Wireless mesh hidden node problem

通过Zigbee和Thread标准小组的市场采用，IEEE 802.15.4标准已经成为低功耗无线通信的基石，无论是消费者还是商业通信。Zigbee和Thread协议都支持无线网状网络，其中Zigbee是专有网络，Thread基于互联网协议版本6(IPv6)。IEEE 802.15.4标准是专门为与干扰者的互操作性而设计的，无论是通过检测另一IEEE 802.15.4载波的存在，还是从另一种带内协议(如Wi-Fi)。干扰避免机制使用CSMA/CA(具有冲突避免的载波侦听多路访问)信道访问机制。利用这种机制，发送设备在尝试发送之前，首先检查信道是否没有干扰。如果它观察到该信道是空闲的，它将开始传输。如果不是，它将在可编程的限制内的随机时间间隔之后重试整个过程。当所有节点都可以相互“监听”时，这种方法工作得很好，这在大型网络中是罕见的。如果节点无法发现其接收器范围之外正在发生的事情，则其传输可能会导致隐藏节点冲突。这种现象被称为隐藏节点(或隐藏终端)问题。增加密集网络中节点的发射功率并不一定解决问题，因为它增加了在网格网络的另一部分中引起隐藏节点冲突的概率。此外，在北美以外的大部分地区，IEEE 802.15.4协议的传输功率仅被限制为10 mW。通过产品制造商的实际部署，可以明显看出，由于隐藏的干扰，IEEE 802.15.4协议在存在干扰的情况下难以应对。以前解决该问题的尝试包括更改干扰检测模式、增加重试次数以及在传输后未收到确认后移动到额外的重试延迟。该项目旨在测量提高接收器灵敏度作为缓解隐藏节点问题的一种手段的效果。

The IEEE 802.15.4 standard has become a cornerstone of low power wireless communications, both consumer and commercial, through market adoption by the Zigbee and the Thread standards groups. Both the Zigbee and Thread protocols support wireless mesh networking, with Zigbee being a proprietary network and Thread being based on the internet-protocol version 6 (IPv6).The IEEE 802.15.4 standard has been designed specifically for interoperability with interferers, both through the detection of the presence of another IEEE 802.15.4 carrier, or from another in-band protocol such as Wi-Fi. The interference-avoidance mechanism uses the CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) channel access mechanism. With this mechanism, a transmitting device first checks whether the channel is clear of interferers, before attempting transmission. If it observes that the channel is clear, it will start transmitting. If not, it will retry the entire procedure after a random time interval within a programmable limit.This approach works well when all nodes can "hear" each other, which is a rare case in large networks. If a node is not capable of discovering what is happening beyond its receiver range, then its transmission can lead to hidden-node collision. This phenomena is known as the hidden node (or hidden terminal), problem.Increasing the transmit power of nodes within a dense network does not necessarily solve the problem, as it increases the probability of causing hidden-node collisions in another part of a mesh network. Moreover, the transmit power of the IEEE 802.15.4 protocol is limited to just 10mW in most of the world outside of North America.It has become evident through real-world deployments from product manufacturers that the IEEE 802.15.4 protocol struggles to cope in the presence of interferes, due to the hidden interferers.Previous attempts to solve the problem include changing the modes of interferer detection, increasing the number of retries and moving to an additional retry delay after not having received an acknowledgement after transmission. This project aims to measure the effect of increasing receiver sensitivity as a means of mitigating against the hidden node problem.