In a Time Division Multiple Access (TDMA) communication network (10), a central office (CO) (11) provides interactive communication between a service provider and a plurality of user terminals (32). To synchronize transmissions of the user terminals, a timebase timer (20) in the CO generates modulo N-bit cyclical reference counts as time markers. In a downstream direction to the plurality of user terminals, time marker insertion units (16) in the CO receive both (a) separate digital TDMA transport streams having a predetermined data rate including data packets and Media Access Control (MAC) packets that are interspersed between the data packets at predetermined intervals, and (b) the time markers generated by the timebase timer, and insert a currently received time marker count into a concurrently received MAC packet. The resultant downstream TDMA transport streams with the inserted time marker counts in the MAC packets are transmitted in continuous TDMA output transport stream to the remote user terminals. The time markers are independent of a data rate, a physical channel, and a channel protocol of the transport stream. In each user terminal, a network interface module (34) synchronizes itself by comparing received time markers and locally generated time marker counts for correcting a local oscillator frequency. Upstream channel units (23) in the CO are synchronized using the timebase timer generated time markers for receiving and processing upstream data packets upon arrival at their preassigned channel unit.