Zigbee | Beacon与功耗

Power and beacons
Ultra-low power consumption is how ZigBee technology promotes a long lifetime for devices with nonrechargeable batteries. ZigBee networks are designed to conserve the power of the slave nodes. For most of the time, a slave device is in deep-sleep mode and wakes up only for a fraction of a second to confirm its presence in the network. For example, the transition from sleep mode to data transition is around 15ms and new slave enumeration typically takes just 30ms.

ZigBee networks can use beacon or non-beacon environments. Beacons are used to synchronize the network devices, identify the HAN, and describe the structure of the superframe. The beacon intervals are set by the network coordinator and vary from 15ms to over 4 minutes. Sixteen equal time slots are allocated between beacons for message delivery. The channel access in each time slot is contention-based. However, the network coordinator can dedicate up to seven guaranteed time slots for noncontention based or low-latency delivery.

The non-beacon mode is a simple, traditional multiple-access system used in simple peer and near-peer networks. It operates like a two-way radio network, where each client is autonomous and can initiate a conversation at will, but could interfere with others unintentionally. The recipient may not hear the call or the channel might already be in use.

Beacon mode is a mechanism for controlling power consumption in extended networks such as cluster tree or mesh. It enables all the clients to know when to communicate with each other. Here, the two-way radio network has a central dispatcher that manages the channel and arranges the calls. The primary value of beacon mode is that it reduces the system's power consumption.

Non-beacon mode is typically used for security systems where client units, such as intrusion sensors, motion detectors, and glass-break detectors, sleep 99.999% of the time. Remote units wake up on a regular, yet random, basis to announce their continued presence in the network. When an event occurs, the sensor wakes up instantly and transmits the alert ("Somebody's on the front porch"). The network coordinator, powered from the main source, has its receiver on all the time and can therefore wait to hear from each of these stations. Since the network coordinator has an "infinite" source of power it can allow clients to sleep for unlimited periods of time, enabling them to save power.

Beacon mode is more suitable when the network coordinator is battery-operated. Client units listen for the network coordinator's beacon (broadcast at intervals between 0.015 and 252s). A client registers with the coordinator and looks for any messages directed to it. If no messages are pending, the client returns to sleep, awaking on a schedule specified by the coordinator. Once the client communications are completed, the coordinator itself returns to sleep.

This timing requirement may have an impact on the cost of the timing circuit in each end device. Longer intervals of sleep mean that the timer must be more accurate or turn on earlier to make sure that the beacon is heard, both of which will increase receiver power consumption. Longer sleep intervals also mean the timer must improve the quality of the timing oscillator circuit (which increases cost) or control the maximum period of time between beacons to not exceed 252s, keeping oscillator circuit costs low.

综上，对于开关与灯的组合，为了保证开关每次都可以准确控制灯，个人建议开关使用ZR，灯可以使用ZED。

原文链接：http://www.embedded.com/design/connectivity/4006430/Home-networking-with-Zigbee

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