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Understanding low-power wireless network standards

Posted: 04 Mar 2008 ?? ?Print Version ?Bookmark and Share

Keywords:low-power wireless network? wireless communications?

By Niek Van Dierdonck
Greenpeak

Wireless products and technology for sensing & control applications are quickly becoming a reality. Numerous analysts, technology providers and product integrators agree that widespread adoption of wireless technology is only a matter of time.

Still, standardization organizations and technology providers have not done a very good job addressing the bewildering assortment of competing solutions and technologies. If anything, many have contributed to the general frustration by being vague about their application scope.

End users and system developers need standardization for a variety of reasons: compliance with global regulations, interoperability across brands, second sourcing availability, competition to drive prices down, and the opportunity to tap into a large body of knowledge. But there is more.

Some technology components are so expensive to develop that they can only generate an economic return through very high volumes. And when volumes need to be large, the presence of a global market is paramount. Standards are an excellent vehicle to generate global awareness and prepare for a global market for ramp up.

Wireless sensor architecture
The basic architecture of a wireless sensor system consists of three layers, as depicted in Figure 1.

Figure 1

The wireless transceiver, at the bottom of the stack, translates digital information into a wireless electromagnetic signal that can be broadcast by the transmitter and reconstructed at the receiver end.

In previous generations of wireless technology, you either had a transmitter for transmission only, or a receiver that was only capable of reception. Nowadays, to improve reliability and performance, technology has shifted to combine receive and transmit devices.

Wi-Fi vs Bluetooth vs 802.15.4
Chip manufacturers need high volume sales to generate meaningful return, and high volumes require global markets. For a global technology market to take off, history has demonstrated that standards are essential.

This was true for both Wi-Fi wireless Internet, technically termed IEEE 802.11a/b/g/n/, and Bluetooth, which is based on a standard defined in the IEEE 802.15.1 specification. It will also be true for sensor networks, which are governed by the IEEE 802.15.4a/b standard (Working Group for Wireless Personal Area) set up in 2003.

All three of these technologies target different applications. Wi-Fi was conceived as an alternative to wired Ethernet PC communication: high data rate networks with a base station at the center and PCs nearby (i.e. a star-network topology). To achieve high data rates in a local area, Wi-Fi consumes a fair amount of power, usually sourced from a laptop battery.

Data rates degrade quickly as distance to the base station increases. Bluetooth was conceived with the mobile phone as the center of the universe: it connects the phone to an earpiece, to a GPS device and to a laptop.

The Bluetooth data rate of 1Mbps is high enough to carry voice, but is at least one order of magnitude smaller than that of Wi-Fi. In return, the power consumption is lower, most often sourced from a mobile phone battery.

In general, the communication range is also smaller than that of Wi-Fi, which reflects the fact that the phone is usually in the vicinity of the earpiece, the laptop and the GPS device.

Sensor applications have totally different requirements, particularly with regard to power consumption: sensors often have to work for years on a coin cell battery, or on energy harvested from the environment through a solar panel or vibration harvester. The battery cannot be recharged like a laptop or a phone battery.

Other sensor-specific requirements are determined by factors such as reliability, communication range, the large number of nodes that may need to be supported in a single network, and the need for automatic network organization. In return, a lower data rate is generally acceptable, as most sensors generate fairly small amounts of data, and generally not on a continuous basis.

For wireless sensor transceivers, the dominant and probably only real standard is the IEEE 802.15.4 specification. The first version was ratified in 2003, with an update in 2006. Several vendors offer transceiver chips.

Some of them are a minimal implementation of the standard. Others offer add-ons which are useful in some application segments. GreenPeak's own GP-2000 transceiver, for example, has many power reducing features optimized for coin-cell and battery-less applications.

Table 1 lists the main parameters of the IEEE 802.15.4 standard and compares them to Bluetooth.

There have been efforts to use Bluetooth and Wi-Fi for sensor applications. In both cases, Bluetooth and WI-FI were used in a non-standard way, weaving the principles of IEEE 802.15.4 into their native implementation. Nowadays, it is widely accepted that IEEE 802.15.4 offers the best solution for wireless sensor applications.

Not all technology suppliers adhere to the IEEE 802.15.4 standard. Some have chosen to build proprietary transceivers, with the goal of reducing complexity and cost. It remains to be seen if these proprietary solutions will achieve the volume needed to actually reduce cost. Moreover, reducing complexity generally goes hand in hand with sacrificing performance, thereby and limiting the range of applications for these solutions.

The network stack
The network stack has two responsibilities. First, it forms and maintains the network. Wireless network stacks, in particular, must be able to cope with the constantly varying quality of the wireless links between nodes.

For example in a building automation application, people moving around (i.e., a person standing between two nodes) can have a formidable effect on link quality. Thus, the network stack must take into account that links can disappear at any moment, possibly isolating a network node or even a whole branch of the network.

In response to interference, the network stack must be able to reroute communication paths and establishing new links that provide uninterrupted connectivity to all parts of the network.

The network's second responsibility is to ensure that messages can travel from source to destination nodes in a reliable and efficient way. Efficiency here means that latency requirementsthe travel time of a messageshould be met and that bottlenecks in the routing of messages should be avoided.

