Echelon LONWORKS, User Manual, Page: 12 / 68

2-2 Theory of Operation
LonTalk Protocol Support for Routers
The LonTalk protocol is designed to provide transparent routing of messages between
nodes that communicate via routers. To increase the efficiency of routers, the LonTalk
protocol defines a hierarchical form of addressing using domain, subnet, and node
addresses. Subnets do not cross intelligent routers, which allows intelligent routers to
make a routing decision based on the subnet component of a node's logical address. To
further facilitate the addressing of multiple dispersed nodes, the LonTalk protocol
defines another class of addresses using domain and group addresses. Intelligent
routers also can be configured to make a routing decision based on the group addressing
component of a message.
In general, a network services tool such as the LonMaker Installation Tool, is
responsible for domain, subnet, node, and group address assignments.
See the LonTalk Protocol engineering bulletin (005-0017-01) for a further description of
the LonTalk protocol. See the L
ON
W
ORKS
Installation Overview (005-0006-01) for a
further description of the installation scenarios.
Looping Topologies
A looping topology is a network topology that has the potential for message loops . A
loop is a path through two or more routers that forwards a message from a channel
back to the same channel. For example, figure 2.1 shows a looping topology with two
channels and two routers. A message on channel A could be forwarded by router 1 to
channel B, then the same message could be forwarded by router 2 back to channel A,
starting an endless loop of forwarded messages.
Router 1
Router 2
Channel B
Channel A
Figure 2.1 A Looping Topology
The LonTalk protocol does not support topologies where loops can occur. However,
looping topologies may be desirable for the following reasons:
• Increased Reliability . Redundant routers may be used to increase system reliability
by providing multiple paths between two channels.