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Hello everybody,

Can anyone please tell me whether the channel width in DSSS is 5 MHz or 1 MHz?

To me, it seems that a DSSS channel is 5 MHz wide, but p. 186 mentions that "Each DSSS channel ... based around a center frequency that uses 1 MHz carrier frequencies, just as with FHSS.", while p. 189 says that "Because the center frequencies are 5 MHz apart, ...".

Thanks for any help.

blueice,

I believe the channel width in DSSS is more like 22MHz. As stated on page 188 of the study guide 802.11 doen't specify a channel width. IEEE 802.11-1999 & 802.11b-1999 specifiy operating channels by their center frequency. These frequencies are specified using MHz resolution just as shown in figure 6.4.

Both standards also speak of a transmit spectrum mask. My understanding of the mask says that at 11 MHz either side of the center frequency the power level should drop to 1/1000 (-30 dBr) of what it was at the center frequency.

802.11-1999 says "In a multiple cell network topology, overlapping and/or adjacent cells using different channels can operate simultaneously without interference if the distance between the center frequencies is at least 30 MHz."

802.11b-1999 says "In a multiple cell network topology, overlapping and/or adjacent cells using different channels can operate simultaneously without interference if the distance between the center frequencies is at least 25 MHz.

I'm not sure if I am saying this correctly and I hope it clears rather that muddies the water:
The channels do not have mutually exclusive sets of MHz frequencies- the channels overlap.

Look again at figures 6.5 and 6.6. Keep in mind that each half-circle in figure 6.6 should look more like the channel diagrammed in figure 6.5. Also look again at figures 6.7 and 6.8. Figure 6.8 demonstrates that the side-lobes of non-overlapping channels do overlap and that the main-lobes of overlapping channels should not overlap.

moe

I agree with your explanation, but it seems that I did not explain my problem clearly.

What I have not understood is:

On p. 181 (FHSS) it is mentioned that "The IEEE 802.11 standard specifies in section 14.6.5, 1 MHz "channels," each of which serves as the center frequency for transmission during a dwell time..."
So, in the frequency range 2.402 to 2.480 GHz, you get 79 possible channels. This is OK.

Now, p. 186 says that "Each DSSS channel is a contiguous band of frequencies based around a center frequency that uses 1 MHz carrier frequencies, just as with FHSS.".

Question 1: what is meant by "a center frequency that uses 1 MHz carrier frequencies"?

Question 2: I can conclude from the definition of non-overlapping channels at the bottom of page 188 that each channel is 5 MHz wide. Now, how does the statement "Each DSSS channel is a contiguous band of frequencies based around a center frequency that uses 1 MHz carrier frequencies, just as with FHSS." on page 186 relate to the 5 MHz channel?

Hope I've described my problem clearly. Can you clear my confusion?

blueice Escribi?3:

Question 1: what is meant by "a center frequency that uses 1 MHz carrier frequencies"?

I spoke with an EE and his response is:

that in spread spectrum the 1 MHz carrier frequency is the same as the 1 MHz center frequency around which the transmission is spread;

blueice Escribi?3:

Question 2: I can conclude from the definition of non-overlapping channels at the bottom of page 188 that each channel is 5 MHz wide. Now, how does the statement "Each DSSS channel is a contiguous band of frequencies based around a center frequency that uses 1 MHz carrier frequencies, just as with FHSS." on page 186 relate to the 5 MHz channel?

Since non-overlapping channels must be 6 channels apart in 802.11 DSSS and 5 channels apart in 802.11b HR-DSSS and 802.11g ERP-OFDM

and the channel centers are 5 MHz apart,

I conclude that the channel widths must be:

closer to 30 MHz than to 5 MHz for 802.11 DSSS;
closer to 25 MHz than to 5 MHz for 802.11b HR-DSSS and 802.11g ERP-OFDM;
and closer to 20 MHz than 5 MHz for 802.11a

Again, the 802.11 IEEE documents do not specify a channel width for DSSS.

Maybe some RFs or EEs could jump in here and try to clarify this for us.

Thanks,

moe

Hi BlueIce and Moe:

I recommend that CWNAv3 study guide page 186, last paragraph, first sentence read: "Each DSSS channel is a contiguous band of frequencies based around a center frequency." The reference to FHSS is a mistake. I will submit this as an erratum.

Regarding "channel width", the IEEE doesn't define channel width but rather channel center separation. In the 2.4 GHz ISM band, the IEEE defines 14 channel centers 5 MHz apart and tells us that two channels overlap unless their centers are at least 30 MHz apart for DSSS PHYs or at least 25 MHz apart for HR/DSSS and ERP PHYs.

This means for example that channels 1, 7, and 13 do not overlap for DSSS PHYs, and channels 1, 6, and 11 do not overlap for HR/DSSS and ERP PHYs.

By the way, figure 6.4 on page 187 should show Channel ID 14 as "Yes" in Japan.

I hope this helps. Thanks. /criss

OK. That's a bit clearer now.
Thanks moe and Criss.