by Tomi Engdahl on Thu Sep 22, 2005 12:31 pm
So called "GSM modems" are pretty different things than those normal telephone line modems. Actually those "GSM modems" are not modems at all in the classical sense, they do not do any modulation/demodulation for the signal.
The GSM modems are more like data format converters / interface converters. They convert the serial communications data between serial port (or USB, IR, Bluetooth etc..) and the data format used in GSM network. A GSM modem with serial interface does around this when transmitting data: receives several characters of data coming from serial ports (amount from zero charaters to maximum that can be put to one GSM data packet, how manu received within set timeframe), put them to GMS data packet (includes adding header information, error correcting code, maybe encrypting/scrambling etc..). Then push it the ready packet to the digital GSM radio circuitry to send it out at right time (within the time slot given by base station system for this phone to use).
On the other end the electronics receiovs the packets, decodes them (removes headers, does error correcting, etc..) and puts received characters to serial port transmitting buffer to be sent out though serial port at needed data rate.
This is how communiction with two GSM modems that have serial ports would do. This kind of processing to both diretions.
When the "GMS modem" need to communicate with a normal telephone modem, the communication works as described above to a serial port on the operator premisses that is connected to an ordinary telephone line modem (in real life systems the "GMS modem" electronics, telephone line modem and controle electronics are all integrated to one single circuit board insalled to equipment rack, possibly one circuit board holdign electronics for many lines).
Traditional DTMF decoding tricks do not work with GSM modems.
So called "GSM modems" are pretty different things than those normal telephone line modems. Actually those "GSM modems" are not modems at all in the classical sense, they do not do any modulation/demodulation for the signal.
The GSM modems are more like data format converters / interface converters. They convert the serial communications data between serial port (or USB, IR, Bluetooth etc..) and the data format used in GSM network. A GSM modem with serial interface does around this when transmitting data: receives several characters of data coming from serial ports (amount from zero charaters to maximum that can be put to one GSM data packet, how manu received within set timeframe), put them to GMS data packet (includes adding header information, error correcting code, maybe encrypting/scrambling etc..). Then push it the ready packet to the digital GSM radio circuitry to send it out at right time (within the time slot given by base station system for this phone to use).
On the other end the electronics receiovs the packets, decodes them (removes headers, does error correcting, etc..) and puts received characters to serial port transmitting buffer to be sent out though serial port at needed data rate.
This is how communiction with two GSM modems that have serial ports would do. This kind of processing to both diretions.
When the "GMS modem" need to communicate with a normal telephone modem, the communication works as described above to a serial port on the operator premisses that is connected to an ordinary telephone line modem (in real life systems the "GMS modem" electronics, telephone line modem and controle electronics are all integrated to one single circuit board insalled to equipment rack, possibly one circuit board holdign electronics for many lines).
Traditional DTMF decoding tricks do not work with GSM modems.
