What I-Am Is
I-Am is a BACnet unconfirmed service broadcast by a device in response to a Who-Is request. It is the device’s way of announcing its presence and identity on the network.
What an I-Am Contains
| Field | Description |
|---|---|
| Object Identifier | Device object type + Device Instance number |
| Max APDU Length | Maximum application protocol data unit the device can accept |
| Segmentation Supported | Whether the device supports segmented messages |
| Vendor ID | Numeric vendor identifier (assigned by ASHRAE) |
How Discovery Works
- Supervisor broadcasts Who-Is (optionally with a device instance range)
- Each matching device broadcasts I-Am
- Supervisor collects I-Am responses and builds its device table
- Subsequent communication uses the device’s address from the I-Am
[!NOTE] I-Am is broadcast — it reaches all devices on the subnet, not just the original Who-Is sender. On BACnet/IP networks, BBMDs forward I-Am broadcasts across subnets.
Common Issues
- No I-Am response — device may be offline, misconfigured, or on a different subnet without BBMD forwarding
- Delayed I-Am — some devices respond slowly; supervisory systems with short discovery timeouts may miss them
- Duplicate Device Instances — multiple I-Am responses with the same instance cause address conflicts