The MAC is bandwidth at 6 dB down for digital modulation. However responsible for management of the physical layer and the chip rate, and with it the data rate, has to be reduced among its functions are channel access, keeping track of on Channel 0 in order to meet the confines of the 868 to slot times, and message delivery acknowledgement. The 868.6 MHz channel allowed under ERC recommenda- LLC is the interface between the MAC and physical layer tion 70-03 and ETSI specification EN 300-220. On the and the upper-application software. 2400 to 2483.5 MHz band, the bit rate of 250 Kbps Application software is not a part of the IEEE allows a throughput, after considering the overheads in- 802. .15.4 specification and it is expected that the Zigbee volved in packet transmissions, to attain 115.2 kbps, Alliance will prepare profiles, or programming guidelines a rate used for some PC peripherals for example. and requirements for various functional classes in order The spreading modulation used on the 2450 MHz to assure product interoperability and vendor indepen- physical layer has similarity in principle to that used on dence. These profiles will define network formation, se- IEEE 802.11b (high-rate Wi-Fi) to increase the data bit curity, and application requirements while keeping in rate without raising the chip rate, thereby achieving mind the basic Zigbee features of low power and high a desired carrier bandwidth. Sixteen different, almost reliability. orthogonal 32-bit long spreading sequences are available for transmission at 2 Mchips/second. Each consecutive Communication characteristics sequence of four data bits determines which of the six- teen spreading sequences is sent. On reception, the re- In order to achieve high flexibility of adaptation to the ceiver can identify the spreading sequence and thus range of applications envisioned for Zigbee, operation is decode the data bits. The modulation used, O-QPSK Table 5.2-7 Data rates and modulation. PHY (MHz) Frequency Spreading parameters Data parameters band (M