Shulou(Shulou.com)06/03 Report--

This article is reproduced from the official account of Wechat, "those things about Wireless CCIE" (ID:passcciew). By Xie Qing.

Correctly understand the function of 802.11ac

The second generation of 802.11ac is making great strides towards the mainstream, and all the technical descriptions about 802.11ac are about the improvement of speed, that is, the rate of data connection. We mentioned in the previous article that an important step in reasonable planning of WLAN is to improve empty port efficiency in the coverage cell of a single channel, that is, to establish a higher data connection rate between the client and the wireless access point. Doesn't 802.11ac technology achieve this goal in this respect?

But the reality is that in the actual design and deployment, simple speed improvement is not a panacea, and the planning, design and deployment problems we face cannot be solved by 802.11ac technology alone.

In order to correctly understand the role of 802.11ac, let's first take a look at the main means by which 802.11ac can improve the connection speed. When the number of spatial streams is constant, it is mainly achieved by the following two means:

More channel bandwidth of ●, from the maximum 40MHz of 802.11n to 80 MHz or even 160MHz today (up to 117% or 333%, respectively)

● 's denser modulation modes, from 64th-order quadrature amplitude modulation (QAM) of 802.11n to 256th-order quadrature amplitude modulation (QAM) today (33% growth in a small range, with the same coverage)

Let's talk about the change of modulation mode first.

Wireless signal transmission in the air medium will be affected by a variety of factors, these factors will destroy the radio frequency signal, especially radio frequency interference, so measures must be taken to minimize the possibility of data damage. For this reason, each data bit is first encoded into multiple bits and then transmitted. This way of adding redundant information to the data can improve the ability of the system to resist data damage, which we call coding. After data coding, the radio transmitter modulates the data to produce carrier signal. The modulation mode can be carried out according to quadrant, frequency and amplitude. 802.11n/ac adopts a more complex modulation mode-QAM.

Popularly speaking, you can regard different modulation methods as different languages. The higher the degree of modulation, the more complex the language, the higher the requirements for the surrounding environment. As you move away from the Wi-Fi radio, complex languages become difficult to understand because of the noisy environment around you. By using a less complex language, you can keep communicating, but only at a lower data rate. This is that the low signal quality is usually equal to the slower speed.

The densest modulation of 802.11n is called 64-QAM. 802.11ac adopts a new scheme, 256-QAM, which contains 33% more data into the signal than 64-QAM.

256-QAM corresponds to two new data connection rates (MCS 8 and MCS 9), which achieves the highest data connection rate than MCS 7 in the same air time. But 256-QAM needs more sensitive equipment to detect different phases and amplitudes in the modulation. So he:

● is more sensitive to radio frequency noise in the environment.

● needs cleaner signal and higher signal-to-noise ratio between the client and the wireless access point.

In actual deployment, the line-of-sight distance between the wireless access point and the client is often required, and the distance is no more than 10 meters before it is possible to establish a MCS 8 or MCS 9 data connection rate. Otherwise, it will drop to the next lower data connection rate, even with MCS 7 modulated by 64-QAM, which is no different from 802.11n.

Let's look at increasing the width of the channel.

There is no problem with channel bundling to achieve a wider channel itself, just like road capacity, a road originally has only one lane, and the traffic flow per unit time is limited. When you widen the road to 2 lanes, 4 lanes, and 8 lanes, the traffic per unit time will increase exponentially, but there is a premise that you have enough land resources to expand the road.

The principle of wireless local area network based on broadband communication technology is the same, the greater the bandwidth that can be used for carrier, the more data is transmitted. But for wireless networks, the spectrum resources that rely on work are very limited, not to mention 2.4GHz. For 5GHz spectrum with relatively abundant spectrum resources, the channel width determines how many lanes you can repair on the road.

As you can see from the above picture, 5GHz channel is currently used in China:

13 available for ◆ 20MHz channel

6 available for ◆ 40MHz channel

3 available for ◆ 80MHz channel

1 available for ◆ 160MHz channel

Therefore, using wider channels in planning, design and deployment is not realistic in the real world:

● needs extremely clean spectrum resources to achieve a wider channel, that is to say, it requires a very high signal-to-noise ratio.

