BACnet Foreign Device

Intro

BACnet/IP networks use UDP to communicate. UDP is a connectionless protocol therefore when a message is broadcasted on one subnet, routers do not forward the message onto other subnets. This means that a BACnet Client will not be able to discover other BACnet servers on different subnets using a UDP broadcast WHO-IS message. There are two solutions to this problem:

  1. BBMD to BBMD registration
  2. A BACnet device on a different subnet can send a Foreign Device Registration to a BBMD on the local network, or a BACnet client device

Although a BBMD to BBMD connection between subnets will always allow for broadcasts and message forwarding, it is not always cost-efficient to install, configure, and maintain a BBMD on a subnet where there are no other BACnet nodes. The BACnet spec provides an alternative to this in the form of foreign devices.

What is a Foreign Device?

A foreign device is a BACnet device with an IP subnet address different from the comprising BACnet/IP network which the device seeks to join (see BACnet Spec J.5.1). The foreign device on a different subnet may send a Register-Foreign-Device Message to the BBMD on the comprising BACnet/IP network to become registered within the BBMD’s FDT (Foreign Device Table) (see BACnet Spec J.2.6.1). Once registered, upon recipient of a local broadcast, the BBMD shall create a forwarded message and send it to each foreign device currently registered in its FDT.

Each device registered in the BBMD’s FDT has a Time-To-Live value in its entry which is the number of seconds within which a foreign device must re-register with a BBMD or risk having its entry purged from the BBMD (see BACnet Spec J.5.2.1). Time-To-Live is two octets and is a parameter of the Register-Foreign-Device message. Upon foreign device registration, a timer is started equal the Time-To-Live with a fixed grace period of 30 seconds. This timer was implemented to conserve the FDT table space resource and purge any Foreign Devices whose Time-To-Live has expired. A device will have to re-register with the BBMD periodically to maintain full participation in the BACnet/IP network.

CAS BACnet BBMD - Would you like to be a Beta Tester?

Visit our BACnet BBMD Page for more information

The CAS BACnet BBMD (BACnet Broadcast Management Device) is a Windows and Linux software application that allows multiple BACnet/IP devices to send BACnet broadcast messages across NAT routers, and BACnet/IP devices with different IP/UDP ports to communicate with each other.

BACnet/IP uses IP/UDP broadcast messages to communicate on local subnet. If two BACnet/IP devices are on different subnetworks (VLans, NAT Routers, etc…) and need to communicate with each other, a BBMD device is needed to facilitate sending and rebroadcasting the messages on the different sub networks.

BACnet Integration Solutions

Chipkin has BACnet integration solutions for almost every situation. We specialize in network protocol communications and have over 20+ years of experience. Click for more information:


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