Adaptive Antennas and Interference

There is quite a nice discussion going on over at GT Hill's blog about Adaptive Antennas.

One part of the discussion centers around interference rejection of such a system, by "controlling" the RF signal. This serves to increase signal gain for the intended client and reduce interference in most other directions (I agree on these points). My contention is that in a multi-cell architecture, as is typically found in most enterprise, education, healthcare, and [especially] high density environments, the increased signal gain also results in increased interference (or more aptly "contention") of other neighboring Wi-Fi cells in the path of the newly shaped signal. And, different than directional antennas, the path changes with every client that the Adaptive Antenna system is transmitting to. So, with an omni-directional coverage area and clients distributed throughout that area, interference will impact all other Wi-Fi cells around it. Not at the same time, but at different points in time depending on traffic patterns downlink to clients.

What I'm getting at is that Adaptive Antennas simultaneously reduce interference in some directions and increase it in others. I'm not saying that one outweighs the other, because I don't think anyone can predict that unless we know the specific deployment scenario and network design. My point is just that we need to acknowledge the presence of both effects (not just one, while ignoring the other).

Hopefully this makes sense. It does in my [admittedly jumbled] mind ;)

I would normally just post this in the comments section on GT's blog, but I would like to include a drawing to help convey what I'm talking about. (I'm a great artist BTW. Can't you tell!)

Adaptive Antenna's Simultaneously Decrease and
Increase Interference In Different Directions

GT - Hopefully this clarifies my opinion. Sorry to take the conversation away from your website.

Readers - please comment directly on GT's blog to keep the conversation thread in one location.

Cheers,
Andrew

Measuring Wi-Fi Performance using Ruckus Wireless Zap

Recently, I was provided a free copy of a Wi-Fi performance testing tool for iOS call ZapPerf for iOS by Zaib (@WLANBook). This tool is a port of the open source Zap test tool released in 2010 by Ruckus Wireless. This spurred me into doing a little performance testing using both the iOS version and the Mac version.

The Zap test tool is a bit different from other performance test tools out there, such as iperf, in that it measures the consistency of performance rather than peak or average throughput. This translates into a better measurement of the usability of a Wi-Fi network to handle real-time traffic that cannot tolerate momentary performance degradation, such as voice and video. For example, streaming a H.264 compressed high-definition Blu-Ray quality video could require a consistent 25-35Mbps of throughput. If performance drops even momentarily the video quality could degrade and result in video pixelation or freezing which would be unacceptable to viewer.

Ruckus published a whitepaper on Characterizing Wireless Network Performance, in which they describe the design of the Zap tool:

Prior to Zap, the focus of existing tools in the market has been on measuring average throughput, not worst-case throughput. Ruckus engineers originally designed Zap to measure and predict what type of performance they could expect most of the time (not just some of the time), using a large number of samples.

The output of the tool is a Cumulative Distribution Function (CDF), which is a representation of the probability of achieving a given throughput level.

Cumulative Distribution Function example shows
the percentage of time the system can achieve a given throughput value

I decided to do some testing with two systems, an iPad 1 and a MacBook Pro (Mid 2010). I also decided to run multiple tests on each system with different QoS value to determine how Wi-Fi quality of service impacts device performance. Both systems were tested with two QoS values: Best Effort (DSCP 0, Zap option -q0x00) and Voice (DSCP 46, Zap option -q0xB8).

Note - The Zap QoS option is a bit confusing, as it requires a hexadecimal value representing the entire 8-bit IP ToS header instead of the only the 6-bit DSCP portion. Hence the hex values above account for the two low-order ECN bits as well.

iPad 1 Results
This is a first-gen iPad 1 which sports an 802.11n 1x1:1 radio chipset capable of 65Mbps raw Wi-Fi speed.

You can see that the performance of the iPad 1 was very similar for both QoS queues. This is likely due to the fact that testing was performed using an 11n access point in a non-congested location with little traffic except for the system under test. The AP and RF spectrum were sufficiently able to deliver a good user experience above 30Mbps at all times (100th percentile).

MacBook Pro (Mid 2010) Results
This is a MacBookPro6,2 Core i7 2.66GHz (Mid 2010) model, which sports an 802.11n 2x2:2 radio chipset capable of 300Mbps raw Wi-Fi speed.

Contrast the performance of the iPad with the MBP. The MacBook is capable of 2 spatial streams and 300Mbps raw speed, which is able to saturate the AP and RF spectrum capabilities more thoroughly. In this scenario, we can see that the Voice queue clearly outperforms the Best Effort queue, all other variables constant. The Voice queue is able to achieve 179Mbps at all times (100th percentile) while the Best Effort queue is only able to match that performance about 90% of the time. The small amount of competing Wi-Fi traffic in the area reduces the guaranteed service level of the Best Effort traffic down to 96Mbps.

Concurrent Voice and Best Effort Traffic
Next, I decided to stress the network a little bit more to see the difference in performance between QoS queues when the network is under heavier load. For this test I used two MacBooks, both capable of 300Mbps, with one in the Voice queue and one in the Best Effort queue.

