High Density Wi-Fi considerations

 

High Density Wi-Fi is becoming more popular by the day.  There are many avenues that can lead you to deploying HD Wi-Fi, and I will try and outline them for you.

Large public venues – pro sporting arenas, large conference halls, Convention centers and theme parks.  Smaller venues, such as libraries, airport terminals, college/university settings and k-12 may still benefit from high density Wi-Fi designs and deployments.

The intended audience for this chapter is for those of you who wish to take a stab at deploying High Density Wi-Fi in your environment.  This is not meant to replace your WLAN vendor’s  professional services –however, it will help educate you and hopefully give you an idea of what the end goal is.  If you are a smaller environment – such as K-12, University, College or other entity and wish to give it a try, this will help arm you with some of the basic strategies and best practices.

You may glean plenty of ideas from the plethora of HD Wi-Fi deployment guides freely available to you on the Internet since this field is changing rapidly.

I am under the assumption you know something about how to build an enterprise network, and have deployed an enterprise class wireless network in the past, or had it done for you via professional services and you feel confident in your abilities and want to make your wireless network better.  You should probably have the skill level CWNA, which is a highly recognized wireless certification.  I'm focusing on the wireless part – not the backend which will also need to be designed properly.

The goals for a HD Wi-Fi environment is to have higher capacity via smaller cell sizes in the 2.4 GHz and 5GHz ranges.  This enables faster communication due to higher data rates.  Higher gain directional antennas create small cells and reduce co-channel interference.  More cells equal more throughput, which means more capacity.  You’ll have to define what your design goals are for your capacity, whether you are a basketball stadium with 16,000 seats, or a library with large seating areas.  For stadiums, I’ve seen design goals aiming to serve 200-300 seats per access point, and large 900 seat auditoriums it is not uncommon to see design goals to serve 100% of the seating area with 25 - 50 clients per access point. This capacity is accomplished with directional antennas, low power APs, higher data rates and restricting the number of SSIDs being broadcast. It is not common to find access points with omni-directional antennas deployed in a high density Wi-Fi environment.  

One thing to consider if you are designing for your public environment is how many SSIDs you might need.  You may need WLANs for the following groups of users – Ticketing, Point of Sale, Admistrative, Press, Inventory, VoWi-Fi and of course, the guests.  You’ll want to restrict the number of SSIDs being broadcast as much as possible.  If the SSID is not required in a certain area, then don’t broadcast is.  Hiding the SSID name is not enough – you’ll want to remove that SSID from the area that it is not needed, and thankfully most enterprise wireless systems have this capability.

When designing your WLAN, it is important to have accurate floor plans.  They will help you visualize where your clients will be, where your RF will be, and where you don't want your RF to be.  Place close attention to that last sentence!  You’ll also want those floor plans for doing your WLAN site survey.

The 2.4 GHz spectrum is home to all kinds of devices that utilize this spectrum, including 802.11b/g/n WLAN systems.  There are so many devices in this, both 802.11 based and not, that it is difficult for these devices not to be stomping all over each other.  802.11 is a polite protocol – if an AP is on channel one and it hears another AP or client on that channel, the AP will wait until channel one is clear before transmitting.  Adding access points with omnidirectional antennas to the ceiling of a large auditorium does not necessarily increase capacity, throughput or the number of users that can be online if many of the access points are on the same channel and can hear each other.

Designing an HD WLAN can be difficult.  There are methodologies that are in place that many of the professional WLAN designers all agree upon.  This list here is by far not all inclusive, as this field is changing all the time.  Here are a few that seem to be pretty common goals and methods:

HD Wi-Fi is designed to minimize the cell size

Antenna placement is important and the best spot is often worth the effort

Deploy more dual band APs than needed and turn off some 2.4 GHz radios.

Use directional antennas in open environments – auditoriums, arenas, conference centers

Omnidirectional antennas may be used in non-HD Wi-Fi areas – as long as they are isolated

 

As an example, imagine an indoor seating chart of a 16,000 seat basketball arena. 
This is a large, open air environment.  We'll aim for 250-500 seats per access point.

If you put three dozen dual band access points with omnidirectional antennas in there, you might find that the Wi-Fi doesn't work that well, even though "the math works out".  Your channel utilization will be excessive – I'll talk more about that later.

