Got my CWNA

CWNA

Passed Cisco Wireless Network Administrator exam, a vendor-independent certification promoted by CWNP Foundation.

My past Wi-Fi experience was 99.9% Cisco and in my opinion Cisco has excellent matherials on how to design, deploy and troubleshoot its solutions, so does it pay great attention to theoretical basics. Cisco solution also provides you with very good set of troubleshooting tools. Whether in the future I still will be deploying and troubleshooting Cisco solutions or not, studying for a vendor independent certifications seemed to be very useful to better understand what I am doing and what is actually going on “in the air”.

May seem trivial but worth repeating: the deeper you go into theory the more developed and solid will be your ideas on how to design, configure or troubleshoot something. CWNA is definitely a good step towards this direction.

Now moving on to reading Certified Wireless Analysis Professional matherials.

Wi-Fi Interference – Microwave Oven

For those who didn’t see, this is how microwave oven looks on spectrum analyzer diagrams taken on a foodcourt, about 15m from kitchens:

Duty Cycle Diagram
Duty Cycle Diagram

Here is an example taken about 1m from working microwave oven:

RSSI and Duty Cycle Diagrams
RSSI and Duty Cycle Diagrams

Duty cycle is close to 100% so it severely affects Wi-Fi channels it crosses.

Cisco 1532 compact outdoor AP

As 1550s APs go end-of-sale in March 2016, two model series are available to choose from Cisco Outdoor portfolio: full-feature 1570 and compact  1530. The latter in turn has two options:

● 15 30I: 3×3 MIMO with 3 spatial streams (2.4 GHz) and 2×3 MIMO with 2 spatial streams (5 GHz)

1530E: 2×2 MIMO with 2 spatial streams (2.4 GHz) and 2×2 MIMO with 2 spatial streams (5 GHz)

(1570 has 4×4 MIMO with 3 spatial streams)

The obvious benefit of 1530 is it’s compact and easy-to-mount design. We have just received one which we are going to install on a bus stop in order to provide internet service for waiting passengers. For this installation we chose the one with internal antennas.

I have decided to post some photos of the AP just in case anyone wants to see it closer. In my opinion it looks very nice.

AP1 AP2 AP3 AP4

AP5

Notice two ports named LAN and PoE. They are for daisy-chaining two APs (when one AP provides wired Ethernet to another). Here is a plug used to seal the network cable (copper is the only option):

Plug1 Plug2

My advice for everyone who acquiring this AP model to consider AIR-ACC1530-PMK2= mount kit that has tilt mechanism so you can direct the AP as needed. The one that comes by default suits you if you want to mount it on a pole vertically. As always – check the radiation pattern.

More detailed information can be found in Deployment Guide, provided by Cisco.

Which channel to place your AP on?

Choosing the lesser evil

Let’s say you are setting up a Wi-Fi access point. You launch some simple analyzer tool to check which channel to configure your AP for and see something like this:

int1

Given the fact that there is no way to avoid any interference, which channel would you choose?

There is a temptation to put your new AP on channel 3 as it looks like it has the lowest Received Signal Strength from foreign APs.On the other hand we have a recommendation to use channels 1, 6 and 11 of the 2.4GHz ISM band for the majority of countries. So maybe we just use channel 1 and hope that our signal will be heard well?

Two types of Wi-Fi interference

If we set our AP to channel 3 the signal spectrum will partially overlap with the AP configured for channel 1. This is called Adjacent Channel Interference:

Adjacent channel interference. The mess between channels 4 and 13 is omitted for simplicity

If we set it to channel 1 then what we will have is called Co-Channel Interference. The whole spectrum of both signals will overlap:

Co-Channel Interference
Co-Channel Interference

Wi-Fi is often called polite protocol. In practice, this means that APs and clients do their best to avoid collisions during data transmission. In short if someone is already using the channel wireless device backs off and tries to transmit its data later. All this works when one wireless device “hears” other devices data (i.e. is able to see the actual Layer-2 frames).

What happens in case of adjacent channel interference is that you have disruption on part of the channel spectrum. The signal from an AP operating on an overlapping channel is perceived as noise. The collision avoidance mechanism don’t work, both APs keep sending data whenever they want, but large part of the data comes corrupt to its destination. AP tries to change data rate and coding scheme to adapt to noisy environment and then resend the data good part of which will probably come corrupt causing the whole cycle to get repeated. More than that, the interfering AP will certainly have periods of more and less intense conversations to its clients. So your AP will try to switch to lower and higher datarates during these periods which will make the situation even worse.

Of course we all would like to operate in an interference free environment where all the channel spectrum is ours, but nowadays it is virtually impossible. So if you have to choose between co-channel and ajdacent-channel interferer, choose the first one. In the presented case channel 1 looks like the best option.

Also keep in mind that there are some other factors that will affect the performance, sometimes even more. For example, how intensely the other AP and its clients utilize the channel, the presence of non-Wi-Fi interference sources and so on.