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Tuning outdoor wireless network performance by Mike Everest

5 basic rules to making the most from class license equipment


One of the really great things about 5GHz equipment is that it is class licensed, which means that there is no need to obtain any special license to operate.

One of the really bad things about 5GHz equipment is that it is class licensed, which means that you have to learn to live with your neighbours in peace and harmony!

The good news is that it is not difficult to follow a few basic rules to make sure that there is enough spectrum to go around for everyone, and to avoid turning the 5GHz band into a useless pea soup of noise!


Rule #1: Know your limits

You are probably already aware that there are limits to the power output allowed for class license equipment transmitters. The limits are carefully determined in order to make sure that the usefulness of the controlled spectrum is maximised for as many people as possible. So long as everyone plays the game, then everyone will be able to enjoy good fast links for relatively low cost.

When installing a new wireless service, one of the very first configuration tasks that you should ALWAYS do is to check that you have set your power transmission levels to match your local regulatory authority. For the 5GHz band in Australia, the regulatory limits for 5GHz bands is as follows:

Band EIRP Limit
5150-5350 MHz 200mW (23dBm) for indoor applications only
5470–5600 MHz 1W (30dBm)
5650–5725 MHz 1W (30dBm)
5725–5850 MHz 4W (36 dBm)

Wireless transmission limits are expressed in terms of Effective Isotropic Radiated Power (EIRP) measured (or calculated) in the SI units of Watts, or in decibel form dBm (decibel milliwatts). EIRP can be readily calculated by expressing the power in terms of decibel (dB) as follows:

transmitter power + antenna gain – cable losses

When the wireless equipment has an integrated antenna (like the MikroTik SXT, Sextant and DuxLink Duo) the cable loss can be considered effectively zero, and so EIRP is simply the sum of tx output power (in dBm) and antenna gain. For example, and SXT has an antenna gain of 16dBi, so;

Tx Power (dBm)
EIRP (dBm)
3
3 + 16 = 19
20
20 + 16 = 36
25
25 + 16 = 41

Referring to the max limit table above, we find that for SXT (for example, with antenna gain of 16dBi), the maximum legal transmitter power level would be:

Frequency Band (MHz)
Max tx setting for SXT
5150-5350
23 – 16 = 7 dBm (indoor only!)
5470–5600
MHz 30 – 16 = 14 dBm
5650–5725
MHz 30 – 16 = 14 dBm
5725–5850
MHz 36 – 16 = 20 dBm

In addition to these basic calculations, it is also important to consider two other factors:

  1. If you are using a dual polarity antenna, and transmitting on both chains, the total EIRP is effectively doubled; i.e. 36dBm in the horizontal plane, and 36dBm in the vertical.
    Therefore, when operating in 2x2 MIMO mode, you need to reduce the max allowed limit by 3 dBm (half of the maximum power in Watts)
  2. If you are using 802.11n extension channel, you will also be transmitting double the radiated output, so again you need to halve the transmitter output power (3dBm lower)

Thus:

Frequency Band (MHz)
Max tx setting for MikroTik SXT
20 MHz Channel
40 MHz Channel
1 chain
2x2 MIMO
3x3 MIMO
1 chain
2x2 MIMO
3x3 MIMO
5150-5350
7
4
1
4
1
-2
5470–5600
14
11
8
11
8
5
5650–5725
14
11
8
11
8
5
5725–5850
20
17
14
17
14
11

To avoid mistakes from arithmetic errors or misreading of the rules, most wireless products feature some method to automatically observe the regulatory limits. In routerOS, set the "frequency-mode" setting of wireless interface to "regulatory-domain" and set the value of country-code and antenna-gain.

For example, for SXT in Australia, use the following config for total peace of mind! ;-)

/int wireless set wlan1 frequency-mode=regulatory-domain country=australia antenna-gain=16

Rule #2: Know your environment

When conducting spectrum analysis, always check for spectrum usage and not just for operating wireless networks. This is because a wireless 'site survey' type scan will only detect transmitters that are using a technology and protocol that is understood by the device making the scan. Only MikroTik devices, for example, can detect other devices transmitting using nv2 protocol. Similarly, using Scan tool from a MikroTik device will not detect other proprietary equipment like Motorola Canopy, or Skypilot or weather radar and so on. The correct tool to use is a spectrum analyser application that will show you the level of activity across the full band of spectrum available.

