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Real world examples and discussion around WiFi channels

Guest Blogger

Question #1:You are deploying your new Aruba WiFi network and you know you should use channels 1,  6, and 11. But do you know why ?

 

Question#2: Are you troubleshooting a WiFi issue and you see the same channel overlap ? What does that mean really? Should I be concerned ?

 

Question#3: Does having two access points on the same channel cause immediate interference rendering the WiFi useless ?

 

Question#4: How much separation is needed between same channel access points to reduce channel interference ?

 

 

This blog post I'm going to attempt to educate the non-wifi user using real world examples and analogies in a George kinda way. 

 

These are questions we've all asked ourselves at one point or another. Lets take a peek and understand the "why", not just understand what the correct answer is.

 

Im an avid boater. If its 70+ degrees and the sea is calm, you can find me cruising the bay and around the island of Galveston,Texas in my 30ft Cruiser, namednone other then (Wi-Fi)shing. 

 

A few months back I invited a customer who was visiting Houston to come out for a day of fishing. My customer was new to the underlying mechanics of WiFi. Over a few cocktails, some great fishing, and a ham and cheese sandwich 5 miles out from shore anyone would die for, the conversation turned to WiFi and channels. In fact, the above questions are exactly what he asked.

 

First we need to understand some basics.

 

WiFi is half duplex medium and this is by design my friends! What does this really mean?

 

As I cast my fishing line, I hear my marine radio go offIt’s the US Coast Guard on channel 16. It was a normal USCG channel announcement. At that moment, I thought, what a great way to discuss half duplex in a real world example! The US Coast Guard monitors marine channel 16. Boaters tune their radios to channel 16 to receive announcements. When in distress, boaters use channel 16 to contact the USCG. 

 

Much like an access point configured to channel 1, your WiFi supplicant auto tunes itself via its roaming algorithm to channel 1 through the use of probing. When both the access point and client radio are on channel 1 and within in cell range, send and receive transmissions can occur through the use of 802.11 protocols. The rule of thumb is you need to be on the same channel to communicate. Again, high overview to a very complex 802.11 discussion. 

 

With that out of the way, on to the half duplex discussion. When you use a marine radio  to contact the USGC on channel 16, your marine radio switches modes from (Rx) receive to (Tx) transmit. It cannot do both at the same time. If it could, we would reference it as FULL duplex. Much like the walkie talkies we used as kids.

 

WiFi is a half duplex medium, very similar to the example above. Only one WiFi transmission can happen on channel. WiFi uses complex 802.11 protocols to manage when devices can transmit. Access points are no different and must abide by the very arbitration rules that wireless clients do. In fact, CSMA/CA is the sauce in 802.11 that manages this process. Think of CSMA/CA as a set of rules that govern WiFi radio transmissions and used by the radio.  When a WiFi radio wants to transmit a frame it must reference these rules each and every time. CSMA/CA is a favorite topic of mine. I am fascinated by the CCA and NAV timer and how they work together to determine if the medium is busy. 

 

If you want to learn more about CSMA/CA, CWNP white paper by Marcus Burton is a must read: http://www.cwnp.com/wp-content/uploads/pdf/802.11_arbitration.pdf 

 

You are probably wondering, George,what happens if two boaters or in my WiFi example, two WiFi devices transmit at the exact same time on the same channel in the same cell. Well my friends, we would call that a collision. Both devices are trying to access the same medium at the same exact time. No one is going to get access to the medium. In fact, if you are the boater in my example, you probably wont realize your transmission didn't even go through. If you are an 802.11 client, you wouldn't receive a ACK in return and normal back off rules would apply to set the stage for a frame retransmission. 

 

So what did we learn from all this, before tackling the questions at hand? WiFi is half duplex and only one device can access the medium at a time. Should more than one device access the medium a collision will occur causing a retransmission. 

 

 

Question #1:You are deploying your new Aruba WiFi network and you know you should use channels 1,  6, and 11. But do you know why ?

 

We're told that we need to have separate channels whereby devices can communicate within a cell. Why should you only use channels 1, 6 or 11? Why not use channel 1,2,3,4,5,6,7,8,9,10 or 11? Let there be no mistake my friend, you can use all those channels. What you need to understand is why you should not! To better understand this subject, we need to look at the 2.4 GHz frequency. 

