283 lines
5.8 KiB
HTML
283 lines
5.8 KiB
HTML
<HTML
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><HEAD
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><TITLE
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>Modems and overseas telecommunications requirements</TITLE
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><META
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NAME="GENERATOR"
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CONTENT="Modular DocBook HTML Stylesheet Version 1.7"><LINK
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REL="HOME"
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TITLE="Remote Serial Console HOWTO"
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HREF="index.html"><LINK
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REL="UP"
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TITLE="Bugs and annoyances"
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HREF="bugs.html"><LINK
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REL="PREVIOUS"
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TITLE="Advanced Power Management"
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HREF="bugs-apm.html"><LINK
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REL="NEXT"
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TITLE="Uploading files from a serial console"
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HREF="upload.html"></HEAD
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><BODY
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CLASS="SECTION"
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BGCOLOR="#FFFFFF"
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TEXT="#000000"
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LINK="#0000FF"
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VLINK="#840084"
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ALINK="#0000FF"
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><DIV
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CLASS="NAVHEADER"
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><TABLE
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SUMMARY="Header navigation table"
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WIDTH="100%"
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BORDER="0"
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CELLPADDING="0"
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CELLSPACING="0"
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><TR
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><TH
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COLSPAN="3"
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ALIGN="center"
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>Remote Serial Console HOWTO</TH
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></TR
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><TR
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><TD
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WIDTH="10%"
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ALIGN="left"
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VALIGN="bottom"
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><A
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HREF="bugs-apm.html"
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ACCESSKEY="P"
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>Prev</A
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></TD
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><TD
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WIDTH="80%"
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ALIGN="center"
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VALIGN="bottom"
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>Appendix A. Bugs and annoyances</TD
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><TD
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WIDTH="10%"
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ALIGN="right"
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VALIGN="bottom"
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><A
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HREF="upload.html"
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ACCESSKEY="N"
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>Next</A
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></TD
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></TR
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></TABLE
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><HR
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ALIGN="LEFT"
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WIDTH="100%"></DIV
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><DIV
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CLASS="SECTION"
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><H1
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CLASS="SECTION"
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><A
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NAME="BUGS-INTERNATIONAL"
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></A
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>A.11. Modems and overseas telecommunications requirements</H1
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><P
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>There is no world-wide approval processes to certify that a
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modem is suitable for connection to the telephone network. This is
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despite the presence of a common set of technical standards that
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modems must meet for use on the global switched telephone network.
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There is little or no recognition of one nation's approvals by
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other national regulators.</P
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><P
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>There are national technical requirements concerning the use
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of modems. Common requirements are to set the modem and its
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software to answer after the second ring and never to dial the same
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engaged or faulty number more than five times in a row.</P
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><DIV
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CLASS="WARNING"
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><A
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NAME="WARNING-APPROVAL"
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></A
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><P
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></P
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><TABLE
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CLASS="WARNING"
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WIDTH="100%"
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BORDER="0"
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><TR
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><TD
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WIDTH="25"
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ALIGN="CENTER"
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VALIGN="TOP"
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><IMG
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SRC="../images/warning.gif"
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HSPACE="5"
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ALT="Warning"></TD
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><TH
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ALIGN="LEFT"
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VALIGN="CENTER"
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><B
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>Telecommunications device approvals</B
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></TH
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></TR
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><TR
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><TD
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> </TD
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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><P
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>Using or importing unapproved telecommications equipment is
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a criminal offense in most countries.</P
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><P
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>Additionally, the operator of some types of equipment may
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require certification.</P
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></TD
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></TR
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></TABLE
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></DIV
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><P
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>Privacy laws may control what can be done with calling line
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identification records.</P
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><P
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>Do not assume that Touch Tone dialling is globally available.
