The new paradigm of Dynamic Spectrum Access (DSA) networks aims to provide opportunistic access to large parts of underutilized spectrum. Majority of research in this area has focused on free-for-all, uncoordinated access methods common in ad-hoc military applications [5, 7, 8, 10, 11]. In this paper, we argue that a simpler, pragmatic approach that offers coordinated, spatially aggregated spectrum access via a regional spectrum broker is more attractive in the immediate future. We first introduce two new concepts, namely, Coordinated Access Band (CAB) and Statistically Multiplexed Access (SMA) to spectrum that form the basis of our work. We then describe their implementation in the new DIMSUMnet network architecture consisting of four elements: base stations, clients, a Radio Access Network Manager (RANMAN) that obtains spectrum leases, and a per-domain spectrum broker that controls spectrum access. We also discuss in detail various issues in the design of spectrum brokers and spectrum allocation policies and algorithms.