OMMUNICATION LINK 32 BITS 8 BITS 8 BITS 127 BYTES MAXIMUM PAN COORDINATOR, FFD PREAMBLE START-OF- FRAME PSDU -- PHY SERVICE DATA UNIT FRAME LENGTH RFD OR FFD DELIMINATOR Figure 5.2-22 Network topologies. Figure 5.2-24 Transmission packet. Short-range wireless applications and technologies CHAPTER 5.2 payload itself, and at the end, two bytes that serve as the greatest distance between any two directly commu- a frame check sequence. Reliability is assured since the nicating nodes does not exceed Zigbee's basic range ca- receiver performs an independent calculation of this pability. For example, in a multi-floor building, sensors on frame check sequence and compares it with the number the top floor can send alarms to the control box in the received. If any bits have been changed by interference or basement by passing messages through sensors located on noise, the numbers will not match. Only if a match every floor and operating as relay stations. occurs, the receiving side returns an acknowledgement to No doubt that there will be competition between the originator of the message. Lacking an acknowledge- Bluetooth and Zigbee for use in certain applications, but ment, the transmission will be repeated until it is suc- the overall deployment and the reliability of wireless cessfully received. control systems will increase. The proportion of wireless In order to avoid two or more stations trying to security and automation systems will increase because transmit at the same time, a carrier sense multiple access the new standard will provide a significant boost in re- with collision avoidance (CSMA-CA) routine is em- liability, security, and convenience, as compared to most ployed, similar to that used in Wi-Fi, IEEE 802.11. The present solutions. Zigbee receiver monitors the channel and only if it is idle it may initiate a transmission. If the channel is occupied, the terminal must wait a random back off period before it 5.2.4 Conflict and compatibility can again attempt access. Acknowledgement messages are sent without 