BACnet supports several data-link options; the most common are BACnet/IP (over Ethernet/UDP, port 47808) for the backbone, and BACnet MS/TP (over RS-485) for local controller sub-networks.

A typical BMS architecture combines an IP backbone running BACnet/IP on the building LAN, with BBMDs (BACnet Broadcast Management Devices) propagating broadcasts between IP sub-networks, and MS/TP sub-networks running RS-485 multi-drop with roughly 32 controllers per segment at 9600 to 115 200 baud. A BACnet/IP to MS/TP router bridges the two. Less common variants include BACnet/Ethernet (direct Layer-2 transport) and BACnet/SC (Secure Connect).

When sizing an MS/TP segment, budget for the worst-case token rotation time rather than the average: an over-loaded segment becomes unusable for COV long before it shows up as packet loss.