Zigbee PHY and MAC (and difference from IEEE 802.15.4)

Zigbee, like Bluetooth, operates primarily in the 2.4 GHz ISM band. Unlike Bluetooth, it also operates at 868 MHz in Europe and 915 MHz in the USA and Australia. Because of the lower frequency, it has better propensity to penetrate walls and obstacles over traditional 2.4 GHz signals. Zigbee does not use all of the IEEE802.15.4 PHY and MAC specifications. Zigbee does make use of the CSMA/CA collision avoidance scheme. It also uses the MAC level mechanism to prevent nodes from talking over each other. 

The security specification of 802.15.4 is slightly modified. The CCM modes that provide authentication and encryption require a different security for every layer. Zigbee is targeted for severely resource-constrained and deeply-embedded systems and does not provide the level of security as defined in 802.15.4.

Zigbee is based on the IEEE 802.15.4-2003 specification before enhancements with two new PHYs and radios were standardized in the IEEE 802.15.4-2006 specification. This implies that the data rates are slightly lower than they could be in the 868 MHz and 900 MHz bands.