Thursday, March 28, 2013

Ekahau Site Survey™ 6.0 Incorporates 802.11ac Channel and Capacity Planning

Ekahau has announced that Ekahau Site Survey™ 6.0 now includes planning capabilities for 802.11ac wireless networks, including support for 802.11ac enhancements such as wider channels, primary and secondary channels (at various channel widths), more spatial streams, MU-MIMO, beamforming, and higher modulation rates. They have also released a companion white paper on planning for 802.11ac adoption.

White Paper: Planning for 802.11ac Adoption with Ekahau Site Survey™ 6.0

In the paper, Ekahau makes some of the very same points that I have made in prior posts about the need for careful channel planning with 802.11ac due to the likelihood co-channel interference at larger 80 MHz and 160 MHz channel widths by ensuring non-overlapping 40 MHz and 20 MHz primary channels.

Wide 80MHz and 160MHz channels improve throughput but only when full channel bandwidth is free from interfering transmissions. In dense network deployments, careful channel planning is critical to ensure interference-free operation. Channels cannot be selected arbitrarily but primary channels must always be selected so that access points within radio range can fall back to use lower non-overlapping channel for simultaneous transmissions. For example, APs on an 80MHz channel can fall back to use 40MHz or 20MHz channel bandwidths as described in Figure 1.

Here is Figure 1 from the guide:

ESS™ 6.0 also offers channel planning capabilities to help WLAN administrators ensure they provide non-overlapping channels to allow simultaneous transmissions.
For 802.11ac, the user can configure the utilized channel bandwidth as well as the allowed channels. The planning algorithm selects frequency channels using the selected bandwidth, including selection of the center channel as well as the primary channel. Channels are selected in such a way, that interference and channel overlap in the network is minimized. The primary channel selection algorithm is optimized for a mixed 802.11ac/802.11n client base and supports parallel non-overlapping 80MHz, 40MHz, and/or 20MHz bandwidth transmissions when full bandwidth is not available.
Here is a screenshot of an automatic channel plan for 802.11ac 80 MHz channels derived by the tool:

Ekahau Site Survey™ 6.0 Provides Automated 802.11ac Channel Planning

One question that I still have, and hope to uncover once I get my hands on the tool, is if ESS™ 6.0 can automatically plan for non-overlapping 40 MHz channels while simultaneously assigning neighboring APs channels such that the likelihood of simultaneous 80 MHz transmissions are maximized? Here is my figure, depicting non-overlapping 40 MHz and 20 MHz primary channels while still allowing 80 MHz channel use on a best-effort basis:

802.11ac Non-Overlapping 40 MHz and 20 MHz Channels

I also really like how Ekahau Site Survey™ incorporates capacity planning into their site survey tool, rather than relying on RF coverage alone. This is a point that I focused on when writing the Aerohive High-Density Wi-Fi Design and Configuration Guide. Today's Wi-Fi networks are growing in size and client density, which requires adequate focus on capacity planning by understanding client and application requirements coupled with traditional RF coverage and channel planning.

The goal of network analysis is to understand network capacity. The ESS capacity estimation algorithm allows user to accurately estimate the capacity of the planned network with a different set of client devices and traffic patterns. ESS models the key parameters of 802.11ac including the MIMO configurations, channel bandwidths, new QAM256 modulation, and frame aggregation just to name few. To support analysis of your own device, ESS includes predefined templates of AP and client devices and allows estimation of network capacity with a user-configurable set of client devices and their applications. This allows estimation of how network capacity differs, for example, between first and second generation 802.11ac devices.

Ekahau will be hosting live webinars on April 15th and April 19th to cover 802.11ac and ESS 6.0™ features. I'm planning on attending to hear about how this tool can help WLAN administrators adequately prepare for 802.11ac and high-density networks.

I have been a user of Ekahau Site Survey™ 5.5, but admittedly I have only used it on rare occasions since I have not invested the time to learn it properly. I'll have to upgrade to version 6.0 and invest more time and effort (perhaps training?), since it appears to take the right approach to WLAN planning that other tools historically have not.


I have no formal affiliation with Ekahau and was not compensated in any fashion for writing this article.

