KNX/EIB KNXnet/IP Telegram/Datagram structure (Length) - KNX load on an IP network

[Fahmi]

Hi, I need to learn some more information about the KNXnet/IP telegram structure (Fields and the length of each field), my ultimate goal is to know the maximum length of a KNXnet/IP telegram length in order to calculate the maximum added load by a KNX system on an IP network.

Would you please send me any information related to this subject to eng_afj@hotmail.com

Thanks and Regards

Fahmi

[makki]

Probably noone being in posession of this this will send you, sorry. The max. payload lenght is 14 byte right now.
Still, ask google for bcusdk, reading eibd-source and the thesis of Fritz Praus together with the keyword Freebus should make it easier to learn about..

Makki

[ThorstenF]

Hi, I need to learn some more information about the KNXnet/IP telegram structure (Fields and the length of each field), my ultimate goal is to know the maximum length of a KNXnet/IP telegram length in order to calculate the maximum added load by a KNX system on an IP network.

Would you please send me any information related to this subject to eng_afj@hotmail.com

The structure is fairly simple. You have IP + UDP + KNXnet/IP, which adds to 16 + 4 + 20 bytes for the packages, plus data payload. On the bus you have a minimum length of 6 bytes plus payload. A byte on the bus is encoded as 11 bits, the bus runs with 9600 bit/s. So you get a maximum telegram rate of 125 telegrams per second (not counting other stuff that might happen on the bus, like collisions). So on Ethernet you need at most 125 transfers of 41 bytes, which equals about 5 Kilobyte of data per second per KNX router or tunnel connected to the network.

Now, knowing that an Ethernet frame has a minimum payload size of 46 bytes, and knowing that on a 10 MBit Ethernet there are at most 14880 frames per second, you can connect 14880 / 125 = 119 KNX bus segments to a single 10 MBit Ethernet.