[afnog] Wireless Network

[Chris Wilson]

Hi Benjamin,

On Tue, 5 Jan 2010, Benjamin Cobblah wrote:

> 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"
>
> I could be grateful if any one could help

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 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 router, 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
   (http://www.cradlepoint.com/knowledge-base/maximum-clients-router).
   Keenan Systems reckons that "Once you have more than 25 clients
   associated most access points start to break down"
   (http://www.keenansystems.com/wififaq.htm). 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.

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

Cheers, Chris.
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