Other posts you might be interested in:

802.11ac Gigabit Wi-Fi Series:
High-Density Wi-Fi Design Series:

Tuesday, March 26, 2013

Video Blog: High-Density Wi-Fi Design Part 2 – RF Planning

In this video, I explain the key RF principles that you should consider when designing a high-density Wi-Fi network. These include:
  • Leveraging the 5 GHz bands to reduce co-channel interference and increase network capacity
  • Appropriate use of 20 MHz versus 40 MHz channel width
  • Using non-adjacent channels to minimize adjacent channel interference
  • Always performing a pre-deployment site survey
  • Appropriate cell sizing to maintain high data rates for clients while minimizing co-channel interference
  • Ensuring a high quality bi-directional link with clients
  • Leveraging facility obstructions for channel re-use
  • Co-locating APs to increase capacity
  • Alternative AP mounting methods and appropriate use of antennas to achieve desired coverage while minimizing co-channel interference

These principles are covered in more depth in the Aerohive High-Density Wi-Fi Design and Configuration Guide.

Read the Entire High-Density Wi-Fi Design Series:


Full Disclosure - This video was created in cooperation with Aerohive Networks, my current employer. 

Friday, March 22, 2013

Safely Using 80 MHz Channels with 802.11ac

The big appeal of 802.11ac is higher bandwidth, which will be accomplished in first generation 11ac products primarily through the use of wider 80 MHz channels. Channel planning in 802.11ac also involves assigning primary channels to allow for dynamic per-frame channel width adjustments to reduce co-channel interference (CCI). In my last post, I recommended that you select only one primary 20 MHz channel at the channel width you can likely "guarantee" is free from CCI. This means that most enterprises should design around 20 MHz or 40 MHz channels since they provide more non-overlapping channels to reduce CCI.

Why shouldn't you plan around 80 MHz channels? There likely aren't enough non-overlapping channels at 80 MHz to reduce co-channel interference, so you are better off planning around non-overlapping 40 MHz channels. 

But many organizations will still want to benefit from the higher peak performance gains that 802.11ac can provide. That's the big appeal, right?! The answer is to take advantage of the per-frame channel width capabilities of 802.11ac to dynamically allow wider channel use when the entire 80 MHz channel is clear (not busy).

Let's demonstrate by using two examples...

Example 1 - Planning around 40 MHz channels
You design your enterprise WLAN around non-overlapping 40 MHz channels because there are a sufficient number of channels for you to safely re-use channels across your environment without creating co-channel interference. You also enable 80 MHz channel width on your WLAN, which will be used on a "best-effort" basis if the entire 80 MHz channel is clear.

Note - One big assumption with this example is that DFS channels are supported by your clients. If not, then you're still best off planning around non-overlapping 20 MHz channels and using both 40 MHz and 80 MHz on a best-effort basis.

You designate primary 20 MHz channels so that it results in non-overlapping 40 MHz channels. If you're in the U.S. you can't use 40 MHz channels 118 and 126 (due to TDWR restrictions), so this results in 10 non-overlapping channels. If you're in the UK/EU you can't use 40 MHz channels 151 and 159 (due to Band C licensing), so this also results in 10 non-overlapping channels.

802.11ac Non-Overlapping 40 MHz Channels
Remember that administrators only configure the primary 20 MHz channel, and the primary 40 MHz and 80 MHz channels are dynamically assigned by the AP. I provide a deeper explanation in my post on 802.11ac Channel Planning. In this graphic, primary channels at various channel widths are denoted with gray and dotted-gray shading.

You'll also need to consider AP channel assignment based on physical AP locations in order to maximize the likelihood that 80 MHz channel widths can be used without co-channel interference. You can accomplish this by skipping one primary 20 MHz channel when assigning channels to neighboring APs. For example, if AP1 and AP2 are neighbors, assign AP1 primary channel 36 and AP2 primary channel 52, skipping channel 44. In this manner, neighboring APs result with different 80 MHz channels which are less likely to interfere with one another.

