First Look - 802.11ac Gigabit Wi-Fi Speed

I just couldn't wait for 802.11ac enterprise WLAN gear to be released, so I went and picked up a consumer router and USB adapter to get a first look at 802.11ac.

Equipment
I picked up the ASUS RT-AC66U and corresponding ASUS USB-AC53 adapter. I decided to go this route because with pre-standard equipment we've historically seen best performance when pairing the AP and client adapters from the same manufacturer.

ASUS 802.11ac Product Lineup

The router supports 3x3:3 MIMO with 80 MHz channels and 256-QAM on the 5GHz 802.11ac capable radio. This results in a raw Wi-Fi data rate capable of 1.3 Gbps. You might want to reference this 802.11ac data rate chart.

The USB adapter on the other hand is a bit handicapped, supporting only 2x2:2 MIMO, 80 MHz channels, and 256-QAM. This results in a raw Wi-Fi data rate capable of 867 Mbps. However, actual throughput performance is further limited by it's USB 2.0 interface which has a raw bus speed of 480Mbps but is limited to somewhere around 60% of that due to USB host controller overhead. So realistically, with this USB adapter I'm going to max out around 288 Mbps of actual throughput. Additionally, it's increasingly hard to pack a 3x3:3 MIMO antenna system into an external USB adapter due to physical size constraints. This is what we've seen with external 802.11n adapters for the most part, and that has continued to be the case with most first-generation external 802.11ac client adapters. We should see more spatial stream support on internal Wi-Fi adapters where the antenna can be integrated into the laptop case in order to create sufficient physical separation.

I loaded the adapter driver and utility into a Windows 7 laptop and it reported the raw speed as 866.5 Mbps, which is equivalent to the adapter's maximum raw Wi-Fi speed based on its specifications.

Windows Connection Details Reported an 866.5 Mbps Speed

Peak Performance
I decided to initially perform a simple file transfer using SMB. To accomplish this, I hooked one laptop to the wired Gigabit Ethernet LAN port on the router as the server and connected the second laptop to the 5 GHz Wi-Fi radio as the client. I pulled a 1.38GB file down from the server to the client over the wireless link. I performed this test over 10 times for reproducibility. The router was configured for 5 GHZ operation with an 80 MHz channel and a primary channel of 48.

File size: 1.38GB / 1,490,209,655 Bytes / 11,921,677,240 bits
Transfer time: 51.0 seconds (avg.)
Application throughput: 233.76 Mbps

Note - remember that this is application layer throughput, which does not include SMB, TCP, IP, or MAC layer overhead.

Next, I performed an iPerf TCP throughput transfer for 60 seconds, modifying the TCP window size to 1024KB. The test was performed ten times, with the average result clocking in at 204 Mbps.

iperf -f m -i 1 -w 1024K -c 192.168.1.33 -t 60

[ ID] Interval       Transfer     Bandwidth
[156]  0.0-60.0 sec  1482 MBytes   204 Mbits/sec

Overall, I was fairly pleased with this result. Although it is nowhere near the Gigabit speeds that 802.11ac is capable of achieving, this is to be expected given the handicapped USB 2.0 adapter that I am using. Given a raw data rate of 867 Mbps peak, I would expect to see throughput around 520 Mbps without the USB 2.0 limitation (or roughly 60% of the peak data rate).

I used MetaGeek's Chanalyzer Pro and WiSpy DBx to record the 80 MHz file transfers. I configured the router to use an 80 MHz channel width, with a primary channel of 48 in the UNII-1 band. The spectrum analysis workstation was located within 5 feet of the router.

Spectrum Analysis of an 802.11ac 80 MHz Channel

You can clearly see FFT waveform across the entire UNII-1 band due to the automatic extension of the primary 20 MHz channel up to the maximum 80 MHz channel width that was configured. As a recap of how the primary and extension channels work, you might want to review my previous post on 802.11ac Channel Planning.

Performance Comparison versus 802.11n
To assess how well the 2 spatial stream 802.11ac client performed, I decided to benchmark it against similarly capable 2SS 802.11n clients. To make the test apples-to-apples, I also turned down the channel width on the ASUS router to 40 MHz, since that is the largest channel width the 802.11n clients support.

I ran the same SMB file transfer test as described previously. The SMB file transfer was run three times for each client and I averaged the results. Each client was placed in the same physical location, approximately 10 feet from the router to assess peak performance.

