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© farang-dreamstime.com Application Notes | September 30, 2013

Secure & Reliable Wireless Sensor Networks for Defense Applications

Sensors provide the real world interface for a wide range of military systems from surveillance and weapons control through to remotely controlled vehicles while behind the scenes large deployments of hundreds or even thousands of sensors monitor military hardware or facilities.
Connecting these large sensor networks securely and reliably has traditionally involved hard-wired installations, but now wireless technology may offer a more flexible and lower cost solution.

Wireless Transmission Challenges

For the purposes of this article, we will narrow the focus to consider the application of “onboard” sensor networks used for health monitoring in aircraft or ships. These networks can be generally characterized as low data rate where the sensors occupy fixed and perhaps inaccessible positions measuring physical conditions such as vibration, temperature, pressure, strain and acceleration.

Existing wired sensor solutions provide some sensor coverage, however both the high installation costs of wires and the added weight associated with cabling conspire to limit sensor usage to only a small fraction of the desired coverage. For a wireless solution to be viable, it must meet the stringent requirements of such a challenging environment.

A typical sensor network will ideally have a 10-year life in an equipment which itself might be refitted or upgraded several times over 30 years or more. Sensors and radio links should be maintenance free and ideally very low power with battery and energy harvesting technology to enable a cable free installations.

Security and safety are naturally very important; if a wireless sensor network is to replace a wired network it must have secure message transfer and be protected against external attack, both from hacking and spoofing.

Reliability of the message transfer is also critical; typical onboard military environments present some significant challenges to wireless transmission with intricate metal compartments and confined spaces. Point-to-point radio communications are notoriously unpredictable and will be impacted by RF interference, multi-path fading, blocked paths and in exceptional cases complete node failure. Networks with nodes organized in a mesh arrangement can address these problems if they use multiple frequency channels to sidestep interferers and can identify blocked paths and missing nodes and reconfigure routing paths automatically.

If these requirements can be met with a wireless sensor network, the benefits will be substantial. The savings in installation costs alone often justifies the usage of wireless, since costs of running wire can exceed 10x the cost of the sensor itself. Furthermore, with increased sensor coverage, machine health monitoring can substantially reduce system down-time, improve mechanical efficiency, and enable predictive maintenance.

Wireless Technology Choices

There are several technology choices to consider; satellite and cellular technologies can be suitable for outdoor applications with large distances between sensors but are not appropriate in the context of a closed onboard system. They also have the highest energy cost per packet and may require data to pass through third party network operators which could compound the security concerns.

Another choice is WiFi (IEEE 802.11b, g), which offers a much lower energy cost per data packet than satellite or cellular but generally requires a higher density of access points for reliable communications from fixed sensors when compared to traditional mobile data usage. This type of solution can be appropriate where a smaller number of higher power nodes are a better trade-off than a large number of low power nodes.

There are also a number of solutions based on the LR-WPAN (Low-Rate Wireless Personal Area Networking) standard IEEE 802.15.4 including the Linear Technology Dust Networks® product line. The 802.15.4 standard is well suited to low power, short-range sensor network operation with relatively low data rate (up to 250kbps) and short packet length.
Various competing IEEE 802.15.4 solutions utilize a network of interconnected nodes in such a way that packets of data can be routed through different paths to increase the reliability of transmissions.

Dust Networks products have improved on the traditional mesh arrangement and pioneered the use of a Time Synchronized, Channel Hopping (TSCH) network protocol, which has become the foundation of prominent wireless mesh standards, such as IEC62591 (WirelessHART) and IEEE 802.15.4E. ZigBee Pro offers an alternative but cannot support a full mesh implementation at the end nodes and the use of CSMA (Carrier Sense, Multiple Access) inherently leads to packet collisions as messages compete in the same time domain.

As networks become larger, this becomes an increasing problem and is wasteful of energy as nodes must re-try transmission after a random delay period. ZigBee Pro is not optimized for ultralow power at all nodes, and as we will see later, alternative solutions called Snap and Digi-Mesh (variations of ZigBee Pro) attempt to overcome this, but cannot match the performance or security of SmartMesh IP solutions.

The Dust Networks Solution

A SmartMesh network consists of a self-forming multi-hop, mesh of nodes, known as motes, which collect and relay data, and a network manager that monitors and manages network performance and security, and exchanges data with a host application.

SmartMesh motes and managers are complete embedded wireless sensor network solutions. They combine hardware based on Dust Networks Eterna® System-on-Chip technology with a time-synchronized, channel hopping link layer and complete networking software to deliver >99.999% data reliability in the most challenging RF environments and >10 year battery life for every node in the network, including routing nodes.


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Low Power

Ideally a wireless sensor should not require an external power feed, thus enabling the notion of a “peel and stick” kind of installation. SmartMesh IP uses centrally managed predefined timeslots of 7.5ms to synchronize data packet communications between nodes. Timeslots are assigned according to the bandwidth requirements of the application, but a >1% duty cycle is typical. Nodes wake up only when scheduled and therefore deliver ultra low power consumption. Routing nodes typically consume <50uA average current, enabling 5 to 10 years operation on two lithium AA batteries.

ZigBee Pro power consumption is higher because the routing nodes are continuously powered when in receive mode, thereby requiring line power levels. Snap and Digi-Mesh offer low power routers but rely on the “beaconing” feature within IEEE 802.15.4 where the entire network sleeps and wakes, leading to severe bandwidth restrictions.

Security

All packets in a SmartMesh network are authenticated on each layer, and encrypted end-to-end. On the Link-layer, packets are authenticated at each hop using a run-time key and a time-based counter. Furthermore, packets are authenticated and encrypted end-to-end using run-time session keys and a shared counter. Together, these layers of authentication guard against replay attacks and man-in-the-middle attacks.

The packet payload is protected from eavesdropping with a session-based, 128-bit AES symmetric key encryption. New nodes use a special join key initially and then multiple keys are assigned to the new node using a random number generator. With these multiple encryption keys, compromise of one node does not compromise the security of the other nodes of the network. SmartMesh security is considerably more robust than Snap and Digi-Mesh solutions which both use only a single encryption key for the entire network.

Reliability

Dust Networks SmartMesh IP™ protocol typically delivers >99.999% data reliability by combining the time slot synchronized message transmission with frequency diversity using 16-channel FHSS (Frequency Hopping Spread Spectrum). Each pair of nodes utilizes a pseudo-random sequence of the 16 channels where channel usage is allocated by the network manager so as not to interfere with adjacent nodes.

FHSS effectively increases the bandwidth by 16 times but also maximizes the chance of message transfer even when the majority of the band is blocked with RF interference. In contrast, ZigBee Pro operates the entire network on a single frequency channel at any single point in time making it susceptible to multi-path fading.

Conclusions

The Dust Networks SmartMesh IP product family builds on the IEEE 802.15.4 LR-WPAN standard with a fully implemented self-healing, self-optimizing time synchronized mesh network with ultralow power consumption.
By combining time-synchronized slots with frequency hopping and strong encryption it is possible to implement a secure wireless network with >99.999% reliable message transfer, making it a realistic alternative to wired sensor networks.

SmartMesh IP products could form the basis of a technology platform for wireless sensors networks in the next generation military equipment infrastructure and offer large potential cost savings, reduced build time and improved operational flexibility.
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Author: Steve Munns, Military & Aerospace Marketing Manager at © Linear Technology Corp.

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