TechSNAP 162 Question on WiFi Roaming

Dear Marlene, Allen and Chris, wifi testing image

Catching up on Episode 162 of TechSNAP I heard Marlene’s question. Doing conference style roaming on a small budget tends to be very frustrating, because there are a lot of moving parts that most WiFi-EAP (enterprise grade) systems are pre-assembled with. When you have to assemble your own components, you start taking your network design very carefully. Rather than recommending any particular products in this post, I will discuss the basic assemblies involved so that you can get a sense for what you would need to assemble.

Your everyday residential WiFi access point provides DHCP for handing out IP assignments. When planning for a wireless environment, consider that your average business or technical attendee will bring three wireless devices. This means three IP addresses. If you expect 200 people to attend (round up for safety) you will need capacity for a minimum of 600 IPs. This is beyond what a consumer AP can handle (which is typically 24-32 stations). Also throw in more headroom for IP leases that are timing out. Plan you lease length to free up your pool as attendees leave. You can increase the lease time if there is less walk through traffic.

We are now looking at a network address mask of /23 or This provides for 510 hosts. You can play with this calculation with the ipcalc utility on any Linux commandline. If you want better roaming, consider setting up DHCP for your wireless network on a network server that is not itself an AP. A PFSense box would work well. If you have a DHCP server for each AP, you might get yourself into a big mess. Most channel hopping between APs does not actually trigger DHCP.

Another service that some people want is a Captive Portal. It is arguable that this is necessary. You do not want multiple captive portals. A captive portal service would be desired if you were obligated to issue Legal Warnings or TOS or other stuff. Rather silly for people using smartphones, because mobile browsers are still not great.

I’ve mentioned channel hopping. This is when a wifi station (a mobile phone or laptop) changes frequency. It will try to change to a stronger frequency with the same SSID with the same authentication scheme. That is, you cannot roam between SSID Burger and SSID Fries, nor could you roam between SSID Burger with WPA-Personal and SSID Burger with WAP-EAP. Effect frequency distribution is when you have middle-strength (not to hot) signal not overlapping with any other active adjacent frequency. So assign different frequencies using something like 12-75% signal power (12% can be plenty effective, you’re going for even coverage, not one large beacon) and move them around while using a mobile WiFi inspector app. At LinuxFestNW, there were about 27 APs setup across the conference hall. There were separate APs for each classroom. No two adjacent classrooms should share the same frequency, and discourage two adjacent frequencies from being next to each other if possible. Do as much in the 5Ghz range as possible. By having smaller wifi ranges, you reduce the transmit collisions per area and improve overall throughput.

Isn’t this a lot to take in? You bet. This is often why facilities invest in commercial WiFi installations and proprietary vendors. If you want to experiment from home, try looking at this nice post by Scott Hanelman on doing roaming with two APs.

Best of luck!
Jed Reynolds
PS If you are interested in enterprise WiFi or wired network testing, please check out Candela Technologies.

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