Centrino is just the Intel brand name for a combination of an Intel mobile chipset and an integrated wireless chip. The Centrino specification doesn't address what speed/standard the wireless chip uses. If the particular wireless chip in your computer is based on the 802.11b standard (11 Mbs maximum under the best conditions, usually much less under real world conditions), then having a newer 802.11g 54 Mbs access point is meaningless. It will slow down to the slowest client, namely the 802.11b client, and all other clients (including any 802.11g clients) will be held to the lower speed too. Then once the maximum speed is lowered, it will continue to renegotiate actual speeds up and down in real time based on actual conditions. There could be changing radio interference conditions or changing data transmission demands. If you are losing connectivity or throughput over time, it could be as simple as you never had the throughput in the first place, but it took some time for the system utility to recalculate the average actual speed. Or there are other things going on in your physical enviroment.
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