 

802.11 standard tells us, WiFi operates in a range of 2.400 GHz to 2.4835 GHz or 83.5 MHz of total frequency. There are actually 14 total WiFi channels, but governing bodies like the FCC in the United States regulate use of channels 1 - 11 or center frequency 2.412 to 2.462. When I explain frequency, I like to use the analogy of a loaf of bread. Imagine a loaf of your favorite bread and there are exactly 83.5 slices of bread in your loaf. Each slice is its own frequency. Now take away slices 2.463 to 2.4835, this is what we have to work with in the US.

 

802.11 WiFi channels were designed and are sometimes referenced by their center frequency. George, what is a center frequency? I’m glad you asked! 802.11 standard tells us that WiFi channels are 22 MHz in width. Real world example, you select channel 1 on your Aruba Access Point, your Aruba radio is using 22 MHz of medium, or 22 slices of bread! To you, channel 1, is channel 1, but in frequency speak you are actually using frequency channel 2.412, or better known as the CENTER CHANNEL. In our channel 1 example, it uses 2.412 (-11) to (+11) = (22). Your channel 1 starts at 2.401 (-11) and ends at 2.423 (+11) from center frequency. Know that when your WiFi client is talking to your Aruba Access Point on channel 1, it is using center channel 2.412. All the other frequency to the left (-11) and right (+11) are in use to modulate data.  Think of having 22 acres of real estate, but you have one home address and that address is 2412. 

 

What is all this 22 MHz mumbo jumbo? 802.11 standard states, 22 MHz of frequency is in use in a given 2.4 GHz channel (we are discussing DSSS / ERP-OFDM. 5 GHz uses 20 MHz widths by default).  This means 22 slices of bread are in use to carry WiFi transmissions !

 

George why so much, why not just 1 MHz slices instead, so we can have more channels ? Excellent question! In fact, there is a legacy standard called 802.11 (thats it, just 802.11) no letters or task groups after it (unlike 802.11g or 802.11n). The 802.11 standard uses FHSS (Frequency Hopping Spread Spectrum). FHSS uses 1 MHz channels and hops around like a crazy man! The problem with FHSS, it’s inefficient and at max you can only transmit at 2 PHY rate or up to 2 million bits of data in a given transmission. Unlike its counter part 802.11g at 54 PHY rate, which can transmit 54 million bits of data in a given transmission or 802.11n which can transmit up to 450 million bits of data in a given transmission. 

 

Lets look at other center channel frequencies:

 

1 = 2412

2 = 2417

3 = 2422

4 = 2427

5 = 2432

6 = 2437

7 = 2442

8 = 2447

9 = 2452

10 = 2457

11 = 2462

 

In our example, we know channel 1 uses 22 MHz or 22 slices of bread. So that means channel 1 uses (-11) / (+11) = 22 MHz or up to frequency 2.423 on the high end. Did a light bulb just go off in your head?

 

Real world example, you have one access point on channel 1 and another access point on channel 2. Channel 2 center frequency is 2.417 (-11) (+11) = 22 MHz or 2.406 to 2.428. Wait a minute! They over lap don't they? 

 

Channel 1 = 2.401 - 2.423

Channel 2 = 2.406 - 2.428

 

There are a total of 3 non-overlapping channels in 2.4 GHz. Channels 1,6, and 11. 

 

Channel 1 = 2.412 2.401 (-11) - 2.423 (+11) 

Channel 6 = 2.437 2.426 (-11) - 2.448 (+11)

Channel 11 = 2.462 2.451 (-11) - 2.473 (+11)

 

2.4GHz channels.png

 

This, my friend, is why channels 1,6, and 11 should only be used. They don't overlap. Shortly, I will explain with a real world example what happens when same channels overlap and what you might expect.

 

 

Question#2: Are you troubleshooting a WiFi issue and you see the same channel overlap? What does that mean really? Should I be concerned ?

 

Have you ever opened up a WiFi analyzer and seen tons of wireless networks and some of these networks/access points are using the same channels? Large WiFi designs require the reuse of channels. With a properly designed and deployed WiFi network you can minimize interference when reusing WiFi channels.  

 

George, what happens if two access points are using the same channel and are in close proximity to each other? You're asking some great questions! 

