The multi-solution approach is built on the hardware foundation of the transceiver, which is often referred to as the radio or RF integrated circuit and the microcontroller (MCU). From this foundation and above the hardware level, various implementations of the physical layer (PHY) are offered. As mentioned, NXP offers the SMAC, a proprietary solution for developing the simplest of networks quickly and easily. The 802.15.4 MAC solution and the ZigBee-compliant solution are based on the IEEE 802.15.4 specification with regard to the PHY and the MAC. ZigBee, however, as defined by the ZigBee Alliance, is a subset of the 802.15.4 specification. The subset was established using features and functions of the 802.15.4 specification, which were best suited to sensing, monitoring and control networks. Also, they were particularly focused on low data rate, small packet transmission, and low power. The ZigBee network, therefore, sits on the top of the 802.15.4 MAC. Above the ZigBee network layer is the application layer, with the ZigBee profiles as defined by the Alliance at the uppermost layer.