In my previous blog on the 11ac series, I explored 80 MHz channel operation in 802.11ac in the context of data rate, OBSS (Overlapping BSS), network throughput, and auto-channel assignment.
In the present post, I explore the other speed factor of 1.33X that shows up in the Wave-1 data rate equation: (2.16 x 1 x 1.33) x 450 Mbps of 802.11n rate = 1.3 Gbps. This 1.33X factor is attributed to the new modulation technique called 256-QAM introduced in 802.11ac (802.11n had only upto 64-QAM). Consistent with the theme of this blog series that the data rate equation does not bring out critical network engineering aspects, this post explores 256-QAM from the enterprise network design perspective.
256-QAM causes step function change in data rate near the AP
There are two newly added MCS’s (Modulation & Coding Scheme) in 802.11ac. They result in respective data rate increase factors of 1.21 and 1.33, over the highest possible data rate in 802.11n for a given channel bandwidth and number of spatial streams.
These two newly added MCS’s use the 256-QAM scheme, which requires about 5 to 7 dB higher SNR (which is a lot given that dB is logarithmic scale) compared to the least SNR at which the best MCS in 802.11n (64-QAM, R 5/6) can work with.
As a result, the 256-QAM can only be used close to the AP. From the network engineering standpoint, the key point to note is that 256-QAM to 64-QAM is step function change, that is, as you move away from the AP, the data rate drops in step function from 256-QAM rate to legacy 64-QAM rate.
This observation is important to quantify cell-wide benefit of 256-QAM.
What is the cell-wide impact of 256-QAM?
In enterprise deployments, clients are distributed throughout the cell. In a sense, this is different from the home networking environment where many clients can be close to the AP. A well-known principle in 802.11 is airtime un-fairness, which means clients away from the AP consume more airtime due to their lower speed compared to those closer to it. By now, you probably can guess what I am getting at.
For illustrative purposes, consider four clients (let us call them C1, C2, C3, C4) at four distances from the AP, respectively, and having data rates (assuming 40 MHz channels and 2 antennas on clients) as follows:
- C1 @ 360 Mbps (256-QAM rate with 1.33X data rate increase),
- C2 @ 270 Mbps (maximum 64-QAM rate),
- C3 @ 216 Mbps (another 64-QAM rate), and
- C4 @ 108 Mbps (16-QAM rate).
I will compare this situation with the corresponding 802.11n data rates (no 256-QAM) at the same distances for the same clients:
- C1 @ 270 Mbps (maximum 64-QAM rate),
- C2 @ 270 Mbps (maximum 64-QAM rate),
- C3@ 216 Mbps (another 64-QAM rate), and
- C4 @ 108 Mbps (16-QAM rate).
Below is the diagram depicting total airtime saved due to the use of 256-QAM for clients close to the AP in the above example. Here, I have avoided using lower rates like 54 Mbps and 27 Mbps (which are for the QPSK and BPSK modulation schemes) for clients further away from the AP to favor 256-QAM. The saving in airtime will be distributed to the clients in proportions of their data rates.
The above example shows about 4% saving in total airtime for the cell when the client close to the AP can use 256-QAM.. Also a point to note here is that actual numbers of data rates and clients are not important and that relative proportions are important. You get the same saving number for the same relative proportions of the data rates.
More clients away than close (Area = Pi * Square of radius effect)
The area of coverage of the cell is proportional to square of distance from the AP (middle-school formula for the area of the circle).
So in reality, there are usually more clients away from the AP than as many close to the AP. This type of client distribution requires computation of weighted proportions of airtime consumption rather than simple proportions as I did above. With weighted proportions, the savings in total airtime due to the use of 256-QAM close to the AP are below 5%.
For example, with one C1-type client, two C2-type clients, three C3-type clients and four C4-type clients, the total airtime saving because of C1 being able to use 256-QAM comes out to be 1.5%.
Airtime fairness feature on AP
APs support airtime fairness feature which tries to prevent higher airtime usage by clients operating at lower data rates. Suppose the fairness feature is configured to equalize the airtime consumption across clients. Then, in the computation above (with simple proportions), without 256-QAM, airtime would have been equalized as 25% each for each of the four clients. When 256-QAM is used, only one of the 25% slices (representing client closest to the AP) see airtime reduction of about 25% (due to 1.33X data rate).
So when normalized over the entire cell, with equal airtime fairness implemented on the AP, the total airtime saving due to the use of 256-QAM near the AP, comes to about 6.25%. As discussed earlier, in general there will be more clients away from the AP than those close to the AP. With weighted proportions computation as above, the total airtime savings is about 2.5%.
New radio implementations
As we can see from the previous examples, raising data rates of only those client that are close to the AP (like what 256-QAM does), results in relatively small total airtime savings (this reminds me of an analogy from popular rhetoric: “what does it mean to the society if the rich become richer”).From the network engineering perspective, the clients that are away from the AP need more help. One hope is that 802.11ac clients may have better radio implementation than the 802.11n clients. This may enable the 802.11ac client at a given distance to achieve better SNR than the 802.11n client at the same distance. Introduction of low density parity check (LDPC) codes introduced in 802.11ac could also help a bit there, but that alone does not seem to be adequate. However, whether the net SNR boost will be adequate enough to raise the client at least one level up in the data rate (i.e., one layer up in MCS), remains to be seen until real life test results are out.
Overall, we see that 256-QAM shows juicy 1.33X gain factor in the Wave-1 data rate equation. However, from the perspective of cell-wide impact, the airtime savings can be much lower. There needs to be a way to raise data rates of all the clients, particularly of those away from the AP, in order to achieve attractive airtime saving (and hence capacity and throughput gain for the cell). In that regards, 256-QAM seems to be better geared towards home networking than enterprise networking.
For enterprise networking, we may have to rely on radio implementation improvements due to hardware and processing techniques enhancements over time, to be able to obtain blanket data rate increase over the cell. Alternatively, one can plan coverage of .11ac cells to raise the minimum data rate at the edge of the cell, but it has cost and co-channel interference considerations.
These network engineering insights are appreciated only if you think outside of the isolated data rate equation!
- 802.11ac (Wave-1): Network Engineering Insights
- Don’t deploy 802.11ac without thorough RF planning
- Abandoning the 2.4 GHz junk band – Moving Wi-Fi to 5 GHz by Lisa Phifer in Webtorial
- The 802.11ac Paradox by Lee H. Badman
- BOM Math for Secure Wi-Fi Deployments
- Wi-Fi networks in 5 GHz: a few observations