Tunnelling FAQs

KNXnet/IP tunnelling is the primary method of interfacing to a KNX system. It allows for Unicast communication from a single external device to the KNX system. This is akin to using a USB or Serial Interface to interface to the KNX system.

What maintains the tunnel connection?

The Control End Point connects, maintains and disconnects the tunnel connection. To be more precise, control of the tunnel connection is the only thing the Control End Point does. The Control End Point sees very little traffic, only two or three messages to connect or disconnect and one or two messages per minute to maintain the connection.

Figure 7.   Maximum Received Packets Que Size
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The Maximum Received Packets Que Size controls the size of the control end point’s receive queue. It defaults two five packets. If this value were too small, there would be an indication of lost frames in the hidden Rx Frames Lost Queue Full communications counter.

Figure 8.   Rx Frames Lost Queue Full
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Which queue is used for data through the tunnel connection?

The Maximum Received Packets Que Size controls the size of the tunnel connection’s Data End Point receive queue. All Group Address messages pass through this queue and End Point so it is much busier than the Control End Point. Because it handles more traffic, its default queue size of 50 is larger than that of the Control End Point’s queue size.

Figure 9.   Maximum Received Packets Que Size
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Unlike the Group Data Operation Queue, the necessary size of this queue does not depend on the number and frequency of control point polling. Instead, it depends on how quickly the KNXnet/IP driver can process incoming messages, which in turn depends on the overall CPU usage in the platform.

You can monitor the success or otherwise of received data by inspecting the Rx Frames Lost Queue Full counter in the hidden communications counters. Depending on the number and frequency of lost Rx frames you can try increasing the Maximum Received Packets Que Size in steps of 50, until no more Rx frames are being lost.

Figure 10.   Rx Frames Lost Queue Full
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