802.11b DSSS or FH ?

Hi All,

Up until a day or so ago I have been of the understanding that 802.11b
used Direct Sequence Spread Spectrum (DSSS).

I set up an Agilent Spectrum Analyser on the 2.400 - 2.450GHz band and
was a little surprised to see that my 4 years old Access Point (Dlink
DI-614+).appears to be using Frequency Hopping.

From my understanding, FH uses a carrier which changes frequency 10s
or 100s of times per second within it's allocated spectrum in a random
manner. Spectral width of the carrier varies according to payload.

A DSSS signal in comparison should be noise like, with a reasonably
rectangular spectrum. From memory this is similar to ATSC TV in the US
and CDMA telephony. Spectrum should not vary significantly with
payload.

We had some analog 2.4GHz phones a while ago and I do remember a
continuous ticking noise in the background while the Wireless LAN was
switched on (this matches the FH measurement). I would have expected a
decrease in Signal to Noise ratio if the analog phone was close to a
DSSS signal source.

Thoughts, comment and observations most welcome.

Regards,

Mark

Jeff,

Thanks, good pointers, the confusion starts to be de-cluttered...

You might be amused at this: http://tinyurl.com/3ddnt5

-Mark



On Oct 25, 2:24*am, Jeff Liebermann <je...@cruzio.com> wrote:
> On Fri, 24 Oct 2008 01:29:44 -0700 (PDT), mark.aren...@gmail.com
> wrote:
>
> >Up until a day or so ago I have been of the understanding that 802.11b
> >used Direct Sequence Spread Spectrum (DSSS).
>
> Nope. *It can also be frequency hopping (FHSS). *Products made by
> Breezecom/Alvarion, Raylink, Proxim (Symphony), Symbol, and others
> used FHSS. *There are advantages to both technologies. *FHSS never
> went faster than 3Mbits/sec connection speed, receiver sensitivity was
> limited, and the FCC rules restricted FHSS, resulting in lousy range
> and speed. *However, reliability and lack of susceptibility to
> interference was a FHSS strength. *Where DSSS and FHSS meet, FHSS just
> slows down a little, while DSSS stops dead.
>
> >I set up an Agilent Spectrum Analyser on the 2.400 - 2.450GHz band and
> >was a little surprised to see that my 4 years old Access Point (Dlink
> >DI-614+).appears to be using Frequency Hopping.
>
> Nope. *It's DSSS. *If you see a classic sin(x)/x "hump" pattern on the
> SA, it's DSSS. *If it looks like a step functions, it's FHSS. *You may
> need ot adjust the sweep rate on the SA to see things clearly. *Also
> note that your DI-614+ is transmitting in bursts (probably just
> beacons), which can look like FHSS is an exessively fast sweep. *Move
> some data through the wireless link and then watch the SA.
>
> >From my understanding, FH uses a carrier which changes frequency 10s
> >or 100s of times per second within it's allocated spectrum in a random
> >manner.
>
> Sorta. *Bluegoof is 1600 hops per second. *802.11 FHSS is somewhat
> faster, but I forgot the number (and am too lazy to RTFM for it).
>
> >Spectral width of the carrier varies according to payload.
>
> Nope. *Occupied bandwidth is the same regardless of the traffic with
> FHSS. *The way it usually works is that data is sent with a 1 MHz wide
> FM signal. *There are 80 channels, each 1 MHz wide. *FHSS is not
> allowed to loiter on any one of these and must randomly hop through a
> minimum of 75 channels before re-using any. *What you'll see on the SA
> should look like a 1 MHz wide signal moving over 83.5 MHz bandwidth.
>
> >A DSSS signal in comparison should be noise like, with a reasonably
> >rectangular spectrum.
>
> Totally wrong. *It should be a sin(x)/x envelope with a hump in the
> middle. *See:
> <http://explorer.cyberstreet.com/wrt54g/WRT54g-spectraloutput.html>
> Ugh. *Those are awful photos but close enough to what you'll see with
> uncontrolled input data. *See photos at:
> <http://sss-mag.com/ss.html>
> for what DSSS and FHSS should look like. *Also see the Yellowjacket
> videos:
> <http://www.bvsystems.com/Videos/videos.htm#yellowjacket-bang>
> (after the sales pitch).
>
> >From memory this is similar to ATSC TV in the US
> >and CDMA telephony. Spectrum should not vary significantly with
> >payload.
>
> CDMA is spread spectrum and uses the same PN (pseudo noise) methods
> used by DSSS. *However, the modulation rates and spectral masks are
> radically different. *With DSSS, the air time (time the xmitter is on
> the air) and modulation rate both vary with payload. *I don't know
> enough about ATSC to comment on it.
>
> >We had some analog 2.4GHz phones a while ago and I do remember a
> >continuous ticking noise in the background while the Wireless LAN was
> >switched on (this matches the FH measurement).
>
> That would be the 802.11 beacon broadcast used to identify the access
> point.
>
> >I would have expected a
> >decrease in Signal to Noise ratio if the analog phone was close to a
> >DSSS signal source.
>
> Only if you were moving data on the 802.11 system. *Also, you won't be
> testing for SNR. *It will be BER (bit error rate). *It also depends on
> the type of cordless phone. *There are analog FM, DSSS, and FHSS
> cordless phones with assorted compatibility issues. *Some cordless
> phone work cross band. *It's impossible to predict the effects without
> known the underlying technology and method of testing.
>
> >Thoughts, comment and observations most welcome.
>
> --
> Jeff Liebermann * * je...@cruzio.com
> 150 Felker St #D * *http://www.LearnByDestroying.com
> Santa Cruz CA 95060http://802.11junk.com
> Skype: JeffLiebermann * * AE6KS * *831-336-2558