On Fri, 8 Feb 2008 16:24:40 -0500, papa
<(E-Mail Removed)> wrote:
>This SNR/sensitivity issure arose when I installed my otherwise, decent
>working Netgear WG311T PCI card in my 3GHz AMD machine - and it promptly
>started freezing.
I've used the WG311T:
<http://www.netgear.com/Products/Adapters/SuperGWirelessAdapters/WG311T.aspx>
in various fast machines without incident. Vista, XT, or Linux? Any
particular flavor of AMD? Latest driver?
>All the usual reinstalls and upgrades did nothing to
>solve the issue, so I popped-in a Linksys WMP54G and all seemed OK.
>Well, not exactly. Although the Linksys card ultimately solved the
>freezing, it was clearly not as sensitive as the Netgear WG311T.
>Needless to say, I'm a little disappointed by this find.
>
>Linksys publishes their SNR and sensitivity specs, but Netgear does
>NOT.
Careful here. Most vendor leak their specs somewhere, but almost all
of them lie. The most common abuse is to specify the native
sensitivity of the chipset used, as supplied by the chipset vendor. I
got fed up and started to tabulate the rx sensitivity numbers:
<http://802.11junk.com/jeffl/rx-sens/receiver%20sensitivity.htm>
I've also done some bench measurements in the distant past (when I had
access to the proper test equipment) and found a rather wide bell
curve variation in sensitivity across a manufacturers production. Your
WMP54G may have been a lemon. The large number of products, which
amazingly have exactly the same RX sensitivity specs as competing
products should give you a clue. The numbers typical vary about +/-
3dB (or worse).
Note that the basic sensitivity is seriously affected by digital and
RF noise coming from the board. This makes the board layout critical
for good sensitivity. Sensitivity can also be ruined by too many coax
cables, connectors, diversity switches, and strip lines between the
antenna and the receiver chip.
You can get a clue as to what's inside the WG311T and WMP45G by
looking at the photos on the FCC ID web pile:
<http://www.fcc.gov/oet/ea/>
which has now mutated into the Equipment Authorization web pile.
You'll find the FCC ID on the serial number tag.
>So, does anyone have or know of the specs (or which wifi card has
>the best SNR & sensitivity)? I'm not to concerned which has the
>greatest transmitting 'power',.. just the best reciever.
I can give you a guess based on the chipset, but do not have accurate
(or repeatable) numbers to justify my guesswork. I'm partial to
various Atheros chipsets as being the best of the bunch for
sensitivity. I've had more experience with Broadcom and they seem to
be all over the map.
While searching with Google, I found this Powerpoint slide show on
receiver sensitivity and MIMO, which should give you a clue as to how
messy it is to measure sensitivity:
<https://mentor.ieee.org/802.11/public/03/11-03-0845-01-000n-11-03-0845-00-000n-receiver-sensitivity-tables-mimo-ofdm.ppt>
Got money? This is what it takes to measure sensitivity:
<http://www.home.agilent.com/agilent/product.jspx?cc=US&lc=eng&ckey=736843&nid=-536900799.536908552.00&id=736843>
Only about $10,000 plus $3,000 for option 103 (Wlan testing). I think
the price tag might be a clue as to why vendors would rather lie than
measure.
However, there are exceptions. Looking at the numbers, I think DLink
actually does measure their products. That's because the variations
in results closely resemble the mess I used to obtain doing the same
measurements.
Incidentally, SNR (signal to noise ratio) is a reference level.
Sensitivity is a signal level at which a specified SNR, BER (bit error
rate), PER (packet error rate), or other reference level is achieved.
--
# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
# 831-336-2558
(E-Mail Removed)
#
http://802.11junk.com (E-Mail Removed)
#
http://www.LearnByDestroying.com AE6KS
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