Here you can see that the greater number of 40 MHz channels reduces CCI when compared to 80 MHz channels. 80 MHz channel width can still be on a best-effort basis, if enabled, but remember that two adjacent 40 MHz channels will still use the same 80 MHz channel width. In this example, channels 38 and 46 would share the same 80 MHz channel 42. We have staggered them in our RF design to decrease the signal strength between the two and maximize the possibility of 80 MHz use, even though we can't guarantee it.

40 MHz Co-Channel Interference is Less Likely
In this manner, we have enabled 80 MHz channel use, but have assured ourselves that we can safely fallback to 40 MHz channel width on a per-frame basis if the larger channel width is busy. This allows us to take advantage of the higher performance that 802.11ac wide channels offer without creating large collision domains and high levels of co-channel interference.

Example 2 - Planning around 80 MHz channels is a recipe for disaster!
You design your enterprise WLAN around non-overlapping 80 MHz channels, even though there is greater AP density than non-overlapping channels. You've decided to take a gamble and see if you can get the higher performance that wider channels bring all the time, at the risk of creating more co-channel interference.

You designate primary 20 MHz channels that result in non-overlapping 80 MHz channels (36, 52, 100, 116, 132, 149). If you're in the U.S. you won't be able to use channel 122 (due to TDWR restrictions), so you're left with 5 non-overlapping 80 MHz channels. If you're in the UK/EU you won't be able to use channel 155 (due to Band C licensing), so you're left with 5 non-overlapping 80 MHz channels as well.

802.11ac Non-Overlapping 80 MHz Channels

However, you have a fairly dense AP deployment, resulting in some co-channel interference between APs. Let's say two APs, both using 80 MHz channel 42 can hear one another and sense that the air is busy. Using the per-frame channel width capabilities of 802.11ac, they attempt to back-down to smaller channel widths. However, there is a problem... since you've designed your primary channels based on an 80 MHz channel width, both APs attempt to back-down to the same primary 40 MHz channel (ch38) and primary 20 MHz channel (ch36). They can't avoid the co-channel interference! This results in both APs sharing airtime and reducing network performance and capacity.

80 MHz Co-Channel Interference is Likely

This happens because when you plan around the larger 80 MHz channel width, the primary channels that are assigned at the smaller channel widths are more likely to result in co-channel interference as well. Therefore, if co-channel interference does occur, the neighboring APs will be unable to back-down to smaller channel widths to avoid the interference.

It would be better to allow them to back-down to non-overlapping 40 MHz channels, breaking apart their collision domains so they can both transmit at the same time and avoid co-channel interference. This is exactly what happens when you plan around smaller channel widths instead!

Final Thoughts
With 802.11ac it may be tempting to use 80 MHz channel widths for peak performance. However, in order to reduce co-channel interference it is recommended that you derive your channel plan using non-overlapping 20 MHz or 40 MHz channels instead, allowing 80 MHz channel width on a best-effort basis. This allows APs to back-down to smaller channel widths that are non-overlapping when 80 MHz CCI is present using per-frame channel width capabilities available with 802.11ac. This allows APs to use the higher peak performance when possible, while maintaining separate collision domains at smaller channel widths when 80 MHz transmissions are not possible.


802.11ac Gigabit Wi-Fi Series:

Wednesday, March 20, 2013

802.11ac Channel Planning

The forthcoming 802.11ac Gigabit Wi-Fi amendment will bring with it support for larger channels at 80 MHz and 160 MHz widths. This is one of the primary drivers behind the increased peak performance and bandwidth of wireless APs and clients. Therefore, careful consideration of channel widths allowed on APs and the channel plan for WLAN deployments must be made prior to an enterprise deployment.

Channel Numbering
First, let's tackle how channels are numbered and referenced in 802.11ac. The standard method to denote 5 GHz channels has been to always use the 20 MHz center channel frequencies for both 20 MHz and 40 MHz wide channels. Starting with 802.11n, 40 MHz channels were referenced as the primary 20 MHz channel plus an extension channel either above or below the primary channel. An example would be a 40 MHz channel consisting of channel 36 (primary) + 40 (extension above).

802.11ac changes how we reference larger channel widths. Instead of continuing to reference the 20 MHz extension channel(s), we will now reference the center channel frequency for the entire 20, 40, 80 or 160 MHz wide channel.

