Abstract: In this paper, we document our design of a MAC protocol, called OSU-MAC, subject to the physical layer characteristics and constraints of a narrow-band wireless modem testbed currently being built at the Ohio State University. The narrow-band wireless modem testbed is expected to support both real-time (bus location tracking) and non-real-time (regular) data applications. A number of techniques are proposed to support QoS imposed by the real-time applications, to deal with the asymmetry on the forward and reverse channels and the half-duplex transmission constraint imposed by the physical layer, and to enhance the error control capability of OSU-MAC. We also present simulation results to demonstrate the key, functional characteristics of OSU-MAC.