Remote Serial Console HOWTO
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A.10. Modems and overseas telecommunications requirements

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.

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.

WarningTelecommunications device approvals
 

Using or importing unapproved telecommications equipment is a criminal offense in most countries.

Additionally, the operator of some types of equipment may require certification.

Privacy laws may control what can be done with calling line identification records.

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.

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.

Intelligent network features such as toll-free numbers are usually not available to calls originating from abroad.

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 1200bps to 2400bps with some errors from satelite. Expect 75bps to 300bps with many errors from HF radio.

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 Zmodem file transfer performance if you disable the HDLC-based error correction in the modems.

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 9600bps. You may be able to program a guard tone to disable analogue conditioning, this will vary by carrier and the commands to send the guard tone vary by modem.

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 silence suppression).

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