Sleepy Nodes and Sleepy Meshes Ultra-low-power Mesh Routing

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1 Application White Paper Sleepy Nodes and Sleepy Meshes Ultra-low-power Mesh Routing Physical nodes in a SNAP based network communicate wirelessly using a full mesh topology. Nodes that are in direct radio range of each other will communicate directly. When nodes are not in direct radio range, intermediate nodes will automatically forward any messages to their intended destinations. This is known as "mesh routing". All SNAP based nodes have mesh routing capability "out-of-the-box". Such a network is self-forming, which means that when a new node is powered-up, it is automatically integrated into the network. The network is also selfhealing, which means that if a node catastrophically fails for any reason, other nodes will automatically route signals around the failed node. In some wireless network deployments, it is possible for all of the physical nodes to be externally powered ("plugged into a wall socket"). In this case, power consumption is not a problem and all of the nodes can be active "talking"/transmitting and/or "listening"/receiving all of the time. In many environments, however, it is necessary for the nodes to be powered by batteries, in which case power consumption can be a significant problem. Even in the case of a low-power Synapse RF Engine module running SNAP, an active module will consume an average of 50 milli-amps (ma). When powered by AA batteries capable of supplying say 2,500 milli-amp-hours (mah), this means that a node can stay active for 2,500 / 50 = 50 hours, which is a little over two days. This is simply not acceptable in terms of resource requirements (someone changing the batteries) and expense (batteries aren t cheap) for the vast majority of installations, especially in the case of networks involving hundreds or thousands of wireless nodes. The solution is for the nodes to 2008 Synapse, All Rights Reserved. All Synapse products are patent pending. 132 Export Circle, Huntsville, Alabama 35806,

2 alternate between being "awake" for a short amount of time and then entering a "sleep" mode in which they consume dramatically less power. Unfortunately, traditional mesh networks find it difficult to fully-implement a network-wide sleep scenario. By comparison, SNAP based network can be configured to implement a "sleepy mesh" that can extend each node's battery life to more than a year. Problems with conventional solutions Traditional wireless networks typically require three different types of nodes as illustrated in Figure 1: Routers, which are used to implement the mesh network; End Devices, which interface with the real-world via sensors and actuators; and a Bridge node, which links the network to some administration application running on a host computer. A popular misconception is that all of the End Device nodes in a conventional mesh topology can forward traffic through the network. In reality, however, each End Device has to communicate with the Bridge node or with a local Router node. Apart from anything else, this means that should a Router fails, any End Devices associated with the failed Router have to be able to access another Router located in close enough proximity Synapse, All Rights Reserved. All Synapse products are patent pending. 2

3 Figure 1. A traditional wireless network with all nodes currently active. (This is a simplified view each router can handle multiple end devices.) There are several problems with this conventional scenario. First and foremost, all of the Router nodes have to remain constantly powered-up so as to be available whenever a message is required to be propagated through the mesh. Thus, even if all of the End Devices are placed into sleep mode, the Routers all have to be externally powered as illustrated in Figure Synapse, All Rights Reserved. All Synapse products are patent pending. 3

4 Figure 2. A traditional wireless network with End Devices in sleep mode (the routers have to remain on external power). Having to maintain the Routers on external power sort of defeats the point of the exercise in the first place, but there are also a number of other issues, which can be summarized as follows: There are no "off-the-shelf" solutions that can handle each target application's unique requirements. This means that the base-level sleep mode in the End Devices will typically have to be customized for each user. In addition to being time-consuming, this requires substantial embedded programming expertise to modify the software, which is typically created in C/C++ or in assembly language. Depending on the particular implementation, it may be that when an End Device enters its sleep mode, all of its output pins are placed in a "floating state". In many cases, this means that external circuitry has to be added to maintain the pre-sleep values on the outputs. Depending on the particular implementation, it may be that when an End Device enters its sleep mode, it is rendered completely out-of-action; that is, it cannot respond to any external events such as a "mission-critical" button being pressed Synapse, All Rights Reserved. All Synapse products are patent pending. 4

5 Depending on the particular implementation, it may be that when an End Device exits its sleep mode, it as to be completely rebooted, which means that it forgets all knowledge of its previous state. The solution: sleepy nodes and sleepy meshes Synapse SNAP based mesh networks offer an innovative solution to the problem of placing the entire mesh in sleep mode. First of all, all End Devices support peer-to-peer communication, which means that they form the mesh network themselves without requiring additional Router nodes as illustrated in Figure 3. Figure 3. A SNAP based mesh network no Router or Bridge nodes required. Similarly, it isn t necessary to have a dedicated Bridge node in order to communicate with the Portal application development and network administration software running on the host PC; whichever physical node is currently connected to the PC assumes the role of a Bridge node. Of particular interest is the fact that SNAP based End Devices can act individually as "sleepy nodes" or in concert to form a "sleepy mesh". In this latter case, all of the nodes can be instructed to enter their sleep modes 2008 Synapse, All Rights Reserved. All Synapse products are patent pending. 5

6 simultaneously (Figure 4). After some pre-determined delay, all of the nodes will wake up again, communicate with each other as required, and then go back to sleep. Figure 4. A SNAP based "sleepy mesh" in its sleep mode. The power savings provided by sleepy nodes in a sleepy mesh can be truly amazing, because Synapse RF Engine modules consume less than 2 microamps (µa) while in sleep mode. For the purposes of these discussions, let's assume that the mesh wakes up only once a minute and stays active for 300 milliseconds. Let's also assume that the active current for a node is 50 ma, the sleep current is 2 µa, and the battery capacity is 2,500 mah. In this case, the average current will be only 252 µa, resulting in a battery life of almost 10,000 hours, or a little less than 14 months! There are several additional advantages associated with SNAP based sleepy nodes implementing a sleepy mesh; these can be summarized as follows: No embedded programming expertise is required to modify the applications running on the nodes forming a SNAP based network. These applications are created in Python, which is a modern, high-level scripting language that can be learned in just a few days. This means that it's easy for users to customize and configure the sleepy mesh so as to optimally handle each target application's unique requirements Synapse, All Rights Reserved. All Synapse products are patent pending. 6

7 When a SNAP based node enters its sleep mode, all of its output pins maintain their pre-sleep values. Prior to SNAP based node entering its sleep mode, it can be instructed to monitor one or more input pins. For example, the node can be instructed to wake up if an operator presses a button or a sensor switch is opened or closed. When a SNAP based node exits its sleep mode, it does not need to automatically reboot itself. Instead, it remembers its pre-sleep state and can continue executing its application script from the point it left off when it went to sleep. In summary, a SNAP based wireless network can be used to implement a "sleepy mesh" in which all of the nodes go to sleep and wake up at the same time. Depending on the target application, the result can be to extend the battery life of each node from a little over two days to more than a year! 2008 Synapse, All Rights Reserved. All Synapse products are patent pending. 7