Design considerations for using Ethernet rings by Mike Jones

Tag : cable identifier

Industrial Control and Automation networks generally use uniquetopology.Unlike typical office Ethernet 'star' networks where amulti-port switch provides point-to-point links to the other nodes,the control layer of an industrial network is usually based on aring layout.A ring or daisy-chain simplifies cabling and can extendEthernet's reach without going to the expense of a fibre-basedsystem. Mike Jones examines the design implications.

Reduction in the required cabling of an Ethernet ring networkprovides benefits welcome to the automotive and aerospace industry.Less cabling means weight reductions which has a direct impact onfuel efficiency and hence, overall costs. It also suits the layoutof the typical automation cell where production tends to take placein a linear progression down the factory floor.

The introduction of Ethernet over lightweight Plastic Optical Fibre(POF) has added a further dimension by lowering of costs both inmoney and weight. An example applications is shown in Fig. 1 using a simple surveillance system distributed around a buildingperimeter. By linking each camera/sensor as a physical network ringaround the perimeter, such techniques will prevent the need for aseries of longer cable runs each routed back to a central switch.It also makes it easier to keep to the 100m link limit specified inIEEE 802.3 section 40.7. The basic building block in a ring networkis the 3-Port switch. As we will discuss shortly the Ethernet ringrequires managing to ensure that one of the links is 'broken' toprevent endless message looping.

Fig. 1.A ring network requires managing against the endless packetloop

However, there are two major issues as a result of implementing anEthernet ring network; management of the loop and increasedlatency. A ring network also limits bandwidth, but this can bedeemed as a minor issue as the 100Mbps offered by Fast Ethernet isusually more than adequate for control-layer networks.

Managing the Ethernet Ring