A real time device polling method for multiplexed transmission of on/off constant bit rate data, such as voice data from a telephone call, over a cable data network is disclosed. The network serves as a shared bus for both the downstream and upstream traffic. The upstream channel is modeled as a stream of mini-slots. A cable modem termination system at the head end supports a number of cable modems attached to the cable network and connected to customer premises equipment. Allocation maps are transmitted on the downstream channel to the cable modems to define transmission opportunities on the upstream channel. The cable modem termination system polls the cable modems in an efficient way such that the overhead associated with the polling is minimized, and the availability of data transmission slots is synchronized with the data packet generation by the codec for the cable modem, which minimizes delay.