The best it can manage is half of theoretically maximum. All depends on condition, distance, things in between. But, 11g has short range than 11b.
As for download speed, you can not blame BT for it because (1) the server your are trying to download maybe busy (sometime I only got 0.1k for some popular server). (2) You are sharing the same bandwidth with 49 other people in your "street" because all consumer adsl services have contension ratio of 50 to 1.
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