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>A.11. Modems and overseas telecommunications requirements</H1
><P
>There is no world-wide approval processes to certify that a
modem is suitable for connection to the telephone network. This is
despite the presence of a common set of technical standards that
modems must meet for use on the global switched telephone network.
There is little or no recognition of one nation's approvals by
other national regulators.</P
><P
>There are national technical requirements concerning the use
of modems. Common requirements are to set the modem and its
software to answer after the second ring and never to dial the same
engaged or faulty number more than five times in a row.</P
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>Telecommunications device approvals</B
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><P
>Using or importing unapproved telecommications equipment is
a criminal offense in most countries.</P
><P
>Additionally, the operator of some types of equipment may
require certification.</P
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><P
>Privacy laws may control what can be done with calling line
identification records.</P
><P
>Do not assume that Touch Tone dialling is globally available.
There is no common standard for decadic dialling: some countries
have the longest sequence for zero, other countries have the
shortest sequence for zero.</P
><P
>There is little coordination of national numbering plans. Be
careful not to call a national emergency services number when
intending to dial the international access code. Common emergency
services numbers are: 112, 911, 000. International access codes
vary by country.</P
><P
>Intelligent network features such as toll-free numbers are
usually not available to calls originating from abroad.</P
><P
>International calls may be routed through fiber optical
submarine cable, satelite or High Frequency radio. The possible
bit rates vary considerably between these options. Expect the
maximum throughput with no errors from fiber optical submarine
cable. Expect 1200<SPAN
CLASS="ABBREV"
>bps</SPAN
> to
2400<SPAN
CLASS="ABBREV"
>bps</SPAN
> with some errors from satelite. Expect
75<SPAN
CLASS="ABBREV"
>bps</SPAN
> to 300<SPAN
CLASS="ABBREV"
>bps</SPAN
> with many
errors from <SPAN
CLASS="ACRONYM"
>HF</SPAN
> radio.</P
><P
>There will be considerable latency depending upon the
distance. If the latency becomes greater than the modem's error
correction window then you will get better
<SPAN
CLASS="PRODUCTNAME"
>Zmodem</SPAN
> file transfer performance if you
disable the <SPAN
CLASS="ACRONYM"
>HDLC</SPAN
>-based error correction in the
modems.</P
><P
>International calls may have their signal altered
considerably. Traditionally, international calls are placed
through analogue conditioning circuits to minimise echo. This
conditioning limits the maximum bit rate a modem can achieve,
probably to less than 9600<SPAN
CLASS="ABBREV"
>bps</SPAN
>. You may be able to program a
<I
CLASS="WORDASWORD"
>guard tone</I
> to disable analogue
conditioning, this will vary by carrier and the commands to send
the guard tone vary by modem.</P
><P
>On some modern international circuits, particularly those
accessed by international calling cards, digital voice compression
is used. No reliable modem connection can be established over
these digitally-compressed circuits. The best current tactic for
identifying these digitally compressed circuits is to listen to the
background noise — when no-one is speaking the real
background noise will be replaced by a synthesized background noise
(a compression technique called <I
CLASS="WORDASWORD"
>silence
suppression</I
>).</P
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