Wireless Body Area Networks (WBANs) are becoming increasingly important for a wide range of applications, from medical to entertainment services. Since 2007, IEEE 802.15 Task Group 6 has been working on a communication standard for low power devices and operation on, in, or around the human body. We consider an IEEE 802.15.6 WBAN where wearable sensor devices are distributed on the body and have to send the measured data to a coordinator. Upon reception of a query coming from the coordinator, nodes of the WBAN compete to access the channel, using the Carrier Sense Multiple Access with Collision Avoidance algorithm defined in the standard, for transmitting their data via a direct link. We evaluate network performance in terms of packet loss rate, delay, and throughput, through simulation. Two different channel models for on-body communication are considered. A comparison with a WBAN based on the IEEE 802.15.4 standard is also provided.