40 MHz throughput comparison against two different 2x2:2 802.11n clients:

Wi-Fi Adapter	Wi-Fi Capabilities	Max Wi-Fi Data Rate at 40 MHz	SMB File Transfer Time	SMB Throughput
MacBook Air Airport (Internal)	802.11n, 2x2:2 MIMO	300 Mbps	1:36.7 sec	123.29 Mbps
Intel 4965ABGN (Internal)	802.11n, 2x2:2 MIMO	300 Mbps	1:15.5 sec	157.90 Mbps
Asus USB-AC53	802.11ac, 2x2:2 MIMO	400 Mbps (limited further due to USB 2.0 bus speed)	1:07.0 sec	177.94 Mbps

Clearly, the peak performance test shows that the 802.11ac client has an edge due to the higher data rates provided at the top-end with 256-QAM modulation.

Rate over Range versus 802.11n
The aggressive 256-QAM modulation may provide higher peak throughput when a client is physically very close to the AP. But will that advantage hold up over larger distances? There has been significant discussion in the industry about the real-world usefulness of 256-QAM, especially at typical client distances of 10-20 feet or greater from an AP. Will clients realistically be able to use such aggressive modulation in practice?

However, the question about the use of higher modulation is not the only one. Newer wireless chipsets should also benefit from better manufacturing processes that improve 802.11ac client receive sensitivity, translating into the use of higher data rates at at any given distance when compared to older 802.11n clients. Put another way, does 802.11ac exhibit better rate-over-range compared to 802.11n?

For the rate-over-range testing, I ran the same iPerf TCP throughput test as described previously, using a 40 MHz channel width at varying distances from the router. The tests were performed in a residential house since that is the only space that I have available at the moment.

- Point #1: 10 feet from the router, 1 wood panel wall in-between
- Point #2: 25 feet from the router, 1 wood panel wall in-between
- Point #3: 30 feet from the router, 1 wood floor and 1 drywall in-between
- Point #4: 40 feet from the router, 2 wood floors and 1 wood panel wall in-between

Additionally, in parenthesis I provide a rough signal strength and data rate used during the tests as reported by the client supplicant/driver. I also provided the test results for the 802.11ac client using an 80 MHz channel width for reference.

Location	Macbook Air  (40 MHz)	Intel 4965ABGN  (40 MHz)	ASUS USB-AC53  (40 MHz)	ASUS USB-AC53  (80 MHz)
Point 1	154 Mbps (-50 dBm, 300 Mbps)	165 Mbps (-50 dBm, 300 Mbps)	204 Mbps  (-40 dBm, 400 Mbps)	204 Mbps  (-40 dBm, 867 Mbps)
Point 2	144 Mbps (-60 dBm, 216 Mbps)	150 Mbps (-60 dBm, 270 Mbps)	180 Mbps  (-52 dBm, 400 and 324 Mbps)	202 Mbps  (-54 dBm, 702 and 585 Mbps)
Point 3	112 Mbps (-65 dBm, 162 Mbps)	102 Mbps (-70 dBm, 180 Mbps)	144 Mbps  (-54 dBm, 243 Mbps)	201 Mbps  (-60 dBm, 526 Mbps)
Point 4	35 Mbps (-80 dBm, 54 Mbps)	61 Mbps (-80 dBm, 120 Mbps)	119 Mbps  (-65 dBm, 216 Mbps)	190 Mbps  (-70 dBm, 468 Mbps)

The 802.11ac client performance beats both 802.11n clients in all tests at all locations. Additionally, the performance gap widens significantly at Point #4, the farthest distance and weakest signal from the router. Clearly, 802.11ac provides significant rate-over-range improvements over 802.11n.

Performance improvement at each location:
- Point #1: 32.5% (vs MBA), 23.6% (vs Intel)
- Point #2: 25.0% (vs MBA), 20.0% (vs Intel)
- Point #3: 28.6% (vs MBA), 41.2% (vs Intel)
- Point #4: 240% (vs MBA), 95.1% (vs Intel)

The peak performance improvement at Point #1 (32.5%, 23.6%) is squarely in-line with 802.11ac's 33% theoretical improvement over 802.11n due to the higher modulation rate of 256-QAM (400 Mbps) versus 64-QAM (300 Mbps).

The receive sensitivity of the 802.11ac client is also better than the 802.11n clients. At most test locations the 11ac adapter exhibited an 8-15 dB signal advantage over the older 11n adapters. This highlights the fact that newer wireless chipsets offer improved hardware quality over older chipsets. Receive sensitivity also appears to be better for the same 802.11ac adapter when smaller channel widths are used. This highlights the fact that as channel width increases, clients will need to maintain a slightly better signal strength to maintain the same modulation rate. In practice, this will mean there is a slight trade-off with decreased modulation rate when increasing channel width, while maintaining all other variables constant (such as transmit power, antenna gain, etc).

