6 S36 register may be incremented up to
4 in order to better survive adverse line conditions.
Level 2
AT S120=2 J6 S36=2
Level 2 will impact interactive performance severely while file
transfers remain at close to normal performance.
Level 3
AT S120=3 J6 S36=3
Level 3 is a last resort setting which will severly impact interactive
performance and limit file transfer speeds to a maximum of 6000 bps.
Data compression, S110=1, can make up some of the difference, but only
if the data has not been previously compressed.
The S121=1 register will enable compensation for echo cancellation on
the circuit, and can be enabled at any of these levels.
If a stand alone modem is in use, the special register commands
should be sent to the modem and saved to Non-Volatile Memory, by
sending the save command "AT&W".
In situations where a mixture of local and long distance calls are
being placed, it is recommended that these special settings be
made in a modem initialization file or that the A/B selection
switch be used.
THE PEP FINE