Cable plants were initially designed for one-way broadcast of analog television signals (from the head-end to the neighborhood). They are now being upgraded to provide an upstream path (from the home to the head-end). New challenges arise in using the upstream channel since the available bandwidth is low and the noise levels are high. Different variations of the reservation Aloha protocol have been proposed as the MAC protocol to efficiently share the scarce upstream capacity. The efficiency of the reservation protocol highly depends on the capacity assigned to the reservation channel. In this paper we present a control mechanism that dynamically adjusts the operating parameters to the current load on the system. The control mechanism does not require any framing structure and is built around a "sea of mini-slots". The performance under both static and highly dynamic loads is close to optimum. The station implementation remains particularly simple and the downstream control structure is also simple. Results are given here for fixed length data units (ATM cells) but the algorithm extends very simply to variable length MAC frames.