BACnet and LonWorks®: A
Several solutions have become available recently which promise to change this situation permanently and dramatically. One such solution is called BACnet: The Building Automation and Controls Network. BACnet is a standard for computers used in building automation and controls systems that has been developed over the past nine years by ASHRAE. In December of 1995, BACnet was also adopted by ANSI, and is now an American National Standard (ANSI/ASHRAE 135-1995). Nearly every major vendor of building automation and controls systems in North America has demonstrated support for BACnet in the form of new products, many of which have been displayed at the annual AHR/ASHRAE show in Atlanta this year. Another completely different solution is called LonWorks® which is a proprietary communications technology which has been marketed for several years by the Echelon Corporation in partnership with Motorola. Various vendors have used LonWorks successfully in recent years to provide solutions for small controls systems applications, in some cases involving multiple vendors.
This paper will explore both of these systems in some detail to help bring into focus the substantial differences between each approach. We will also focus on various popular myths in order to dispel some of the confusion and misinformation that has surfaced as these different solutions have been introduced to the marketplace.
WHAT IS BACNET?
BACnet is an American National Standard. This is literally a book which describes in great detail how to create an automation and controls system which may interoperate with other BACnet systems. In BACnet terms, interoperate means that two or more BACnet-speaking computer systems may share the same communications networks, and ask each other to perform various functions on a peer-to-peer basis. Although BACnet does not require every system to have equal capabilities, it is possible for designers of system components at every level of complexity to have access to functions of other automation system peers. In the BACnet world, there is no class distinction between large controllers, small controllers, sensors, actuators and operator workstations or host computers.
There are two key concepts in BACnet that are critical to understand. First, is the idea that BACnet is applicable to all types of building systems: HVAC, Security, Access Control, Fire, Vertical Transport, Maintenance, Waste Management, Lighting, and so forth. The same mechanism that gives BACnet this flexibility has two other important benefits: vendor-independence and forward-compatibility with future generations of systems. This is accomplished using an object-oriented approach for representing all information within each controller. The second key idea is that BACnet uses any combination of five types of local area network or LAN technology for transporting BACnet application messages. These five types of LAN choices give the system designer or owner significant flexibility in choosing the best fit among price/performance options that suits each situation. Four of the five LAN options are ANSI or international (ISO) standards. Since BACnet is based on standards, it provides maximum benefits for both the vendor who designs BACnet systems, and the specifier or owner of those systems.
BACnet provides a sophisticated model for describing automation systems of all types. This model is based on the idea that for systems to be truly interoperable, there must be some agreement about various aspects of the overall operation and the individual systems themselves. BACnet organizes its model into these component parts:
Each of these components delivers important benefits to purchasers and specifiers of BACnet systems.
BACnet is not a guarantee that forces all systems to be the same, or even an assurance of interoperability. BACnet does provide the mechanism to allow cooperating devices to interoperate if they choose to.
Internetworking addresses each of these issues. One of BACnet's key strengths is the sophistication and flexibility of its internetworking that is achieved at only a modest cost to most BACnet devices. Typically internetworking is accomplished in BACnet using special devices called routers which couple two or more different networks together. Although BACnet routers can use the same LAN technology on each end of a router, more typically routers are used to couple different LAN types together. The important thing to keep in mind is that a BACnet router couples together two or more BACnet LANs.
It is also possible to have devices called gateways which couple a BACnet LAN to a non-BACnet LAN, perhaps using a proprietary communication scheme. Gateways are inherently more complex than routers because in addition to equalizing the differences between LAN types, gateways must also manage the usually vastly different conceptual model of one application protocol with another. This task may range from straightforward to very difficult or impossible in some cases.
BACnet defines six broad categories of conformance classes that describe general features of BACnet which a device of that class must provide. Generally speaking, a higher numbered class means that more features are implemented. However, class is not sufficient by itself to specify a BACnet device. For example, a class 2 BACnet device must provide both the ReadProperty and WriteProperty services, but only one standard object (the Device object) is required. In practice, you would want to specify also those BACnet standard object types that you were looking for, so class by itself doesn't do the job. BACnet also describes what are called functional groups which are collections of BACnet features (services, objects and required properties) that are required to realize certain common functions, such as Alarm Reporting or File Transfer or Virtual Terminals.
