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KNX protocol stack-PHY,datalink,routing,transport,application layers

This KNX tutorial page covers KNX protocol stack basics. The KNX protocol layers include physical layer,datalink layer,routing or network layer,transport layer and application layer.
Refer KNX basics➤➤.

Figure-1 below depicts KNX protocol stack layers. Refer OSI protocol stack layers which mentions basic and generic functions of each of the layers used in KNX.

KNX protocol stack

KNX Physical layer

The KNX physical layer is specified in Vol.3, Chapter 3/3/1. KNX protocol stack uses any of the following physical layers.
• Twisted Pair (KNX-TP)
• PowerLine (KNX-PL)
• Radio Frequency (KNX-RF)
• KNX-IP

KNX-TP uses CSMA/CA mechanism at MAC layer using inductive coupling. The Bus delivers 21-29 Volt power supply. Low power KNX nodes using the twisted pair and draw power of about 150 mWatt.

KNX-PL version PL110 uses FSK modulation. Each of the PLC line will have about 64 devices. As PLC is a broadcast open media, the separation of domain is done using 48 bit domain address.

KNX-RF physical layer uses 868 to 870 MHz frequency band.

KNX-IP uses IP protocols as medium of communication. It uses binary or XML encoded PDUs.

KNX protocol-datalink layer

KNX Telegram format

Various Physical layers will use different telegram formats in KNX protocol stack. Figure-2 depicts KNX telegram formats for TP(Twisted Pair), PL(PowerLine) and RF (Radio Frequency) PHY layer modes.

This telegrams are transmitted octet by octet. These are acknowledged by recipient node.

KNX routing or network layer

Each of the KNX node has 2 byte unique address assigned. It is used as source of telegrams or used for configurations. Source address = { Area Identifier(4 bits),
Line identifier(4 bits
), device number ( 8 bits) }

Source address here is used as physical address. It is configured during installation. Here device number 0 is used for backbone and line couplers.

Destination address can be used either as logical group address(bit 17 = 1) or as physical address (bit 17=0). Group adress 0000h will address all the KNX nodes. The KNX installation will have 64K space. All the individual as well as group addresses are stored in the address table of the KNX device.

Total 4 priority levels are supported by KNX protocol and encoded as specific values of control bits in the frame.

KNX protocol stack-transport layer

The transport layer of KNX protocol stack provides connection oriented peer to peer communication service. It provides connect primitive, disconnect primitive, TL-ACK, Sequence counter, time out management(about 6sec for configuration mode). It is defined in Volume 3, Chapter 3/4.

KNX application layer

It is defined in Vol. 3, section 3/3/7. It defines group objects and exchange of object values through service requests. It also defines "property value write" service. This service is used to set values and also to configure parameters to KNX device for interfacing objects.


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