POWER ELECTRONICS FEATURE


group, and have been proven in some of the toughest environments around. TSCH is already a foundational building block of existing industrial wireless standards, such as WirelessHART (IEC62591), and is an enabling piece of emerging Internet Protocol-based wireless sensor networks standards. In a TSCH network, each node has a


common sense of time that is accurate across the network to within a few tens of microseconds. Network communication is organised into time slots to enable low-power packet exchange, pair-wise channel hopping and full path diversity.


Low-power packet exchange - The use of TSCH allows motes to sleep at ultralow power between scheduled communications. Each device is only active if it is sending a packet or listening for a potential packet from a neighbouring device. Even more importantly, since each node knows when it is scheduled to wake up, each node is always available to relay information from its neighbours. Therefore, TSCH networks often reach duty cycles of <1% while keeping the network completely available. Furthermore, since each packet transaction is scheduled, there are no in-network packet collisions in a TSCH network. Networks may be dense and scale without creating debilitating RF self-interference.


Pair-wise channel hopping - Time synchronisation enables channel hopping on every transmitter-receiver pair for frequency diversity. With a TSCH network, every packet exchange channel hops to avoid inevitable RF interference and fading. In addition, multiple transmissions between different device pairs can occur simultaneously on different channels, increasing network bandwidth.


Full path and frequency diversity - Each device has redundant paths to overcome communications interruptions due to interference, physical obstruction or multipath fading. If a packet transmission fails on one path, a mote will automatically retry on the next available path and a different RF channel. Unlike other mesh technologies, a TSCH network does not require powered routers and time-consuming path rediscovery. TSCH-based networks are successfully


deployed today in such applications as smart-parking applications , computer data centres to monitor energy efficiency and in industrial plants. Many applications, such as pipeline monitoring, structural monitoring of bridges and tunnels, as well as power transmission line monitoring require the WSN to span long distances. And yet, the ability to


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101) are within range of the three IDs lower and higher. For example, device 50 is within range of devices 47, 48, 49, 51, 52 and 53. In this topology, the minimum number of transmissions (hops) required to reach device 101 is 32, although in practice most packets take more hops. At the time this article was written, this


establish and successfully maintain a low-power, reliable wireless network over such distances represents one of the more challenging network topologies. By definition, a deep-hop network


means that the messages from nodes furthest away need to traverse many hops to reach their destination. While this enables a single network to cover a large geographic area with relatively low-power transceivers, it sometimes raises the concern of whether a long network can successfully sustain regular data traffic from all its motes, and do so with acceptable latency and current consumption.


CASE STUDY – A DEEP HOP MESH NETWORK To characterise such a network, a 100- mote, 32-hop deep network was constructed and measured using Dust Network’s SmartMesh IP network. Each of the 100 motes generated and sent a data packet every 30 seconds with the expectation that each packet is received within 30 seconds latency (i.e. before the next packet from the same node is generated).


The deep network is constructed from real wireless devices in which seven devices (given IDs 1 through 7) communicate directly with the manager. Devices 8 through to 10 communicate through the first seven motes, and the remaining devices (devices 11 through to


Figure 4:


Average mote current – even the most heavily loaded routers in this deep network consume only a few hundred microamps


Figure 3:


Packet latency - packets in a deep network are delivered reliably within the targeted 30 second latency


network has been running continuously for 52 days. In total, 17 million data packets have been collected, requiring a sum of over 400 million individual transmissions, due to the hop depth and retries. Of the 17 million packets sent, none were lost, yielding a data reliability of 100%. About 25 thousand of these packets are “health reports” – diagnostic information sent periodically by the nodes.


ANALYSING LATENCY & CURRENT CONSUMPTION Each packet is timestamped when it is generated at the sensor mote and again when received at the manager, so the latency of each packet can be monitored. Over a period of 90 minutes in this network, the data distribution is plotted in Figure 3. As expected, motes with the higher IDs, which are deeper in the network, have longer latency and more variation per packet, as the route options increase exponentially with depth. Despite this, data packets from the furthest mote (ID 101) all arrived at its destination in less than the targeted 30 second latency. All motes keep an internal count of the battery charge consumed and report this information in periodic reports to the manager. From this information, the average current throughout the network can be plotted as in Figure 4. The motes with low ID numbers show the highest current consumption, because they carry the traffic from motes further away. But as can be seen, even the most heavily loaded routers in this 32-hop deep network had average current consumptions of a few hundred microamps. At such low current consumption, the routing nodes can be powered with a pair of lithium D-cell batteries yet last over 15 years. SmartMesh IP networks, based on Time Synchronised Channel Hopping, routinely deliver >99.999% data reliability and very low power consumption in challenging applications. With 10-15 years of operation on reasonably small lithium batteries, wireless sensors can be pragmatically placed anywhere, enabling real city-scale IoT applications.


Analog Devices Ltd. www.linear.com www.analog.com T: 01628 477 066


ELECTRONICS | FEBRUARY 2018 17


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