Introduction to Industrial Automation

8.3.3   Network  Devices

Due to various factors, industrial networks (as well as networks in general) have a limited capa-

bility in terms of both the number of interconnected stations as well as the maximum length 

of interconnecting network cables. Therefore, problems are often created when it is necessary 

to extend a network of PLCs or to interconnect an existing network with another one. To 

overcome these limitations, four network devices are used, namely the repeater, the bridge, the 

router, and the gateway. With the help of these devices, it becomes possible to extend a PLC 

network, either in terms of the number of connected stations, or in terms of geographic area 

and range. Also, with the help of these devices, the communication between heterogeneous 

networks of PLCs is possible.

Repeater. The repeater is used only in homogeneous networks, i.e., networks that follow the 

same exact communication protocol. The repeater just regenerates and forwards bits from one 

sub-network to another, making two sub-networks look like one. Great distances are covered by 

concatenating two or more network segments via a repeater. Repeaters do not need software to 

operate, since they just copy and regenerate bits blindly, without any further processing on the 

repeated data packets.

Bridge. The bridge performs the connection of two networks which have different data link 

layers (data frames with different media access control addresses) but the same network layer as 

in the OSI model. The data frames from one network segment, reaching the bridge, are analyzed. 

After checking and filtering the destination address, they are forwarded to the second network 

segment. Bridges, unlike repeaters, use software for their operation. They are programmed for 

exchanging certain data between subnets and generally for carrying out the necessary modifi-

cations during their operation. As an industrial example, by using two CAN@net II/Generic 

bridges by the HMS/IXXAT Company, a CAN-Ethernet-CAN* bridging can be implemented. 

This bridge allows the exchange of CAN messages between two separate CAN networks via the 

Internet, where filter tables can be defined.

Router. A router can connect two dissimilar networks. In terms of the OSI model layers, a 

router connects two networks that differ in the three lower levels. This means that the router 

is used when two networks have the same transfer layer but different network layers of the OSI 

model. The router, unlike the bridge, may have its own address in the network. Some routers are 

also used as industrial secure devices protecting control networks from critical assets.

Gateway. The gateway is the most general type of device for interconnecting heterogeneous 

networks. It has the ability to interlink completely different networks in terms of communication 

protocols, even networks that are different in all the seven layers of the OSI model. The gateway 

is also used for the interconnection of networks that do not follow the OSI model. Gateways are 

generally more complex than bridges or routers. Figure 8.8 shows the use of the above devices 

*

 

Controller area network (CAN) is a well-known industrial network.

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Industrial Networks of PLCs 

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to create an integrated interconnection of different or similar networks, called a “multi-network 

communication structure”.