Engineers may be concerned over the crowded 2.4GHz band. As shown in this slide, 802.11b, Bluetooth® and 802.15.4 are just a few of the solutions that share this channel. Note that the 802.11b spectrum occupancy is shown by the red arcs and the 802.15.4 channels are shown by the blue bars. While 802.15.4 utilizes interference avoidance techniques, such as listen before transmit, Wi-Fi environment channels 15, 20, 25, and 26 can be selected to reduce interference.