To facilitate the description of a BACnet implementation, either one a vendor has made, or one that a purchaser would like to buy, BACnet defines the Protocol Implementation Conformance Statement or PICS. The PICS defines the information that must be provided to identify all of the key features of a BACnet device. The PICS identifies the manufacturer, make and model, the conformance class, which functional groups are supported, which standard objects are present, which optional properties of those objects are implemented, the acceptable range of values for writable properties, what type of LANs are supported with what types of media, etc.
Although not required by BACnet, the astute specifier or purchaser should also require that all of the objects and properties of every BACnet device should be documented, especially proprietary ones. This documentation should include not only the object type and property identifiers, but the range of acceptable or producible values and a description of the operation and meaning of each property and what it represents or controls.
Summary: What is BACnet?
BACnet systems are realized by sending and receiving messages across various types of transport mechanisms (LANs) using a common communications protocol.
Nearly every major vendor of building automation and controls systems in North America has participated in the development of BACnet, and have either demonstrated or are actively selling BACnet-based controls today. Given that the standard has only been "official" for six months, this is a strong indication of what to expect going forward.
WHAT IS LONWORKS?
LonWorks is actually a family of products originally developed by the Echelon Corporation. At the core of this technology is a proprietary communications protocol called LonTalk. In this context, the term "proprietary" means that the technology was initially owned by a single proprietor, namely Echelon. A communications protocol is a set of rules that describe methods which can be used to manage the exchange of messages between cooperating devices that implement the protocol. The LonTalk protocol uses some advanced ideas that are unique to Echelon and their products. Because of the complexity of some of these ideas, Echelon's designers decided to develop a special type of communications "chip" which was uniquely well suited to implementing LonTalk. Using this chip, and the appropriate software, much of the burden of implementing LonTalk can be absorbed completely by the communications chip, freeing the rest of the system for application tasks. This communications chip is called the Neuron®. Echelon eventually licensed the chip design to Motorola under a royalty agreement. Motorola has since become a substantial investor in Echelon. As is their standard practice for most chip designs, Motorola has a cross-licensing agreement with another chip manufacturer, Toshiba. Together, Motorola and Toshiba market the Neuron world-wide to various types of OEM developers who apply the LonTalk technology in different markets and applications.
LonTalk is like a very simple mailing system that provides system designers with some basic mechanisms for transporting messages between systems. In and of itself, LonTalk does not define what these messages contain. Like the U.S. Postal system, LonTalk merely provides a way to send a "message" to another recipient. Various options for sending may be used, but the "postal system" doesn't really care what the message says, or whether the recipient can even understand it.
For the message system of LonTalk to be useful in a given application, the sender and receiver need to agree on the content of these messages. Since Echelon's designers had a fairly good idea of some of the applications that the Neuron and LonTalk might be used for, they were able to develop a second protocol that could be used to define the content of application messages. This "one size fits all" protocol represents the session, presentation and application layers of LonTalk and is often referred to as LonWorks.
In this paper we will make the distinction between the lower layers of LonTalk and the upper layers by consistantly calling the upper layers "LonWorks."
Controllers that make use of LonWorks can communicate with each other through what LonWorks calls "Standard Network Variable Types" or SNVTs (pronounced "snivets"). The SNVT method is a different approach to defining data objects that requires a detailed knowledge on the part of the sender and receiver of what the structure of each SNVT is. SNVTs are identified by a code number that the receiving controller can use to determine how to interpret the information presented in each SNVT.
The open-ended nature of SNVTs is both a strength and a liability. The liability comes from the fact that since LonWorks does not define what a particular SNVT code represents, it is possible, and regrettably commonplace, for LonWorks systems from different vendors to use the same SNVT code to mean different things. At best this causes confusion when these systems are coupled together, and at worst causes inappropriate actions to be mistakenly taken. To help solve this problem, a consortium of Echelon vendors was formed to try to agree upon some rules for how LonWorks should be used, and to agree on a common set of SNVT codes and their associated meanings. This group is called The LonMark® Consortium, and the documents that they have produced are also commonly referred to as LonMark. In theory, controllers that only use the LonMark subset of LonWorks capabilities can be made to interoperate with each other, and at least not interfere with each other's proper operation. The LonMark Consortium members must pay substantial membership fees for their on-going participation in LonMark. Only the highest paying tiers of members may vote on extending LonMark capabilities.
