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.
the 614+ has a mini PCI card with an FCC ID KA2ACX100. The operational description:-
Are you sure you were not looking at the signal from a bluetooth device or a wifi keyboard? For a bluetooth display look at the bluetooth waveform in the airmagnet flash at
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.
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.
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).
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.
Totally wrong. It should be a sin(x)/x envelope with a hump in the middle. See:
Ugh. Those are awful photos but close enough to what you'll see with uncontrolled input data. See photos at:
for what DSSS and FHSS should look like. Also see the Yellowjacket videos:
(after the sales pitch).
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.
That would be the 802.11 beacon broadcast used to identify the access point.
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.
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