The RouterOS the "Frequency Usage" function is one such tool – it will present a break-down of frequencies in the operating band into as narrow as 5MHz intervals, with a reading of 'noise floor' for each frequency interval. The important distinction between this method and the standard 'site survey' type report is that the frequency usage will help you identify ALL transmitters and not just those using protocols recognised by your own devices.

When you are using dual polarity antennas with dual chain MIMO radio equipment, make sure that you run your scans independently for each polarity, and compare results for each. Wireless transmissions are almost always polarised in a given plane, especially in outdoor applications, and most antennas have a very high attenuation between planes – typically more than 40 dB! Therefore, any given frequency channel that is virtually clear in one polarity may have very high noise floor in the other plane.

Also, always run your scans at BOTH ends of the link. It is quite possible that a wireless transmitter /behind/ one end of the P2P link might be virtually indistinguishable to that end, yet introduce significant noise to the remote end of the link. It is quite typical for the activity landscape at each end to be quite different.


Rule #3: Pick your target

Once you have built up an accurate understanding of the ambient wireless activity that might potentially affect your link performance, you need to determine what kind of link parameters that you will be able to work with. If you want to attemot maximum speed throughput, using full 2x2 MIMO operation with 40MHz channel width, you need to find a 40MHz wide gap in the background activity – in BOTH vertical and horizontal planes!

Analysing the background noise-scape, you might discover that you can only find a 20MHz gap in both polairities, or perhaps there is a 40MHz gap in one plane only. It is not uncommon that you will discover that you will achieve better results turning off the extension channel and reverting to 20 MHz only. Similarly, it is quite possible that you will get improved throughput by using just one polarity!


Rule #4: Play Nice

An important feature of many digital wireless communication protocols is that they are designed to share spectrum with other devices. Before beginning a transmission burst, wireless network devices will always first 'listen' to the airwaves to discover whethere some other nearby system is already transmitting. If another transmission is detected, a device will delay it's own transmission to allow the other to complete. In this way, it is possible for many wireless networks to enjoy relatively clear transmission activity using the same physical spectrum.

In order for wireless devices to recognise other transmissions, however, they must be operating on exactlyh the same center frequency, and they must be operating using the same protocols. For example, an 802.11n base station transmiting on a 20MHz wide channel centered on 5500MHz will not be detected by a wireless client set to operate on a 20MHz channel centered on 5505MHz. The same will be true when the wireless client is set to a 20MHz channel on 5500 MHz, but operating in nv2 protocol instead of 802.11n. In both these cases, the transmission of the base station will be treated purely as background noise, and the client transmission will proceed as if there are no other transmissions in progress. The result, of course, will be risk of interference to the active transmission by the base station to it's network peer.

Further to this, note that when centered on 5500MHz, a 20MHz channel will extend from 5490 MHz to 5510 MHz. Thus if there is another transmitter centered on 5505MHz, extending between 5495 and 5515 MHz, a large potion of these channels overlap – thus concurrent transmissions will experience potentially significant interference across 75% of their channel widths.

Due to this principle, you will often discover that you can achieve significant improvement to wireless data throughput rates by selecting a channel EXACTLY the same as another device already operating, than trying to 'squeeze' between two other networks.


Rule #5: Do unto others

Although it is often very tempting to turn up the transmitters as high as possible, and to use antennas with as high gain as you can get, doing so will not only help to ruin the spectrum for other legitimate users, but can also lead to potential prosecution by the spectrum regulator which carries quite hefty fines in most countries including imprisonment penalties in many countries including Australia. (In Australia, fines up to $165000 are applicable for individuals, higher for organisations)

Even setting aside legal aspects, it makes good sense to follow the old 'do unto others' principle, and consider how you would enjoy it if someone else sets up a wireless transmitter that renders your own links unusable. In fact it is quite likely that this will be exactly the result of an over-powered transmitter: If someone is transmitting too high, the only way for others to get ANY link happening is to turn up their own. Then when other link transmitters are ramped up, everyone else effected has to bump up their own power. And on it goes…


5 basic rules, 5 ways to improve your network performance, 5 ways to avoid causing problems to others. In a perfect world, there is plenty of class license spectrum to go around. Do your part to keep it that way, and others will remain more likely to do so!

Comments, feedback, corrections, further questions are always welcome!  Please feel free to contact us :-)

 

 

 

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