r firm- initial frequency-hopping synchronization. There are also ware for the purpose of acquisition, addressing, and flow NULL and POLL packets that have an access code and control. Packet bits are also subjected to data whitening header, but no payload. They're sent when slaves are (randomization), error-correction coding, and encryption being polled to maintain synchronization or confirm as defined for each particular data type. Figure 5.2-19 packet reception (in the case of NULL) in the piconet shows the standard packet format. but there is no data to be transferred. The access code is used for synchronization, d-c level compensation, and identification. Each Bluetooth device Error correction and encryption has a unique address, and it is the address of the device acting as master that is used to identify transmitted The use of forward error correction (FEC) improves packets as belonging to a specific piconet. A 64-bit syn- throughput on noisy channels because it reduces the chronization word sandwiched between a four-bit header number of bad packets that have to be retransmitted. In and four-bit trailer, which provide d-c compensation, is the case of SCO links that don't use retransmission, FEC based on the master's address. This word has excellent can improve voice quality. However, error correction correlation properties SO when it is received by any of the involves bit redundancy SO using it on relatively noiseless piconet members it provides synchronization and posi- links will decrease throughput. Therefore, the applica- tive identification that the packet of which it is a part tion decides whether to use FEC or not. belongs to their network. All message packets sent by As already mentioned, there are various types of members of the piconet use the same access code. packets, and the packet type defines whether or not FEC ACCESS CODE HEADER PAYLOAD 72 BITS 54 BITS 0 TO 2745 BITS Figure 5.2-19 Bluetooth packet. Short-range wireless applications and technologies CHAPTER 5.2 is used. The most redundant FEC