• What bandwidth limit for my project?
• I have XXX group addresses in my KNX project. Which solution of your products do I need?

Answer

• The bandwidth limit of the KNX TP medium is limited to 9.6 kbits/s. This means that there can be transfered between 20 – 40 telegrams per second on each single KNX TP line. An exact value will depend on the size of telegrams. An example project of 2400 data points with an average send interval of 120 seconds causes 20 telegrams per second, which again defensively calculated would be already 100% bus load. At peak times this would cause telegram loss and with it the loss of data. Therefore, NETxAutomation recommends increasing the number of  KNXnet/IP interfaces and the use of Ethernet as backbone. A project of 4800 data points equally split on 4 KNXnet/IP interfaces with a similar average send interval would cause 10 telegrams per second on the TP side. This equals a bus load of 50% and should leave enough buffer for peak loads.
• In general, it is better avoiding a three level structure if possible. TP main lines are bottlenecks, if all group telegrams are forwarded from the lines to the main lines and then to the IP backbone. NETxAutomation suggests using a two level structure (i.e. KNX TP lines and then IP main lines). Since KNXnet/IP Tunneling is used in our products to connect to the KNXnet/IP routers or interfaces, it can be communicated directly with the line. Thus a bottleneck at the main line is avoided.
• In addition filtering shall be enabled within the couplers and IP routers to reduce the traffic. Please keep in mind that if a three level structure is used and the OPC server is connected to the backbone, it has to be ensured that the required KNX group telegrams are forwarded through the couplers and routers. This can be done by using dummy devices in the ETS or by setting “Pass through Line Coupler”.