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There is no common standard for decadic dialling: some countries
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have the longest sequence for zero, other countries have the
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shortest sequence for zero.</P
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><P
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>There is little coordination of national numbering plans. Be
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careful not to call a national emergency services number when
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intending to dial the international access code. Common emergency
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services numbers are: 112, 911, 000. International access codes
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vary by country.</P
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><P
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>Intelligent network features such as toll-free numbers are
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usually not available to calls originating from abroad.</P
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><P
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>International calls may be routed through fiber optical
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submarine cable, satelite or High Frequency radio. The possible
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bit rates vary considerably between these options. Expect the
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maximum throughput with no errors from fiber optical submarine
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cable. Expect 1200<SPAN
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CLASS="ABBREV"
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>bps</SPAN
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> to
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2400<SPAN
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CLASS="ABBREV"
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>bps</SPAN
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> with some errors from satelite. Expect
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75<SPAN
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CLASS="ABBREV"
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>bps</SPAN
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> to 300<SPAN
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CLASS="ABBREV"
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>bps</SPAN
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> with many
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errors from <SPAN
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CLASS="ACRONYM"
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>HF</SPAN
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> radio.</P
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><P
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>There will be considerable latency depending upon the
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distance. If the latency becomes greater than the modem's error
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correction window then you will get better
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<SPAN
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CLASS="PRODUCTNAME"
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>Zmodem</SPAN
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> file transfer performance if you
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disable the <SPAN
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CLASS="ACRONYM"
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>HDLC</SPAN
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>-based error correction in the
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modems.</P
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><P
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>International calls may have their signal altered
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considerably. Traditionally, international calls are placed
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through analogue conditioning circuits to minimise echo. This
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conditioning limits the maximum bit rate a modem can achieve,
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probably to less than 9600<SPAN
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CLASS="ABBREV"
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>bps</SPAN
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>. You may be able to program a
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<I
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CLASS="WORDASWORD"
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>guard tone</I
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> to disable analogue
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conditioning, this will vary by carrier and the commands to send
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the guard tone vary by modem.</P
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><P
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>On some modern international circuits, particularly those
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accessed by international calling cards, digital voice compression
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is used. No reliable modem connection can be established over
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these digitally-compressed circuits. The best current tactic for
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identifying these digitally compressed circuits is to listen to the
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background noise — when no-one is speaking the real
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background noise will be replaced by a synthesized background noise
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(a compression technique called <I
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CLASS="WORDASWORD"
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>silence
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suppression</I
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>).</P
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></DIV
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><DIV
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CLASS="NAVFOOTER"
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><HR
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ALIGN="LEFT"
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WIDTH="100%"><TABLE
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SUMMARY="Footer navigation table"
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WIDTH="100%"
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BORDER="0"
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CELLPADDING="0"
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CELLSPACING="0"
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><TR
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><TD
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WIDTH="33%"
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ALIGN="left"
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VALIGN="top"
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><A
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HREF="bugs-apm.html"
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ACCESSKEY="P"
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>Prev</A
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></TD
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><TD
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WIDTH="34%"
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ALIGN="center"
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VALIGN="top"
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><A
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HREF="index.html"
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ACCESSKEY="H"
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>Home</A
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></TD
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><TD
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WIDTH="33%"
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ALIGN="right"
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VALIGN="top"
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><A
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HREF="upload.html"
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ACCESSKEY="N"
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>Next</A
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></TD
|
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></TR
|
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><TR
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><TD
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WIDTH="33%"
|
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ALIGN="left"
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VALIGN="top"
|
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><SPAN
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CLASS="PRODUCTNAME"
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>Advanced Power Management</SPAN
|
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></TD
|
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><TD
|
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WIDTH="34%"
|
|
ALIGN="center"
|
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VALIGN="top"
|
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><A
|
|
HREF="bugs.html"
|
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ACCESSKEY="U"
|
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>Up</A
|
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></TD
|
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><TD
|
|
WIDTH="33%"
|
|
ALIGN="right"
|
|
VALIGN="top"
|
|
>Uploading files from a serial console</TD
|
|
></TR
|
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></TABLE
|
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></DIV
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></BODY
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></HTML
|
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> |