Initially, various vendors appeared in the marketplace with products implemented based on LonWorks. Many had bought into the promise of automatic interoperability as a "free" consequence of using the Neuron and LonWorks. As previously mentioned, this turned out to be problematic. The formation of LonMark, though envisioned as a solution to this problem, has brought forward various new problems. While LonMark has created an industry consortium to attempt to standardize on implementations of LonWorks, there is still a lot of confusion and very little actual interoperability.
Let's focus on several myths and misconceptions about LonMark, LonWorks, LonTalk and their relationship to BACnet.
One major problem is that LonMark is not extensible without the agreement of "gold level" LonMark members. So vendors who need to add functionality to their systems may not do so without violating their agreement to adhere to LonMark (therefore losing the right to bear the LonMark mark), or convincing the golden members to bless their extensions. For purchasers or specifiers, this restricts your choices to "gold member approved" features and functions (of course provided only by their companies). Since LonMark is not a standards body, features may come and go as members elect, so no assurances of forward compatibility can be made in good faith by any LonMark member, since future changes or enhancement may easily compromise backward compatibility.
A second problem is that not all systems are LonMark systems. It is both possible and probable that LonWorks will continue to be used by some vendors who do not wish to, or cannot afford to, buy in to the LonMark consortium. Since the primary justification to applying LonWorks is the alleged attraction to integrating communications with an application in the same device (i.e. Neuron) to lower costs, then development cost, manufactured cost and ongoing support cost are clearly important issues.
BACnet LANs Revisited
This table briefly summarizes each technology. The system cost per node represents the cost of using this technology in a real system including issues like wiring costs, installation costs, the need for repeating devices etc.:
Although it is misleading to generalize about the pros and cons of these LAN types, it is useful to point out some of their individual limitations and issues that affect their cost when applied in real situations for automation systems. Both Ethernet and LonTalk are non-deterministic technologies which means that because of the way they work, there is no way to guarantee how long it will take for a message to get from one node to another under any circumstances. LonTalk's approach improves on the Ethernet scheme by attempting to predict potential collisions thereby improving, so it is claimed, confidence in assuring delivery times. Both schemes suffer degraded performance when the network becomes busy, potentially interfering with automation functions as traffic increases. ARCNET and MS/TP use a deterministic scheme that makes it possible to determine the worst case performance of a particular network configuration, which may be desirable in some instances.
Although each LAN has distance limitations, Ethernet, ARCNET and LonTalk are potentially much more limited because they can employ higher communication speeds. Above about 156Kbps, the maximum distance of an unrepeated network segment for any of these LANs drops dramatically. For Ethernet, the maximum distances are about 1000' for thick wire (10Base5), 600' for thin wire (10Base2) and 300' for twisted pair (10BaseT). Similar restrictions apply to ARCNET and LonTalk, depending on the media used. ARCNET, LonTalk and MS/TP can employ EIA-485 baseband signaling at speeds below about 156Kbps over up to 4000' of twisted pair wiring.
ARCNET and LonTalk have the disadvantage that they require the use of a dedicated special communications chip from a single source manufacturer. Although Motorola and Toshiba are both "sources" for Neuron chips, they are in fact produced by the same facilities and simply distributed through the two organizations. In contrast, Ethernet chips are available from many different sources worldwide. This impacts the cost and supportability of end devices made with these technologies, and directly drives the cost and availability of third party devices such as repeaters, bridges and routers, not to mention diagnostic equipment.