The wireless sensor application space is broad, with widely varying requirements that call for flexibility in the communication technology. Hardware alone cannot provide this flexibility. It requires a programmable stack that reduces up-front investment cost and enables suppliers to make a healthy return with lower volumes. Today, a number of standard network stacks are emerging, with others underway, all built atop the IEEE 802.15.4 foundation.

Figure 2: A view on the most prominent sensor network stack standards.

The impact of the ZigBee Alliance
The ZigBee Alliance is an independent standards organization driven by a large group of technology providers and OEMs. The group's most recent milestone, achieved at the end of 2007, was to finalize the specification for two network stacks: the ZigBee network stack and the ZigBee PRO network stack.

From a usage standpoint, the ZigBee stack is well suited to residential "home" networks, which typically contain from ten to a few hundred devices. ZigBee PRO, a superset of ZigBee, adds functionality that makes it possible to scale the network and better cope with wireless interference from other technologies.

These features make ZigBee PRO well suited to large applications such as commercial building spaces. For now, this functionality requires a larger a larger program memory size that increases cost and limits the applicability of ZigBee PRO for many consumer markets.

However, thanks to the ever decreasing cost of silicon, we predict that the cost difference between ZigBee and ZigBee PRO will soon become negligible and that most applications will adopt ZigBee PRO.

The ZigBee Alliance does not explicitly rule out industrial applications. However, a number of large industrial automation companies have identified the need for extra features that are not on ZigBee's top priority list. The two most important "industrial" features are deterministic latency and deterministic reliability.

Latency is the time a message takes to travel from source to destination. If the source is a PLC and the destination is a machine, it is essential to have tight control over latency.

That is why the standards that explicitly target industrial automation exploit an IEEE 802.15.4 feature called Guaranteed Time Slots, which makes it possible to ensure a worst-case message latency. Presently, ZigBee does not exploit Guaranteed Time Slots. Deterministic reliability refers to the ability to provide a guaranteed communication path between two wireless devices.

The chief enemy of reliability is wireless interference from other users of the same wireless frequency band. In the case of IEEE 802.15.4 devices, which operate in the 2.4GHz frequency band, the most notable interferers are WI-FI transceivers. Most interferers do not fully block out IEEE 802.15.4 devices.

But they do cause some wireless packets to get lost, regardless of the network stack operating atop it. To mitigate the impact of these packet losses, the wireless standards used for industrial applications provide a mechanism that allows packet losses to be evenly spread over time, thereby making transmissions more predictable and reliability.

Automation standards
ISA-100 and Wireless HART are the two driving industrial wireless automation standards. ISA-100 is the brainchild of the Instrumentation, Systems and Automation Society (ISA), a non-profit technical society focusing on industrial automation. ISA-100 is expected to deliver a standard specification in the course of 2008-2009.

Wireless HART is not a full industrial sensor protocol, but an add-on to the old but very popular HART industrial (wired) bus standard for industrial automation. In essence, Wireless HART provides an alternative to the wired message transmission protocol of HART.

As ISA-100 and Wireless HART fundamentally solve the same problems, they have recently joined hands in an effort to examine whether both standards can be merged into one. The first version will most likely not be interoperable and require a network bridge, a translator between the two systems. A follow up version might define a common language.

The enhancements offered by industrial standards can also be advantageous in commercial building automation, but are generally not essential. Meanwhile, they add substantial cost that limits their feasibility for many residential and commercial applications.

Table 2 lists some of the features of the commercial and industrial standards discussed.

Proprietary wireless technology
In addition to the standard wireless sensor technologies, there are proprietary technologies tied to individual companies. Proprietary does not necessarily mean that the specification is closed.

It means that a single company controls the direction of the technology, effectively leading to a monopoly. Proprietary standards are often created to address a single application or a limited set of applications.

In practice, proprietary technologies can be developed much faster than standard technologies because there is no need to reach consensus among different companies. Proprietary technologies may also be superior to standard technologies for a limited application set.

The two most notable proprietary wireless sensor technologies are Zensys' Z-Wave and Cornis' Wavenis. Z-Wave targets residential automation, as exemplified by its support for a maximum of 237 nodes.

This number is sufficient for homes but not suitable for larger commercial installations such as hotels and office buildings. Wavenis, meanwhile, has achieved traction in Automatic Meter Reading applications, though it is also marketed for other applications.

Advances
Even within the boundaries of standards, there are many opportunities for technology differentiation. GreenPeak, for example, offers IEEE 802.15.4-compliant transceivers and stacks that provide additional functionality for ultra-low-power applications.

This technology enables wireless systems to live off of a coin cell battery, or even energy harvested from the environment through a solar cell, vibration energy harvester or other environmental energy converter.

GreenPeak has also developed low power routing (LPR) technology that is likely to make its way into standards in the near future. In an LPR network, battery-powered devices can receive messages from nearby devices and forward them down a longer communication chain.

Today's standards are only able to offer this functionality for mains-powered devices. This is because they require devices to be in a continuous listening state, which consumes a significant amount of power.

LPR, by contrast, eliminates this always-on power consumption by adding a time synchronization mechanism that allows devices to wake up and initiate communications simultaneously.

About the author
Niek Van Dierdonck
is the VP for strategy and product management at Greenpeak. A pioneer in wireless sensor communication, he founded Ubiwave in 2003 where he remained as CEO until 2007 when Ubiwave was acquired by Xanadu Wireless and renamed to GreenPeak.




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