We present an architecture for implementing a wireless physical layer in a packet-based network simulator. We have integrated this architecture in the popular ns-2 network simulator and used it to implement an impulse-radio ultra-wide band (IR-UWB) physical layer. Contrary to the current wireless physical layer implementation of ns-2, in our case a packet is fully received by our physical layer before being delivered to the MAC layer. A packet detection and timing acquisition model has been implemented. Further, for each packet, a packet error rate (PER) can be computed as a function of the received power, interference from concurrent transmissions and thermal noise. This architecture is quite generic and allows for the simulation of any physical layer where an accurate model of interference is of high importance, e.g., IR-UWB or CDMA. Our implementation for IR-UWB models transmissions with different time-hopping sequences (THS). The underlying modulation is binary phase shift keying (BPSK), followed by a variable-rate channel code. Our implementation is the first available that allows for the simulation of IR-UWB networks. It is modular and can thus be easily modified and extended.