Large BACnet/SC projects with more than 100 devices and up to ~1000 devices need multiple hubs. The hubs must be hierarchically structured. One hub must be a superordinate hub, i.e. a hub of hubs.
Topology of a large BACnet/SC project
Desigo CC must connect directly to every floor and plant hub and not to the building hub. This avoids funneling all traffic indirectly through the building hub and prevents the building scope hub from being overloaded by traffic.
How is the BACnet/SC topology structured?
Typically, the BACnet/SC structure will be set up to resemble the building structure. This simplifies commissioning and maintenance in the future. In order for communication to function properly, some rules must be followed:
- If data exchange is required in different floor networks, they must be connected in a building hub network, e.g. central function, weather station, outside air temperature, human centric lighting.
- Structure the BACnet/SC network in such a way that network traffic can also be reduced.
- Each floor hub has its own BACnet/SC network number.
- Desigo CC requires an individual port for each hub connection.
Large floors may require multiple hubs per floor.
| Description |
|---|---|
| Device is a hub. In a BACnet/SC environment, the name changes to a building hub (one building hub in a project) or a floor hub. The functionality is the same as a hub. |
| Device is a failover hub. Note: A failover hub is always a node too. |
| Device connects to a hub. |
| Device is a node with a failover hub defined. |
| Desigo CC links to all floor hubs or failover hubs. Do not connect Desigo CC to the building hub. |
| A floor hub link to the building hub. |
Guidelines for structuring BACnet/SC topologies
Contrary to BACnet/IP, BACnet broadcast commands must be resolved in BACnet/SC directly within the BACnet layer itself. This means that typical broadcast commands like group commands lead to a high traffic load at the hub, as the hub must process the broadcast commands as a sequence of unicast commands to its members.
While the Desigo architecture tries to automatically optimize the traffic, consider the following guidelines:
- Don't use flexible room application on BACnet/SC if not really necessary.
- If you use flexible room application on BACnet/SC, strictly follow the maximum remote segment limits (BACnet/SC system limits).
- The hub-of-hub architecture is currently limited to a two level hierarchy: the superordinate building scope hub and all other subordinate hubs. This means that floors that are larger than the maximum number of nodes that a single floor scope hub can address, directly lead to several floor scope hubs that cannot communicate between each other.
For more information on setting up large BACnet/SC projects, see "Creating a BACnet/SC network topology with building- and floor hub" in the ABT Site online help.
Mixed BACnet/SC and BACnet/IP topology
Projects with BACnet/IP and BACnet/SC are possible. The following topology shows a small building with 3 floors with a mixed primary and room usage. BACnet/IP is already set up and working. The second and third floor have been extended with BACnet/SC. BACnet/IP communication has been kept on the first floor.
Key
1 | Desigo CC is connected to both the BACnet/IP and BACnet/SC networks. |
2 | Mixed BACnet/IP and BACnet/SC topology. The building hub also routes to the room control with central functions. |
3 | BACnet/SC topology with PXC7 and DRA controllers. This is a floor hub connected and routing to the building hub. |
4 | BACnet/IP topology with DRA room control. Only relevant for routing purposes. |
5 | BACnet/IP topology with primary plants. |
Guidelines for mixed BACnet/SC and BACnet/IP projects
Migration and extension projects have both BACnet/IP and BACnet/SC data links.
Consider the following guidelines:
- Keep the total amount of traffic that crosses data link or network borders to a minimum.
- Avoid excessive BACnet routing between BACnet/IP and BACnet/SC.
For example, migrating only half a floor of room automation devices to BACnet/SC and keeping the other half on BACnet/IP leads to a split, where a group of devices partially spans across BACnet/IP devices and partially across BACnet/SC devices. This constitutes a bad structure, as each group command also requires heavy BACnet routing between both sides of the BACnet network. Migrating all room devices of one floor to BACnet/SC with its dedicated floor scope hub avoids such a load on the hub and router devices and is therefore preferred.