Best WAP For Dense Crowds? 178
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by
timothy
from the well-phrased-and-specific dept.
from the well-phrased-and-specific dept.
An anonymous reader writes "A local community organization has asked me to help them set up Wi-Fi access for an upcoming event, with some unusual (to me) requirements. All users (up to 500 people) will occupy a relatively small area and more-or-less have line-of-sight to the WAP, so issues like signal strength and wall penetration don't matter. Security also does not matter, as we plan to open this to anyone wanting to connect. Cost always matters, but we realize a $50 Linksys or three won't cut it here.
In the past, I have used Cisco AP1200s for a few dozen users to great satisfaction, but they only handle 50 connections at a time, and practically count as antiques at this point anyway. My research on the matter tells me that 802.11n performs far better in this regard, but I want to support 802.11g as well. I have no objection to using two APs to split those apart (with n limited to 5.8GHz, as per the suggestion of several comments in a recent Ask Slashdot), but physical constraints make it preferable to minimize the total number of APs needed — Ten WRT54s might cost about the same as one Aironet, but I only have three good places to mount these.
I welcome any suggestions and real-world experiences with similar situations, including the ever-popular Ask Slashdot refrain of 'What kind of idiot would do it like that, when you can just do this?' Ideally, I would like to know model numbers and how well they held up under real-world loads comparable to my situation."
What's the event? (Score:4, Insightful)
Will all 500 users connect at the same time and continuously (like some type of LAN party w/o the LAN) or is this much more haphazard and random with far less users at any one time?
Re:how cheap? pfsense? (Score:3, Insightful)
Use simultaneous dual band APs. Push everyone possible to 5Ghz.
Re:Not cheap, but... (Score:5, Insightful)
The parent means Xirrus will cause the event organizers to mortgage a house. Still, Xirrus can have tons of radios in one device, all with segmented antennas, and they really are a good fit for this kind of stuff. They even have a pole/tripod mounting option where you can set up more if need be. See about the sponsoring or maybe renting.
Alternatively, get external 60 degree segment antennas for something like Cisco 1250s and do hexagonal cells, like wireless carriers do. For dual band MIMO you need six antennas per AP, so it'll get out of control mighty fast.
Worst case, get a bunch of APs, have three of them use the three 2.4 GHz channels with MIMO (but no channel bonding!) and as many 5 GHz ones as you can, since you have many more non-overlapping channels to work with. Chances are that anyone stuck on 2.4 GHz is going to hate life. Plan power levels as well, and don't run radios hotter than they need to be, despite the temptation.
Also, very, very important: DISABLE LOW DATA RATES. Mandate 5 or 11 Mbps as the lowest supported rate at all the radios. Otherwise the 1 Mbps Nintendo DS's and phones will eat up all the airtime and starve everyone of access. If you can get away with turning off 802.11b support and only offering 802.11g on 2.4 GHz, do so.
Finally, ignore any comment suggesting consumer gear.
Re:airport extreme (Score:2, Insightful)
Re:how cheap? pfsense? (Score:5, Insightful)
And that's a pretty serious problem. In my experience (which admittedly is mostly as a user, not an admin), most OSes aren't happy with high-latency wireless networks. With a fast, low-latency network with no packet loss, it can take a couple of seconds to do a DHCP request. With a slow, high-latency link with packet loss, I've seen it take a couple of *hours* to do a successful DHCP request. Sadly, such connections are easy to get when you have a few dozen people on an AP downloading porn^H^H^H^Hmovies^H^H^H^H^H^Hlegitimate software bits.
Something you might do to alleviate this is to use the 10 network for all your access points. Never reuse IPs and set your DHCP server to cache IP assignments and always ack when a host requests to extend its assignment. Oh, and set the lease time to a month or something. Doing this should reduce the number of DHCP packets that have to be sent. IIRC, for a re-request, you're down to one packet in each direction instead of (at least) two in each direction. Of course, if you can't get the response back within about two seconds, the client is likely to give up and fall back to a full-blown DHCPDISCOVER....
You should probably use a fast switch with a fast backbone between the APs and your core router/DHCP server to minimize latency between the AP and your DHCP server.
I would not use the same machine for the upstream router and the DHCP server. By keeping those separate, you are further reducing the wired portion of your latency because your DHCP discover/request packets aren't getting backed up behind outbound network traffic on the wire. Be sure to use a reasonably fast box for the DHCP server and a FAST box for your router/firewall/NAT box. Do not, under any circumstances, use the NAT built into any consumer router boxes.... The CPUs just aren't anywhere near fast enough.
Get several radios going, crank the gain down as much as is practical without losing bars of signal as seen from the devices, use directional antennas to dice up the space into as many distinct zones as possible, and organize the zones to maximize the distance between APs on the same channel. I'd probably put external antennas spaced periodically down each wall in alternation, forming a series of alternating cone-shaped zones. The exact distances depend on the spread angle of the antenna and the width of the room. Alternatively, you might consider hanging them from the ceiling pointing down, spaced in a grid formation.
If you can, try to make your APs give top priority to DHCP messages, thus minimizing the number of these packets that get dropped before they make it out (in either direction).
Oh, yes, and turn off 802.11b support if you can. Allowing 802.11b means that every packet sent at high speed requires additional crap before and after it so that the 802.11b radios don't choke. If that's not possible, set up a separate segregated network for legacy 802.11b clients and stick it on its own channel---probably one AP for the whole room.
Finally, if at all possible, make sure your DHCP server sends ACK using unicast where possible. AFAIK, every major OS should be able to handle this. IIRC, broadcast packets on 802.11 are particularly expensive. The more you can minimize them, the better off you are. While you're at it, crank up the multicast rate (basically, the minimum signal level that a client must maintain before the AP throws you out). This will force clients to associate with new stations more frequently, but should increase network performance and decrease latency under (particularly multicast/broadcast) load.
Oh, and one more thing. I'd like to echo the comments about not using home router gear. Get yourself Cisco APs. Most home routers just don't have the CPU to keep latency low enough when routing that much traffic, and many don't have sufficient control over power levels, external antenna jacks, etc.
Re:WAP? (Score:3, Insightful)
Re:p.s. if running cables is a problem... (Score:3, Insightful)
they have been getting best-in-class service.
I hate this phrase. Is the service class they are getting First Class, Coach, or Baggage? Every MarketingDroid who uses this phrase never says which one...