December 2004
  
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ZigBee or Not ZigBee
In order to standardize the implementation of digital radio telemetry across many markets, including commercial HVAC controls, the ZigBee organization has formed.

  Guy Zebrick, Kele Inc

Guy Zebrick
Account Manager
Kele, Inc

The building automation market has seen the emergence of a new class of affordable, reliable, commercial-grade, RF (Radio Frequency) enabled devices. These wireless devices can reduce traditional electrical contracting costs associated with difficult installations of HVAC I/O (Heating, Ventilating, and Air Conditioning Inputs and Outputs), and by using digital technology, these new systems eliminate many of the headaches and uncertainties associated with older radios. In order to standardize the implementation of digital radio telemetry across many markets, including commercial HVAC controls, the ZigBee organization has formed. Will ZigBee standardization fulfill every promise offered by the latest in radio technology?

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Belimo

Digital this and digital that

Prior to the emergence of digital radio, analog narrow-band FM technology was frequently used in commercial radio applications. Narrow-band FM is prone to interference and drift, and is also power-hungry. High-tech digital radios were primarily found in government and military applications, where interference-free, jam-proof requirements justified the once higher cost of this computerized radio technology. New digital telemetry systems are now available to the building automation market at reduced costs. This has become possible through microelectronic manufacturing techniques that combine powerful computers and radios in a single, low-cost, integrated circuit chip.

In order to truly be wireless, a device must have an on-board power source. Battery life has always been a limiting factor for applying wireless in commercial applications. One common wireless feature, made easier with the integration of processor and radio, is the ability of the on-board computer to dramatically extend battery life by shifting the electronics down to an extreme low-power mode between radio transmissions. This works for simple devices such as space temperature sensors and radio transmitters, but still negates the practical use of battery power for current-hungry HVAC applications such as pressure, flow, CO and CO2 sensing, signal repeating, or applications where frequent data updates are required. All battery-powered transmitters are designed with a compromise between battery life, transmission interval, duration, and power, regardless of technology used or frequency operated. Digital radio technology provides an excellent way to stretch battery performance.

Standard network protocols

A common communication protocol is required for each device on a network in order for the devices to successfully communicate. For example, a defined protocol exists for Internet communication between hosts – TCP/IP (Transmission Control Protocol / Internet Protocol). TCP/IP allows different computers, from different manufacturers, to interconnect and communicate on the same pipe without interference. Note that TCP/IP does not guarantee that useable data will be able to flow between computers, only that different computers can recognize and cooperate with each other on the network without interference. It is up to the developers of the applications to provide solutions that can actually work together. In addition, even though TCP/IP is commonly associated with the Internet, the actual physical network used for sending TCP/IP packets could be a local Ethernet network, a telephone modem connection, or even Wi-Fi wireless. The physical medium is not specific to the protocol.

There has not been a single recognized standard for formatting the flow of data between low power RF-enabled I/O devices. Manufacturers have had to develop their own protocols and radio technology for RF communication between transmitter and receiver. In addition, there are various ways of transmitting data, as well as a variety of radio modulation techniques. Today, the assortment of available commercial-grade products operate on different radio frequency bands and use different protocols, with some still using old-fashioned narrow-band FM radios.

Enter ZigBee

The ZigBee alliance, an association of companies working on an agreed-upon standard for the transmission of low-power digital data, has formed with the intent of bringing to market state-of-the-art, RF-enabled devices that all share a common protocol and radio technology. ZigBee radios may one day be embedded in consumer electronics, automotive, home and building automation, industrial controls, PC peripherals, medical applications, and even electronic toys and games.

Automated Logic Corporation ZigBee is targeted for low-cost, low-data rates (20 - 250 kbps) and low-power telemetry applications. ZigBee-compliant radios may operate on one of three different radio bands: the 800 MHz, 900 MHz, or 2.4 GHz frequencies. The protocol includes very advanced security encryption options, a robust implementation of digital DSSS (Direct Sequence Spread Spectrum) technology for reduced interference and improved sensitivity, plus support for multiple MESH network operating modes optimized for speed, reliability, or efficiency. ZigBee follows the internationally recognized radio standard IEEE 802.15.4 WPAN standard for the Media Access Control (MAC) and Physical Layer (PHY).

In simpler terms, many manufacturers have agreed to standardize a common low-power and short-range, but very high-tech radio technology, all operating on the same groups of frequencies, and all using the same language, or format, for the data packets. The radio is already well defined as IEEE 802.15.4. ZigBee adds the application and network protocols to 802.15.4 to ensure interoperability between vendors.

