Large Wireless Networks

By Chris Wilson on 05 January 2010

I saw an interesting request on the AfNOG mailing list:

How does one determine the number of users,  a wireless network can support. I need to buy a wireless router to support 2000 users within an organization. The problem is how do I determine this capability given the specs of the wireless router.

To put it in a better way "what determines the number of users a wireless router can support"[?]

Although I'm not an expert on wireless networks, I have worked with them a bit, and I sent a reply that might be useful to others (I hope).

I'm not sure there's an easy answer to that question. Some factors that may influence the decision are:

• The total bandwidth available to a single wireless access point (AP), e.g. 54 MBps for an 802.11g router. This also depends on the level of 802.11 that the clients support. An 802.11b client will use much more airtime per packet than an 802.11g client, so if most of your clients are 802.11b then you won't get more than 11MBps per AP, regardless of the theoretical maximum of the AP.
• The frequency space available. There are only three non-overlapping 802.11b bands (maybe fewer for 802.11g), so no matter how many APs you have, the most bandwidth you could get in a given spot cannot be more than three times the bandwidth of one AP. Also, if they form a contiguous roaming network (same SSID and key) you have little or no control over which one a client will associate with, so you can't evenly divide the available bandwidth between the three that you can see.
• The guard time between different transmissions and for RTS/CTS round trips. This will cut your available bandwidth at least in half from the theoretical maximum, and more if you have hidden nodes (which is close to inevitable with thousands of clients, unless they are all in the same room).
• The maximum number of clients that can associate with a given router. Most APs don't publish this number, but Cradlepoint routers can handle between 4 and 64 clients per router. Keenan Systems reckons that "Once you have more than 25 clients associated most access points start to break down". I'd guess that Cisco kit has the highest limit, especially the professional versions (not Linksys branded) and el cheapo generic Chinese kit has the lowest.
• If the AP is serving DHCP and running NAT (acting as a router as well as an AP) then the translation and DHCP tables of the router will be a limit. Some router DHCP servers only allow class C subnets, with a maximum of 253 usable client IP addresses per AP. It's probably more advisable to use a real machine (with a hard disk) as a DHCP server.
• Similarly, if you don't do NAT on the AP, then whatever handles the NAT on your Internet gateway will see the IPs of the individual machines, and will therefore need to be able to handle however many simultaneous IPs your clients have, and connections that they make.
• Whatever your DHCP server, the number of IPs available in your network subnet will limit the number of clients who can have a valid unique IP address at one time.
• The bandwidth of your Internet connection. The minimum that I've seen working at all is 3kbps per client, or 6 MBps with 2000 clients. That should be real bandwidth, not contended upstream by the ISP, otherwise multiply by the contention ratio. Don't forget to include your fixed clients as well.

The best advice I can give you, never having built a wireless network this large myself, is to:

• Grit your teeth and buy the best kit you can find on the market. Be prepared to pay through the nose, e.g. $1000 per AP or more.
• Talk to the manufacturers about the maximum number of associated clients, and get assurances in writing that their kit can handle the load. Preferably get them to propose a solution for 2000 clients, also in writing.
• Use small cells with directional antennae and lots of APs in areas where you expect more than 10 clients at peak times.
• Try to scale your network up smoothly rather than buying a complete solution in one go. Don't try to support 2000 clients in the first year, let alone the first day.
• Monitor and graph the performance of the network, particularly bandwidth, wireless contention, number of errors and number of associated clients, and identify hotspots.
• Keep one or two APs spare, and deploy them in the areas that are seeing the most activity.

Sunday Folayan wrote:

Must this network be implemented with JUST ONE wireless router? With one router ... If you run 802.11bg at 2.4ghz, you have just about 2Mbps of bandwidth to play with, from one AP. If you deploy 802.11a at 5.8Ghz, you should get better than 10Mbps. If any of the clients is 802.11bg, the AP will default to 802.11bg, even if it is capable of 802.11a. With 2000 users, that is an average of 1Kbps or 5kbps at the best per subscriber! Could this be what you want?

To put it in a different way ... One single AP cannot do it.

And Hervey Allen wrote:

From what I've experienced wireless router specifications and claims often do not match what you will experience in real-world use. I know of several large-scale installations (10,000+ users and above) who ended up using Cisco Aironet series routers with Power over Ethernet capabilities (PoE).

I will double-check, but last time I was on-site the upper limit for one of these wireless routers was around 50 concurrent users with light to moderate use. That is, a single user running a torrent can make an access point almost unusable for the other 49 potential users...

It would be interesting to hear from others on the list who have large wireless installations what their experience has been, and what hardware they have used.

Issues of giving out addresses, roaming, recapturing addresses, etc... are quite important.

Patrick Okui wrote:

Joel Ja did a pretty good presentation on what he's learned from setting up wifi installations for the various meetings/events at NANOG27. A few things have changed in the wifi world since 2003 but the concepts are still valid.

Hamish Downer wrote in a comment to this post:

This page has some good answers. It is about tech conferences, but the basic problem of getting lots of people on wifi in a single space is covered by the solutions.

I fully agree with Hamish, the page has excellent advice from people who have actually done this, unlike me.

Finally, Mark Tinka replied:

I generally wouldn't recommend vendors on a public mailing list in such variable matters as wireless deployments, but given the scale you're considering, Aruba came to see me once (uninvited, as usual), and they seemed to have some rather interesting things to say re: their wireless product portfolio, with particular regard to large scale installations.

You might want to add them to your shopping list, but my guess is the price point is way-up-there, what with their controllers and all.

But be careful about "buying" everything they tell you (same goes for other vendors). As others have mentioned, binding assurances from them as well as PoC's (proof of concept) before you sign would be great!

I hope this helps someone. Please let us know how you get on.