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From: Gord Hannah
To: All
Date: 2010-10-15 01:00:06
Subject: [11 of 12] Comm Primer

   Standard transmit levels for domestic (US/Canada) modems are
   approximately -10 dB, although V.34 negotiates these levels during the
   initial connection attempt.  Receiving levels can vary widely, depending
   on the conditions on your local phone line, the line at the remote
   modem, and any long-distance or inter-office carrier facilities.

   Typical receiving levels range from -40 dB at the low end, to -15
   dB at the high end, with figures in the -20dB to -35dB range being
   most common.  Extreme values in either direction probably indicate
   a problem in the connection from your modem to your local phone
   company, which in some cases the phone company may be able to
   adjust.

   However, be aware that Ma Bell and the long distance carriers are
   not required by law, statute, or tariff to "fix" this
"problem" on
   unconditioned voice grade lines, because it is not really a
   "problem"!

   Why does it get bad?

   Simple line impairment.

   Variations in line quality are typically the culprit for low
   connect rates.  Line impairments can result in link timeouts (when
   the error control protocol does not receive a block of data within
   its expected time frame), link naks (when the error control
   protocol requests retransmission of data), blers (block errors;
   errors in received error control protocol or data blocks), and
   resent data blocks. 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 19.2kbps, you will want to investigate the line quality of
   your connections.

       *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.

       *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.

       *Try eliminating all telephone extensions, phone line surge
        suppressors, line switches, utility monitoring devices
        connect to the phone line, and anything else on the line with
        the modem.

       *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.

Question: Why is it that the phone company and some of my friends say that
it is impossible for me to operate my 14400 baud modem on a normal phone
line? Do I really need on of those costly special lines?

   Answer: This question arises from the improper usage of the term
   baud in reference to the DCE rate of a modem. It is quite correct
   that a 9600 -baud- modem will not operate within the bandwidth of a
   common phone line. It is quite another matter when referring to a
   14400 -bps- modem (that operates at 2400 symbols/sec), which
   certainly will.

Question: I just bought a GENERIC XPRESS V.32bis 14400bps modem but can't
connect at 14400 with a system that is running a USR HST 14400 modem,
shouldn't I be able to connect at 14400?

   Answer: No, you will only be able to connect at 300, 1200 or
   2400bps because Bell 103, Bell 212A, and V.22bis are the only
   mutually supported protocols the modems have. HST is a proprietary
   protocol which is only available on some of US Robotics' modems.
   You should be able to connect at 14400bps with any other modem
   which is ITU-T V.32bis compliant however.

Question: I just bought a Generic Xpress-Lite 2400 modem with V.42bis, and
the package it came in says it can achieve 9600bps throughput. but when I
call a system I know operates at 9600bps, I only connect at 2400bps, what's
going on, shouldn't I connect at 9600? I also notice that my transfer
speeds are only slightly higher than they were with my Generic Lite 2400
that didn't have the V.42 stuff (about 285cps using Ymodem-g versus the 238
got with Zmodem-MobyTurbo with the Generic Lite 2400), shouldn't I get the
1100cps or so that other folks do with 9600bps modems?

   Answer: No, you will only connect at 2400bps because your modem is
   a 2400bps DCE device. You will get 9600bps DTE data transfer rate
   between you computer and modem by locking your serial port at
   9600bps, but -only- achieve modem to modem effective throughput of
   9600bps if the data you are transferring is in non-compressed form.
   Note that your base link rate remains 2400bps, but that the data
   compression of V.42bis is capable of effectively quadrupling data
   throughput. Regarding transfer speeds, you are doing the best you
   can do with a modem which operates at a 2400bps DCE. The folks with
   9600bps modems typically operate with 19200 or 38400bps DTEs and
   have real 9600bps DCEs.

Question: While I was glancing through the manual that came with my Generic
Xpress, I noticed it saying something about "Locking" my serial port,
enabling CTS/RTS flow control, and changing all of the speed entries in my
dialing directory to 38400 or 19200, how do I do this, and why should I?

   Answer: In order to derive the enhanced throughput benefit offered by

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