Echelon LNS - Manual (Page 167)

LNS Programmer's Guide 153
network variable. This can be used to avoid joining or creating groups, as shown in
figures 7.1 and 7.2 later in this section.
For monitor and control applications, the benefit of this approach is that the monitoring
tool does not have to join the group. The existing group of nodes 1,2,3 will remain as it is,
and node 1 will also propagate network variable updates to the monitoring tool using a
unicast connection with Subnet/Node ID addressing. Each time the output network
variable is updated, two messages will be sent: one to the group connection, and one
using subnet node addressing. The benefit is that address table space on the monitoring
node is preserved (becoming a member of a group requires an address table entry to
accommodate the group membership information).
The lcaAliasForUnicasts option can also be used to avoid multicast connections. The
building automation example described later in this chapter introduces a problem that
can be avoided by splitting a multicast connection into multiple unicast connections.
Note that there are only 256 distinct groups that can be used for multicast addressing
per network, and that by using unicast addressing you can conserve group usage. Group
addresses also consume address table entries in both the receiving and transmitting
devices involved in the connection, and most application devices are limited to 15 address
table entries. Network Service Devices that use standard network interfaces are limited
to 15 address table entries for use with group addressing. However, unicast connections
may consume more address table entries on transmitting devices.
Typically, multiple unicast messages create extra network traffic compared to a single
multicast connection, as all network variable updates will require the transmission of
multiple messages onto the network. In contrast, a network variable update in a group
connection would only require one message to be sent that would be received by all
members of the group.
Note that you can use the AppDevice object’s AliasCapacity to determine the total
number of aliases supported by a device, and you can use the AliasUseCount property
to determine the number of aliases on the device that are already being used by existing
connections.
Broadcast Options
The BroadcastOptions property determines when a connection should use broadcast
addressing. This can be set to any of the following values: lcaBroadcastNever,
lcaBroadcastGroup, and lcaBroadcastAlways,
The lcaBroadcastGroup value requires some explanation. The lcaBroadcastGroup
value allows LNS to automatically use broadcast addressing when a multicast connection
is required, and no group identifiers are available. This requires that the connection use
a messaging service type acceptable for use with broadcast addressing, such as the
unacknowledged messaging service or the unacknowledged/repeat messaging service.