What this tool estimates
BACnet Bandwidth Inputs
Use realistic averages. For busy buildings, plan conservatively and validate with packet captures.
Example Data Table
A realistic starting point for a mid-size building automation segment.
| Scenario | Transport | Link | Devices | Msgs/device/min | Payload (B) | Overhead (B) | Peak | Retries | Estimated Utilization |
|---|---|---|---|---|---|---|---|---|---|
| Office tower floor controllers | BACnet/IP | 100 Mbps | 80 | 18 | 180 | 60 | 1.5 | 2% | ~0.01% (typical planning result) |
| Legacy fieldbus trunk | BACnet MS/TP | 76.8 kbps | 50 | 25 | 120 | 25 | 2.0 | 5% | ~20–40% (depends on polling and frames) |
Formula Used
- Msgs/sec = Devices × (Messages per device per minute) ÷ 60
- Adjusted Msgs/sec = Msgs/sec × (1 + Broadcast%/100)
- Bytes/message = Payload bytes + Overhead bytes
- Raw bps = Adjusted Msgs/sec × Bytes/message × 8
- Required bps = Raw bps × (1 + Retries%/100) × Peak factor
- Utilization% = Required bps ÷ Available bps × 100
- Effective target% = Target utilization% − Reserved margin%
- Safe device limit ≈ Current devices × (Effective target% ÷ Utilization%)
How to Use This Calculator
- Select the transport for the segment you are sizing.
- Enter the link speed using the correct unit.
- Count all devices sharing the same segment or trunk.
- Estimate average messages per device per minute.
- Use realistic payload and overhead estimates.
- Add broadcast and retry percentages for real conditions.
- Set a peak factor for start-up and alarm bursts.
- Choose a utilization target and reserve extra margin.
- Calculate and review utilization, headroom, and device limit.
Professional Article
1) Purpose of BACnet bandwidth planning
Building automation networks carry control reads, alarms, trends, and supervisory commands. When traffic exceeds practical capacity, response times grow, timeouts increase, and operators see “offline” points. This calculator estimates required bitrate so design teams can segment networks early and avoid late commissioning rework.
2) Message-rate drivers you can measure
A useful planning metric is average messages per device per minute. For example, 80 controllers at 18 messages/device/min produce 1,440 messages/min (24 messages/s) before applying broadcast allowance. Polling schedules, trend intervals, and alarm bursts can double this during morning start-up, so a peak factor is included. Supervisory servers may also execute graphics refreshes every 2–5 seconds, and analytics jobs can create short bursts, so conservative averages reduce surprises during handover and testing.
3) Frame size and overhead assumptions
Payload bytes represent the NPDU and application data for common services such as ReadPropertyMultiple and COV notifications. Overhead bytes represent lower-layer headers, routing, VLAN tags, and framing. Typical planning overhead is about 60 bytes on IP-based segments and about 25 bytes on MS/TP trunks, but real values vary by stack and topology.
4) Interpreting utilization targets with margins
A 100 Mbps BACnet/IP segment usually has ample headroom for hundreds of controllers, but shared enterprise links still benefit from targets such as 10–30% utilization. MS/TP links are slower (often 9.6–115.2 kbps, with 76.8 kbps commonly used), so practical targets around 20–40% help maintain responsiveness under token passing and retries.
5) Practical actions when utilization is high
Start by reducing aggressive polling and consolidating reads using bulk services. Use COV subscriptions where supported, and stagger trend uploads. Segment by floor or system, add routers, or upgrade trunk speeds when devices allow. Finally, validate assumptions with packet captures during commissioning and refine the message and size inputs for as-built performance.
FAQs
1) What should I enter for messages per device?
Use an average across normal operation. Count scheduled polls, trend reads, COVs, and alarms. If unsure, start with 10–30 messages/device/min and adjust after reviewing trends and supervisory workstation behavior.
2) How do I choose payload and overhead bytes?
Payload is the typical application content size. Overhead is headers and framing. For IP segments, 40–80 bytes is a common planning range. For MS/TP, 15–35 bytes is a reasonable starting range.
3) Why include broadcast and retry percentages?
Broadcasts cover discovery, Who-Is/I-Am, and some supervisory chatter. Retries represent resends from timeouts, collisions, or congestion. Both can rise sharply during commissioning or when devices misbehave, so reserving capacity improves stability.
4) Is low utilization always safe?
Not always. MS/TP performance depends on token passing, device turnaround time, and frame timing. Even with low average load, bursts can cause delays. Use the peak factor and validate with captures for time-sensitive control loops.
5) What utilization target should I use?
For shared IP segments, 10–30% is conservative. For fieldbus-style trunks, 20–40% often balances performance and cost. If expansion is expected, add reserved margin and design segmentation for growth.
6) How accurate is the safe device limit?
It is a proportional estimate based on your current inputs and effective target. Real limits depend on device behavior, routing, token timing, and supervisory patterns. Treat it as a planning guide, not a guarantee.
7) How can I reduce bandwidth without changing hardware?
Reduce polling frequency, use COV for stable points, batch reads, stagger schedules, limit unnecessary broadcasts, and tune workstation graphics refresh rates. Also fix devices that repeatedly retry or flood the network.