I recently met with Rick Bahr, VP of engineering at Atheros, and was able to get clarity on a few of the questions that frequently come up here about wireless performance.
Q: DOES THE MERE PRESENCE OF AN 802.11B DEVICE SLOW DOWN AN OTHERWISE ALL-G OR ALL-N NETWORK? A: YES[/B] THIS IS OF COURSE ALREADY WELL KNOWN, ALTHOUGH THE EXACT IMPLICATIONS ARE OFTEN MISUNDERSTOOD. THE PRESENCE OF AN 802.11B DEVICE ON AN G OR N NETWORK CAUSES THE NEWER DEVICES TO HAVE TO RESORT TO SOME KLUDGY BEHAVIOR TO MAKE SURE THAT THE B DEVICES DON'T TRANSMIT WHEN THE G/N DEVICES ARE USING THE AIRWAVES, AND TO MAKE SURE THAT BOTH THE B AND G/N DEVICES CAN SEE THINGS LIKE BEACON PACKETS. THE EXACT IMPACT ON THROUGHPUT IS HARD TO ESTIMATE GENERALLY, BUT IT WILL NOT \"SLOW THE WHOLE NETWORK TO 802.11B\" AS IS OFTEN STATED. THERE IS HOWEVER A SIGNIFICANT SLOWDOWN IMPOSED BY THE MERE PRESENCE OF A B DEVICE, EVEN WHEN IT IS NOT ACTIVE. WE (SLIM) DID SOME TESTING OF THIS A COUPLE YEARS AGO AND FOUND THAT USUALLY THE THROUGHPUT BETWEEN THE G DEVICES DROPPED BY 30-50% (EG FROM 20MBPS TO 10MBPS), BUT NOT NEARLY AS LOW AS THE SPEED OF A B-ONLY NETWORK (5MBPS IN THE SAME ENVIRONMENT). *THE THEORETICAL MAXIMUM THROUGHPUT ON 802.11G IS 23 MBPS WITHOUT ANY B DEVICES ASSOCIATED, AND 14MBPS WITH. So upgrading any 802.11B devices on your network will most definitely improve the throughput of your wireless SB3 or Transporter, along with any other G or N devices. The above is not really new information but it was nice to get an authoritative answer that agreed with what we've found in practice. However the next answer is more interesting and contradicts some oft-stated myths that were a holdover from the 802.11B issue. Q: WILL 802.11G DEVICES SLOW DOWN AN ALL-N NETWORK? A: NO*, EXCEPT INSOFAR AS THE AIR-TIME THAT THEY TAKE WHEN ACTIVE WILL BE AT THE G THROUGHPUT LEVEL AS OPPOSED TO THE N LEVEL. I.E. THE DEVICES STILL EACH COMMUNICATE AT THEIR OPTIMAL RATE IN EACH TIME SLICE. UNLIKE IN THE 802.11B BACKWARD COMPATIBILITY MODE, *G DEVICES DO NOT IMPOSE ANY PERFORMANCE-DEGRADING BEHAVIOR ON N DEVICES* IN ORDER FOR THEM TO BE BACKWARD COMPATIBLE. 802.11G DEVICES ARE ABLE TO RECOGNIZE THE 802.11N PREAMBLE, AND THEY PLAY NICELY IN TERMS OF KNOWING WHEN ONE OR THE OTHER IS TRYING TO TRANSMIT. THE PREAMBLE TELLS WHICH MODULATION SCHEME WILL BE USED, SO THE N DEVICES CAN SPEAK N, WHILE G DEVICES CAN SPEAK G. THEY DON'T HAVE TO RESORT TO \"ESPERANTO\" AS WITH B IN ORDER TO COOPERATE. THIS MEANS THAT WHEN THE G DEVICE IS ASSOCIATED BUT NOT ACTIVE, IT HAS NO IMPACT AT ALL. WHEN THE G DEVICES ARE ACTIVE THEY WILL CONSUME AIR TIME ROUGHLY IN PROPORTION TO THE AMOUNT OF DATA BEING TRANSFERRED. THIS AIR TIME WOULD OF COURSE BE AT THE G RATE AS OPPOSED TO THE N RATE, SO IN THE EVENT THAT THE AIRWAVES ARE FULLY SATURATED (EG BY A LOCAL FILE TRANSFER), THERE WOULD BE SOME REDUCTION IN THE TOTAL MBPS ACHIEVABLE BY ALL DEVICES COLLECTIVELY, BUT THERE IS NO PENALTY FOR HAVING THE G DEVICES ASSOCIATED. *Q: IS HAVING A (DRAFT) 802.11N ACCESS POINT ADVANTAGEOUS, EVEN IF MOST OR ALL CLIENTS ON THE NETWORK ARE 802.11G? YES*, PRIMARILY BECAUSE 802.11N RADIOS HAVE THE BENEFIT OF MORE SOPHISTICATED MULTIPATH RECEPTION CAPABILITY. THEY CAN THEREBY EXTEND THE RANGE AND THROUGHPUT AVAILABLE TO G DEVICES TO SOME DEGREE. *Q: WHY DO SO FEW NEW DEVICES (ASIDE FROM APS) FEATURE 802.11N? A: SEVERAL REASONS: - Many of these applications would not benefit at all from having higher throughput. - Since G plays well with N, there would be little benefit from the network's perspective. - N chips are more power hungry, reducing battery life - N chips are more expensive - The N standard is new and is not yet finalized [b]In conclusion:* - DO: Upgrade to an 802.11N access point - DO: Phase out any B devices - DO: Turn off B compatibility in your access point to make sure. - DONT: worry about G devices on your N network - DONT: wait for the N version of your favorite gizmo as you could be waiting a long time! -- seanadams ------------------------------------------------------------------------ seanadams's Profile: http://forums.slimdevices.com/member.php?userid=3 View this thread: http://forums.slimdevices.com/showthread.php?t=42853 _______________________________________________ discuss mailing list [email protected] http://lists.slimdevices.com/lists/listinfo/discuss
