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Calculations of Throughput

Unanswered Question

I have some basic questions about the 340 and 350 series that I was hoping to get answered:


1) When I look at the results of the linktest I get signal strength in dBm in % and I get quality in %. I understand what strength in dBm, I want to know when you are reporting strength in % are you taking the max power possible (100mW or 20dBm) and dividing it by the received strength? And how do you measure signal quality. What does that number actually mean, is it number of packets received/number of packets sent etc.

2) What is the expected bit rate when I have less than perfect signal

levels. I have yet to see anything close to 11Mb/s I typically get rates around 900kb/s with signal levels around -68dBm and quality around 60%. Do you have a way to calculate the expected rate based on signal level and

quality.



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Here are my thoughts:


(1) I've always thought it was a 'max. power possible' percentage, but I hope to hear other comments about this. 20 dBm/100mw is the max. TRANSMIT power, though, so the "maximum" would likely be based on a recieved signal strength threshold, say, -50 dBm. I've always thought that Signal Quality was either a S/N Ratio or BER.


(2) You'll never see anything close to 11 Mbps. That's the raw, physical data rate. For a STRONG signal, one model (Prasad, "WLAN Systems and Wireless IP for the next generation", pg. 220") explains that collissions and overhead for MAC & TCP frames, PHY overhead, and idle time slots will only net throughput of just over 5 MBps. Another model, pg. 221, with RTS/CTS enabled show 4.2 MBps as the theoritical maximum. We know wired 10 MBps Ethernet can get to ~8Mbps, but remember 802.11b is also CSMA/CA (note: "CA"-collission avoidance, not "CD"-collission detection like in wired Ethernet). So right off the bandwagon we're maxed out at 4-5 MBps and this doesn't accound for retrys/errors prevalent in the wireless channel (errors in wired Ethernet are usually due to colissions). These errors are unavoidable as the signal level drops, and inevitably degrades throughput.


If you have a signal level of -68 dBm and a signal quality about 60%, you are most likely getting RF interference which is causing the degradation. Make sure adjacent access points are on different channels and that no other 2.4 GHz ISM equipment is causing this.


Hope this helps...


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