A reality check on high speed roaming over muni networks

What do you need to take into account when building a citywide wireless network if you are planning to use mobile voice over Wi-Fi? What issues arise when you plan for full roaming capability?Many of the RFPs for municipal wireless networks have requirements for mobility from automobiles driving through cities at up to 60 miles per hour. To make full use of the network, especially for public safety applications, this level of roaming is certainly valuable. You need to be aware of some issues, though, when attempting to support these requirements.

At the recent MuniWireless conference in Dallas, Ebrahim Keshavarz, VP New Services Innovation at AT&T, explained that roaming at driving speeds is not mature enough for municipal networks. He further mentioned that AT&T is not guaranteeing mobility from cars and that they are marketing their municipal Wi-Fi solution with a focus on portability. I agree somewhat with Ebrahim.

The IEEE 802.11 standard, which is the basis for Wi-Fi, was developed to support vehicular speeds, but I’ve found through testing installed municipal networks that driving at even 30mph causes a typical a laptop equipped with Wi-Fi and downloading a large file to lose connectivity and have trouble reconnecting with mesh nodes. The cause of the dropped connection is generally due to the radio in the client device (laptop in my case). As you roam through a city, many Wi-Fi radios tend to stay connected to a particular mesh node until communications get really bad before connecting to a closer mesh node (one whizzing by) that supports better communications. Wi-Fi phones generally do a good job with roaming, but radios in laptops and PDAs generally don’t do well at all.

Keep this in mind when deploying high speed mobile applications. It’s always advantageous to do some experimental testing with the mobile applications you plan to use while roaming. Also, focus on identifying radios that roam well in higher speed scenarios and find ways to maintain session persistence if the radios lag behind as you’re cruising through the city.

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Jim Geier is an independent consultant and founder of Wireless-Nets, Ltd (www.wireless-nets.com), a consulting firm assisting municipalities, enterprises, hospitals, airports, and equipment providers with the development and deployment of wireless networks. Jim is the author of several books, including Deploying Voice over Wireless LANs (Cisco Press), Wireless LANs (SAMS), Wireless Networks ‚Äö?Ñ?¨ First Step (Cisco Press), Wireless Networking Handbook (Macmillan), and Network Reengineering (McGraw-Hill).