[ 4 pages—2,831 words]


INTRODUCTION

A local area network (LAN) is a private network usually confined to one plant. Virtual LANs (VLANs) allow a single physical LAN to be partitioned into several smaller logical LANs. VLANs limit the broacast domain, improve security and performance and are ideal for separating industrial automation systems from information technology systems.


Structured Wiring

One of the advantages cited for migrating to Industrial Ethernet from fieldbus technology is found in the comment "our plant is already wired for Ethernet. I do not need to run specialized wiring since twisted-pair wiring is already in place." This could be true since Ethernet cabling installations typically follow structured wiring standards such as TIA/EIA-568-A Commercial Building Telecommunications Cabling Standard. Following the standard, end stations at each work area would be wired to patch panels in a wiring closet (see Figure 1). The cross connection between end stations and hub ports are made with short patch cords. Out of each wiring closet is a single connection to a cascaded hub located in an equipment room. All wiring closet feeds go to the equipment room, but it is the intent of the standard to limit the number of levels of hierarchy. It is quite possible that the plant floor is wired in a similar fashion and, in this way, all stations within the plant share the same LAN.



Figure 1 — Structured wiring creates a hierarchy of hubs.

Sharing the same LAN may not always be a good idea. LANs are typically maintained by the information technology (IT) department that has become increasingly more interested in a secure network than maximizing up-time. Disconnecting a user suspected of having a faulty station by removing a patch cord is typically done and is treated as an inconvenience to the user. However, the same action done to a device on an industrial control system could be disastrous. Therefore, it has been suggested to have two LANs—one for IT and one for industrial automation systems. This would certainly remove the security concerns of the IT department, but segregating the physical wiring may not be possible nor convenient.

There is another reason to separate the information technology LAN and the industrial automation system LANs. A LAN is considered a single broadcast domain. This means that broadcast messages (messages destined to all stations) will be sent to every station on the LAN. This is usually true for multicast messages (messages destined to many, but not all stations). If the exact location of stations that are to receive a multicast message is not known, than all stations will receive the message. Industrial automation protocols frequently use the producer/consumer model in order to improve real-time response. In the producer/consumer model, one originating message that is produced by one station is consumed by several stations called consumers. With Ethernet, this generates many broadcast and multicast messages that can consume the total bandwidth of the LAN. Is there another way of retaining the same physical network, but allowing separate LAN functionality? Yes, there is, and it is called virtual local area networks (VLANs).

(No part of this article may be reproduced without the written consent of the Industrial Ethernet University.)