I was expecting to see a significant performance gap between the Voice and Best Effort queue, highlighting the benefit of QoS traffic prioritization. Here's what resulted:

My results matched my expectations. The stream in the Voice queue significantly outperformed the stream in the Best Effort queue. The Voice stream was able to achieve over 212Mbps for 98% of the time and resulted in worst-case performance of 179Mbps. You will also notice that the performance remained very consistent throughout the test, as indicated by the relatively flat line across the graph.

The Best Effort stream on the other hand saw a more varied performance, indicated by the sloped line in the graph. This stream was able to achieve 146Mbps for 98% of the time, but only 124Mbps worst-case.

Revolution or Evolution? - Andrew's Take
When it comes to Wi-Fi and preparing a network for rich multimedia applications such as video streaming, testing consistency of performance becomes more important than measuring peak or average performance, which can masque temporary degradation when sampling a large amount of data. Using the Zap tool by Ruckus Wireless offers a method to measure the sustainable throughput of a Wi-Fi network that more closely resembles the expected user experience.

Cheers,
Andrew

Articles Worth Reading: 12/30/2010

Here are a collection of articles from the past week that I have found useful, interesting, or enlightening. As always, for a complete list of articles check out my shared article feed from Google Reader.


Unplugged – Show 4 – Too Many iPads – My Etherealmind: 

"#Content

* 802.11n adoption in the enterprise may be driven by iPads and iPhones today, and Android in the future. This may drive the greater of adoption of wireless networking generally.

* Will the prevalence of personal hotspot devices end up ruining enterprise guest networks? Guest networks are becoming important systems for external consultants and contractors and the wireless networks need to be more reliable. Discussions of some of the challenges around this.

* Fast roaming and why it’s important in an enterprise environment, especially with 802.1x/EAP authentication; forthcoming Wi-Fi Alliance Voice Enterprise certification.

* Keith R Parsons moving to Ruckus."

2010: The Enterprise Wi-Fi Year in Review - www.enterprisenetworkingplanet.com: 

"Thanks to 802.11n, there's a good chance 2010 was the year a wireless LAN became your default network access method. This retrospective from Wi-Fi Planet looks at the major deals, improvements in technology and changes in the market that pushed Wi-Fi ahead in the enterprise."

* I had the honor of contributing to this great article by Lisa Phifer.


Breaking GSM Security With a $15 Phone: 

"Whatever assurances have been given about the security of GSM cellphone calls, forget about them now."

802.11: Safety equipment for outdoor WLAN deployments: 

"Deploying outdoor network is hard. It is hard technology wise. One is bound by the limitations of the technology (the 802.11 standard), regulatory restrictions (FCC and the likes) and the environment itself (buildings, trees, hills, weather changes, etc). And once you have designed the network, you have to deploy the network."

dailywireless.org » AT&T Expands Public WiFi: 

"AT&T will expand Wi-Fi hot spots in New York’s Times Square just ahead of New Year’s celebrations and is also deploying its first hot spots in a public, outdoor area of San Francisco, the Embarcadero waterfront district."

How to Get Your Idea Approved - Amy Gallo - Best Practices - Harvard Business Review: 

"When you have an idea, proposal, or recommendation that you believe in, it's easy to presume that getting it approved will be a breeze. If you see how great the idea is, won't everyone else? However, whether an audience accepts an idea is often less about the idea itself than about how you present it. When you need approval, don't assume that just because it's brilliant, others will see it that way — convince them."

Layer 2 vs Layer 3 in Wireless Mesh: Do You Have to Choose? « Mesh Without Wires: 

"Layer 2 vs Layer 3 benefits and trade-offs have been a topic of discussion for a couple of decades as both approaches have their benefits and drawbacks."

Cheers (and happy reading),
Andrew

Packet Pushers Unplugged #4 - Too Many iPads

Enterprise support for consumer devices is definitely one of the hot topics these days for Wi-Fi engineers and IT departments. Check out the latest Wireless Unplugged episode of Packet Pushers Podcast, hosted by Greg Ferro (@etherealmind), and featuring the expert opinions of Jennifer Huber (@jenniferlucille), Chris Lyttle (@wifikiwi), and myself (@revolutionwifi).

In this episode we discuss the iPad as well as other relevant wireless topics, including:

- 802.11n adoption in the enterprise may be driven by iPads and iPhones today, and Android in the future. This may drive the greater of adoption of wireless networking generally.

- Will the prevalence of personal hotspot devices end up ruining enterprise guest networks? Guest networks are becoming important systems for external consultants and contractors and the wireless networks need to be more reliable. Discussions of some of the challenges around this.

- Fast roaming and why it’s important in an enterprise environment, especially with 802.1x/EAP authentication; forthcoming Wi-Fi Alliance Voice Enterprise certification.

- Keith R Parsons moving to Ruckus.

Packet Pushers Podcast: Wireless Unplugged Show #4 - Too Many iPads

Thanks to all participants for the great discussion! It's always an honor.

Cheers,
Andrew