The valid channel numbers for various channel widths are:

Channel Width Valid Channel Numbers
20 MHz 36, 40, 44, 48, 52, 56, 60, 64, 100, 104, 108, 112, 116,
120, 124, 128, 132, 136, 140, 144, 149, 153, 161, 165, 169
40 MHz 38, 46, 54, 62, 102, 110, 118, 126, 134, 142, 151, 159
80 MHz 42, 58, 106, 122, 138, 155
160 MHz 50, 114

This results in channel numbers that may look unfamiliar to most WLAN administrators. Simply remember that channel numbers increment by one for every 5 MHz increase in frequency. This will probably be easier to reference through a graphic for most people. In the graphic below, identify the center of each 80 MHz and 160 MHz channel block, follow it up to the 20 MHz IEEE channel numbers, then split the difference between the two 20 MHz channel numbers that it falls between. For example, the 80 MHz channel block in UNII-1 is centered between channels 40 and 44; splitting the difference gives us channel 42.

5 GHz Channels, with DFS and TDWR Restrictions
Non-Overlapping Channels
As I previously detailed in my post on the impact of 802.11ac on enterprise networks, these wide channel widths may not be realistic to use in an enterprise environment where multiple access points are deployed on non-overlapping channels and co-channel interference must be minimized. To recap:
  • 80 MHz wide channels allow for five (5) non-overlapping channels in the U.S. and five (5) in the UK/EU (channels 149 and higher require light licensing for outdoor use only) when DFS is used, but only two (2) channels in the U.S. and one (1) in UK/EU without DFS.
  • 160 MHz wide channels allow for one (1) non-overlapping channel in the U.S. and two (2) in the UK/EU, with DFS being mandatory for their use in all circumstances.
Note - In the U.S. channels 120-128 are prohibited due to TDWR restrictions, and in the UK/EU channels in Band C (equivalent to UNII-3) require "light" licensing an are restricted to outdoor use.

I'm purposely going to skip the 160 MHz wide channels that are possible using 80+80 MHz discontiguous channels for simplicity at this point.

It's clear that 80 MHz channels will be hard to implement in an enterprise setting that requires high capacity due to issues with channel re-use and minimizing co-channel interference. Even when DFS is used, only 4 or 5 non-overlapping channels will be available. And forget about using 160 MHz channels in the enterprise... leave those for home use where only one AP will be deployed (and hopefully you're neighbors don't live too close to cause interference)!

However, it's not quite as dire a situation as that. There is a saving grace that will allow enterprises to take advantage of these wider channels on a "best-effort" basis. Let's step back for a moment - with 802.11n, 40 MHz channels were an all-or-nothing proposition. The APs channel width was statically set at 20 or 40 MHz.  On the other hand, 802.11ac allows per-frame channel width and bandwidth signaling. Practically, this means that WLAN administrators can allow the use of wider channels by APs and clients when all of the constituent smaller channels are clear. If a portion of the large channel is busy at the point in time when a frame needs to be transmitted, for instance a neighboring AP or WLAN is actively using a 20 or 40 MHz portion, then the AP or client can simply back down and use the primary 20 or 40 MHz portion of the larger channel that is clear. For the next frame transmission, if the entire 80/160 MHz channel is clear then the AP or client can ramp back up and use the full channel width.

Critical to this dynamic per-frame channel width procedure is the notion of the primary and secondary channels. The WLAN administrator must designate which 20 MHz segment within a 40, 80, or 160 MHz wide channel is the primary 20 MHz channel. This channel forms the core frequency segment that the BSS (basic service set) or AP radio operates on. Based on the channel blocks depicted in the table above, the BSS will then automatically designate the primary 40 MHz and primary 80 MHz channels by extending the primary 20 MHz channel (moving downward through the table). Only the 40 MHz and 80 MHz channels pictured in the table are allowed. For example primary 20 MHz channel 56 can only be expanded into 40 MHz channel 54 (combining channel 56 and 52); combination with channel 60 is not allowed. For easy reference, just use the channels as depicted :)

802.11ac Primary and Secondary Channels
(Image from 802.11ac: A Survival Guide)

