Wireless Data over the GSM Voice Channel

Bernhard M. Bradatsch

For many users in Europe, mobile wireless modems are still a thing of the future. But an inventive German company called TLK Computer (Munich) has a modem that, in connection with a digital mobile phone, is able to transmit data across Europe over a network that is currently used for voice traffic, no matter what your location. TLK's transmitting system works over the GSM-Network (Groupe Speciale Mobile), which is a digital cellular mobile telecommunications service.

Initially, the D-net, which is the GSM-Network's digital cellular network, was designed for speech transmission only. Both telecommunications companies in Germany, Telekom (i.e., German PTT) and Mannesmann, have stated that it is impossible to transmit data via the voice ch annel of the D-net. But thanks to a special error-correction method and modulation scheme, TLK has achieved what those companies said couldn't be done.

Due to TLK's modulation scheme, to transmit data over the D-channel, you need a TLK GSM modem on both ends of the connection. But it makes no difference if the connection is established between a GSM phone and a regular one. The modems will cost about $1600.

Presently, the modems transmit at approximately 1000 bps. This is because they must share slices of time with other users on the D-net. TLK (+49 89 45 85670; fax +49 89 44 86297) is investigating various compression techniques that it will use in the next version of the modem (due sometime this year).