Best WAP For Dense Crowds?

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."

Comments

Best WAP For Dense Crowds?

[theolein]

You don't have to hit 'em, mate. Just find another crowd that's brighter.

how cheap? pfsense?

[itzdandy]

consider running a small pfsense box with a number of wifi adapters. You could pick up some cheap directional antennas to help limit connections to any one radio somewhat. Alternatively you could just run 4 sids and do a script to hide a sid when the user count got so high so the next users would only see the less loaded ones.

Re:how cheap? pfsense?

[itzdandy]

I might add that you are going to be stuck with 4 channels ( 1,4,7,10 ) which means that 500 people will be hard to support without highly directional antennas. Maybe try to split the space into 4 with directionals.

Re:how cheap? pfsense?

[Anonymous Coward]

I did a little googling because I was worried about the number of clients. 802.11 uses CSMA which means that every client must wait for every other client to go silent before transmitting.

That means that you would have to take the minimum latency and multiply it by 500 since all clients will be equals. That puts you into 500ms of theoretical latency per packet.

What this means practically is that with 500 clients using all roughly the same bandwidth at 54Mb (unrealistic BTW) you would have just 110Kb per second available to each with 500ms+ latencies, which will compound exponentially.

Though on paper you might be able to show that ability to connect this many clients but realistically, on HIGH end hardware your are going to have a 50 client MAX simply because of CSMA requiring everyone to take turns but less any bandwidth sharing.

To make things worse, the amount of data having to be moved just to keep everyone connected and to communicate who is 1st,2nd,3rd, etc in line to speak is going to cut your bandwidth to a tiny fraction of the link speed.

I highly suggest that you take one of the early poster's advice and drag some cat5e around. You might have some lucky with 'CELLS' of WRT54g type routers with a carefully selected channel scheme where a set of 4 routers would have channels 1,4,7,10 and the next closest 2,5,8,11 and the next 3,6,9 and then start over. The channels will overlap somewhat but having 11 SSIDs for 500 people even with some channel interference would get you to somewhere around 50.

you could extend that to put some 5Ghz band routers in each router bunch and hope that people are fairly evenly split between G and 5Ghz N

Re:how cheap? pfsense?

[dgatwood]

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.

you will need more than 2 APs

[jeffstar]

there was a slashdot the other day about the wifi at a python conference.

any AP is only going to handle 50 users or so because 802.11x is contention based.

So go ahead and get yourself 10 APs, spread them out, and make sure the ones near eachother are on different channels.

Re:you will need more than 2 APs

[Vancorps]

Or buy two Xirrus units which are all in one turn-key arrays of access points all that will auto-tune for you. They have a 16 access point and an 8 access point versions that would handle this setup without any problem.

Re:WAP?

[KiwiSurfer]

WAP = Wireless Access Point.

Mikrotik a possible choice?

[lordsilence]

Even though they're suspected GPL offenders (opinions differ) I still have to put in my word for mikrotik. These guys know how to build wifi in rural areas with plenty of subscribers, stable hardware and good software at low cost. Even their cheaper products are very well up to the task and can be expanded upon with different wireless-transmitters and antennas. If that is not enough you can always look at their more "enterprise:ish" products. I've only good things to say about them, and we used their products for well over 5 years when we still ran a WISP.

Re:What's the event?

[MichaelSmith]

And why wasn't I invited?

Meru Networks

[zerofoo]

Not the cheapest stuff, but Meru's access points and controllers will allow you to run all the APs on one channel, and the controller "load balances" the users across the available access points within reach of the client.

We use them at my place of employment (6 APs scattered throughout the building servicing around 200 laptops), and the performance is quite good.

-ted

Not cheap, but...

[mmccarn]

Xirrus 'Arrays' are designed for what you're doing. I've used 2 4-radio Xirrus arrays to serve 240 users in a single ballroom. http://store.xirrus.com/SearchResults.asp?Cat=4

Re:Not cheap, but...

[Kizeh]

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.

airport extreme

[saleenS281]

It won't have anywhere near the granularity in configurations, but I will say apple airport extreme's tend to "just work". They support both g and n operating at the same time since they have multiple antenna's, and they also have a sort of sandbox guest environment you can set.

If you want fall-down easy to setup and manage, they'll get the job done. If you want granular control, don't waste your time. I got sick of trying to make dd-wrt work with WAP, wireless-n and g at the same time a year ago, and just bit the bullet on the apple units. I can say it's been one purchase I don't regret.

consumer equipment is the wrong answer

[Anonymous Coward]

Background on me to qualify my comments: I am a cisco engineer specialising in wireless and security. My product recommendations later come from this experience but there are other products capable of the same performance such as the aruba equipment which would be my close second recommendation but i have no specific product knowledge.

I think you need to refine your requirements. It is highly unlikey that a crowd of 500 people will create 500 connections. You will probably end up serving 100-150 clients simultaneously but not all of them requesting data at the same time unless there is something specific that all users need to connect to at the same time throughout the event.

Without much better information everyone is just throwing out a product, not a design. And as you clearly are not a wireless expert (as you asked for 802.11n "as well as .11g) i would recommend finding someone who is to consult properly.

And for those suggesting consumer products, your dreaming. Without some form of spectrum management in this situation the asker is doomed to provide a very poor service with no roaming and massive 2.4ghz congestion. In addition, those people recommending wired access, WTF? You very clearly do not understand what you are talking about. Are you expecting 500 desks with RJ45 ports, or multiple 48 switches places around the room for people to huddle around with their laptops (and only laptops as no mobile device even has an RJ45 port). This is clearly a fallacious argument.

Answer the following questions and we can all get very specific.

3 points to place APs. Is this to physically mount or a cabling limitation? Can you mount more but have no cabling? Un-manged switches can help with this for less than $50 each. If only to mount then you are stuffed, There is nothing out there that will handle 500 clients with any useful service. It's not a limitation of the products it's the contention of the medium as mentioned earlier.

What services are they accessing? Are they local or is it just the internet? If the internet, what is the upstream bandwidth available? If local access at high speed (100Mb/s +) then you will end up with contention issues. If it is the internet and the pipe isn't fat you are not looking at contention issues you are looking at number of users connected. Most modern APs do not have practical limits of associated clients but most recommend around 25 per AP.

What is the nature of the event? Basically, are you providing a service that is required constantly throughout the event leading to 100% of attendees connecting all the time. Also, are users accessing a high bandwidth service (streaming video for example) all the time or things like static web pages delivered via http? The later will deliver small amounts of data to each person but will then take time to read by the attendees al will also be cached locally meaning subsequent connections will require even less bandwidth. If streaming video, someone should have though of this earlier and you will need a consultant/engineer 100% or expect to fail.

An off the cuff answer without the above knowledge assuming http type data required, cabling limitation not mounting, the more realistic 150 simultaneous users and internet link at less than 30Mb/s:

1x Cisco 2112 Controller (100Mb ports not important as limited upstream)
5-9x Cisco 1142 APs (very nice 802.11n dual band with the ability to force people to move to 5Ghz if they have it 6.0+ code)
3x gigabit unmanaged switches (something like dlink DGS-1005D)

It would not be far fetched to contact decent size Cisco/Aruba/VendorX partner and get loan equipment for a price + a consultant as part of the deal.