KNX Data Link Layer

Following the OSI network layer architecture model, KNX defines its Data Link Layer between Network Layer and Physical Layer.

Data Link Layer Services

KNX Data Link Layer can be in either "normal" or Bus Monitor mode. The available services in the data link layer change accordingly.

Data Link Layer services can also be considered either remote or local, depending on whether an actual frame is transmitted between the originating data link layer and a remote data link layer. Certain data link layer services only affect the local (originating) data link layer and therefore have no corresponding remote (receiving) data link layer.

Data Link Layer also defines device addressing.

	Individual Addresses
	Group Addresses

Service	Distribution	Availability
Data Service (L_Data)	Remote	Normal Mode
Poll Data Service (L_Poll_Data)	Remote	Normal Mode
Service Info Service	Local	Normal Mode
Bus Monitor Service	Local	Bus Monitor Mode

Reliability and Retry Policies

KNX Data Link Layer attempts to implement a reliable transport of frames between devices in the subnetwork. In case of failures, retries to transmit the frame are made. A level of flow control is implemented in case of a slow or overwhelmed receiver if the receiver indicates it cannot receive more frames (buffers are full). Retry policy can be configured in the common EMI control fields for remote data service frames.

On receiving incoming frames, KNX Data Link Layer ensures the frame has not been corrupted while transmitted on the physical media. It will also check the destination address matches to that of the receiving data link layer and sends acknowledgements on successful frame receives to the transmitting data link layer. Acknowledgment policy can be configured in the common EMI control fields for remote data service frames.

Frame Priorities

Link layer frame priority can be set in common EMI frame control field.

A frame priority is a 2 bit value with the following semantics:

Priority	Value	Description
Low	0x3	Low frame priority. This is mandatory for long (extended) frames where the Application Protocol Data Unit (APDU) is longer than 15 bytes. This priority is also recommended for burst traffic. See the Common EMI frame for details on standard (short) and extended (long) frames.
Normal	0x1	Default priority for short (standard) frames. See the Common EMI frame for details on standard (short) frames.
Urgent	0x2	Frames using urgent priority should not exceed more than 5% of total traffic during any one minute period in the network.
System	0x0	This priority is reserved for frames used in system configuration and maintenance.