Complex sentence: 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.
Split into simpler sentences: 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.