THE NEED FOR SPEED: Rate My Wi-Fi

With Wi-Fi, we obsess over maximum data rates, MIMO radio chains, and bandwidth. But we seldom talk about the mechanism that switches gears. Like cars, Wi-Fi devices have transmissions too.  The transmission is called dynamic rate adaptation.

Rate adaptation is the function that determines how and when to automatically change to a new data rate. When tuned properly, a good adaptation algorithm finds the right data rate that delivers peak AP output in current RF conditions, unstable as they are.

CHOOSING A RATE: Ethernet vs. Wi-Fi 

On a wired Ethernet link, endpoints connect and auto-negotiate the fastest mutually supported signaling rate. Because Ethernet link conditions are static, the rate remains the same. Simple.

But with Wi-Fi, link conditions change more often than HP changes CEOs. To find the best data rate in an undulating (we've been looking to use that word forever) sea of unlicensed spectrum caused by mobile clients, transient devices, RF interference, temporary networks, bursty traffic, etc., smart rate adaptation is essential and ongoing. Instead of selecting the fastest mutual speed, 802.11 stations attempt to find the best speed, based on a tradeoff of reliability and performance.

WHAT IS A DATA RATE?

The term data rate is used to indicate the speed of a wireless connection. Data rates are determined by a number of variables, including modulation, coding, channel bandwidth, and spatial streams.

• Modulation—the process of changing the properties of a carrier wave to represent information bits.
• Coding—also known as forward error correction, coding is the process of adding redundant information bits to a data stream to improve reliability over unreliable mediums. 

Modulation and coding schemes (MCS) have orders of complexity. It’s important to understand that higher data rates are more efficient than lower data rates, but they are also more “complex” and require better signal quality.

With lower data rates, the modulation and coding mechanisms are simplified, which makes them less efficient, but more reliable. Fundamentally, rate adaptation is an unending adjustment to find the most efficient data rate with an adequate level of reliability. This is especially true with 802.11n that adds a bunch more data rate choices.

The 802.11 specification introduces the term dynamic rate switching and acknowledges the fundamental issue: with multiple data rates, there is a need to dynamically adjust based on RF conditions. But, they don’t lend any help. So if you look at ten different Wi-Fi companies, you’ll see ten different rate control algorithms, some better than others.

BEING SMARTER ABOUT RATE SELECTION

Ruckus approaches rate selection with a unique focus. Instead of using unreliable signal measurements to hope for the best data rate, our rate selection algorithms are statistically optimized, which is our engineer-chic way of saying that we adapt the data rate based on historical, statistical models of known-to-be-good methods for each client.

Without the right algorithm, the optimal rate for any client is a crapshoot. And when you’re guessing, the safest guess is to err on the side of reliability, which sacrifices throughput and capacity. In common practice, errors lead to data rate downshifts. Subsequent frames use lower data rates, which occupy the medium for longer durations; thus they are more likely to experience interference, leading to another rate downshift. Eventually, throughput for that client tanks and network capacity suffers as a result.  Click on the chart below to see why.

In other words, purely reactive algorithms are sub-optimal in their myopia. Math is the better way. Statistics tell us more about the implications of transient interference and long-term trends. Accordingly, we can adjust—or perhaps more importantly, not adjust—the data rate to optimize both for performance and capacity, based on previous experience. 

Ruckus is also focused on thorough data rate validation. Because of our custom AP hardware and software, we test and test and test everything some more. One such monotonous test is for data rate performance. Believe it or not, we test every individual MCS rate at different ranges and conditions to ensure that our performance is a bulwark of reliability.

And this is no small feat with 802.11n MCS tables. The end result of this focused engineering and testing is stable connections.

Economy cars are everywhere. But if you want a racecar—at economy car price—you can get it. We obsess. We nitpick. We are anal retentive about the details. Yes, we have issues.  But your Wi-Fi applications and users will thank us. 

Comments

[Dave Wright (@wifidave)]

Marcus,

Very nice post. You guys are definitely anal retentive about RF, but I'll consider that a fully acceptable character quirk in a Wi-Fi vendor. I think the graphics contrasting interference impacts with a reactive -vs- adaptive/smart algo were quite compelling. You might want to step thru those a bit more for people new to the field. It would also be nice if the smart algo graphic was expandable in a new window like the reactive one is.

Thanks, Dave

[andrew slivkovski]

Great Post!!
The following is meant in the spirit of provoking dialogue ... Feel free to moderate.....
"Even If I read this article a thousand times...and understood networking performance.... It wouldn't help figure out why my WIFI can't update my son and daughter's ipad and my iphone to IOS6 ... Last check the download is 386 MB.... I feel like I am going on a long road trip to Maine ... "Daddy...are we there yet..(is done yet)".....
"I thought to myself ... I have good performance on my wired network connections 2-5 MB/Sec.... It would be great if I could someone connect my devices to the wired connection...and I would be confident that it would download in a 5-10 minutes... switch the device out ..."
" I knew I was spending too much time with this when I starting watching the 1:00 patriots game ...and I finally got my daughters updated when 60 minutes started"
Look forward to when you might come out with the "Super Party Version" ... everybody will be wanting good wifi for their device .... "QoS guaranteed for your WIFI service" .... please let Acela Amtrack know ... it would make ride to Boston to new York trip a lot faster .