Modification of the basic random multiple-access model for the case of a centralized reservation-based data network is considered. Substance of the modification is the following. The channel time is composed of equal intervals, which are called frames. Each frame consists of some consequent mini-slots for the transmission of requests, used for the channel reservation, and consequent slots for the actual packets transmission. Upper and lower bounds of Tsybakov’s capacity are estimated for the system. Problem of the optimal choice for the numbers of mini-slots and slots per frame is analyzed. It is shown that these values do not depend on the ratio between the duration of request and packet transmission.