In this article, we present a novel discrete-time Markov chain model of book-ahead bandwidthsharing mechanisms. We use this analytical model and a simulation model to understand the benefits of book-ahead (BA) bandwidth-sharing when compared to the immediate-request (IR) call-blocking mode of bandwidth-sharing in circuit-switched networks. We study two different BA schemes, BA-all, in which the caller accepts any set of available timeslots, and BA-n, in which the caller specifies n call-initiation time options. Numerical results show that the BA-all achieves 95% utilization with a call-blocking probability of only 1%, while in the IR mode, call blocking probability is 23% even when utilization is only 80%. The BA-n schemes perform as well as the BA-all scheme if the call-initiation time options are restricted to fall on timeslot boundaries separated by the minimum call holding time. The length of the advance-reservation horizon, K, is shown to increase linearly with call holding time,...