However, if you were able to mount APs in the ceiling structure (roughly 50 feet above the seats) with a maximum 30 degree beam width antennas aiming down at the seating areas you might find that co-channel interference (CCI) was reduced drastically.  Notice I stated "from the ceiling structure above", and not "from behind the seating area" which could be a lot of concrete and steel.  The reasoning behind this is that when your venue is full of people, the signal is attenuated more when the antennas are behind the seating areas.  APs communicate to clients at 93 feet when the antennas are behind the seating area – however it does not work that well.

The other issue with CCI is that signals come from your non-bowl/arena access points.  You've changed out all those omnidirectional antennas for directional ones, but the signal still leaks in from the lobby and concourse areas.  The key here is to use patch antennas in those areas, aiming away from the seating area.  The more "directionally contained" your signals are, the less CCI you will have.

Keep in mind that there's other Wi-Fi out there.  You might have Wi-Fi in the parking lot outside your facility for the hot dog vendors, ticketing, etc.  The best thing to do here is to use directional antennas when possible to minimize CCI.  At least in those areas you won't have as many users as you might have inside the seating area of your basketball arena.  

Other Wi-Fi exists that you might overlook.  Moveable partitions slide to the side and open up your environment, changing the dynamics of your RF propagation.  Glass walls in press boxes slide open and let RF spill out into a seating area if you are using an omnidirectional antenna.  I cannot stress enough the need to properly survey your area and to use patch/directional antennas!

Let’s think of another example.  A large library on a University campus.  This is a large, open air environment.  In this case, we have rows and rows of books, offering lots of attenuation, and then seating areas where three hundred students sit and do their homework.  Each student may have a smart phone, a laptop, and maybe even a tablet.

 

If you install twelve dual band access points with omnidirectional antennas on the ceiling in there, you might find that the Wi-Fi doesn't work at all after an hour into the day.  Your channel utilization will be excessive – and if you use a protocol analyzer you may find that all the clients are communicating at the lowest mandatory data rate configured on your WLAN controller.

Instead of mounting the APs in an open ceiling environment, you can reduce the coverage area of an access point using patch antennas (a form of directional antenna) mounted against a pillar, which attenuates the back lobes dramatically and allows you to bathe a specific area with RF, and restrict it from covering other areas.  Just like the basketball arena example, it is important to isolate the higher density WiFi areas from the non HD WiFi areas.  You don’t want the signal to “leak in” from the hallways, classrooms, and other areas.  As before, the more "directionally contained" your signals are, the less CCI you will have.

You may encounter “adjacent WiFi” in these areas, where coffee shops, etc, may have WiFi for vending purposes.  If at all possible, arrange with the vendor to broadcast their SSID on your WLAN equipment on areas that they require.

We're going to start to dive into some of the nuts and bolts of HD Wi-Fi.  Starting with why we want to minimize the RF propagation of the cell.  By limiting the mandatory and supported data rates and by limiting the power output of the transmitter, we keep the cell size small.  When we do that, we can keep the CCI low and re-use the three channels in the 2.4 GHz spectrum.  The channels I speak of are 1,6, and 11.  If you use anything other than that, you risk overlapping

In Wi-Fi, if a frame didn't get to the recipient (the transmitter didn't receive an ACK frame from the recipient) it will try and retransmit the frame and/or lower the data rate and try again.  This leads to an even busier channel!  Think of it as trying to talk to your best friend sitting right next to you at a Van Halen concert.  You have to repeat yourself three times, and then say it slower, then reduce yourself to charades in order to communicate.  Next thing you know, all clients are using charades and your Wi-Fi is unusable.  In other words, all of the WLAN clients are now communicating at the lowest configured data rate.  This is what happened in the library example above.

If you are planning an HD deployment for an existing facility, I highly recommend assembling a professional WLAN site survey toolkit.  At a minimum, you’ll need a portable computer, WLAN Site Survey software (Ekahau ESS, for example), an access point and several antennas to test your design.  Be sure the WLAN network adapter supports all of the channels you plan to use in your WLAN.  Most of the WLAN survey kits are homemade at best, so be creative and don’t be afraid to keep modifying your setup.  As for antennas that are on the market today, new ones appear every time I look.  You can now purchase overhead directional antennas, handrail antennas, under seat antennas, patch antennas, etc.

