ETHERNET • 115
Network Architecture – Principles and Regulations
Cabling guidelines
"Structured Cabling" specifies general guidelines for network architecture of a LAN, establishing maximum cable lengths for the grounds area, building and floor cabling.
Standardized in EN 50173, ISO 11801 and TIA 568-A, "Structured Cabling" forms the basis for a future-orientated, application-independent and costeffective network infrastructure.
The cabling standards define a domain covering a geographical area of 3 km and for an office area of up to 1 million square meters with 50 to 50,000 terminals. In addition, they describe recommendations for setting up of a cabling system.
Specifications may vary depending on the selected topology, the transmission media and coupler modules used in industrial environments, as well as the use of components from different manufacturers in a network. Therefore, the specifications given here are only intended as recommendations.
5.2.3 Coupler Modules
There are a number of hardware modules that allow for flexible arrangement for setting up an ETHERNET network. They also offer important functions, some of which are very similar.
The following table defines and compares these modules and is intended to simplify the correct selection and appropriate application of them.
Module |
Characteristics/application |
ISO/OSI |
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layer |
Repeater |
Amplifier for signal regeneration, connection on a physical level. |
1 |
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Bridge |
Segmentation of networks to increase the length. |
2 |
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Switch |
Multiport bridge, meaning each port has a separate bridge |
2 (3) |
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function. |
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Logically separates network segments, thereby reducing network |
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traffic. |
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Consistent use makes ETHERNET collision-free. |
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Hub |
Used to create star topologies, supports various transmission |
2 |
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media, does not prevent any network collisions. |
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Router |
Links two or more data networks. |
3 |
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Matches topology changes and incompatible packet sizes (e.g. |
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used in industrial and office areas). |
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Gateway |
Links two manufacturer-specific networks which use different |
4-7 |
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software and hardware (i.e., ETHERNET and Interbus-Loop). |
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Tab. 5-2: Comparison of Coupler Modules for Networks
WAGO-I/O-SYSTEM 750
ETHERNET TCP/IP
116 • ETHERNET
Network Architecture – Principles and Regulations
5.2.4 Important Terms
Data security
If an internal network (Intranet) is to be connected to the public network (e.g., the Internet) then data security is an extremely important aspect.
Undesired access can be prevented by a Firewall.
Firewalls can be implemented in software or network components. They are interconnected in a similar way to routers as a switching element between Intranets and the public network. Firewalls are able to limit or completely block all access to the other networks, depending on the access direction, the service used and the authenticity of the network user.
Real-time ability
Transmission above the fieldbus system level generally involves relatively large data quantities. The permissible delay times may also be relatively long (0.1...10 seconds).
However, real-time behavior within the fieldbus system level is required for ETHERNET in industry.
In ETHERNET it is possible to meet the real-time requirements by restricting the bus traffic (< 10 %), by using a master-slave principle, or also by implementing a switch instead of a hub.
MODBUS/TCP is a master/slave protocol in which the slaves only respond to commands from the master. When only one master is used, data traffic over the network can be controlled and collisions avoided.
Shared ETHERNET
Several nodes linked via a hub share a common medium. When a message is sent from a station, it is broadcast throughout the entire network and is sent to each connected node. Only the node with the correct target address processes the message. Collisions may occur and messages have to be repeatedly transmitted as a result of the large amount of data traffic. The delay time in a Shared ETHERNET cannot be easily calculated or predicted.
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Fig. 5-6: Principle of Shared ETHERNET |
G012910e |
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WAGO-I/O-SYSTEM 750
ETHERNET TCP/IP
