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.

 

 

 

 

 

Certified Wireless Network Expert

Certified Wireless Network Expert (CWNE)

CWNE.png

The purpose of the blog post is to discuss my journey towards becoming a CWNE and the  common questions I get asked.

 

CWNE certification is the finale step in the CWNP certification track. For more information about what CWNP offered and the CWNE certification requirements I have added the link below.

https://www.cwnp.com/certifications/cwne

I have done up some pervious blog posts on my journey with the CWNA and CWNP certifications, links are posted below.

https://wordpress.com/post/wifiburns.wordpress.com/281

https://wordpress.com/post/wifiburns.wordpress.com/686

In April 2018  after a gruelling  4months of waiting since submitting my CWNE application, I received the email congratulating me that I have successfully become CWNE#269, once I read the email I jumped out of my chair and repeatedly fist pump the air.

Screen Shot 2019-01-04 at 11.00.05 am.png

Why did I decided to embark on this journey?

  • It made perfect sense given my passion for wireless, experience, current job role and responsibilities.

How long did the process take?

  • Having achieved certain requirements already, such as vendor certifications, experience etc. It took around 12 months to complete the exams and application. Normal process is around 3 years minimum.

What order did I complete my exams in?

  • CWNA, CWDP, CWAP, & CWSP. There is a lot of talk about doing it in the following order CWNA, CWAP, CWSP & CWDP, and there is logic to this, but I did it in the order of topics I found the most interesting.

What exam did I find the hardest?

  • CWSP, I didn’t find the exam hard, just trying to stay motivated to study for it was difficult.

When did I start working on my CWNE application?

  • When I started studying for my last exam (CWSP).

What did I write my essays on?

  • Analysis, design and security, this seems to be the normal approach

How many words where my essay’s?

  • Keeping your essay in the minimum and maximum word count is difficult. I included pictures which added more pages to each essay. But overall I think they were around 1500 words, there is talk that it should be less.

What  vendor certifications did I use as part of my application?

  • CCNP R & S
  • CCDA

How did I submit my application?

  • I submitted my application via email, by attaching the individual sections of the application. Note there is a lot of people on the CWNE board so your formatting or structure of your application may not be preferred by some members.

Would I recommend CWNE certification?

  • It depends, on you own personal circumstances, such as experience, career goals,  current job etc. For me it made perfect sense and has greatly benefited my career.
    • I do strongly believe anyone working with 802.11 networks should hold a CWNA certification.

Any misc. tips?

  • Get a CWNE mentor, I was extremely lucky to have the opportunity to work with a 2xCCIE and CWNE over the period of 2017. In Australia CWNE’s are rare as hens teeth.
  • Get fellow peers to review your application.
  • Don’t rush it.
  • Prepared to wait a few months to get the final response on your application. The CWNE board is volunteer based.
  • If you don’t get an email saying your application has been received email until you do.
  • Get solid referrers:
    • I think it should be mandatory that one is a CWNE, that can validate your work ethic and knowledge.
  • Remember your application needs to sell you as a wireless expert.
  • Be active in the Wireless community, The wireless community in Australia is tiny and its even smaller when you exclude the wireless people that have no idea what they are doing 🙂 so  reach out, get involved in
    • Twitter
    • Slack
    • Wireless LAN association
    • Forums
    • Blogs
    • youtube

Hope this post has been able to provide some answers to the common questions I get about the CWNE certification.