The site survey will give you the ability to test the coverage patterns of the antennas you would like to deploy.  You may go through several different antenna types before you are happy with your selection.  Keep in mind that surveying a venue full of people is challenging, and that your site survey's data will change dramatically when you compare an empty auditorium, arena or library and a full one.  I also recommend purchasing a spectrum analyzer for your laptop, along with a protocol anaylzer.  Ekahau ESS has a specrum analyzer that integrates with their software, and AirMagnet Wi-Fi Analzyer is one of the best protocol analyzers around.  All will prove themselves valuable over time  Check out Keith Parsons' "Seven Rules for Accurate Site Surveys".  

Set your survey access point to the same settings (power, data rates) you want for your production HD WLAN environment.  Industry experts recommend using the same type of access point and antennas for your survey as you will deploy when you are finished with your design.  I recommend surveying at lower power settings with a higher minimum data rate of 18 Mbps.  That means that 1,2,5.5,6,9,11 & 12 are disabled.  Keep in mind you are going to be using higher gain, directional antennas which will increase the power in your small cell.

If you are designing Wi-Fi for an existing facility that may already have Wi-Fi installed for ticketing or Point of Sale, we recommend planning your new system to cover those areas for them and decommission the other system.  After all, you don't want competing WLANs in the air space.  I wouldn't mention this if I haven't come across it several times.

Why do we want to limit the number of SSIDs being broadcast?  Every SSID requires all the management overhead traffic associated with it.  This means the more SSIDs you have, the less airtime you have for your client devices.  If you have older, existing 802.11b ticketing and point of sale systems with 802.11b and other lower data rates enabled, the beacons will all transmit at the lowest data rate – using up valuable airtime.  One of the best practices in my opinion is to limit your high density environment (basketball stadium, auditorium, or library) to a single SSID.  Other areas in the facility can broadcast up to four, but by all means limit them to the best of your ability.  The next best practice is to disable 802.11b entirely, but not before you have researched existing Wi-Fi clients to ensure 802.11g or greater compatibility.  Upgrading existing 802.11b WLAN clients will be worth the expense, if you discover they’re out there.

Know what type and how many WLAN clients you will be supporting.  Ensure that your WLAN’s back end will support them.  This means that if you are using a controller based architecture, that your system will need to support the quantity of APs you will deploy, along with the number of concurrent client connections, frequency channels and desired transmit power levels.  Your sports complex or conference hall may see a very large number of WLAN clients associate in a small window of time, so be sure that your DHCP and DNS servers can handle the load.  I recommend an Enterprise server at least, ignoring the temptation to use the DHCP server built into your WLAN controller or switch.  In order to serve that many clients, you may want to configure your DHCP server for four hour leases or less to accommodate the ebb and flow of client devices.

To recap some of our network design and configuration recommendations, I will list them here for you.

·         Keep WLANs being broadcast down to one WLAN in HD areas, and four or less in your non-HD areas.

·         Design redundancy – you don’t want a failed component to take your system offline

·         Network sizing – if using a controller, make sure your controller can handle all those clients

 

·         Aim for single digit Channel Utilization when your building is empty

 

·         Remove lower data rates on both 5GHz and 2.4 GHz

 

·         Use 5GHz for mission critical WLANs

 

 

Regularly use your spectrum analyzer to sweep the channels you are using to see if there are any interference sources.  I have seen wireless camera systems on three different occasions wreak havoc on production WLANs.

A number of Wi-Fi clients present in production environments do not actually support connections in the UNII-2 Extended Band. This allotment of channels ranging from 100 to 140 is essentially invisible to older Wi-Fi clients created before this band was made available for Wi-Fi by the FCC.  Many HD Wi-Fi environments do not have any control over the client base, so you may want to keep that in mind.

Let's setup a controller and see how we can tweak it to work for an HD environment.  This post is going to concentrate on the 2.4 GHz band - we'll delve into the 5GHz configuration in a later post that will be similar to this one.

