Problems, diagnostics, symptoms, causes and troubleshooting methods for data transmission over a telephone line Dual-UP
I encountered the following problems when organizing connections between branches using dial-up channels of telephone lines:
1. no dial tone (hum)
2. no dialing after the very first digit - call reset with characteristic tones
3. dialing occurs - no connection
4 there is a dial tone, there is dialing, there is a connection, but extraneous noise is heard
5. everything is fine - but Dual-up either lasts a couple of seconds or the connection is not established at all
6. when connected to the line - the parallel phone works quieter, and generally puts the line into "busy" mode
what could be the cause and how to overcome them?
1. no dial tone (hum)
check the cable connection - try connecting a phone instead of the modem
2. no dialing after the very first digit - call reset with characteristic tones
try changing the dialing method to pulse or tone mode
The phone set has a special switch
And in the dialed number settings, add the English letter P before the number being dialed, for example p234233
Check the line type, it is possible that if the connection is through a PBX - then to make calls you need an outside line access code - for example 9, example 9234233
or the so-called "capacity-less number" is used - that is, a PBX on the side of the telephone line operator - you can call the city or the same number, but you cannot call to it
Try replacing the modem, on both sides of the call, if everything has been tried - call the phone operator with the problem
3. dialing occurs - no connection
try calling from a phone instead of the modem
try setting various data transfer protocols in the modem initialization string
try replacing the modems
if everything has been tried - call the phone operator with the problem
4 there is a dial tone, there is dialing, there is a connection, but extraneous noise is heard
try replacing the modems or, if possible, other telephone lines
if everything has been tried - call the phone operator with the problem
5. everything is fine - but Dual-up either lasts a couple of seconds or the connection is not established at all
try replacing the modems or, if possible, other telephone lines
if everything has been tried - call the phone operator with the problem
6. when connected to the line - the parallel phone works quieter, and generally puts the line into "busy" mode
possibly a short circuit in the modem, for example from a high voltage surge, or a lightning strike on the phone cable... replace the modem or reinstall the drivers with a system restart
To increase connection stability, initialization strings are used, with which you can select a lower data transfer protocol and other specific modem parameters. For example, to select V.32 modulation, the initialization string looks like this:
AT+MS=V32
To select the dialing type, tone or pulse, the letter t or p respectively is placed before the number being dialed, for example like this:
p1234567 or t12345678
the choice of dialing type depends on what PBX you have, some PBXs do not understand tone dialing. If you need to dial 9 in the number in case you have a mini-PBX, or 8 if your caller ID (ANI) does not work, and billing is based on the phone number, the symbol W from the English wait will help you, here is an example:
p8w04412345678
this is done so that the modem waits for a dial tone after dialing 8 or 9.
There are also Dual-up Modem initialization strings
+MS=V22B;+A8E=,,,0 - connection is practically impossible
+MS=V32;+A8E=,,,0 - very poor quality
+MS=V32B;+A8E=,,,0 - poor quality
+MS=V34,1,300,21600,300,21600 - below average
+MS=V34,1,300,24000,300,24000 - average
+MS=V34,1,300,28800,300,28800 - slightly above average
+MS=V90,1,300,31200,300,38667 - unstable line
+MS=V90,1,300,31200,300,44000 - good line
+MS=V92,1 - excellent line
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