The primary hub outlined in the previous chapter is central to BACnet/SC networks. BACnet/SC does have an option for a failover hub as a backup. The failover hub acts as a hot standby for the nodes anytime the primary hub is unavailable, but is limited to backing up only the BACnet/SC area of a site. Under a mixed scenario (BACnet/SC and non-BACnet/SC networks), the site forms two distinct BACnet internetworks on failover. Consequently, failover hubs only make sense in the long term when BACnet/SC alone operates entire sites. Nevertheless, network engineers should plan for a failover hub on a BACnet/SC network (and properly test redundancy), since the failover hub assumes operations in the event of downtime to the primary hub (within the BACnet/SC area of the project).
IT administrators intentionally design local area networks (LAN) to operate a smaller number of devices of ~200 devices for cybersecurity. Although technically superior and capable of connecting any number of IP subnets, it is always a good practice to follow IT design principles whenever large numbers of devices are involved:
- Synchronize BACnet networks to IP subnets: BACnet/SC can extend to multiple subnets, but it still makes sense to retain most of the traffic within natural boundaries (applies to both the BACnet network layers and IP network layers).
- Divide large IP subnets (i.e. in excess of 200 devices) into multiple smaller BACnet/SC networks (and/or BACnet/IP) networks.
- Design BACnet networks to retain most of the traffic on individual networks (building, floor, plant), while limiting traffic routed across networks.
On large projects, the principles result in multi-hub architectures for BACnet/SC networks that are very similar to BACnet/IP networks.
See BACnet/IP networks.
The BACnet/SC hub is typically located where the BACnet Broadcast Management Devices (BBMD) are depicted in BACnet/IP topologies. Due to their similarities, BACnet/SC and BACnet/IP topologies simplify phased migration of an installed BACnet/IP base to BACnet/SC. Finally, this eliminates the problem with the UDP broadcasts in the BACnet/IP world.
The BACnet/SC standard does not define how to design and balance traffic loads in large systems as this document does, for example, in the section on asymmetric BBMDs in BACnet/IP. Extensions are planned for future market deliveries that can be easily applied to such topologies.