802.11b & 802.11g Channels

jamarchitect

Hi. I was wondering if 802.11b & g channels are exclusive? Meaning, can I use 802.11b channel 1 next to 802.11g channel 3 next to each other without interference?

This is a decision I had to make on the transcender practice exam.

The example is:

• SSID: Students - 802.11b using channel 1
• SSID: Faculty - 802.11b using channel 6
• SSID: Guest - 802.11g using channel 9

The want me to add another 802.11g network. Three choices are given: channel 3, channel 5, and channel 9.

Since the guest SSID is already G on channel 9, I ruled out channel 9-11. However, the first time I answered this question, I happened to choose channel 3. The answer was incorrect. I found out that channel 5 was the correct answer.

All three channels are on the same frequency. The only difference I can see is the b & g.

So my question is, why is it ok to use channel 5 on 802.11b 2.4Ghz network, but not channel 3 on an 802.11b 2.4Ghz network?

If channel 5 on G doesn't overlap with channel 6 on B, then why does channel 3 on G overlap with channel 1 on B?

I hope this makes sense to you all.

jamarchitect

While lying in bed last night, I mentally review the channel ranges, which I believe are:

1-3, 4-8, and 9-11 (US).

Yup. All three channels would have an over-lap problem with a fourth AP. (Location/distance of fourth AP was not in the problem.) So it must have something to do with the B and G, even though the both use the same frequency.

Can someone help me out with this one? Why would 802.11b channel 1 interfere with 802.11g channel 3, but 802.11b channel 6 won't interfere with 802.11g channel 5? All of the choices given are 2.4 Ghz.

Can someone help me out here. There's something I'm not getting here.

Thanks, and happy new year.

DoubleNNs

Idk. Doesn't make sense to me.
Maybe copy the question verbatim, in case there's something being missed.

Trifidw

Why are you using b?
Why are you separating b and g?
Why do you have your guest network on a separate channel? (instead of on the same AP and channel but different SSIDs and VLANs)

The non overlapping channels are 1, 6 and 11 and you want to ensure the APs that are on the same channel are a good distance apart so they don't cause interference with each other.

Darril

You mention that this is a Transcender practice exam. I haven't used them in a long time but remember that they always had excellent explanations with the questions (at least before they were purchased by another testing company). What does the explanation say?

jamarchitect

I'm going to run into that test question again in the next couple of days, so I'll copy it to paste here then.

The tests are good, and I've got a 97% average across the first three domains. Now I'm testing for the fourth domain. The only thing that isn't that great is that the questions don't change. The order does, but the actual questions don't.

That's OK, because most of them are things you either remember or you don't (port numbers, acronyms, cable speeds & lengths, etc.), but it would be nice to have multiple subnet questions to work out.

After I've finished domain 5, taken a test with all the questions, and passed to my satisfaction, then I'll start hunting around for other practice questions before my test date.

So, I will post the question (and read the explanation, as you suggested) in the next couple of days.

Darril

Good plan. That's the same with any quality practice test questions. The explanations give you insight into why the correct answers are correct and why the incorrect answers are incorrect and many times it's just a subtle change of wording. However, when you understand the correct and incorrect answers, you'll be able to answer the questions on the live exam no matter how CompTIA words them.

Good luck.

jamarchitect

OK. Here is the exact question as it appears on the Transcender practice exam:



Your organization deploys three wireless networks in close proximity. The configuration of the three wireless networks is as follows:


SSID: Students - 802.11b using channel 1
SSID: Faculty - 802.11b using channel 6
SSID: Guest - 802.11g using channel 9


You have been asked to deploy a new wireless network for the Research department. This wireless network should only support 802.11g wireless devices and must use a non-overlapping channel that does not interfere with the other wireless networks. The network should be named Research and should not be advertised.


Answer:


Network Mode - g-only - (Got this correct)
SSID - Research (Got this correct)
Channel - 5 - 2.432GHz (AAAAAHHHHH!!!!!!!)
SSID Broadcast - Disable (Got this correct)

Since both G and B are on the 2.4 Ghz frequency, and the channel ranges are 1-3. 4-8, and 9-11, I don't understand why 5 is the correct channel. The choices I was given were 3, 5, and 9. 3 overlaps with 1. 5 overlaps with 6. 9 overlaps with 9.

So why is 5 the correct answer when it's right next to 6, and not 3? In fact, all the choices given overlap with a channel, so how can any of them be correct without using 802.11a?

Notice that the spacing and placement of the three APs are not mentioned...so I can calculate which one to use based on placement either.

Either I'm really dumb, or there is something very odd about this question...and it's probably me!

DoubleNNs

The question doesn't have an explanation included along w/ the answer?

zidian

jamarchitect wrote: »

The choices I was given were 3, 5, and 9. 3 overlaps with 1. 5 overlaps with 6. 9 overlaps with 9.

Channel 5 doesn't make any sense as an answer. Of the choices given, I would say 9 is definitely the correct answer. Wireless Access Points are able to see the bandwidth on the spectrum and the point of using non overlapping channels is to prevent noise. Clear signals on the same channel are better than noise on one channel off. Noise causes problems whereas clear overlap allows the algorithms to work and share the bandwidth. Channel 3 and 5 are just absolutely terrible choices.