For example, consider a 160 MHz channel in UNII-1 / UNII-2 where the WLAN admin has selected channel 60 as the primary 20 MHz channel. The primary 40 MHz channel will be 62, and the primary 80 MHz channel will be 58. If any portion of the secondary 80 MHz channel (ch42) is busy then the frame can use the primary 80 MHz channel (ch58). If any portion of the secondary 40 MHz channel (ch54) is busy then the frame can use the primary 40 MHz channel (ch62). And if any portion of the secondary 20 MHz channel (ch64) is busy then the frame can use the primary 20 MHz channel (ch60). This allows an AP or client to dynamically fallback to narrower channel widths in the presence of co-channel interference or noise that only affects a portion of the larger channel.

This has some interesting implications for channel planning!

Developing a Channel Plan
Since eliminating co-channel interference is one of the main objectives in designing a WLAN channel plan, you'll want to carefully consider how you select the operating channel width and primary 20 MHz channels for APs in order to avoid co-channel interference.

First, determine if DFS channels can be used in your environment based on proximity to radar systems and client device support. DFS client support with 802.11a/n has been spotty at best. Hopefully 802.11ac clients will support DFS channels since 5 GHz support is mandated by the amendment and the FCC has eased DFS band adoption by clients since they no longer have to implement radar detection if they passively scan (listen-only) for an AP before transmitting any frames. In essence, they are relying on the AP to perform the radar detection and begin operating on a DFS channel if allowed. But until the time comes when a majority of clients support DFS channels, administrators must verify what channels the client devices on their network support so they don't cause coverage holes by having APs operate on channels that clients can't use.

Second, determine the channel width that you want to attempt to "guarantee" to client devices, which needs to be free of co-channel interference as much as possible. This needs to be based on the density of your AP deployment as well as client device capabilities. Remember - 802.11ac certified clients must support 80 MHz channel width, while 160 MHz channel width is optional. The key is to ensure that every AP has fewer neighboring APs within radio range than non-overlapping channels available.
  • In high-density areas, this should be 20 MHz (stadiums, large event centers, urban areas with many neighboring WLANs, etc.).
  • In normal-density areas, this is likely to be 40 MHz channels (large building office space). 
  • In low-density areas this could be 80 MHz channels (small buildings, few neighbors, etc.).  
  • In single-AP areas, this could be 160 MHz channels (such as homes or very small offices).

Third, make a list of the acceptable primary 20 MHz channels that results in non-overlapping channels at the "guaranteed" channel width. This will maximize the likelihood of transmitting at the wider channel width without causing interference with other APs. For example, if attempting to guarantee non-overlapping 80 MHz channels, limit the allowed subset of primary 20 MHz channels to 36, 52, 100, 116, 132, and 149. The specific 20 MHz channels could be different than I have listed in this example, but the key point is that only ONE primary 20 MHz channel be allowed within each "guaranteed" wider channel width.

Finally, channels will be configured by WLAN administrators in two steps:
  1. Select a channel width (20, 40, 80, 160 MHz) for AP or WLAN operation
    This should be the "guaranteed" channel width, at minimum. It could be larger than the "guaranteed" channel width if you want to allow APs and clients to achieve higher peak performance when the network is fairly idle. Since this can be done on a per-frame basis and clear-channel assessment is performed prior to transmission, allowing dynamic use of wider bandwidth shouldn't result in significantly more collisions. But it will result in more co-channel interference (sharing of bandwidth) between neighboring APs. The exact impact of per-frame channel width and co-channel interference in a multi-AP environment will require more in-depth testing once 802.11ac equipment is released. I would stick with using the "guaranteed" channel width until you are able to test in your own environment.

