The upcoming Ultra-wide-band (UWB) radio technology holds great promise for revolutionizing wireless communications. UWB radios transmit using precise, very short (e.g. picosecond) impulses spread over a very large bandwidth (up to a few Ghz). The significant advantages of this technology are low-power operation, mitigated multi-path fading effects, high bit-rates and unique precise position/timing location ability. However, one of the drawbacks of this technology, in its current state, is the high channel acquisition time, i.e. the time for a transmitter and receiver to achieve bit synchronization. This tends to be quite high, of the order of a few milli-seconds. Hence, it is important for current medium access control (MAC) protocol design to consider the impact of acquisition time. In this paper, we study the performance of two standard MAC protocols - the distributed CSMA/CA protocol and the centralized TDM protocol in the context of UWB wireless local area networks. We study effects of varying packet frame sizes and packet arrival rates and present a quantification of the impact of acquisition time on overall performance.