To answer the questions surrounding the use of 256-QAM at distances greater than 10-20 feet, I've found that the usable distance and signal strength required to use 256-QAM data rates is around 25-30 feet (in my case with one light wall in-between) and around -52 dBm signal strength. I note multiple data rates being used for the 802.11ac client at Point #2 because the client appeared to be data rate shifting during the test, likely unable to sustain 256-QAM modulation at times and shifting to a lower rate. These values are almost certain to vary between client adapters based on receive sensitivity, but this should provide a rough estimate for WLAN administrators.

Results Recap

Let's add up the 802.11ac test results:

1. Decent peak performance at 80 MHz channel width, although it we should see double this performance with integrated adapters or USB 3.0 external adapters.
2. Better peak performance than 802.11n at comparable 40 MHz channel width due to the use of more aggressive 256-QAM at relatively close distances to the AP.
3. Better rate-over-range performance than 802.11n, especially as distance from the AP increases and signal level deteriorates.

Final Thoughts
It was great to get a first-look at 802.11ac equipment, even though the currently available client adapters are a bit disappointing from a peak performance standpoint due to their reliance on USB 2.0 bus speed. You might consider waiting for integrated 802.11ac client adapters or external USB 3.0 adapters to hit the market, which should be capable of supporting the full throughput that 802.11ac offers. A few are already out there; I found this 802.11ac hardware wiki that seems to be keeping track of consumer equipment.

Even though you might not see awe-inspiring peak performance for any single client with this early release equipment, 802.11ac still stands to improve the aggregate performance and capacity of the network through more efficient use of airtime. WLAN administrators should expect to see this aggregate increase in network capacity even with 802.11ac capable mobile devices such as tablets and smartphones. Since they will be using higher Wi-Fi data rates, they will be getting on and off the air quicker for a given application throughput level than they would by using 802.11a/g/n. This will translate into the ability to support more clients or higher throughput per-client, and will be a big boost for enterprise WLAN capacity!

Cheers,
Andrew


Read the Entire 802.11ac Gigabit Wi-Fi Series:

• The Impact of 802.11ac Gigabit Wi-Fi on Enterprise Networks
• 802.11ac Channel Planning
• Safely Using 80 MHz Channels with 802.11ac
• Ekahau Site Survey™ 6.0 Incorporates 802.11ac Channel and Capacity Planning

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.

Cheers,
Andrew


802.11ac Gigabit Wi-Fi Series:

• The Impact of 802.11ac Gigabit Wi-Fi on Enterprise Networks
• 802.11ac Channel Planning
• Safely Using 80 MHz Channels with 802.11ac
• Ekahau Site Survey™ 6.0 Incorporates 802.11ac Channel and Capacity Planning
• First Look - 802.11ac Gigabit Wi-Fi Speed

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.

Cheers,
Andrew


802.11ac Gigabit Wi-Fi Series:

• The Impact of 802.11ac Gigabit Wi-Fi on Enterprise Networks
• 802.11ac Channel Planning
• Safely Using 80 MHz Channels with 802.11ac
• Ekahau Site Survey™ 6.0 Incorporates 802.11ac Channel and Capacity Planning
• First Look - 802.11ac Gigabit Wi-Fi Speed

The Impact of 802.11ac Gigabit Wi-Fi on Enterprise Networks

802.11ac is being hyped as a dramatic improvement in performance over existing 802.11n equipment. While this will be technically true once all of the capabilities of 802.11ac are available, this is going to take time. Realistically, within the next year, first generation 802.11ac equipment will only offer marginal improvement over 802.11n equipment for enterprise networks. Separating the hype from real-world impact to enterprise WLANs has received little attention.

I recently had the opportunity to provide commentary on the subject for an article written by Lee Badman in Network Computing. Lee did a superb job with the article, as he always does, and provides great insight for corporate WLAN network managers looking for advice.

I'd like to expand a little bit deeper on my viewpoints expressed in his article with this post, and to provide an assessment of the "real-world" impact that most enterprises should be considering.

802.11ac - Wi-Fi Just Keeps Getting Better!
The 802.11ac standard includes complex technology that will eventually allow multi-Gigabit data transfer, but not all aspects of the specification will be available on day one. Similar to 802.11n, which began with two spatial stream devices capable of 300 Mbps and eventually saw maturation to three spatial streams capable of 450 Mbps, 802.11ac will see an initial first wave of products that are capable of 1.3 Gbps with future maturation possibly up to 6.9 Gbps. Whether or not we will actually see 802.11ac products capable of 6.9 Gbps is dependent on hardware enhancements on both the access point and client that are not certain.