  2. Assign the primary 20 MHz channel to each AP (or allow auto-assignment)
    The primary 40 MHz and 80 MHz channels will be determined automatically based on the primary 20 MHz channel selected. If "Auto" channel planning is used, which is common in enterprise WLAN equipment, ensure the subset of primary 20 MHz channels allowed to be assigned to APs is limited to those in your list.
Final Thoughts
802.11ac offers exciting prospects for "Gigabit" Wi-Fi. However, most of the benefit of the first wave of products centers around the use of ever-wider channels. Two barriers to the use of these wider channels exist for enterprise WLANs:
  1. Limited spectrum, resulting in insufficient channels to facilitate a re-use plan that effectively allows wider channels without excessive co-channel interference.
  2. Greater reliance on DFS channels to provide more spectrum and channels, which many Wi-Fi clients do not support today.
Luckily, the engineers designing 802.11ac learned from 802.11n's shortcomings and devised a clever method to minimize co-channel interference through per-frame channel width adaptation and the designation of primary channels. This presents a fundamental shift in how administrators should approach WLAN channel planning. Administrators should be careful in selecting primary 20 MHz channels that result in non-overlapping channels at the larger channel widths that they wish to use in their environments. Also, a heavier reliance on DFS channels is required to realize the benefits that 802.11ac has to offer in an enterprise environment. Enterprises will need to evaluate what devices are in use on their WLANs to determine if 5 GHz DFS channel use is feasible. This can be especially problematic with personal devices where the organization has little control over the devices being used. However, consumer device lifecycles are typically shorter than enterprise lifecycles, so the adoption of 802.11ac capable client devices should occur relatively quickly (~2 years).

From an implementation perspective, most enterprises should plan around non-overlapping 40 MHz channels, or even 20 MHz channels in high-density areas. If the FCC frees up an additional 195 MHz of shared spectrum in late 2014 or early 2015 then designing around non-overlapping 80 MHz channels (or possibly even 160 MHz channels) in the U.S. will become much more practical.


802.11ac Gigabit Wi-Fi Series:

Monday, March 18, 2013

Video Blog: High-Density Wi-Fi Design Part 1 - Forecasting AP Capacity

In my previous post, Design your WLAN for High Capacity, I outlined the increasing demands being placed on modern enterprise WLANs caused by the growth in the number of Wi-Fi connected devices, the proliferation of mobile devices and BYOD, and the increasing reliance on the WLAN as the primary network for users in the enterprise. As described in the Aerohive High-Density Wi-Fi Design and Configuration Guide, the key to supporting this increased demand is to design the WLAN for capacity rather than simply coverage.

The first step in designing a WLAN to meet capacity demands is to perform adequate requirements gathering. This starts with a proper understanding of client device capabilities. Because RF is a shared environment, the capacity is determined by the capabilities of the AP infrastructure and the client devices, application bandwidth requirements, and the resulting airtime utilization that results from their unique combination.

In this first of three videos on high-density Wi-Fi design, I describe how these variables interact and can be used to derive a preliminary forecast of the required AP capacity to support the intended network load. I also walk through a few examples to highlight how to apply this method to both homogenous and heterogeneous client environments.

The resulting AP capacity forecast is a starting point to aid the RF design and site survey process. The value in deriving the AP capacity forecast is to ensure that capacity needs are properly accounted for in the site survey process. For example, even though one AP may provide adequate coverage in a university lecture hall, several more APs may be required for capacity. Historically, RF site surveys have only focused on providing adequate RF coverage for the physical area, which may provide sufficient signal in all desired locations but lack AP and channel capacity to successfully support the client and application load.

Once you’ve watched the examples in the video, walk through a few of your own scenarios using the requirements gathering worksheets in the appendix of the Aerohive High-Density Wi-Fi Design and Configuration Guide.

Stay tuned for the remaining two videos in this series, where I’ll cover key RF design and network configuration principles for high-density networks.

Read the Entire High-Density Wi-Fi Design Series:


Full Disclosure - This video was created in cooperation with Aerohive Networks, my current employer. 

Saturday, March 2, 2013

Wi-Fi Sightings: "Motel Wi-Fi"

I've seen plenty of Wi-Fi in hotels, but in a Motel? Well now I have seen that too! This free Wi-Fi access was found at a Best Western in Christchurch, New Zealand.

It appears Wi-Fi in hospitality is continuing the trend of "free" access as an service to attract customers at lower-end locations, while higher-end locations "charge" for access as an amenity for their ostensibly richer clientele.

Wi-Fi Sightings: "Phone Booth Wi-Fi"

Found in Christchurch, New Zealand on 2-Mar 2013