The importance of physical AP separation

The following information is to highlight the negative impact that occurs when AP’s are installed next to each other <3m, as It is not uncommon in today’s wireless deployment, to see these types on installations.  As the old saying goes a picture paints a thousand words, so I have tried to minimise technical jargon, by using pictures.

Figure 1, shows the spectral mask (Shape) of an OFDM modulation pattern, which will be represented throughout. An OFDM spectral mask, is approximately 20MHz wide, and is based on the centre channel. In this example it is based on channel 6.

 

Screen Shot 2019-01-30 at 10.37.04 am.png

Figure 1: OFDM Spectral mask

 

The Spectral mask of any 802.11 modulation contains certain key areas,

  • Peak power
  • Shoulders
  • And where it tapers off into background noise

 

All have values which are described as decibels relative to peak power (dBr) of the centre frequency

Good design practice is to ensure only non-overlapping channels are used. In the 2.4GHz band to be consider non-overlapping it must be separated by 5 channels or 25 MHz, in the 5GHz band it is 20MHz separation from the centre frequency.

Not only is having non-overlapping channels critical to any good wireless design, so is the importance of AP separation. If AP’s are not physically separated >3m or have some form RF isolation method than interference will occur. This also applies to AP placement when located next to objects, but sometimes this is unavoidable, so correct antenna selection is crucial.

To demonstration this I have setup the following equipment in a small lab scenario as shown in Figure 2, AP1, 2 & 3 at approximately less than 30cm apart, configured on static channels 1, 6 & 11, at transmit power of 20dBm This testing is relative to the equipment used) Spectrum analyser sitting next to the AP’s to view the layer 1(RF) information

Not shown in this picture are test clients, 3x 1×1 Apple IPad mini and Apple MBP that will be used for throughput testing and capturing information.

 

Screen Shot 2019-01-30 at 10.38.04 am.png

Figure 2:  Lab setup

Shown below in Figure 3. Is the current 2.4 GHz band utilisation.

Screen Shot 2019-01-30 at 10.38.12 am.png

Figure 3: 2.4GHz band before testing

AP1 is enabled on Channel 1 with a client device conducting a throughput test, as shown in figure 4.

Pay particular attention to;

  • The shoulders of the OFDM spectral mask, notice how it bleeds over into adjacent, and non-overlapping channels.
  • Notice the channel utilisation, for adjacent and non-overlapping channels.

Screen Shot 2019-01-30 at 10.38.20 am.png

Figure 4:  AP1 on Channel 1 with client running throughput test

I will now disable AP1 and repeat the same test again this time with AP2 on channel 6

Screen Shot 2019-01-30 at 10.38.27 am.png

Figure 5:  AP2 on Channel 6 with client running throughput test

This time leaving AP 2 still enabled, I will re-enable AP1. Notice that the shoulders of both the spectral masks have bleed together.

Screen Shot 2019-01-30 at 10.38.32 am.png

Figure 6: AP1 on channel 1 & AP 2 on Channel with throughput testing being conduct with Clients

Now compare the channel utilisation from figure 5 to figure 6, notice the increase that has occurred.

AP 2 is disable and AP3 is enable on channel 11 also with a client conducting a throughput test

Screen Shot 2019-01-30 at 10.38.36 am.png

Figure 7: AP1 & AP3 with throughput testing being conduct on each channel

Figure 7 shows utilisation occurring on channel 6 regardless of any stations. Take note on the channel utilisation in figure 7 and compare it figure 5. The utilisation on channel 6 in figure 5 is almost the same as figure 7.

This time I will re-enable AP2 with client. Take note of the overall increase in 2.4GHz band utilisation and the shoulders of the individual spectral masks.

Screen Shot 2019-01-30 at 10.38.45 am.png

Figure 8: AP1, 2 &3 enabled with clients running throughput tests

Now let’s compare the difference when AP’s are physically separated.

AP1 and AP3 are separated by 5meters, the spectrum analyser is located in the middle

Screen Shot 2019-01-30 at 10.38.51 am.png

Figure 9: AP 1 & 3 separated by 5m

Notice the spectral mask shoulders and overall channel utilisation of the band compared to previous test shown in figure 6.

This time I have changed the channels but the distances remain the same.

Screen Shot 2019-01-30 at 10.38.56 am.png

Figure 10: AP 1 & 3 enabled with throughput test

Compare figure 10 against figure 6. Massive difference. All 3 AP’s are now separated by distance of 5meters.

Screen Shot 2019-01-30 at 10.43.11 am.pngFigure 11: AP1, 2 & 3 enabled with throughput test an AP’s

Notice the difference in overall utilisation when figure 11 is compared to figure 8.

Hopefully this shows that physical separation is extremely important to minimise interference, however testing should always be performed as the distance required is dependant to the AP, antenna and EIRP selected. To determine the required separation  those factors must be account for.

As mentioned this was a small test lab, if this was in production the impact would have been magnified.

 

 

 

 

 

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s