First generation 802.11ac products will achieve 1.3 Gbps through the use of three spatial streams, 80 MHz wide channels (double the largest 40 MHz channel width with 802.11n), and use of better hardware components that allow higher levels of modulation and encoding (256-QAM). The 802.11ac amendment also simplifies implementation of standards-based beamforming for manufacturers by focusing on a single form of explicit beamforming, and eliminating the complex number of beamforming methods detailed in 802.11n. This should allow AP and client manufacturers to align on a single interoperable method.

Future releases of 802.11ac will enable even higher bandwidth by allowing up to eight spatial streams, 160 MHz wide channels, and simultaneous transmission to multiple clients by an access point, called Multi-User MIMO (MU-MIMO). In a few years, the realistic benefits for enterprises will likely be four and five spatial stream products with network designs still primarily based around 20 MHz and 40 MHz wide channels. MU-MIMO deserves special attention because it will mark a significant milestone in wireless technology that will allow greater performance through the use of parallel transmissions to two different receivers from the same transmitter. For example, an AP that is capable of 3 spatial streams (3X3:3) could transmit 1 spatial stream to three different clients that are only capable of 1 spatial stream each, concurrently. This will allow enterprises to better-serve large client populations in high-density environments. As described in the Aerohive High-Density Wi-Fi Design and Configuration Guide, the increasing reliance on WLANs as the primary method of network connectivity and for mission-critical services is shifting the focus from designing Wi-Fi networks for coverage to designing them for capacity. MU-MIMO will be a critical enhancement that will allow Wi-Fi networks to scale larger and provide greater capacity to support growing demand.

Real-World Benefits
Most of the current discussion on 802.11ac focuses on large bandwidth improvements that will not be available for several years to come. The short-term improvements are of bigger benefit in small WLAN deployments, such as SMB and consumer homes, where only a single Wi-Fi AP will be able to take advantage of the much wider channels.

First-generation enterprise 802.11ac products that will be available in 2013 represent an incremental evolution of Wi-Fi above current 802.11n products on the market. These first-generation products will outperform existing 11n products, but only marginally, especially in multi-AP enterprise deployments. This is because the majority of bandwidth gain with first generation 802.11ac products will rely on the use of wider channels, up to 80 MHz. Enterprises need to be cognizant of the limitations in spectral capacity when designing and deploying an enterprise WLAN network. Enterprises must be careful to deploy access points on non-overlapping channels, with sufficient signal attenuation between adjacent access points to prevent co-channel interference.

Here is a look at the data rates available with first-generation 802.11ac products capable of 1, 2, and 3 spatial streams. Note that 160 MHz channels are shown only for reference, and will not be available with the first wave of 11ac products.

First-Generation 802.11ac Data Rates
(Note - 160 MHz channels will not be available in first wave of products)

You can find a complete listing of 802.11ac data rates, including 4-8 spatial streams, in the appendix of the Aerohive High-Density Wi-Fi Design and Configuration Guide.

Arguably, the biggest benefit of first generation 802.11ac will be the adoption of 5 GHz bands by mobile devices. This will enable enterprise WLANs to serve mobile devices in greater quantity with better performance due to the mandatory support of 5 GHz frequency bands by all 802.11ac compliant equipment. Today, enterprise WLANs struggle to provide the capacity required to support the large influx of mobile devices like smartphones and tablets. Once mobile device manufacturers begin deploying 802.11ac capable devices, existing 802.11n and new 802.11ac WLAN deployments will be able to provide significantly better services to mobile devices. Due to the enormous popularity and market growth of mobile devices, they now represent a significant portion of the user-base in many enterprise WLAN networks. However, most mobile devices today are limited to the 2.4 GHz band, which is cluttered with interference and offers minimal capacity. This has resulted in under-performing WLANs and an often poor user experience. With the adoption of 802.11ac, mobile devices will be able to take advantage of the cleaner spectrum and the additional capacity available in the 5 GHz bands. In addition, chipset enhancements should allow mobile devices to operate at higher bandwidth and performance levels with better battery life.

