We are interested in the design of a MAC layer for systems that use ultra-wide band (UWB) communication and transmit very little power. We first explore the design space and find, that, for such networks, the optimal MAC should allow concurrent transmissions on as many channels as there are destinations; this is in sharp contrast to the established way of designing MAC protocols. Further, power control is not optimal: sources should always send at full power when they have something to transmit. Sources should constantly adapt their rate, either at the physical layer (which is difficult for the UWB channel) or at the MAC layer, using an increase/decrease protocol and incremental redundancy. Second, we explore the implications of a MAC layer with concurrent transmissions. We find that, in practice, because of nodes that can do only one thing at a time, there is a need for a ``private MAC" protocol. Also, broadcasting to destinations that can listen to many channels is highly non deterministic. This makes network coding an attractive alternative to flooding for protocols, such as the address resolution protocol (ARP) and routing, that need broadcasting.