Demystifying Polling Control in Zigbee Networks
2018年2月27日
ThreeZigbee® wireless protocols— Zigbee 3.0, Zigbee PRO and Green Power — recently earned the SEAD Connected Efficiency Award for 2017, because of their inherent energy efficiency. In particular, Zigbee 3.0 provides application-layer functionality that is focused on energy management, including support for power configuration, polling control, device aging management, battery monitoring and network traffic reduction strategies aimed at saving energy.
Understanding the polling features in the Zigbee standard can help to optimize the power consumption of your Internet of Things (IoT) product. This blog post takes a closer look at the polling mechanism in Zigbee and its use in a fire alarm sensor application.
What is polling?
轮询是Zigbee网关持续检查相关终端传感器设备状态的机制。低功耗的Zigbee传感器被称为睡眠终端设备（SED），只有“唤醒”才能发送传感器生成的事件。它们还通过定期醒来向网关报告来保持传感器网络的心跳。
Apoll is a media access control (MAC) data request generated by an SED. A gateway in a Zigbee network acknowledges this request by sending an ACK (acknowledgement). The gateway may also decide to extend the polling to send any messages stored for the SED or read out sensor events.
Polling types in the Zigbee standard
The Zigbee standard defines two fundamental types of polling:
- Short poll
- 长投票
SEDs use extended polling schemes like快速投票andcheck-ins从网关发送或接收消息。
的术语表
- 确认：确认
- 雨衣：media access control
- SED:sleepy end device
- ZCL:Zigbee群集库
短轮询
The SED may startshort polling,where it performs a high duty cycle MAC data request to retrieve messages requested by it from other end devices. In the figure below, SED1 sends a data request to SED2 via the gateway. SED1 then continuously polls the gateway to receive the response from SED2. The SED1 is now said to be in“fast poll mode,”which lasts until the expiration of the“fast poll timeout”or when the SED receives the“快速轮询停止”command from the gateway.
长轮询
长轮询allows an SED to periodically report to its gateway. These are low duty cycle MAC data requests. They also provide an opportunity for an SED to retrieve a message from the gateway. The gateway:
- 将此消息存储7.68秒
- 通过在ACK high中设置data pending标志指示它有SED的消息，如下图所示
- 然后将消息作为MAC数据包发送到SED
签入事件
Acheck-in event是一个非常低的占空比事件，由Zigbee的应用层通过Zigbee集群库（ZCL）事务生成。在签入过程中，SED可以从网关接收数据，网关通常驻留的时间比定义的7.68秒长。然后网关可以启动fast poll模式，它可以发送数据，直到快速轮询过期（或直到网关发送“快速轮询停止”command). The gateway can also update the poll control parameters during the check-in event. Check-in events are ideal for performing firmware updates or collecting diagnostic data from sensors. As shown in the figure below, the gateway sends a ZCL read attribute command during the fast polling phase of the check-in. The SED responds with the“ZCL读取响应。”
投票是如何工作的？一个例子
让我们看一个使用火警传感器的真实例子，智能选择轮询可以拯救生命。商用Develco烟雾报警器产品SMSZB-120的技术手册为其传感器提供了以下默认轮询控制设置：
轮询控制属性 |
默认轮询控制设置 |
值机间隔 |
1hour |
长投票interval |
7.5 seconds |
Short poll interval |
1second |
快速轮询超时 |
5 minutes |
Let’s use an example of two Zigbee-based fire alarm sensors placed in the kitchen and the bedroom, which we’ll call SED1 and SED2. Both the sensors are communicating to a Zigbee gateway (in this case, a smart thermostat). SED1 detects a rapid rise in heat and smoke in the kitchen. It sounds the alarm and updates the gateway about the situation developing in the kitchen. Meanwhile, SED2 in the bedroom still doesn’t detect the heat or smoke from the kitchen. How quickly can the gateway update the fire alarm in the bedroom to sound the alarm?
当SED1检测到火灾时，它会立即唤醒并向网关发送警报事件。此消息将在网关上存储7.68秒。当网关收到来自它的长轮询数据请求时，它可以向SED2发送更新。使用上述Develco火警示例的默认设置，SED2可以在不到7.5秒内发出卧室的警报。网关可以在每次长轮询事件时请求SED1更新状态，并将其中继到SED2。
Zigbee: Helping to improve energy efficiency
The polling mechanism in Zigbee is just one component that helps Zigbee-based IoT systems save energy. Stay tuned for more posts, which will explain the battery monitoring and child aging mechanism in the Zigbee standard.
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