Wireless PLC Network

Hi

I had previously bought a cheap router ($50) so I can setup a wireless network in the plant.

The plant had say 4 PLCs and 4 SCADA clients and a few HMI displays. I wanted a wireless network so I can take my laptop around freely.

When I connected the router to the network I was struggling to ping any PLC. Sometime I get latency of 2000ms, sometimes 5ms and sometimes I couldn't ping anything. I decided to disconnect two of the PLC to see if too much network traffic is affecting the wireless router. Turns out it was the issue, the wireless router couldn't switch fast enough. I went and exchanged this router for a much more expensive one ($200+) and everything worked fine.

I would like to learn are there any key specs I need to focus on when buying a router that will be used in a heavy traffic network? Instead of focusing on buying an expensive one.

Allen

I would guess it had/has more to do with the RF output of the WiFi modem in the router. With the amount of equipment you listed I wouldn’t think that it would be too much traffic for a device that should be capable of up to 20Mbps. Most WiFi radios have an auto throttle feature so you can set it up to find the best possible speed. When they slow down the RF output (usually) goes up and the receiver sensitivity goes up as well. With a weak transmitter (20mW?) to get any sort of range it will have to throttle down quite a bit to make a connection to your laptop (maybe all the way down to 1Mbps). With a more expensive router the WiFi modem may possibly have a more powerful transmitter meaning it can go further at higher speeds.
There are lot of other possibilities but one thing to remember that even at $20 they are still making a profit so the build cost is probably around $1 or $2. You can’t build a well designed and powerful transmitter (and all of the other components that makes up the router) for that little money.
I’ve been in the industrial automation wireless data communications for almost 20 years and I see this again and again, cheap comes out expensive (I’m a little surprised you got it working for $200).

Allen,

In a factory environment, you may deal with many different types of interference, so the reliability of your network is dependent upon the hardware, hence the problems you experienced.

Below is a link showing a network topology that you may want to consider.

http://www.eternity-sales.com/WirefreeCNC/WirefreeCNC.htm

The network contains a wireless access point and data relay points. This will most certainly be more expensive than what you currently deployed, but if it isn't doing what you need it to do, then it doesn't matter how inexpensive it is.

Disclosure - ESI is an authorized distributor for WireFreeCNC.

If you have any questions, feel free to post, pm or email.

The easiest way to find a good, inexpensive router with a ton of features is to see what is being recommended by the 3rd party router firmware people. Tomato and OpenWrt being the major players. A really good one for very little money is the ASUS RT-N16.

http://wiki.openwrt.org/toh/buyerguide
http://www.flashrouters.com/blog/category/best-tomato-routers/

It's possible that the router was overloaded by attempting to handle traffic on the wired side, not on the wireless side.

Are you using any protocols like Rockwell's EtherNet/IP in muticast mode on your wired Ethernet network ?

A few years ago I did an informal evaluation of some wireless network access points and routers for use in EtherNet/IP networks and was dismayed to find that the popular Linksys WRT54G could actually be bricked by being exposed to high volumes of UDP multicast traffic.

I have ran into trouble with DNS also. When using a router as a wireless access point, make sure to turn off all DNS related settings.

The problem with evaluating most networking equipment is that the specs they list are basically bullet point list of features supported. These are important to determine interoperability and basic needs, but do little to show the quality of the device, or what it can handle.

Very rarely do they include things like packets per second, the max size of their arp table, or how many simultaneous clients they support, that are indicators of performance under load. While a more expensive Access Point is not always better, if an AP seems like a bargain it probably isn't worth it.