But just what is a MESH network? And most importantly, what exactly does “interoperability between vendors” mean?

A fine MESH

A MESH network is analogous to a fish net with each intersecting crossover in the net capable of functioning as an intelligent repeater or router. A signal transmits, starting at one point on the net, and is received at the nearby intersections. The point located at the intersection functioning as a repeater/router decodes the signal, analyzes the routing requirements, and if needed, transmits the packet to the next repeater/router sending it from one to another until it arrives at its destination. The routing is dynamic and intelligent. Not every point in the network will act as a router/repeater for every message, only those that are required to get the data to its destination.

With ZigBee there are three types of devices, or “nodes,” on the MESH network. A simple source-only node is designated as a RFD (Reduced Function Device). RFDs only start or receive a signal, and do not function as a repeater/router. A node that includes the ability to repeat and route is a FFD (Full Function Device). If an FFD fails in routing a message or goes off-line, another nearby FFD node can quickly pick up the transmission routing duties to send the data to its final destination. In this way, a ZigBee MESH network may be self-configuring and self-healing. ZigBee also includes full hand-shaking capability between nodes to assure that a signal accurately and successfully arrives at the proper designation. Finally, there is support for a third type of ZigBee node, the network master or gateway node, for interfacing a ZigBee network into an external system or to coordinate MESH routing when required.

A MESH Network

ZigBee allows for two distinctly different implementations of MESHing. One method provides for an autonomous self-configuring MESH network where devices all cooperate automatically. They route data dynamically based on origin and destination address and easily accommodate new devices onto the network. The second method is the Beacon mode, where a network master node is required. The network master is programmed to assign specific time slots and routing configurations to all nodes. The Beacon mode helps assure that time-critical data is processed as a priority in a network, and may be able to route traffic more efficiently through specific nodes. A ZigBee light switch networked to a ZigBee ballast is an example where time-critical network processing is required. A delay of even a second or two between turning on a switch and activating a light would not be tolerated.

MESH routing, though an integrated part of the protocol may not always be needed. In fact, a ZigBee network could consist of only a source and a destination node operating peer-to-peer without any intermediate FFD routing nodes or network coordinator, though the ZigBee vision often includes a large multitude of devices all MESHed together.

In a multiple node MESH configuration, FFD nodes may function as both I/O points and intelligent MESH routers. Each node is able to process its own I/O point, as well as monitor the network communication in order to act as a repeater/router when needed. In such a network, the indoor operating distance of each single ZigBee node might, by design, be a relatively short range (1 to 10 meters). If devices throughout the MESH network are spaced close enough, nearby FFDs could help the data packet hop from one node to another, until arriving at the designated destination node. The advantage is that the source node could be designed to operate using very low power and still be able to propagate a signal throughout a facility.

In theory, in an all-ZigBee MESH network, a transmission from one manufacturer’s RFD could be received by a FFD node of another manufacturer and routed to a device of yet another manufacturer. ZigBee devices could all share the same RF pipe, similar to TCP/IP devices sharing the same internet. They do not interfere with each other, and in this case, RF network traffic is automatically negotiated between devices so they are able to help each other route the data packets. Imagine a ZigBee thermostat sending packets throughout a facility by hopping onto the ZigBee fluorescent ballast network.

Semi-wireless wireless

KMC Controls Unfortunately, in most MESH applications, it is not practical for a battery-powered RFD node to function as a MESH FFD router/repeater in a large system, since MESH FFDs must be on at all times, receiving and processing all network traffic and functioning as a router when required. Even though there is an optional Beacon mode that uses a network master to coordinate and assign time slots for all ZigBee devices on a network, theoretically allowing even a FFD to nap, it seems unlikely to be practical for most HVAC applications. With no down time for a FFD MESH router/repeater, each device will require a connection to a constant power source. FFD MESH nodes are, in effect, semi-wireless, since they still require local power wiring. The added installation costs required to provide local electrical power to any on-all-the-time FFDs must be taken into account when evaluating any MESH network system.

The very short range of low power RFD transmitters must also be considered. The range could be a problem if an application does not easily allow for the placement of nearby FFDs for routing/repeating. A large lobby, glass atrium, or open church congregational area requiring wireless remote temperature sensors that can send data 100 meters or more is a typical HVAC application. A short-range MESH device, which can only transmit from 1 to 10 meters, may not be appropriate.

