Saturday, April 2, 2011

Industrial Ethernet Reliability and Performance: Multicasting

Industrial Ethernet works on the same principles and protocols as any other Ethernet network.  In a nutshell, devices place units of data on the wire called packets.  Packets generally have a source and a destination.  When one device sends a packet directly to a second device, the process is called unicasting.  When a device sends a packet to all other devices on the network, the process is called broadcasting.  Finally, when a device sends a packet to specific group of other devices on the network, the process is called multicasting.
You might be asking yourself:  “where would multicasting be used?” One example that is often used in the IT world is a live camera feed.  If twenty people want to view a live camera feed, the camera shouldn’t have to manage twenty individual conversations, so instead, the twenty devices that will be showing the feed subscribe to a multicast group associated with the camera.  The camera sends packets to a multicast group destination address and the devices subscribed to that group receive and process those packets.
So, where does this apply in an industrial controls network? The first example that comes to mind is the use of producer and consumer tags on Rockwell Automation’s Controllogix platform.  One processor is a producer while one or more are consumers.  The consumers subscribe to a multicast group served by the producer.
The potential problem with all of this is that many switches handlemulticasts as broadcasts which creates a large volume of traffic on the network and can cause performance problems due to the packets being forwarded to every port on the network.  The simplest solution to this is to implement IGMP (Internet Group Messaging Protocol) snooping.  This is a feature that is available on some managed industrial Ethernet switches and on many standard duty managed switches.  Once IGMP snooping is enabled, the switch will remember which ports have devices that are members of a particular multicast group and will forward multicast packets only to the devices that should be receiving them.  This will greatly reduce network traffic especially if you have a large number of devices utilizing multicasting on your network.

[ Original Post by Jed Leviner]

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