Protocol Sychronizes Wireless Networks

One of the hottest topics in process control right now concerns the type of wireless sensor network (WSN) that will meet the most requirements of process control engineers.

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One candidate is Time Synchronized Mesh Protocol (TSMP), from Dust Networks Inc., a Hayward, Calif., wireless networking supplier. TSMP is one of several protocols under consideration by companies and committees, including the the Instrumentation, Systems and Automation Society’s ISA SP100 Wireless Standards for Automation committee, and the Hart Communications Foundation’s Wireless Working Group.

Following is a primer on the technology, with technical information provided by Dust Networks.

TSMP provides redundancy and fail-over in time, frequency and space to ensure high reliability in challenging radio environments. TSMP also provides the intelligence required for self-organizing, self-healing mesh routing. Dust says the result is a network that installs easily with no specialized wireless expertise, automatically adapts to unforeseen challenges, and can be extended as needed without sophisticated planning.

Wireless challenges

While wireless provides advantages in cost and flexibility, it also brings along a host of challenges. Specifically, point-to-point radio communication links are notoriously variable and unpredictable. A link that is strong today may be weak tomorrow due to environmental conditions, new obstacles, unanticipated interferers and myriad other factors. Because there is no way to predict what interferers will be present in a facility at a given location, frequency and time, a reliable network must be able to continually sidestep these interferers on an ongoing basis.

TSMP provides a mechanism for WSN intelligence. By defining how a wireless node utilizes radio spectra, joins a network, establishes redundancy and communicates with neighbors, TSMP forms a solid foundation for WSN applications, its backers say.

TSMP is a media access and networking protocol that is designed specifically for low-power, low-bandwidth reliable networking. It is a packet-based protocol in which each transmission contains a single packet and acknowledgements (ACKs) that are generated when a packet has been received unaltered and complete. Mechanisms are in place to transport packets across a multi-hop network as efficiently and reliably as possible.

All node-to-node communication in a TSMP network is transacted in a specific time window. Commonly referred to as Time Division Multiple Access (TDMA), synchronized communication is a proven technique that provides reliable and efficient transport of wireless data. A critical component of any TDMA system is time synchronization. All nodes must share a common sense of time so that they know precisely when to talk, listen or sleep. This is especially critical in power-constrained applications such as WSNs in which battery power is often the only option, and changing batteries can be costly and cumbersome.

A common sense of time enables many network virtues: bandwidth can be pre-allocated to ensure extremely reliable transmission and zero self-interference; transmitting nodes can effectively change frequencies on each transmission, and the receiving node can keep in lock-step; and bandwidth can be added and removed at will in a very predictable and methodical way to accommodate traffic spikes, among other advantages.

In addition to slicing the wireless media across time, Dust points out, TSMP also slices it across frequency. This provides robust fault tolerance in the face of common RF interferers, as well as providing a tremendous increase in effective bandwidth. Commonly referred to as Frequency Hopping Spread Spectrum (FHSS), hopping across multiple frequencies is a proven way to sidestep interference and overcome RF challenges with agility rather than brute force.

All TSMP messages are encrypted and include a network identification (ID). The network ID is used to bind nodes together into a network, allowing multiple TSMP networks to operate in the same radio space without the risk of sharing data or misrouting messages. If a mote hears a node with a network ID that does not match its own, then it will not initiate joining but will continue unsynchronized listening until it hears the right ID.

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