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OS/2 Shareware BBS: 36 Tips
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connect.288
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1995-02-05
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* Originally By: support@usr.com, of (1/31)
* Originally To: william driskell
* Originally Re: Re: 288 Connects
* Original Area: MATRIX*Internet Replies
* Forwarded by : Blue Wave v2.12 OS/2
From: "Customer Support" <support@usr.com>
@Message-Id: <9500127899.AA789949440@robogate.usr.com>
@To: William.Driskell@f914.n109.z1.fidonet.org (William Driskell)
@Subject: Re: 288 Connects
Troubleshooting 28.8 Connects
_____________________________
Tips for High Speed Connections with V.34V.FC Modems
____________________________________________________
Very few people can get consistent 28,800 bps connections. Speeds of 28,800bps
require pristine line quality along the entire length of the connection.
However, V.FC and V.34 modems are capable of pushing the limits of analog phone
lines, commonly offering connect speeds of 21.600, 24,000 and even 26,400 bps.
Line Impairment
_______________
Variations in line quality are typically the culprit for low connect rates.
Everyone occasionally gets "a bad line" and has to hang up and call again to
get
a better connection. However, if you find that you never or rarely connect at
rates above 19200 bps, you will want to investigate the line quality of your
connections. Begin with the following:
*Try calling a different location. Line quality differs from region to
region, and it may be a problem with the lines or modem at the other end of a
particular call. The U.S. Robotics BBS (708 982-5092) is a good one to try
*Try connecting with a local call. Sometimes the connections within a long
distance call can cause impairments. (If this isolates the problem, you can try
switching long distance companies.)
*Try plugging the modem to a different phone line or wall jack.
*If you know someone else In your area with a high speed modem, ask what
type of connections they make. Try making the connection from their location.
If you encounter the same low connection rates, the problem may be resulting
from impairments along the lines running to the local telephone company or
within your home or office. Your telephone company or a private consultant may
be able to help. Information on how to troubleshoot a line for specific
problems
is provided at the end of this document.
Software Setup and Modem Configuration
______________________________________
In addition to line impairments, modem configuration and software setup can
affect connection rates and throughput. The Courier's default configuration
allows 28,800 connections with other V.34 and V.FC modems. Information on
software setup is available through the U.S. Robotics fax-on-demand (FOD)
system
or through your software's support service.
New Code (Courier V.34 modems only):
Download the latest code from the U.S. Robotics BBS (see Upgrades in your
user's
manual or FOD document #565). The upgrade is free and the latest code revision
can improve performance.
Dropped Connections:
Dropped connections can occur when there is a sharp decrease in line quality
during a call. V.34 modems will switch to rates as low as 4800 bps to
compensate for these changes. If the loss of quality is extremely severe, they
will drop the connection.
Dropped V.FC Connections:
V.FC connections can only switch rates down to 14.400 bps. If you connect using
V.FC and line quality drops below that allowable for a 14,400 connection, the
modems will disconnect. If this occurs frequently for a particular call, you
will want to disable V.FC before calling that modem again. A different
modulation protocol (V.32 bis, for example) will be established and will allow
the modems to switch to lower bit rates as line quality warrants. Send the
modem
the following command to disable V.FC:
ATS56=128
Remember to return the modem to its original configuration after the call is
completed by resetting the modem or entering ATS56=O.
Some V.FC modems from other manufactures do not support rate switching. These
connections are more likely to drop. For these calls, you can force a lower
connect speed by locking the modem to a lower link rate via the &N command, or
disable V.FC by entering ATs56=128. Remember to reset the modem or return it to
its original configuration after the call is completed (AT&NO or ATS56=O)
Bandwidth Requirements
______________________
This information is provided for consultants or experienced modem users only.
V.34 and V.FC connection rates are based on the available bandwidth over the
phone line. The modems test the phone lines before establishing a connection
rate and will select the highest symbol rate allowable. In general, a higher
symbol rate allows greater speeds, but requires greater bandwidth.
The approximate bandwidth requirements for each symbol rate are shown in the
chart below. Note that when using V.34, the Courier modem can operate at
either
of two frequency ranges for any of the given symbol rates. This allows it to
select the frequency range of better quality for that call.
Symbol Carrier Bandwidth Maximum
Rate Protocol Frequency Requirements Bit Rate
2400 V.34 1600Hz 400-2800 Hz 21600
V.34/VFC 1800Hz 600-3000 Hz 21600
2743 V.34 1646 Hz 274-3018 Hz 24000
V.FC/V.34 1829 Hz 457-3200 Hz 24000
2800 V.34 1680 Hz 280-3080 Hz 24000
V.FC/V.34 1867 Hz 467-3267 Hz 24000
3000 V.34 1800 Hz 300-3300 Hz 26400
V.34/V.FC 2000 Hz 500-3500 Hz 26400
V.FC 1875 Hz 375-3376 Hz 26400
3200 V.34 1829 Hz 229-3429 Hz 28800
V.FC 1920 Hz 320-3520 Hz 28800
3429 V.34 1959 Hz 244-3674 Hz 28800
NOTE: Minimum bit rates. V.34 will connect at speeds as low as 4800
bps with any of the above symbol rates. V.FC does not support rates below
14,400
bps. If the bit rate is much lower than the maximum bit rate supported by the
symbol rate, the phone line has lots of noise or other impairments on it.
Determining a Connection's Bandwidth
____________________________________
The symbol rate and carrier frequency used during a connection is displayed on
the modem's I11 screen. (The I11 screen is an undocumented USR AT command).
Using the above table, you can determine the approximate bandwidth detected by
the modem. V.34 can be disabled by setting s56=64. V.FC can be disabled by
setting S56=128. (S-Register 56 is an undocumented AT command).