wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

KNX 42-3
Domain A
Backbone line
Domain B
Area BC
Main line
LC
LC
LC
B
Line
Line
Line
FIGURE 42.1
Topology.
In addition to individual and group addresses, KNX also defines the use of polling addresses allowing
a device to request status data from up to 15 other nodes with minimal protocol overhead. This
communication scheme is useful for high-frequency node liveness checking and rests upon the service
L_Poll_Data. It is limited to a single physical segment. Finally, to avoid interference between different
KNX installations on open media, KNX supports the use of domain addresses.
The network layer (NL) uses the services provided by the DL and offers four different NL services:
a unicast service (N_Data_Individual), a multicast service (N_Data_Group), a domain-wide
broadcast service (broadcast within a single domain; N_Data_Broadcast), and a system broadcast
service (broadcast across domain borders; N_Data_SystemBroadcast). For all four services, the
individual address of the sender is used as the source address. The destination address type depends on
the used service: for unicast communication, the individual address of the receiver is used, while for
multicast communication, the group address of the destination group is used. The broadcast services are
implemented by using a DL group address of “0.” Additionally, the NL introduces a hop count, which is
decremented and examined by routers and repeaters to perform filtering based on the amount of elapsed
hops of a packet.
The TL uses the NL services and enriches them by providing a connection-oriented unicast service
(T_Data_Connected). Using this service, a device is able to establish a reliable unicast connection
to another device. The state machine used implements an acknowledgment mechanism where data
packets are retransmitted in case of negative or absent acknowledgments. The other NL services are
transparently passed through (T_Data_Individual, T_Data_Group, T_Data_Broadcast,
and T_Data_SystemBroadcast).
The AL on top of the protocol stack supports a multitude of AL services. Generally, these services
can be divided into two different service classes. The first one is dedicated to exchanging
process data (process data communication). Beside other rarely used opportunities to exchange
process data in KNX (e.g., using the polling mechanism), the most important ones are the group
communication services (A_GroupValue_Read and A_GroupValue_Write). Since they
are based on T_Data_Group, they are multicast services. The second class of services is used
for configuration and maintenance tasks (management communication). It includes services for
uploading user applications, assigning individual and group addresses and accessing diagnostic
information.
KNX defines different frame formats depending on the communication medium and service. The
most commonly encountered format by far is the twisted-pair standard data frame L_Data, shown at
the top of Figure 42.2.
The frame starts with the control octet. This octet indicates the frame format (Frame Type) and
contains the priority field. It also holds a repeat flag that is set for retransmitted frames. The following
© 2011 by Taylor and Francis Group, LLC