User Manual

Network architecture
BACnet architecture (MLN & ALN)
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192 | 351 CM110664en_07
Designation Description
Max APDU Length Accepted Maximum length of application message (Application Protocol Data Unit) supported for this
device. The length depends on the transport medium used, and the capacity of the device
buffer. The length of the APDU must always be less than the length of the smallest NPDU
(Network Protocol Data Unit) between the different bus subscribers.
Beispiel There are two IP networks linked by a PTP connection. The two IP bus subscribers could
have a maximum APDU length of 1476 octets. However, since the maximum NPDU length of
the PTP connection is 500 octets, the maximum APDU length of both devices must be set to
480 octets.
Values for LonTalk Range: 50/128/206 Octets
Default value: 206 Octets
Values for MS/TP Range: 50/128/206/480 Octets
Default value: 480 Octets
Values for IP (equal for IPv6)
APDU Segment Timeout
Range: 50/128/206/480/1024/1476 Octets
Default value: 1476 Octets
Timeout of an APDU segment (= part of an APDU). This value must be identical throughout
the internetwork.
Range: 1000...5000 ms
Default value: 2000 ms
APDU Timeout Timeout for an acknowledged message. This value must be identical throughout the
internetwork.
Range: 1000...5000 ms
Default value: 3000 ms
Number of APDU Retries Number of retries in the event of an APDU or APDU segment timeout. This value must be
identical throughout the internetwork.
Range: 1...5
Default value: 3
Window size
To transfer large data packs, BACnet uses the windowing algorithm. Windowing means that instead of
acknowledging individual segments separately, the acknowledgement applies to a specific number of
segments, referred to as a window.
Definitions for Desigo
The window size is permanently set to four for all Desigo devices, so that for segmented messages, only
every fourth segment is acknowledged.
Network layer
The most important information in the network layer is the network number of the BACnet network.
Network number
The network number is the unique identification of the BACnet network. There are stationary and
temporary networks:
● Stationary networks are defined during commissioning and then remain unchanged.
● Temporary networks are created when a tool (e.g., XWP/ABT) dials into a network via PTP.
Application Layer
Network Layer
PTPIPLonTalk MSTP