Prior to CSD, data transmission over mobile phone systems was done by using a modem, either built into the phone or attached to it such systems were limited by the quality of the audio signal to 2.4 kbit/s or less.
Data transmission over mobile phone systems was done by using a modem. This was prior to CSD. Data transmission over mobile phone systems was either built into the phone. This was prior to CSD. Data transmission over mobile phone systems was attached. This was prior to CSD. It s or less. Such systems were limited by the quality of the audio signal to 2.4 kbit/data.