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Guide to low-power wireless network specs

Posted: 01 Jul 2008 ?? ?Print Version ?Bookmark and Share

Keywords:low-power wireless networks? Wi-Fi wireless Internet? Bluetooth? sensor networks? IEEE 802.11a/b/g/n/?

Wireless products and technology for sensing and control applications are quickly becoming a reality. But for a global technology market to take off, history has demonstrated that standards are essential. This was true for both Wi-Fi wireless Internet (IEEE 802.11a/b/g/n/) and Bluetooth (IEEE 802.15.1). It will also be true for sensor networks (IEEE 802.15.4a/b).

All three technologies target different applications.

Wi-Fi offers an alternative to wired Ethernet PC communication: high data rate networks with a base station at the center and PCs nearby (star-network topology). To achieve high data rates in a local area, Wi-Fi consumes a fair amount of power usually from a laptop battery.

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

The Bluetooth data rate of 1Mbit/s is high enough to carry voice but is at least one order of magnitude smaller than that of Wi-Fi. Thus, power consumption is lower, often 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, laptop and GPS device.

Powering sensors
Sensor applications have 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 standard is IEEE 802.15.4. The first version was ratified in 2003, with an update in 2006. 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. Today, it is widely accepted that IEEE 802.15.4 offers the best solution for wireless sensor applications. Yet 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 needed volume to actually reduce cost. Moreover, reducing complexity generally goes along with sacrificing performance, thus limiting the range of applications for these solutions.

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

In a building automation application, a person moving around or standing between two nodes can impact link quality. Thus, the network stack must consider 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 establish 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 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, several standard network stacks are emerging, with others underway, all built atop the IEEE 802.15.4 foundation, as shown the figure.

Zigbee stack
An independent standards organization driven by a large group of technology providers and OEMs, the Zigbee Alliance finalized in 2007 the specification for two network stacks: the Zigbee network stack and the Zigbee PRO network stack.

From a usage standpoint, the Zigbee stack is tailored for home networks, which typically contain from 10 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 fit for large applications such as commercial building spaces. For now, this functionality requires a larger program memory size that increases cost and limits the applicability of Zigbee PRO for many consumer markets. But with decreasing silicon cost, the cost difference between Zigbee and Zigbee PRO will soon be negligible and more applications will adopt Zigbee PRO.

Extra features
The Zigbee Alliance does not explicitly rule out industrial applications. However, some large industrial automation companies have identified the need for extra features that are not on Zigbee's top priority list, the top two of which are deterministic latency and deterministic reliability.

Latency is the time a message takes to travel from source to destination. If the source is a programmable logic controller 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 use an 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.

Several standard network stacks are emerging, with others underway, all built atop the IEEE 802.15.4 foundation.

Deterministic reliability is 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. For 802.15.4 devices operating 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 cause some wireless packets to get lost, regardless of the network stack operating atop it. To mitigate this impact of these packet losses, wireless standards for industrial applications provide a mechanism that allows packet losses to be evenly spread over time, making transmissions more predictable and reliable.

Wireless automation
ISA-100 and Wireless HART are the two driving industrial wireless automation standards. ISA-100 is expected to deliver a standard specification around 2008-2009.

Not a full industrial sensor protocol, Wireless HART is an add-on to the HART industrial wired bus standard for industrial automation. Wireless HART provides an alternative to the wired message transmission protocol of HART.

As ISA-100 and Wireless HART solve the same problems, both are being examined if they can be merged into one. The first version will most likely not be interoperable and will require a network bridge, a translator between the two systems. A follow up version might define a common language.

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

- Niek Van Dierdonck
VP for Strategy and Product Management
Greenpeak Technologies





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