The wider channel of ● reduces channel reuse and increases co-channel interference, which has been discussed in detail in previous articles.

The wider channel of ● also increases the probability of adjacent frequency interference.

The lack of ● client support reduces the efficiency of network usage.

The picture above shows our data analysis through wireshark grab bags in a large airport terminal. Wireshark is used to grab bags for 12 times at different times of the day for 10 minutes each time. As you can see, even if the wireless access point only uses the 40MHz channel, enabling the 40MHz channel without the support of the client is a complete waste of spectrum.

About spatial flow

It is also important to point out that although 802.11ac can reach 8 spatial streams by standard (theoretically, it can achieve 100% growth compared to the four spatial streams in the 802.11n standard). However, when converting the standard into a product, there is no manufacturer to realize it, but the reason is that the cost is too high, and it is a huge price to work together with 8 transmitting and receiving wireless! Even if a product is made, how many users can buy it? The reality is that from the wireless infrastructure side, 3 to 4 spatial streams are mainstream products, and from the client side, 1 (smartphone) to 2 (tablet, mainstream laptop) spatial streams are mainstream products.

Multi-user multiple input and multiple output (MU-MIMO)

Another important point to understand is that the multi-user multiple-input multiple-output (MU-MIMO) introduced by the second generation of 802.11ac products is not used to improve the connection speed, but the transmission efficiency!

At the same time, we do not need to myth MU-MIMO, it does help to improve the overall efficiency of the cell in the actual use of deployment, but how much help depends on how many of the following conditions are met:

◆ MU-MIMO requires support from both wireless access points and wireless clients to work, and the following combination does not take advantage of MU-MIMO

◆ first generation 11ac wireless access points and second generation 11ac wireless clients

◆ second generation 11ac wireless access points and first generation 11ac wireless clients

There is no denying that there will be more and more wireless clients supporting MU-MIMO, but it is impossible for all clients to be updated to support second-generation 802.11ac, just like the evolution of 11a/g to 11n and 11n to 11ac. The final deployment environment you face must still be a mixed client type scenario.

◆ MU-MIMO only works on downstream data traffic, and the future 802.11ax technology will consider MU-MIMO in the upstream direction.

Because ◆ uses redundant antennas for beamforming, and the number of clients performing MU-MIMO operations is less than the number of wireless access point antennas, if you see a manufacturer saying that my 4x4:4 802.11ac wireless access point can support 4x4:4 MU-MIMO, please laugh it off.

◆ client locations need to be separated spatially

◆ in order to combine the transmission of clients, some clients need to reduce the data rate instead.

To sum up, in today's actual deployment environment, with high-capacity access as the goal, the distance between wireless access points is much closer than the previous way for coverage, and the efficiency of channel multiplexing will not be very high due to the limited spectrum resources.

We said earlier that reasonable planning and design should reduce media competition and radio frequency interference, that is, efficient channels are used to minimize empty port sharing between wireless access points. This is the premise, and then it is to increase the efficiency of air port utilization in the coverage cell of a single channel, that is, to establish a higher data connection rate between the client and the wireless access point. In the case that the premise cannot be realized, there is no point in simply improving the latter!

The most common data connection rate you see in actual deployment is that 1 or 2 spatial streams (mainstream clients) are based on 11n data connection rates.

Finally, send Cisco's best deployment practices for 802.11ac wireless networks for reference.

The previous review:

(1) adhering to correct planning and design is the first step towards successful deployment.

(2) pay attention to key performance indicators (part I)

(3) pay attention to the key performance indicators (part two)

(4) the best guarantee of 5GHz spectrum, and the best effort of 2.4GHz spectrum.

(5) any design plan that ignores client capabilities will fail in deployment!

(6) correctly understand the function of 802.11ac.

(7) it is not easy to achieve high quality wireless coverage.

(8) the location and method of wireless access point deployment should not be compromised.

(9) We should both talk on paper and keep our feet on the ground.

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