The topics of IP networks and BACnet merge when networking BACnet networks. Standard knowledge of planning and implementation suffice for small to mid-sized projects. Expert knowledge of BACnet (and not necessarily IP networks) is required to plan, size, and implement large and complex BACnet/Desigo projects.

Desigo is based on that standard BACnet communication protocol. Services are required to communicate between BACnet devices. The following can be used to exchange data, for example, to transmit a demand signal from the room and its evaluation and forwarding to the primary plant:

  • A Unicast service continuously queries and evaluates the state using a cyclical read (ReadProperty).
  • A Unicast service that subscribes to the demand signal (SubscribeCOV) for a specified lifetime and receives an update if the value or its state changes.

This exchange of data between both BACnet devices can be labeled as highly targeted. The communication protocol also supports broadcasts to all BACnet devices. Example:

  • A BACnet client determines at start up all pending alarms and events to display them in a current alarm list (GetEventInformation).
  • A generic operator unit learns at start up with a broadcast who the other participants are.

(BACnet) broadcasts on a large IP network would take down the entire network.
Layer 3 switches (IP router) block broadcasts to prevent this.

BBMD functionality is available in BACnet to nevertheless allow the use of BACnet broadcasts over multiple IP segments.

A BBMD (BACnet Broadcast Management Device) is set up on each IP segment. The BBMDs recognize one another through a table.

The applicable BBMD recognizes a broadcast triggered in a segment and distributes it as Unicast in a targeted manner to all other BBMDs that are entered in the Broadcast Distribution Table BDT of BBMDs. The broadcast is forward in other segments using their BBMDs.

Only one BBMD per IP segment is permitted. One BBMD must have a fixed IP address and use a UDP port. BBMD is not a separate device, but rather auxiliary functionality on a BACnet device.

In the use cases to date (Desigo PX in multiple IP segments with the same UDP port) a symmetrical BDT is always used. In other words, all BBMDs are entered in the Broadcast Distribution Table and distributed in this manner over the bypass to all other BBMDs in the various IP segments. Mid-sized to large Desigo projects, especially with Desigo room automation, operate with asymmetrical BDT entries. To this end there is a designated BBMD (Building Scope) that knows all other BBMDs, but where the other BBMDs only know these special BBMD.

For detailed information, see Desigo Ethernet, TCP/IP, MSTP and BACnet (CM110666) and the following sections and practical examples.

Local and global broadcasts are used on a Desigo room automation project with PXC3 and DXR2 components.

  • The interaction between room and room segment are based on local broadcasts.
  • Central functions use global broadcasts.

The following graphic shows the interaction on one BACnet internetwork with different IP networks.

A broadcast is sent when starting up a BACnet client, for example, Desigo CC. This is necessary since this BACnet participant is located in a remote IP segment and must registered on the network as a third-party device.

Its broadcast is recognized on the BBMD for the new network, packed, and sent to all participants entered in the BBMD.

BBMDs in other IP segments recognize this and send the Unicast as broadcast to all participants in their respective segment.

Flexible room management uses, in other words, communication in one room over multiple room segments, local broadcasts. Lots of devices on an IP network cause lots of broadcasts.

As result, large, complex projects must:

  • Supply BACnet devices over layer 3 switches in its own IP segment (VLAN).
  • Compile BACnet devices from the building, floor and plant scope, each on their own IP segments.
  • Configure asymmetrical BDTs.
  • Use a powerful PXC5/PXC7 for BBMD functionality in the building scope.