All MESHed up

ZigBee is defined to operate on any one of three available radio bands. The 800 MHz band is used mainly outside of the United States; the 900 MHz band is used mainly in the United States, while the 2.4 GHz band is in use worldwide. The 2.4 GHz microwave band, though more universal and potentially higher in speed, offers less ability to penetrate commercial buildings where microwave signals are easily absorbed by various building materials. This band is also shared with Wi-Fi applications, increasing the potential for reduced signal throughput due to radio crowding. Before interoperability between ZigBee radios can even be considered, all ZigBee devices in a network must at least be operating on the same radio band.

Even if a common band is selected, in order for multiple ZigBee devices to help each other route signals, they must all be able to use a compatible network configuration. The general mode operates like a bus line, picking up packets at all destinations and delivering them anywhere, any time. The network-master-coordinated Beacon mode operates much like an express bus route with limited operating times and stops. Individual nodes are assigned a specific transmission time and routing slot. Situations could occur where it might not even be possible for a device operating in the general ZigBee mode to jump onto the express Beacon line!

Squeeze onto a crowded bus

The ZigBee alliance states that over 65,000 nodes may be part of a single ZigBee network, however this must be qualified. The total number of nodes actually possible in a typical HVAC application may be much less than that. The maximum number of packets that can be processed within any time period is limited, based upon the total size of each data packet, the baud-rate of the transmission, the processing time for each packet, the network routing and communication overhead, and the total number of transmissions being generated on a network within a given time period. For example, if each transmission takes 30 ms to complete, and there is a 100 ms delay for node-node processing, routing, and traffic management, then the maximum total number of transmitters, sending data once per minute, would be limited to only 461 nodes (60 seconds / .130 seconds). This assumes a duration time of 130 ms for complete processing of each node’s transmission. In practice, the actual number of ZigBee nodes, updating data once a minute, that could squeeze onto a ZigBee RF Bus may be even lower. Regardless of the network, protocol, or medium, the maximum number of nodes sharing the same pipe is always a function of transmission duration and interval. If too many nodes try to communicate too often, the pipe will clog, collisions may occur, and there will be no more room on the bus. At this point, the network will fail.

Passing and sharing / Interoperability in the real world

Control Solutions, Inc In a multi-manufacturer ZigBee MESH configuration, devices should be able to cooperate with each other on a single RF network, routing and repeating data as needed to assure reliable communication without interference. RFDs and FFDs would work together automatically and seamlessly, reliably passing data packets throughout the network from source to destination, (providing they are not all simple, battery-powered RFD devices designed without routing capability).

Unfortunately, even though a node from one manufacturer would be able to process data from another manufacturer in a ZigBee network, it may not be able to actually read or use the data. Without agreed upon standards for the actual content of the data payload, there can be no guarantee that data sharing would be possible. A ZigBee-enabled room sensor may be capable of sending a block of data representing room temperature to a different manufacturer’s ZigBee VAV box controller, but for a destination to be able to receive and actually use this data, the exact format of the data must taken into account. How is this point identified? Is the analog sensor data a 12-bit value? 16-bit? Is the value in Celsius? Fahrenheit? Where is the decimal point? Is there any additional data included in the packet like setpoint or occupancy status that must be ignored? Standards must be agreed upon at all levels, from the processing of basic analog and digital values, to common application definitions such as room sensor and VAV box controller, in order to achieve true multi-manufacturer interoperability. Interoperability may also require some sort of common network management tool to facilitate the linking of data points between various manufacturers.

There’s ZigBee, and then there’s ZigBee

It is the intent of the ZigBee alliance to provide seamless plug-and-play functionality between devices from different vendors. However, it remains to be seen if true plug-and-play interoperability will ever be achieved. The recent examples of BACnet, XML, and Echelon LON interoperability may provide some history on the likelihood of a single, universal, agreed-upon standard for network management, point, and device profiles. Once ZigBee devices are available, compliance to the ZigBee standard will also have to be formalized. Some sort of independent testing and certification system should also be in place to assure seamless interoperability between all devices. ZigBee certification may even allow for different levels of compliance, further complicating interoperability efforts.

Get off my bus!

There is another issue regarding cooperative MESH networking beyond the technical and market challenges. Once an installing contractor designs and establishes a finished ZigBee network, perhaps very carefully optimizing device transmission times and node routing and locations for a specific application, it might be unreasonable to expect that a second contactor would even be welcomed to hop onto the installed MESH. The addition of unexpected ZigBee devices suddenly jumping onto an existing ZigBee network could be very problematic, especially if the original network suddenly starts to have trouble and finger pointing begins.