Let’s look back to an earlier example of the marine radio using channel 16. All the boaters in range of the tower on channel 16 shared the same medium. Regardless of their radio type or radio make. Regardless of their boat make or boat size. They all shared one thing in common, and that my friend is the frequency, channel 16. WiFi is no different. If you have two access points on the same channel, in our example channel 1. You have effectively extended, made larger, increased in size, your WiFi cell. 

 

Some of you may say, thats a good thing. Lager cells YEAH!! NO! NO! NO! and NO some more! Remember, WiFi is a half duplex medium. If we increase the size of the cell by adding nearby access points on the same channel in close proximity you have effectively extended and increased in size the area of contention

 

Real world example here, Play along with me on this one. Lets assume you have 3 access points in your office. You know nothing about channels and you think for the sake of improved wifi performance ALL access points should be configured to the same channel. You configure all your access points to channel 1.  What just happen right there ?! You created 1 large contention cell! 

 

Remember, WiFi is half duplex. We just discussed that at the top of this blog post! 

 

Simply put,Kobe who is directly under AP#1 triggers a youtube video, Kersten who is under AP#2 triggers an FTP download, and Tanner who is under AP#3 triggers a music stream. 

 

Kobe, Kersten, and Tanner are all fighting over the same medium. Only one device can transmit at a given time on one channel. The user experience would be mixed and confusing! They would complain that the WiFi is slow, but they have full bars because they are right under the access points.

 

In contrast, if AP#1 was on channel 11, AP#2 was on channel 6, and AP# 3 was on channel 1 would they be in contention for the medium? The answer is simple, NO.

At what distance will access points on the same channel not interfere with each other? I will cover that at the end of this blog post.

 

Question#3: Does having two access points on the same channel cause immediate interference rendering the WiFi useless ?

 

The answer to this question is NO. But, George, you just said access points that share the same channel cause interference. Surely my wireless will be impacted. 

 

You can have 2 access points operate on the same channel in close proximity and rarely see issues or you could experience the complete opposite and have nothing but problems.

 

Let me share with you WHY. If you have two access points on the same channel and only one device is connected and transmitting data, it is highly unlikely you would see any direct performance hit. Why? No other radio is using the medium to cause contention. Sure, you have an adjacent access point trigger beacons, but thats not enough to cause a performance hit in all likelihood.

 

Real World Example -- Step back to the 80s. I can vividly recall when my parents got our first cordless phone. I can also recall listening in on my neighbors conversations on a few rare occasions. At the time, I had no idea what was actually going on. It wasn't till years later I learned more about RF and WiFi that I understood. 

 

My cordless phone was using the same frequency range as my neighbor. Early cordless phones lacked sophistication unlike units today. On a few occasions, I would be talking to a friend and all of a sudden I was on a party line with my neighbor. In this example, say all 4 people are on the call. Now assume, my friend and I are the only ones talking. Is there any interference yet or performance hit to our conversation? Nope, not yet! As I'm in a middle of a sentence, my neighbor starts to talk to his friend.What just happen there ? My neighbor caused interference with me. This caused me to back off until they stop talking and for me to repeat what I said when they are finished. Hence my "connection" or conversation just experienced a slow down.

 

So you can see. You can have access points on the same channel and they may not see immediate interference. The bigger problem is when everyone wants to talk.

 

 

Question#4: How much separation is needed between same channel access points to reduce channel interference ?

 

George, what is the proper distance between same channel access points? I like to use a general rule of thumb which is 20 dBm difference. Meaning, if you are next to an AP that is heard at -40 dBm at your mobile device, your next closest AP on channel 1 should be at -60 dBm or greater to lessen interference. Why is this? That, my friend, is a LONG and complex discussion on CCA, NAV, and radio driver secret sauce. I am going to state this loosely so don't beat me up on specifics. Drivers in general will not set their CCA to busy when frames are heard from adjacent cells at -20 dBm or greater. Hmmm that sounds like a great topic for my next blog post right there! 

 

When deploying access points that share the same channel you should have -20 dBm or greater of separation to minimize same channel interference. 

 

Thanks for reading!

 

Comments
New Member

Excellent post!  That cleared up some questions I have had for a while.

New Member

Thanks


Execellent Post.

 

 

 

Occasional Contributor I

One of the best posts I have come accross!

Guest Blogger

Thank you .. Its cool how one can litter the innerwebs with knowledge and even a few years later its still valid content ..

 

Thank you! 

 

 

 

 

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