BAS communication standards - Intelligent Buildings and Building Automation - page 87

BACnet adopts only the subset of LonTalk, not all layers. Only the LonTalk
Physical Layer and LonTalk Data Link Layer are adopted as one option of
its six alternative transportation technologies. The other layers of BACnet
and LonWorks are completely different. Therefore, BACnet devices and
LonWorks devices are not interoperable.
The reason for the lack of interoperability is not difficult to understand as
can be seen in Figure 5.5. BACnet only adopts the LonTalk Physical Layer
and Data Link Layer as its transportation mechanism. The upper layers are
completely different. It is similar to the way that letters written in different
languages can be put into envelopes; the envelopes can be the same but the
letters are in different languages. Only BACnet devices can read BACnet mes-
sages. Similarly, only LonWorks devices can read LonWorks messages.
5.7.2 IP compatibility of BACnet
BACnet devices are easy to connect to an intranet or the Internet. They have
various choices of devices, such as BACnet router, BBMD, BACnet PAD
and even BACnet devices themselves (native Ethernet 802.3 devices, native
BACnet/IP devices).
There are two kinds of technologies for BACnet to realize an IP connection.
One is BACnet/IP Packet-Assembler-Disassembler (PAD) router technology,
which is specified in Annex H.3 of ANSI/ASHRAE Standard 135-2001.
A PAD router is placed on every BACnet network that is to be connected
over an IP network to another BACnet network. When it receives a BACnet
message for a device on another BACnet network which is reachable only
through an IP inter-network, it puts the BACnet message into a UDP/IP
message with the IP address of the PAD device on the destination BACnet
network and sends the message over the IP network. The receiving PAD
extracts the BACnet message and transmits the message on the local network.
Since the PAD sends the message directly to a known IP of another PAD, this
requires that the PAD maintain a table of its peer PAD devices. Unfortunately,
tunnel routers have several significant configuration issues and require a lot
of expert manual configuration (maintaining the tables) to work correctly.
Another problem is that the addition of a single BACnet device to a BACnet
system via an IP inter-network would require either that the device itself
provides the PAD service or the addition of another device that provides the
PAD service. This would significantly increase the cost.
BACnet has improved significantly, overcoming these limitations by
adopting the second technology for BACnet to realize an IP connection; that
is, BACnet/IP defined in Annex J of ANSI/ASHRAE Standard 135-2001.
Basically, BACnet/IP devices communicate using IP messages in lieu of
BACnet messages, allowing the devices to be added easily to any IP inter-
network. This is implemented by defining a new protocol layer called the
BACnet Virtual Link Layer (BVLL). Broadcast management is accomplished
by defining the capabilities of a new device called a BACnet Broadcast