BAS communication standards - Intelligent Buildings and Building Automation - page 81

￿ Broadcast address: A broadcast address identifies all devices with a sub-
net, or all devices within a domain.
5.3.3 Message services
The LonWorks protocol offers three basic types of message delivery service
and also supports authenticated messages. An optimized network will often
use all of these services. These services (listed below) allow trade-offs between
reliability, efficiency and security:
￿ Acknowledged messaging provides end-to-end acknowledgement. If
acknowledgements are not received, the sender times out and attempts
the transaction again. The number of retries and the time-out are both
configurable.
￿ Repeated messaging causes a message to be sent to a device or group of
any number of devices multiple times. This service is typically used instead
of acknowledged messaging because it does not incur the overhead and
delay of waiting for acknowledgements.
￿ Unacknowledged messaging causes each message to be sent once to a
device or group of any number of devices and no response is expected.
This messaging service has the lowest overhead and is the most typically
used service.
￿ Authenticated service allows the receivers of a message to determine if the
sender is authorized to send that message. Thus, authentication prevents
unauthorized access to devices and is implemented by distributing 48-bit
keys to the devices at installation time.
LonWorks technology is now a popular LAN technology used in the BA
industry, although it is based on proprietary chips. This popularity is due to
the fact that although it was born as a proprietary technology of a particular
company (later a group of companies), the technology is open and accepted
as a standard technology in many cases. The LonTalk/LonWorks LAN capac-
ity well fits the BA network needs at automation and field network levels.
It can be implemented on most physical media thanks to the transceivers
available. Although the LAN is developed on proprietary chips, those spe-
cific networking support chips with supporting software tools provide great
convenience for the BA and control system suppliers to develop network
technology acceptable by end-users.
5.4 Modbus and its features
Modbus is one of the popular protocols used in industrial automation.
Supporting traditional serial protocols of RS232/422/485 and Ethernet
protocols allows industrial devices such a PLCs, HMIs and meters to use
Modbus as their communication method.