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

Wireless network has become closely related to people's life, work, entertainment and study, so the users of wireless network default that wireless network should have the same performance and even user experience as wired network. no matter what type of wireless terminal the application runs on, they should have the same experience.

But in terms of performance, evaluating the performance of a wireless network is very different from evaluating the performance of a wired network. For example, if your laptop is connected to a switch on a wired network through a network cable, and the data connection rate of the link between them is 1000Mbps, then you will get basically the same result when you use performance testing tools to test the throughput of the physical link. And this performance is exclusive to the wired client.

Conversely, if your laptop is connected to a wireless access point through WiFi and the data connection rate of the link between them is 1300Mbps (assuming media conditions permit), what is the result of testing the throughput of this physical link using a performance testing tool? It's about 50% or 65% of 1300Mbps. And this performance throughput is only the peak throughput of the wireless access point. Once multiple clients are connected to the same wireless access point, the aggregate throughput of all clients will continue to decline with the increase of the number of clients. The reason for this phenomenon is related to the use of air-shared media in wireless networks and the working mechanism adopted by the 802.11 protocol when the default media is unstable and unreliable. Whether it is 802.11n or 802.11ac, it is always subject to this limitation.

Because the air medium is affected by many factors (busy degree, temperature, humidity, etc.), the 802.11 protocol uses an Ack acknowledgement mechanism similar to that of a walkie-talkie to ensure that the receiver can receive the message from the sender. If the receiver receives the message from the sender and the verification is not damaged during transmission, the receiver will send an acknowledgement message to the sender, if the sender does not receive the message. Then the sender will retransmit until the upper limit of the number of retransmissions is reached, and the sender will discard the message. In addition, the air medium is a shared medium, so clients and wireless access points in the wireless network need to compete for the right to use the media, which results in a significant increase in system overhead. The 802.11 protocol actually trades system overhead for reliable data transmission.

In addition, the diversity of clients can also lead to huge differences in system throughput. Assuming that you are deploying 802.11ac (first-generation) wireless access points that support three spatial streams and work on the 80MHz channel (of course, no one in a real enterprise deployment would be so stupid to use the 80Mh channel width), you can get the data connection rate of 1300Mbps when clients (often laptops) that also support three spatial streams connect. When a client that supports a single space stream (smartphone, tablet) connects to the wireless access point, you can only establish the data connection rate of 433Mbps. Obviously, the huge difference in peak throughput between the two will affect the performance of the whole system.

Finally, not that the client can establish the highest data connection rate anywhere in the coverage cell generated by the wireless access point, taking into account the unstable characteristics of the air medium, the client will dynamically adjust the data rate established with the wireless access point at different locations away from the wireless access point to adapt to the changing physical media conditions, the farther away from the wireless access point. The lower the data rate established between the client and the wireless access point, the more redundancy rather than performance is considered in the modulation mode.

In short, to ensure the performance of the wireless network and user experience. Its planning, design, deployment and maintenance are very different from the wired network!

Since there is a big difference, which key performance and user experience indicators of wireless networks should we focus on? Is it the data connection rate, peak throughput, or aggregation throughput of wireless access points associated with the client? Is the client receiving signal strength (RSSI) or the number of wireless clients associated with the wireless access point at the same time? Neither!

For individual clients, the key index to evaluate their performance is SNR-Signal to Noise Ratio (signal-to-Signal to Noise Ratio). Other indicators can pay attention to the received signal strength and retransmission rate in turn.

The signal-to-noise ratio is the difference between the signal and the bottom noise. the greater the difference between them, the better the signal quality. For example, if the signal strength received by the client is-65dBm, the bottom noise is-92dBm, and the signal-to-noise ratio is-65dBm-(- 92dBm) = 27dB. If the signal strength received by the client is-60dBm, the bottom noise is-80dBm, and the signal-to-noise ratio is-60dBm-(- 80dBm) = 20dB. Although the received signal strength of the latter is stronger, the signal quality is worse.

We often encounter a misunderstanding that the stronger the signal, the better the signal quality, which is a very wrong idea. The stronger the signal is, the weaker the noise is. Often, the noise is also very strong at this time. As a result, the signal-to-noise ratio is poor and the signal quality is poor.

The signal-to-noise ratio determines what kind of channel bandwidth can be used between the wireless client and the wireless access point, how many spatial streams can be realized, and what modulation mode can be used to determine the high data connection rate to be established. The requirement of signal-to-noise ratio for using 80MHz channel width is much more stringent than using 20Mhz channel width, the signal-to-noise ratio of establishing three-space flow communication is higher than that of establishing single space flow, and the signal-to-noise ratio of complex modulation is higher than that of simple modulation. For traditional 802.11a/g clients and wireless access points to establish high data connection rate, generally need the signal-to-noise ratio of 20dB; for 802.11n, it needs a signal-to-noise ratio of 25 dB; for 802.11ac, it needs a signal-to-noise ratio of more than 30 dB.

Due to the different RF chips and system architectures used by each manufacturer in the industry, the signal-to-noise ratios required for establishing a given data connection rate (Data rate-for 802.11a/b/g) and using modulation and coding scheme (MCS-for 802.11n/ac) are different, even among different products of the same manufacturer.

To ensure the performance and application experience of the wireless network, you first need to determine which applications to use on the network so that you know which data connection rates you need to support such applications. Once you know what data connection rates the clients on the network need to support, you can determine the signal-to-noise ratio required to operate at these data connection rates.

In general, you can follow the following criteria to judge whether the signal-to-noise ratio is good or bad:

1. > 40 dB-the signal quality is very good, and the client can always associate with the wireless access point to establish the highest data connection rate

2.25-40 dB-good signal quality, clients can always associate with wireless access points and establish high data connection rates

3.15-25 dB-good signal quality, client can always associate with wireless access point, and good data connection rate can be established

4.10-15 dB-average signal quality, clients can be associated with wireless access points and can only establish low data connection rates

5-10 dB-low signal quality or no signal, client and wireless access point cannot be associated

How to effectively achieve high signal-to-noise ratio, so as to achieve high data connection rate of the client, so as to get high performance and good experience? Blindly increasing the transmission power of wireless access points is obviously not the best way, the most effective way is to reduce noise.

Where does the noise come from? There are mainly three aspects: co-frequency interference from the wireless network itself, adjacent frequency interference and interference from non-WiFi devices.

For the same frequency interference and adjacent frequency interference from the wireless network itself, it needs to be reduced through reasonable planning and design. Interference from non-WiFi devices needs to be identified, found, removed and mitigated by system-level intelligent spectrum management solutions.

Having said so much, how to check the signal-to-noise ratio of the client?

It generally needs to be viewed from this side of the wireless network infrastructure, because the RF chips used in the wireless infrastructure are sensitive and strictly calibrated.

Historical SNR information for clients can be viewed from the Cisco Prime Infrastrucure:

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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