The Massive Shift to 5 GHz and the Impact to Enterprise WLANs
802.11ac operates exclusively in the 5 GHz unlicensed bands. In the U.S. there are a total of 25 non-overlapping 20 MHz channels in the 5 GHz bands. While this appears sufficiently large to handle channel re-use in large enterprise deployments, it can be deceiving. When channel size is increased, the number of available channels decreases, constraining channel re-use and making the risk of co-channel interference and WLAN performance degradation higher. Only 10 non-overlapping channels will be available with 40 MHz channel width (similar to the 9 channels available with 802.11n; the difference is due to the addition of channel 144 with 802.11ac), and only 5 channels will be available with 80 MHz channel width. When DFS channels are avoided, which are not supported by a large percentage of client devices and are at higher risk of causing network stability issues, the number of remaining channels dwindles down the only 2. Therefore, it will not be practical for most enterprises to use of 80 MHz wide channels because it will significantly constrain channel re-use that is critical to a high-performance WLAN. This will limit practical performance of first-generation 802.11ac in enterprise environments to 600 Mbps using 40 MHz channels, a far cry from the 1.3 Gbps advertised.

Spectral Capacity versus Channel Width

Enterprises in multi-tenant buildings or in dense urban areas will likely see increased utilization of the 5 GHz spectrum bands, which could cause greater levels of interference and degrade WLAN performance. This is of significant concern if enterprises deploy 802.11ac equipment with 80 MHz wide channels, without recognizing the impact to neighboring businesses.

802.11ac also threatens to accelerate the utilization of 5 GHz spectrum bands by a large majority of enterprises. This could be a double-edged sword, providing the promise of increased performance for individual organizations, while simultaneously congesting the once interference-free 5 GHz bands. This may expose the need for more unlicensed spectrum sooner than anticipated. The timing is impeccable, as the FCC and Congress are currently considering allowing unlicensed use of two additional 5 GHz bands, devising rules for spectrum auctions in 2013-2014 of the 600 MHz TV white spaces, and spectrum-sharing plans in the federal 3550-3650 MHz band that would provide additional unlicensed spectrum for general use. While there may not be an "unlicensed spectrum crunch" today, there very well could be one in the not-to-distant future. More information on the current spectrum policy discussions can be found in my previous blog post about the need for a balanced spectrum policy.

When does it make sense to deploy 802.11ac?
Enterprises that have deployed the latest generation 802.11n equipment pervasively throughout their network can be confident in the investment they have made; first generation 802.11ac only offers incremental benefits over 3 spatial stream 802.11n.

First generation 802.11ac products will be of greater interest to enterprises that are purchasing a new "greenfield" WLAN deployment, growing an existing WLAN deployment with additional APs, or are running on older legacy WLAN equipment. 802.11ac is backwards-compatible with all previous versions of Wi-Fi, so it will be able to supplement existing WLAN deployments seamlessly while providing higher performance and investment protection versus 802.11n equipment. Enterprises that were early adopters of 802.11n may see greater appeal in moving to first generation 802.11ac because their existing 802.11n equipment has already been depreciated over a number of years and they have received their return on investment. In addition, 802.11ac can offer a substantial upgrade in performance to 600 Mbps over two spatial stream 802.11n (300 Mbps), allowing the enterprise to increase performance, capacity, and services offered over the WLAN.

Revolution or Evolution? - Andrew's Take
I'm bullish on 802.11ac. The technology holds a lot of promise to improve enterprise WLAN performance and capacity, especially for mobile devices which are accounting for a larger and larger percentage of our client base with BYOD and Consumerization of IT.

However, we need to temper our short-term expectations. The first wave of 802.11ac equipment will likely not prompt upgrades to existing WLAN deployments unless it is replacing older gear; it just won't provide enough value to justify the replacement of the latest generation of 802.11n equipment. Enterprises will also need to be careful to deploy 802.11ac equipment correctly to avoid harmful interference to their own network and neighboring networks - most notably limiting channel width to 20 MHz in high-density areas and 40 MHz maximum in other common-use areas.

Just as with 802.11n, we will see subsequent releases of 802.11ac that implement additional technology improvements detailed in the standard. This will include 4+ spatial streams (possibly up to 8 eventually), 160 MHz wide channels, and MU-MIMO.

Lastly, we need to ensure the value of unlicensed spectrum is recognized by regulatory agencies throughout the world. The 2.4 GHz unlicensed band is often joked about as being the "junk" band today due to rampant interference and over-crowding. The massive shift to 5 GHz is already underway which will only be accelerated by 802.11ac adoption. We run the risk of 5 GHz over-crowding very soon if more unlicensed spectrum isn't made available.

Cheers,
Andrew


802.11ac Gigabit Wi-Fi Series:

• The Impact of 802.11ac Gigabit Wi-Fi on Enterprise Networks
• 802.11ac Channel Planning
• Safely Using 80 MHz Channels with 802.11ac
• Ekahau Site Survey™ 6.0 Incorporates 802.11ac Channel and Capacity Planning
• First Look - 802.11ac Gigabit Wi-Fi Speed