802.11ac is the latest, ratified standard in wireless networking and it is not the future. It is here, now and it rocks.
There are a number of improvents to the existing 802.11n standard that we have been enjoying since it’s ratification in 2009 in 802.11ac. Not the least of which are 256QAM (Quadrature Amplitude Modulation), MU-MIMO (Multi User – Multiple Input, Multiple Output) and, the use of current 80MHz channel widths with 160MHz channel widths coming soon. These three technology refinements make for incredibly efficient use of airtime and allow for massive amounts of data to be pushed over the WLAN. While variations in the number of streams in both access points and client stations, environmental considerations and more significantly affect the potential throughput, we live in an age where it’s possible to push over a Gbps wirelessly!
In this blog post, let’s first look at modulation techniques and advancements and we will address the other benefits of the standard in future installments.
The easiest way to define the benefits of 256QAM is to first look at other, older types of modulation. If you were to go back to the beginnings of Wi-Fi (802.11-Legacy, you would find BPSK or Binary phase-shift keying. When data was transmitted, there were only two, hence binary, symbols or bits that could be sent at any given time. They were either a one or a zero. The process of modulation was much like a man in a crowd translating the 0s to “A” and 1s to “B” and calling out, or broadcasting those translations. The demodulation process was much like another listener hearing, or receiving those As and Bs and translating them back to 1s and 0s. Information is able to be passed but, it’s slow because individual bits are transmitted one at a time.
Next up came QPSK or quadrature phase shift keying which added the ability to not just look at a binary or on/off state but, amplitude as well. When compared to the example above, it would look much like the same man broadcasting, at any given time a “A”, an “a”, a “B” or a “b”. These would then be demodulated not as single symbols or 1s and 0s but as two symbols. An “A” may represent 00, an “a” could represent 01, “B” could represent 10 and, “b” would, in turn, represent 11. There was no more effort or time required to modulate the As and Bs so, we could double the throughput or data carrying capacity of BPSK. Make sense?
As technology continued to be refined and both phase and amplitude were manipulated to send data, we later achieved 16QAM. This was a radical advancement in how data is moved across the Wi-Fi medium of air. It allowed for four symbols to be transmitted at any given time. The easiest way to understand is to study constellation diagrams which are easily found on Wikipedia but, to summarize, using 16QAM, the system is able to send, for example, a single transmission that could represent four digits, like 0101 or 0100. Essentially any variation of 2^4th power. Using our example of a man speaking or broadcasting, it would be like adding to “A”, “a”, “B” and “b” the ability to adjust pitch or frequency. The demodulating listener would hear the difference between a high pitched, squeaky “A” and a low pitched, baritone “A”, allowing for a greater combination to be translated into 1s and 0s. Once again, I ask, does this make sense?
Now that we have covered how the modulation/demodulation process looks in layman’s terms, extrapolate through 64QAM which allows for six symbols or bits (a la 001100 or 010101, or 2^6th power variations) to be transmitted at a time and through to 256QAM which allows for 8 symbols (a la 00110011 or 01010101 or 2^8th power number of variations). Because the system is passing longer variations of bits in the same time frame as older technologies, it’s makes for a far more efficient use of resources. In the case of Wi-Fi, these resources include airtime and, consequently, you’ll see a benefit in battery life of mobile devices like laptops, phones and tablets. There are significant increases in speed of file transfers which are, for all intents and purposes, what every single data transaction in computing is. For example, loading a web page on your tablet’s browser is really only a request by your device to a server asking for files that reside there to be transferred to your device for viewing. Get it? I thought so.
Now, the last bit on this subject which must be addressed is ECC or Error Correction Codes and they’re effect on MCS (Modulation Coding Streams). Because we’re now packing denser bits of data into transmissions (see Wikipedia for constellation diagrams) and pushing said data at much higher rates, the potential for errors is increased. In wireless systems, these errors can depend on many factors including too high a signal or RSSI (Received Signal Strength Indication. Yes, you read that right…too high. Think in terms of distortion in an audio system when speakers are overdriven), low signal or RSSI, reflections of signal causing “garbled” transmissions and more. I will touch more on MCS rates in the last installment of this series but, suffice it to say that when many different potential bit variations are packed into small spaces, the potential for the receiving device to confuse them is higher, requiring correction or retransmits and, essentially, affecting the efficiency of the system. I mention this because many people new to wireless assume that the data throughput rates marked on the box or in marketing materials are to be expected in real world environments and, in truth, they couldn’t be more wrong.
So, this may be the first time you’ve ever considered the “geeky” nature of modulation rates but, I haven’t even touched on modulation techniques or “vehicles” like DSSS (Direct Sequence Spread Spectrum) or OFDM (Orthogonal frequency-division multiplexing). The point being that when dealing with something as mysterious as an invisible means of moving data, as important as Wi-Fi has become to virtually all of us, as ubiquitous as our wired Ethernet networks, it is a very complex subject and to be examined in it’s constituent parts. What could be more fun than that?