With the Cisco WLAN controller's defaults applied to all WLANs, I used AirMagnet Wi-Fi Analyzer to look at channel one's channel utilization.  This is a snapshot of the CU when five APs are currently set to channel one in an HD Wi-Fi environment in an empty area.  The APs are in the default AP Group, and there is one client device - my laptop.  The CU is going to change drastically for the better and I'm going to show you how to do just that.

We will use WLAN Profiles to fix this high channel utilization.  I'm going to focus on the 2.4 GHz WLAN since that is the WLAN where we are seeing most of our wireless clients at sporting events. 

Here's a screenshot of the most important WLAN when it comes to a basketball game, and a few "before and after" shots as well.  Below is the HD WLAN Profile for 2.4 GHz in the basketball arena.  It has no security, is mapped to a VLAN outside the network and is meant for fast Internet access.  It also has a high channel utilization!

 

 

It is tempting to turn on Application Visibility on this WLAN for statistics, but we chose not to. 

 

We will create access control lists to cut down on Apple's Bonjour traffic.  We're doing this to cut down on the Bonjour traffic since most of the clients devices in our HD Wi-Fi environment have that feature enabled.

So lets create two ACLs.   Here we go:

 

Apply those two ACLs to your HD WLAN.

 

 

Now here's the "secret sauce" for decreasing that channel untilization and making your HD environment a lot better for your clients.  We're going to use the Cisco's RF Profiles along with the AP Group feature to override those 2.4 GHz default settings.  I'm going to create an RF Profile then an AP group and call it Basketball_seating and lastly, apply it to all of the access points inside the arena.  Keep in mind when you do this, the arena APs all reboot.

Since this post is focusing on HD Wi-Fi, I'm going to concentrate on the basketball's 2.4GHz RF Profile.  Create an RF Profile.  Use a naming convention that makes sense for your environment.  Mine are "RFP" for RF Profile, <name of WLAN> <coverage area> <24=2.4GHz>.  You'll thank me later for this advice.

 

Here is where we override the 2.4 GHz defaults and disable 802.11b and make our lowest supported 802.11g rate 18 Mbps.

We're using the RRM tab to override the controller's defaults to limit the power thresholds in our basketball arena.

We didn't change anything on these last two tabs.

 

Now create an AP group for the HD Wi-Fi area.  I cannot stress enough to use a proper naming convention so you can figure out what it is for months from now.

Edit the AP group after it is created, and use a proper description so you can figure out what it is for a few months from now.

AP Groups allow you to only broadcast the WLANs you want.  For the basketball arena we only want to broadcast one WLAN.  If we used the default AP group, we would automatically broadcasts all WLANs.

Then we apply the RF profile we created in the earlier step, overriding the global 802.11b 2.4 GHz parameters.

Use the APs tab to add the arena's access points to the newly created AP Group.  They'll reboot.

After the APs reboot, they come up and I let my AirMagnet Wi-Fi Analyzer run for about ten minutes.  The CU is now down in the single digits!

So, to recap, this is what we did to make our HD Wi-Fi exponentially better.

 

1.Replace omnidirectional antennas with directional ones and created small cells

2.Create ACLs to block Bonjour

3.Create RF Profiles to limit data rates and transmit power

4.Create AP groups to limit number of WLANs being broadcasted

 

Here are a few screenshots from AirMagnet Survey Pro.  These are the coverage patterns from a walkabout through the bleachers with directional antennas mounted up high, aiming down at the seats:

This is the walkabout with all 2.4 GHz channels shown.  Notice how we're not getting all the way down to the front seats.  This is because those seats are the farthest from the antennas.

 

 

Here's channel one.  The light blue areas are where the signal is strongest, which is where the antennas are mounted.  This walkabout was done when the venue was empty.  This changes when full.

 

 

 

Here's channel six.

 

 

 

Channel eleven:

 

Here's channel 149.  I decided to show one 5GHz channel just so you could see the coverage pattern differences.  The cell is much smaller than the 2.4GHz channels - but that's fine because we have more 5GHz antennas online than 2.4GHz.  The APs are all dual band, however we shut some of the 2.4GHz radios off.

Many of these High Density Wi-Fi concepts, such as RF Profiles (and the tweaks within), for example, can be applied to higher density deployments, not just sporting venues.