Digital data bi-directional communication method and apparatus

Apparatus and a method of bi-directional communication of digital data between a plurality of remote users and a base station using a shared communication channel and a time-division protocol in which the communication time is divided into a plurality of time slots is provided. The method includes transmitting packets of data according to a TDMA (Time Division Multiple Access) protocol in which each user is allocated a dedicated time slot in the shared communication channel; and transmitting other packets of data according to a time-division ALOHA protocol in which each user is allocated a time slot in the shared communication channel on an "as needed" basis, competing with other users for access to the shared communication channel, and subsequently re-transmits those packets of data where no acknowledgement is received until an acknowledgement is received.