Unlike ARCNET, LonTalk can be implemented in Neurons along with small applications. So in some instances it is possible to implement both the communications system and the application programs within the same chip. In these cases, the Neuron may provide an economical alternative to an ARCNET solution that would require an additional microprocessor chip to handle the application. In these same instances, a typical single chip microprocessor could be used without a Neuron to implement MS/TP for similar, if not lower, cost. When the size of the application becomes more demanding, the Neuron simply does not have enough resources to handle both jobs at the same time, therefore requiring the addition of a separate microprocessor. In these instances, the MS/TP approach is always more cost effective if you can live within the limits of MS/TP in terms of speed. The typical microprocessor solution would be hard pressed to provide MS/TP above about 76Kbps. For speeds in excess of 76Kbps, coupled with larger application requirements, either LonTalk or ARCNET become preferable. At 156Kbps, using baseband EIA-485, the cost is similar between LonTalk and ARCNET. At speeds above 1.25Mbps, or when deterministic response is needed, ARCNET is the clear choice. ARCNET chips are also available which include an additional 8031 microprocessor suitable for small applications, although the cost of these chips is slightly higher than Neurons.
LonTalk has the unique distinction that it is the only LAN technology in this group that really requires special proprietary development tools. Developers who are in the business of making microprocessor-based controllers will already have all the development tools they require to create solutions using the other technologies. The proprietary development aspects of LonTalk have a direct impact on purchasers and specifiers, because these costs will be distributed to them in terms of higher product costs, or long term service fees, or both.
Does Echelon Cost Less Than BACnet?
There are two possibilities for the first case. Either the application is small enough to fit into a Neuron along with LonMark, or it isn't. If the application fits in a Neuron along with LonMark, then the Neuron serves the same function that a single chip microprocessor in a "BACnet using MS/TP" device would. From a pricing standpoint, Neurons are not substantially less or more than average microprocessors, falling in the middle of the pricing pack. The alleged low price of Neurons is only available in very large quantities, where regular microprocessors are available at substantially more attractive discounts. If both solutions use baseband EIA-485 signaling below 156Kbps, the cost of transceivers or line drivers is the same. Current actual implementations of BACnet and MS/TP on real off-the-shelf systems show conclusively that the memory requirements in terms of RAM and ROM are similar. If the application doesn't fit in the extra space in the Neuron, you are forced into using an outboard microprocessor, and the cost comparison is clearly in favor of the BACnet approach. So if cost is the issue, LonMark/LonWorks/Neurons is not an advantage. If you factor the added costs of development systems and debugging tools into unit costs over, let's say, the first 100,000 units, BACnet is clearly the winner. LonTalk has some distinct advantages in terms of speeds higher than 156Kbps and media flexibility which were pointed out earlier, but cost isn't one of them.
The second issue deals with cost for BACnet devices only. The question is whether using LonTalk as a transport medium costs more or less than other alternatives such as MS/TP and ARCNET. This is a similar comparison to the one we just looked at above, but even more severe. In this case, we would be trying to fit LonTalk and a BACnet application layer and the application program into the confines of a Neuron. This can definitely be done. For example, Staefa has developed BACnet devices which have been demonstrated with these capabilities. As in the previous example, once the application presses the limits of what can be managed within the Neuron, an outboard microprocessor is required and the cost war is over. Like the previous example, there is no cost advantage to using a Neuron with LonTalk over using a conventional microprocessor with MS/TP as a vehicle for BACnet. If speed is the issue, but cost is still a concern, then the roughly 76Kbps speed limit to MS/TP may rule out this option. In this case, ARCNET at 156Kbps using EIA-485 is a viable option to LonTalk. If the Neuron by itself cannot contain the application and BACnet and LonTalk and higher speed is required, then ARCNET is definitely an option since the cost of the ARCNET outboard chip is similar to the Neuron in equivalent quantities.
In any of these examples, the other costs are more or less equal. Whether you use Neurons or ARCNET or conventional microprocessors, EIA-485 circuitry costs the same amount. With or without magnetically and optically isolated power supplies, with or without transformer coupling, the signaling circuits have the same costs for each of these LANs (LonTalk, MS/TP, ARCNET) for the same media.
