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Setting up WLAN for a student hall (40 simultaneous connections)

I'm trying to set up a wireless network in an old motel that's been converted into a student hall. The building is roughly 65 x 25 feet -two stories high. There are forty rooms altogether. Assuming two students per room, that should be 40 simultaneous connections at peak. We're planning to subscribe to a DSL connection and share the connection via a wireless network. Could you advise on what is the best way to do this? We're considering Wi-Fi. However we're not sure how well the signal will propagate through walls and ceilings. Could you also suggest some hardware for our kind of application that will be able to cater to roughly 40 users at peak? Also, is there any way of assigning a rule such that users who've used up a lot of bandwidth will have lower priorities? Thanks!

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My guess is that you need two APs, depending upon your building's RF characteristics and your users' performance requirements. Suppose you place two 802.11b APs about 17 feet from each end-wall. If each AP has an 11 Mbps footprint of just 40 feet due to RF absorption by all of your walls, this provides high-speed coverage for everyone. Twenty users per AP would share roughly 6 Mbps of effective bandwidth ? that's about 300 Kbps/user for simultaneous downloads.

Note that I said guess. It isn't possible to know without a site survey and a sense of your application and throughput requirements. You need a survey to identify RF obstacles like steel posts and plumbing and to measure actual signal loss due to your walls and first-floor ceiling. According to Planet3 Wireless, dry wall cuts effective range in half, but a solid-core wall (or ceiling) drops strength to 15%. If your users need higher sustained throughput, you could deploy 802.11a or 802.11g instead of 802.11b. Another consideration: If one AP is down, do you need distant students to be able to use the other AP at a lower rate?

You could hire a local network integrator to perform a site survey, refine your requirements and install the AP(s) you need. Or you could conduct a site survey yourself: buy one AP and card, put the AP where you think you might want it, and use a laptop with monitoring software that comes with the card or NetStumbler to take some measurements. Map out where link speed drops from 11 to 5.5 to 2 to 1 Mbps by walking very slowly away from the AP, standing in the corners and center of each room, etc. Relocate the AP if necessary to eliminate important RF deadspots. Most APs can support 20 associations, but your real limitation probably will not be connection limit. Throughput requirements will dictate what kind of link speed you need to deliver per user and therefore how many APs you will need.

As for hardware, you could buy a broadband router/firewall with integrated DSL modem, hanging APs off the router's Ethernet. Or you could buy a basic DSL modem from your telco and a separate SOHO/wireless firewall, hanging APs off the firewall. Why bother with a separate firewall? Because some firewalls provide bandwidth management features that let you limit per-station or per-group usage - for examples, see the NetScreen-5XT, WatchGuard V60L, Bluesocket WG-1000 SOE and ReefEdge CS-50. When selecting APs, give some thought to security - if you need to keep users from breaking into and reconfiguring your APs, steer clear of entry-level APs that lack security features like configurable admin logins, port numbers and interfaces and support for secure management protocols.

This was first published in April 2003

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