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

LonWorks 41-3
Channel 1
Subnet 2
Channel 4
Repeater
Channel 3
Group 1
1/3
2/3
2/10 2/11
3/3
Channel 2
1/2
2/2
3/2
1/1
2/1
Subnet 4
3/1
Subnet 1
Subnet 3
s/n ... Subnet/node address
4/12
Router Router Domain ID of all nodes: 0×53
Router
Channel 5 (backbone)
FIGURE 41.2
Physical and logical segmentation in LonWorks systems.
The physical and logical view of a LonWorks network example is shown in Figure 41.2. The network
physically consists of several channels connected by routers and repeaters. Logically, it is separated
into subnets. One domain can consist of up to 255 subnets with a maximum of 127 nodes each, which
results in a maximum of 32,385 nodes per domain. Additionally, nodes can be grouped. Per domain 256
groups can be defined. The example contains one group, which spans three nodes in different subnets
and channels.
As mentioned before, different transceivers support different communication media. A common
medium is twisted pair with a data rate of 78.kbps, where the signal is coded as voltage difference
between the two conductors. A compatible variant of twisted pair is Link Power, which also provides
the power supply (42.4.V DC) for the nodes over the twisted pair medium. For backbones, twisted pair
can also be run with 1250.kbps, but only with a reduced maximum distance of 130.m among the nodes.
The topology of twisted pair networks does not have to be a strict line or star. Networks can also
be built as free topology containing arbitrary branches and loops. However, the maximum distance
decreases when using free topology. So, for example, a line bus with short branches allows a maximum
distance of 2700.m among the nodes, whereas in a free topology, it is limited to 500.m.
Other supported media are power line with up to 10.kbps, fiber optics (1.25.Mbps), radio frequency
(up to 19.5.kbps), and the above-mentioned LonWorks/IP (100.Mbps with current IP routers).
To increase the maximum distance of LonWorks networks, repeaters can be used. They simply refresh
all incoming signals and send them to the other network segment, which has to be the same medium
and has to use the same message coding.
Since LonTalk implements the OSI reference model, LonWorks networks can be connected at higher
layers, too. Bridges, working on the data link layer, can be used to connect networks based on different
media and coding. They also ensure that only valid messages are forwarded to the other side.
More common than pure LonTalk bridges are routers, which work on the network layer. Routers have
the advantage that they do not only increase the range of the signals or connect different media, but
they also separate the network traffic of different subnets because routers only forward those messages,
whose destination is on the other side of the router. Therefore, routing tables are used. They describe
which subnets or groups are connected on which port of the router. There are two methods how routing
tables can be obtained: In configured router mode, the tables have to be defined by the system integrator
and in learning router mode, the network traffic is used to determine which subnets are connected to
which port. LonWorks routers can also operate in bridge and repeater mode.
© 2011 by Taylor and Francis Group, LLC