ASHRAE Journal - November 2010 - B9

other devices on the same LAN? My computer is on an Ethernet LAN, and I can communicate with devices all over the world!” That may be true but to do so, it sends its messages destined for computers not on your LAN to a special computer that is connected to both your LAN and some other network. Also, your message has to contain some kind of address for the destination machine that the special computer can use to send your message. If the address is an IP address, then the “special computer” on your LAN is called an “IP Gateway.” Note that to communicate with the IP gateway, its IP address has to be known and added to your IP protocol information at configuration time. To find the LAN or “Medium Access Control” (MAC) address of the IP gateway, your computer uses the broadcast mechanism mentioned previously, which in the case of an IP device on an Ethernet network, is called the “Address Resolution Protocol” (ARP). This protocol is also known as RFC 826 (sidebar on Creation of the Internet Protocol). Increasingly, for mobile computers such as laptops, another well-known broadcast method is used called the “Dynamic Host Configuration Protocol” (DHCP). In this case, the initiating device does not know its own IP address or IP gateway address. It broadcasts a message saying, essentially, send me my IP configuration details and, if there is a DHCP server that hears the request, a message is sent to the requesting device that allows it to configure itself.

creation of the Internet Protocol
The Internet was created in the late ’70s and early ’80s by a group of engineers and computer technicians who were working on projects for the government’s Defense Advanced Research Projects Agency. They needed to collaborate and, although they had computers, they had no established way of networking them. So this early work to establish networking standards took the form of discussions by mail, telephone, fax and face-toface meetings that led to varying degrees of consensus on how to solve the problems. This, in turn, led to someone taking on the task of writing up the various possible solutions in the form of specifications known as a “Request for Comment” or RFC. The RFCs were then circulated within the community and, eventually, became the law of the Internet land. The Internet Protocol itself, for example, is also known as RFC 791 and was adopted in 1981. Other protocols mentioned in this article are also RFCs: the Address Resolution Protocol (ARP) is RFC 826; the Dynamic Host Configuration Protocol (DHCP) was originally RFC 1531 but has been superseded twice. The latest version is RFC 2131. All of these can be found on the web by entering “RFC” into any browser or at www.faqs.org. But RFCs are not exclusively about 1’s and 0’s. Every so often one encounters a philosophical insight worth pondering. For example, the problem statement in RFC 826 contains this gem: “The world is a jungle in general, and the networking game contributes many animals.” No wonder networking can be confusing at times! with 0 in the destination field must be processed by all machines on the LAN. LonTalk and MS/TP also use 8-bit addressing with the broadcast address being 0 and 255, respectively. If a BACnet system consists of only a single network, life is simple. Device-to-device communication uses the unicast address and broadcasts use the broadcast address. But what if there is a need to interconnect multiple networks? How do
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bacnet networks
Now, let’s talk about BACnet. At the BACnet committee’s early meetings in the late ’80s we had intentionally deferred the question of how BACnet messages would be conveyed. We focused, rather, on the content and format of the messages since we knew that networking technology was changing rapidly. The discussions on message transport continued up until the last moment. When BACnet was finally published in 1995, it included clauses on how to use four different LANs: Ethernet, ARCNET, LonTalk and Master-Slave/Token-Passing (MS/TP). The use of IP as a “virtual LAN” was not completely specified until Addendum a, which was also published in 1995, shortly after BACnet itself. The important thing to understand is that a BACnet “internetwork” is a collection of two or more of these LANs, which may consist of different LAN types, each of which is called a BACnet “network.” Each LAN type has its own address format and protocol control information (PCI), the latter being items such as message length, error check digits, and so on. On Ethernet LANs, for instance, each device has a 48-bit unicast address that ranges from 0 to approximately 2.8 × 1014. The address, made up of 48 1-bits, is the broadcast address for Ethernet. On an ARCNET LAN, a unicast address is an 8-bit number between 1 and 255. Address 0 is interpreted to be the broadcast address and messages
November 2010

B A C n e t ® To d a y | A S u p p l e m e n t t o A S H R A E J o u r n a l

ASHRAE Journal - November 2010

Table of Contents for the Digital Edition of ASHRAE Journal - November 2010