User Manual

Network architecture
BACnet architecture (MLN & ALN)
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186 | 353 CM110664en_07
Large networks are structured, that is, divided into several (logical) network
segments for reasons of security, performance, size and (limited) address range
of network devices. The segments must then be connected to routers of the
corresponding transport protocol (e.g., LonWorks router, IP router).
In most cases it is not necessary to divide a BACnet/LonTalk network into
several LonWorks segments (ALN). However, if it does prove necessary, it is
not possible to use a LonWorks router, because this limits the length of the
data packets. An L-switch (Loytec) can be used as a router on the ALN.
BACnet MS/TP networks cannot be segmented, because there are no
associated routers.
With BACnet/IP some IP segments may be connected by IP routers. Since the
IP router prevents broadcasting, the connection must be activated with the
BACnet Broadcast Management Device (BBMD).
Physically, (cable) networks cannot be expanded as desired. Depending on
electrical transmission properties and the data link layer based on it, repeaters
must be added at specific cable lengths to amplify the signal. This divides the
network into multiple, physical segments. A repeater does not impact the
transmission protocol; it merely electrically connects two physical networks.
Dividing up the network into several physical segments may be necessary in
LonWorks technology.
The physical segments are connected with physical or logical repeaters. Due to
the limited buffer size of logical repeaters, only physical repeaters may be used
on the ALN. Only one physical repeater may be located between any two nodes.
MS/TP is transmitted on a two-wire cable as per EIA-485/RS-485*. The length
of the physical segment can be max. 1,200 m and can be extended with EIA-485
repeaters.
*TIA standard (Telecommunications Industry Association): TIA-485-A Electrical
Characteristics of Generators and Receivers for Use in Balanced Digital
Multipoint Systems (ANSI/TIA/EIA-485-A-98) (R2003)
A site is an independent and self-contained logical entity within the building
automation and control system. This type of structuring is not defined by
BACnet, and is therefore largely independent of the BACnet network topology.
The BACnet devices bound to a site can therefore be placed anywhere within a
BACnet internetwork. A site cannot extend across a PTP connection.
Communication occurs only within the site, but data can be exchanged with any
device on the BACnet internetwork.
Only automation stations (PXC/PXC3) and LonWorks system controllers
(PXC...D+PXX-L11/12)) are assigned to the sites, by special structuring of the
Device ID and Device Name. These products are considered third-party devices
for the purposes of a site.
Protocol layer model
Desigo supports:
● BACnet/IP
● BACnet/LonTalk (LonWorks technology)
● BACnet/PTP
● BACnet MS/TP
● BACnet/IPv6 (only via PXG3 BACnet router)
Segment
Physical segment
Desigo site