Rate My Wi-FiRuckus Wireless | White Paper Finding the right balance between optimum performance and reliability with adaptive data rate algorithmsWhen buying a sports car, we often focus on engine size, top Instead of selecting the fastest mutual speed, 802.11 stationsspeed, horsepower, and 0-60 time. But utilizing those capabili- attempt to find the best speed, based on a tradeoff of reliabil-ties requires a well-designed transmission. ity and performance—note that uplink and downlink conditions are different, so the AP and client have data rate autonomy.In Wi-Fi, we often focus on maximum data rate, MIMO configu-ration, channel size, and fancy antennas (guilty as charged). Werarely talk about the mechanism that switches Wi-Fi gears. What’s a data rate? Thanks to marketing departments, Wi-Fi speeds and feeds areLike a car, Wi-Fi devices have a transmission too. In Wi-Fi, it’s fairly well known by Wi-Fi people. However, fewer people inti-called dynamic rate adaptation (aka. rate control, rate switch- mately understand why there are different data rates and whying, or rate selection). dynamically changing data rates can improve communications.Rate adaptation is the function that determines how and when Fundamentally, it’s critical to understand that higher data ratesto dynamically change to a new data rate. When it’s tuned are more “complex” than lower data rates. With lower dataproperly, a good adaptation algorithm finds the right data rates, the modulation and coding mechanisms are simplified,rate that delivers peak AP output in current RF conditions – which makes them less efficient, but more reliable.unstable as they are. Though often ignored, rate adaptation isa critical component to any high performance system. Each data rate is the product of some specific combination of modulation and coding—as well as other factors like channel bandwidth and spatial streams.Choosing a rate: Ethernet vs. Wi-FiOn a wired Ethernet link, endpoints connect and auto-negotiatean interface speed at the fastest mutually supported signaling Modulationrate. Because Ethernet link conditions are static, the rate remains Modulation is the process of changing the properties of a car-the same. Simple. rier wave to represent information bits. There are three basic types of modulation: amplitude, frequency, and phase.In Wi-Fi, link conditions change more often than HP changesCEOs. To find the best data rate in an undulating sea of Figure 1 shows a simplified concept in which each modulationunlicensed spectrum (mobile clients, transient devices, RF change represents a single bit of data (the baseband signal).interference, temporary networks, bursty traffic, etc.), smart We could also visualize modulation on a constellation map, asrate adaptation is essential. Wi-Fi systems must handle chang-ing conditions in stride, adapting communication rates basedon a complex set of variables.
Page 2Rate My Wi-FiFIGURE 1: Basic Types of Modulation FIGURE 3: Phase and Amplitude Modulation with 16-QAM 1 0 1 1 0 1 0 0 16-QAM Q b0b1b2b3 00 10 01 10 11 10 10 10 Baseband Signal TIME Q +3 de itu 00 11 01 11 11 11 10 11 pl Phase° Am +1 I Amplitude Shift Keying (ASK) -3 00 01 -1 01 01 +1 11 01 +3 10 01 I -1 00 00 01 00 11 00 10 00 -3 Frequency Shift Keying (FSK) 16-QAM (quadrature amplitude modulation)—4 bits per symbol—is shown in Figure 3. By now, I’m sure you can see the Phase Shift Keying (PSK) complexity and efficiency pattern emerging. Higher-order modu- lation is more efficient—more bits of data per sample. But, betterFIGURE 2: Phase Modulation with BPSK and QPSK signal quality is required for reliable mapping by the receiver. QPSK 64-QAM—6 bits per symbol—is also used in 802.11a/g/n, and Q BPSK Q Q b0b1 b0 802.11ac will introduce 256-QAM—8 bits per symbol. 01 11 +1Phase° +1 0 1 I -1 +1 I -1 +1 I Coding 00 10 -1 -1 Another important mechanism that controls efficiency and 1 Bit — 180° Phase Shifts 2 Bit — 90° Phase Shifts reliability is the coding rate. Also known as forward error cor- rection, coding is the process of adding redundant information bits to a data stream to improve reliability over unreliablein Figure 2. In Figure 2, we’re focused on phase shift modula- mediums. In other words, x number of data bits are convertedtion, which is common in Wi-Fi. When the receiver receives into y number of coded bits to improve error recovery. Wea modulated signal, the phase of the signal is aligned with a express this as a ratio of data bits to coded bits.constellation (i.e. the red dots) on the map and represents aspecific bit pattern (e.g. 00, 01, 10, 11). More complex modula- Data Bits Coded Bits Coding Rate Efficiency Reliabilitytion types have more bits on the map. 1 2 1/2 Less More 2 3 2/3With BPSK (binary phase shift keying), there is only one bit of 3 4 3/4information. The receiver detects the phase of the signal and 5 6 5/6 More Lessmatches that phase with a bit pattern (either the right or left ofthe constellation map): either a 0 or a 1. So the margin for error As with our modulation methods, the tradeoff for coding isis quite large, making this a highly reliable modulation method. efficiency versus reliability. The benefit of data redundancyHowever, one bit per symbol is inefficient. over noisy RF channels is a priority, but the goal is always to find the right balance.With QPSK (quadrature phase shift keying), there are fourpossible constellation points—2 bits of data. Compared withBPSK, the receiver must detect the signal’s phase with more Modulation and coding schemesprecision. Thus, the signal quality must be higher, but the tech- We use the term data rate to indicate the speed of a wirelessnique is more efficient. connection. Data rates are determined by a number of vari- ables, but the primary elements that we can dynamically controlTo add efficiency, the next higher order of Wi-Fi modulation are modulation and coding schemes. The table on the nextuses both phase and amplitude shifts, as in Figure 3. page shows how the data rate increases or decreases based on the efficiency of the modulation and coding methods.