One comparison not often discussed is the issue of software costs. In the case of ARCNET, sample software to provide the transport function is available free from the manufacturer and numerous public domain examples exist, though none specifically targeted to BACnet (that I know of). Most of the functionality for ARCNET is inside the chip and not changeable anyway. There is no special licensing fee for using ARCNET. In the case of MS/TP, the developer must write their own software or purchase it from a third party, so the development costs must be amortized over some number of units, adding to the cost of that solution. In the case of LonTalk, while it is theoretically possible to implement your own chips to perform LonTalk functions, no one has done so to date. Typically Neurons are purchased and software is written to make the Neuron implement LonTalk. The developer may undertake this themselves or license this software from Echelon. In either case, special development tools are required which may only be purchased from Echelon. Opinions will vary regarding the cost to develop communications software. However, for a vendor with a serious (10K-100K units) commitment to development, the difference in cost between developing your own BACnet software, such as MS/TP, and licensing LonTalk for the Neuron in those quantities is about 20 times less to make your own. If we factor in the added cost of special development tools the cost of LonTalk is much higher still.
LONmark: A Little System
Here are two examples of common everyday problems in building automation systems networking which are impossible to solve using LonWorks or LonMark:
Using BACnet, problem 1 is solved in both cases by a PTP half router. Many larger BACnet controllers already incorporate such features as built-in capabilities. Ask two such routers to dial each other, and you're done. Using LonMark, there is no capability for dial-up, no capability for internetwork routing with session establishment, no capability for eliminating circular routes, etc. This type of function would have to be provided in some non-standardized way using proprietary dial-in gateways. What standard protocol would they use to resolve these problems? Just exactly how does a LonMark device cause dialup to a remote LonMark network to occur? Answer: it can't.
Using BACnet, problem 2 is solved using Ethernet routers, for example ARCNET to Ethernet or MS/TP to Ethernet (or LonTalk to Ethernet!). The coupling is automatic and invisible to nodes on either end. To date, the most ubiquitously available BACnet routers are Ethernet to X. Using LonMark, there is simply no way to provide routing across Ethernet, since the LonTalk Network Layer does not provide the mechanisms necessary to make this happen. It is theoretically possible to make some kind of tunneling gateway to provide this feature in a proprietary device, but no such devices are available with or without LonMark sanctioning.
These, and many other examples, are due to the limited capabilities in the LonTalk strategy for internetworking, which never envisioned this type of application.
LonTalk: The Common Link?
LonMark and/or LonWorks devices cannot and
do not interoperate with BACnet devices!
The technical reasons for this are simple. Even if a BACnet device uses LonTalk as its LAN, each message sent by that device in a LonTalk "envelope" will be expressed in BACnet "language." This BACnet language is totally different from and incomprehensible to a LonMark or LonWorks device which is expecting LonTalk envelopes containing "LonWorks language."
As this drawing shows, the "B" boxes are sending BACnet messages over a LonTalk network to other "B" boxes. The "L" boxes are sending LonWorks messages to other "L" boxes. Although these messages do not interfere with each other, the "L" boxes are not able to interoperate with the "B" boxes because the contents of the messages are incompatible.
The incompatibility arises from the fact that the application "languages" in the BACnet scheme are based on ideas and concepts that are very different from the LonWorks scheme. Not only are the concepts different, but the method of encoding these concepts into numeric codes is also very different. If one wanted to have LonWorks devices and BACnet devices interoperate, it would be necessary to have a gateway device between them which understood both schemes and had a method for converting from one to the other.
As it turns out, this is a fairly complex problem. Many of the ideas in BACnet simply have no equivalent concept in LonWorks. As a result, many of the capabilities which are expected of a BACnet device would have to be emulated by the gateway. Since BACnet devices may contain arbitrary extensions which are proprietary, and easily accessed by other BACnet devices, some or all of these extensions may have no equivalent in LonMark. The consequences of these and other technical issues make the construction of any form of off-the-shelf BACnet-LonMark gateway a practical difficulty, for which no solution currently exists.
It is certainly possible to build a device which could understand both LonWorks and BACnet at the same time. Clearly such devices would require additional resources which would generally add significant extra cost to these traditionally cost-sensitive devices.
BACnet and LonWorks: Summary
Buyers and specifiers of building automation and controls systems don't all want the same things. There is no "one size fits all" solution, and each available option has tradeoffs associated with it. However, BACnet is the only serious choice if you are specifically concerned with these issues:
If none of these issues are of concern to you, then you have a much more difficult task ahead.
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