Where is ZigBee now?

The final ZigBee specification has yet to be released, but the ZigBee organization does have broad support from a variety of manufacturers, is working hard, and looks likely to complete the wireless standard in the near future. I/O products using the ZigBee protocol are currently being developed, with some devices already being shown today as “ZigBee-ready.” At this time, finished ZigBee-enabled HVAC products are not currently available. It is expected that Version 1 of the specification will be finalized in the second half of 2004 with the first generation of ZigBee I/O and HVAC devices to follow sometime in 2005. These initial products are likely to include very short-range battery-powered sensors/transmitter RFDs and semi-wireless I/O FFD nodes. Adherence to common HVAC profiles or gateway devices for common hard-wired HVAC networks may not be included with initial product releases.

The question remains

In summary, ZigBee is primed to emerge as the standard for low powered, short-range radio communication between I/O devices within the next few years. It offers three major advantages over today’s convoluted wireless I/O market:

  1. A common standard, agreed upon by many manufacturers, should make it technically possible to use cooperative routing on a MESH network.

  2. MESH networking, coupled with selectable operating modes, will make it possible to create large networks of short-range devices with communication optimized for speed, reliability, or flexibility.

  3. Common adaptation of direct sequence spread spectrum with advanced security encryption will provide improved immunity against interference, as well as excellent process gain for increased sensitivity of weak signals.

Reliable Controls As good as ZigBee sounds, the very low power transmitters, which are likely to be introduced in the first wave of commercial products, may not have sufficient transmitting range to be practical in many HVAC applications. A ZigBee transmitter operating at .1 mW (0.0001 watt) may only reliably transmit within a 10-meter range indoors. Higher-powered 50- or 100-mW ZigBee devices may need to be developed in order to achieve practical, real-world, 100-meter indoor and 1000-meter outdoor operating ranges. In addition, even though ZigBee is likely to emerge as the standard for wireless I/O in the next year or two, its ultimate promise, as a fully interoperable solution for wireless applications, may not be realized, at least not in the earliest releases.

A real MESH

Right now, state-of-the-art proprietary systems provide the best, and in some cases, the only practical solution for many wireless HVAC I/O applications. The very latest of these also share many of the same technical advantages of ZigBee, including interference-free direct-sequence spread-spectrum transmission and even MESH and MESH-like repeater network protocols. These digital systems provide reliability, performance, and useable range, along with advanced transmitters capable of a two- to five-year battery life. Another plus is that many of the current wireless systems include gateway devices for interfacing into today’s most common HVAC communication protocols including, but not limited to, Echelon’s LON, Modbus or BACnet.

ZigBee, once available, should make possible some level of interference-free and cooperative communication between and through multiple manufacturers, but there is no guarantee that devices provided by different manufacturers will be able to share data. Trying to get a multitude of manufacturers’ ZigBee devices, installed by various contractors, operating on many different radio bands, and using different networking modes to all work together and share data on a single radio network could wind up being a real MESH.

Acronym Index

BACnet

A network communication protocol

CO

Carbon Monoxide

CO2

Carbon Dioxide

DSSS 

Direct Sequence Spread Spectrum

FCC   

Federal Communications Commission

FFD 

Full Function Device (ZigBee)

FM

Frequency Modulation

GHz

Gigahertz

HVAC   

Heating Ventilation and Air Conditioning

I/O   

Input and Output

ISM

Industrial Scientific and Medical (radio bands)

IT

Information Technology

LON

A network communication protocol

MHz

Megahertz

Modbus

A network communication protocol

PLC

Power Line Carrier

RF

Radio Frequency

RFD

Reduced Function Device (ZigBee)

TCP/IP

Transmission Control Protocol / Internet Protocol

VAV

Variable Air Volume

Wi-Fi

Wireless Local Area Network products based on IEEE 802.11

WPAN

Wireless Personal Area Network

 


About the Author

With over 20 years of experience in the building automation and controls industry, Guy Zebrick is currently an Account Manager for Kele, Inc. in Memphis, Tennessee. In that capacity, Guy maintains customer relations for the Eastern region of the United States and has led several professional development training sessions on Kele's Frontier wireless system. He earned a Bachelor of Science degree in Organizational Management from Crichton College in Memphis and an associate degree in Electronics Technology from Delgado Community College in New Orleans, Louisiana. In addition, Guy holds his FCC Second Class Radiotelephone License and Advanced Amateur Radio License.

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