# Forum

## Channel Separation Question

• If, according to the Sybex book, 802.11g aka: ERP-OFDM only requires 20 MHz of channel separation like 802.11a....does that mean that channels 1 and 5 could be considered non-overlapping for a "g" only network and that the required spacing is only 4 instead of 5 channels ?

I hope this hasn't been asked before and I've missed it...

Thanks,
Bob

• Unfortunately, you can not do that. The side bands of channels 1 and 5 that face each other land on 2.422 GHz and would interfere, although to a lesser degree than the DSSS channels. 2.422 GHz is also the center frequency for channel 3. You would also be setting you WLAN up for potential problems from neighboring devices the other adjacent overlapping channels.

• Thanks for the reply, I appreciate you taking the time. However, I'm still confused.

on p. 175 of the book it says....."802.11g Extended Rate Physical - OFDM (ERP-OFDM) channels require only 20 MHz of separation between the center frequencies to be considered non-overlapping because the channel widths comply with 802.11a OFDM channel parameters...."

So if 25 MHz separation is still required, what is the previous sentence trying to tell me, or why does compliance with narrower channel widths not equal less required separation ?

I'm studying for round 2 of the CWNA, so I'm trying to *really* get this straight in my head !

• The last sentence of the first full paragraph on page 175 states that "Under the 802.11g amendment, channels 1, 6, and 11 are considered non-overlapping for both HR-DSSS and ERP-OFDM." There is no mention of using 1 and 5 as non-overlapping. Just do some math. If channel 1 uses 2.412 GHz as its center and is 20 MHz wide that means that the side bands move up 10 MHz to 2.422 GHz. If channel 5 uses 2.432 GHz as its center its side bands move down 10 MHz to 2.422 GHz. There is a degree of overlap but not nearly so much as the DSSS channels. There is 5 MHz seperation between channels 1 and 6 when using ERP-OFDM. As a point for the exam, you are WAY overthinking this. Just remember that in 2.4 GHz you can co-locate 3 AP's in the same physical area without channel reuse if you are using 1, 6, and 11. The modulation and coding do not matter within this context. Best practice is to use 1, 6, and 11.

• Thanks, I get it now. I appreciate you getting me back on track and letting me know I was getting bogged down there. Sometimes I find it hard to let go of something.

Regards,
Bob

• Make sure you are studying for the right exam, PWO-100 or PWO-104. Study from the posted exam objectives. You may need to go beyond the book. The authors have posted updates on the Sybex site as well as a look into the new 802.11n chapter.

Good luck on the test.

Page 1 of 1
• 1