The Wrong Switch Can Spoil The Show


A few weeks ago, I was out of town attending a trade show and doing what most people do in that situation: spending eight hours a day on the show floor and trying to keep up with email in the evening. If I ate dinner and then went back to the hotel for my online work, my email client would randomly announce that it couldn't connect. When I was connected, my zippy WiFi downloaded files as though I were on dialup. However, if I went to the hotel directly after the show, I could check my email and download files, all very quickly and easily until about 8:30pm. Then it all went bad again, and a better use of my time was to head out for a late dinner. What was going on?

Simple — everyone else was back from dinner and online at 8:30pm, and the hotel's wireless network couldn't handle the traffic. Congestion was crippling the network.

Congestion can cripple a show's Ethernet network, too, if the network is not planned properly and equipment is picked that is not suitable for the demands of live performance. Picking the right Ethernet switch, in particular, can be important for any lighting control system running ACN (ANSI E1.17) or other Ethernet-based control protocols. There are few switches made specifically for the entertainment market, so people usually pick equipment designed for office environments. “You get what you pay for,” but, paradoxically, neither the cheapest nor the most expensive equipment is likely to be right for the job. Ethernet switches are available at extremely low prices — a $20 switch is connected to my computer right now — but really low cost equipment is unlikely to be able to handle the volume of bursty data that goes with a live show environment. On the other hand, an enterprise-level switch, suitable for an IP phone system in an office building, probably isn't the best choice either.

An Ethernet switch is not a passive device. It actively reads incoming messages into memory and then sends each message out the port that is connected to the intended destination device. It is possible to have a device on port 1 sending a message to a device on port 3, while something on port 4 is talking to something on port 2. Multiple messages may be passing through the switch at one time, but only one message at a time is going to be sent out from a port to a destination device. If a multicast message comes in on port 5 and is intended for all the connected devices at the same time the other ports are busy, it's going to have to wait in memory until the ports are free. Thus, the throughput of a switch — its ability to handle congestion — is determined not simply by the speed of the Ethernet links, but also by the processing power of the switch. Twenty-dollar switches work well for the price, and an occasional few-second delay in a home network isn't likely to bother anyone, but a $20 switch most likely isn't the best choice for a show's lighting control system.

While putting a $20 switch at the hub of a lighting control network probably isn't wise, putting a really expensive switch there might not be good either. The $20 switch is the bottom of the home/small office range. The next range up for switch manufacturers is the enterprise level range. These are designed for complex systems, and the hardware is certainly capable of handling the data required to run a large show, but the technician who has to install and configure the switch might have difficulty dealing with it. A plethora of features make these complicated, and they usually don't come with a default plug-and-play setting. If there are defaults, they may not be ones you want. For example, a popular feature is IGMP snooping. A switch with IGMP snooping monitors the multicast traffic and shuts off ports connected to devices that are continuously spewing data to every other device. In an office environment, this data torrent could be caused by a computer stuck in a loop or a denial of service attack. However, in a show environment the device most likely to be spewing data is called “the lighting control console,” and we don't want its port shut off.

Configuring enterprise-level switches is a job better left to IT network managers. If you don't have one on staff, stay away from these switches. Pick equipment at the high end of the home/small office range or, better yet, equipment that is specifically made for the entertainment market. If you are going with equipment manufactured for the office environment, advice on what has been proven to work reliably is available from vendors of entertainment lighting control equipment.

Karl Ruling is technical standards manager for ESTA.

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