Page 3Rate My Wi-Fi Data Rate (Mbps) Modulation Coding Rate OFDM Subcarriers Coded Bits per Coded bits per Data bits per subcarrier OFDM symbol OFDM symbol 6 BPSK 1/2 48 1 48 24 9 BPSK 3/4 48 1 48 36 12 QPSK 1/2 48 2 96 48 18 QPSK 3/4 48 2 96 72 24 16-QAM 1/2 48 4 192 96 36 16-QAM 3/4 48 4 192 144 48 64-QAM 2/3 48 6 288 192 54 64-QAM 5/6 48 6 288 216When we look at the guts of modulation and coding, it How do we know whether a few errors are an anomaly or abecomes clearer why rate adaptation is necessary. Access predictor of normal conditions for this network? The realitypoints are responsible for choosing the best combination of is that we don’t. Every RF environment is different, so simplemodulation and coding at any point in time for each connected thresholds are a poor answer to rate shifts. Based purely ondevice. Again, it’s always a tradeoff of efficiency (higher data assumptions, the best reaction to errors and retries is unknown.rates) and reliability (lower data rates). In fact, the best response might be to use higher data ratesThe 802.11 specification introduces the term dynamic rate because they occupy the wireless channel for a shorter periodswitching and acknowledges the fundamental issue: with mul- of time and are less likely to be corrupted by momentary inter-tiple data rates, there is a need to dynamically adjust based on ference. Commonly enough, a data rate downshift causes moreRF conditions. But, they don’t lend any help. So if you look at errors, which causes another downshift. And suddenly, the safeten different Wi-Fi companies, you’ll see ten different rate con- and reactionary rate switch leads to a rate shift sinkhole–gob-trol algorithms. bling capacity as it tanks to the bottom. In other words, purely reactive algorithms are sub-optimal inWhat’s so smart about Ruckus? their myopia. Math is the better way. Statistics tell us moreWi-Fi engineers have been led to believe, and—for better or about the implications of transient interference, short-term hic-worse—site survey software validates the belief, that data rates cups, and longer-term trends. Accordingly, we can adjust—orcan be reliably predicted based on a metric like RSSI or SNR. perhaps more importantly, not adjust—the data rate to opti-And some product manufacturers use simple metrics like these mize both short-term and long-term performance and capacity.to determine the right rate. This is also why spectrum analysis doesn’t help much. Identify- ing an interference source does not readily tell us how exactlyRuckus approaches rate selection with a unique focus. Instead that source will impact our network. Even the best heuristicsof using unreliable signal measurements to hope for the best aren’t as accurate as statistics.data rate, we focus on the math. Our rate selection algorithmsare statistically optimized, which is our engineer-chic way of For delay- and jitter-sensitive applications, the best data ratesaying that we pick the best data rate based on historical, sta- is also the one that consistently delivers the frame to its des-tistical models of performance for each client. tination in the shortest amount of time. Our statistical rate selection model ensures that too.Without the right algorithm, the optimal rate for any client atany given moment in time is a crapshoot. And when you’re Another unique advantage with Ruckus is our test and valida-guessing, the safest guess is to err on the side of reliability, tion rigor. Because of our custom AP hardware and software,which sacrifices throughput and capacity and causes other we test and test and test everything some more. One suchunwanted problems. monotonous test is for data rate performance. Believe it or not, we test every individual MCS rate at different ranges andLet’s look at an example. The normal thought process in conditions to ensure that our performance is a bulwark of reli-Wi-Fi is that frame corruption and Layer-2 errors should lead ability. And this is no small feat. 802.11n MCS options are farus to downshift to a more reliable data rate. It’s a reasonable more complex than 802.11a/g.assumption. However, interference is bursty and transient bynature, so the best response is not necessarily to downshift.