Echelon LNS - Manual (Page 171)

LNS Programmer's Guide 157
unicast connections will be processed as separate transactions, causing more network
traffic than a single multicast update would have produced.
Solving Problems With Your Connection Scenarios
When planning connections for large systems, or for systems accommodating challenging
scenarios, the system integrator might still face difficulties when managing network
variable connections. This includes problems such as shortages of group identifiers,
aliases, and address table space. The following sections discuss some of these difficulties
and their possible remedies.
Shortage of Groups
A single domain has up to 256 distinct groups. However, each group identifier can be
used with multiple connections, as long as the connections remain unambiguous. This
technique, known as group overloading, allows for more than 256 groups to be used
based on the 256 distinct group identifiers. Your application can query the number of
group identifiers currently allocated in the network by reading the properties of the
NetworkResources object, which can be obtained from the System object’s
NetworkResources property. Consult the LNS Object Server Reference help file for
more information on this.
To conserve group usage, your application can do the following:
• Replace group connections using the acknowledged messaging service with multiple
unicast connections by setting the AliasOptions property to lcaAliasForUnicasts.
This requires sufficient address table and alias table space on the transmitting device.
You should note that splitting a multicast connection into multiple unicast connections
extends the total transaction time and the total packet count, compared to a single
multicast.
• You could combine group connections using the unacknowledged/repeat messaging
service into a single connection that uses the unacknowledged/repeat messaging service
and subnet broadcast addressing. However, all destination devices in the connection
must belong to the same subnet. The source device does not have to belong to the same
subnet. This configuration allows up to a 127 devices, the subnet maximum, to
participate in the multicast connection. You can also assign a subset of the nodes on a
given channel to a specific subnet to facilitate subnet broadcast addressing.
• Polling fan-in connections always require a group connection. Such connections should be
avoided. You should use polling strategies only after careful consideration.
• If group addressing is required (for example, for routing purposes), the existing
connections can be reconsidered. You might be able to identify an existing group (e.g. an
acknowledged multicast connection) that you could change to use the
unacknowledged/repeat messaging service with subnet broadcast addressing, or
individual unicast connections as described above, thus freeing group identifiers.
Shortage of Address Table Space
Addressing types with broad scopes generally consume fewer address table entries than
addressing modes that target only a single destination device. For example, if a device