Except for Smart Energy Profile 2.0, Zigbee devices are required to conform to the IEEE 802.15.4-2003 Low-rate Wireless Personal Area Network (LR-WPAN) standard. The standard specifies the lower protocol layers—the physical layer (PHY), and the media access control portion of the data link layer. The basic channel access mode is carrier-sense multiple access with collision avoidance (CSMA/CA). That is, the nodes communicate in a way somewhat analogous to how humans converse: a node briefly checks to see that other nodes are not talking before it starts. CSMA/CA is not used in three notable exceptions: