Signals Can Be Too Strong
An Atheros card's radio reciever can become "saturated" if the signal is too strong. Signal levels as high as -22dBm, as seen by the 'other' receiver, and/or signal quality reported from iwconfig above 60/94 are too strong. Signals that are "Too Strong" yeild actual transmission rates as low as to 5-11Mbps with a net TCP throughput of about 600KB/s.
This commonly occurs if the station device is too close (1-2m) to the access point,
Simply lowering the output power on the AP, increasing the distance between the AP & station, or attenuating the signal by using a lower Db gain antenna brings the bitrates back up to 48-54Mbps and TCP throughput to normal rates (ie: ca. 2.5MB/s). In any case, you're "too close".
Why This Happens
(Thanks to Jim Thompson)
You're probably running outside the 802.11 spec.
Consider that you may have a card putting 15dBm (32mW) into a 2.2dBi antenna (on the AP), and that your card probably has an effective 2dBi gain antenna on it.
Operating at 2.4GHz, at 1m, there is 41dB of path loss between the two units. At 2m, you'll have another 6dB of path loss, for 47dB.
So the signal leaves at 17dBm (from the AP), you lose 41dB due to LOS path loss @ 1m, and the signal arrives at the antenna in your laptop at perhaps -24dBm, and is raised to -22dBm by the antenna. This is *quite* close to the maximum signal level of 802.11g. The IEEE standards state:
For 802.11b: 18.104.22.168 Receiver maximum input level The receiver shall provide a maximum FER of 8×10–2 at a PSDU length of 1024 octets for a maximum input level of –10 dBm measured at the antenna. This FER shall be speciﬁed for 11 Mbit/s CCK modulation.
For 802.11g: 19.1.2 Operational modes ... The changes to the base standard required to implement the ERP are summarized as follows: a) ERP-DSSS/CCK 1) The PHY uses the capabilities of Clause 18 with the following exceptions: iii) The maximum input signal level (see 22.214.171.124) is –20 dBm. b) ERP-OFDM 1) The PHY uses the capabilities of Clause 17 with the following exceptions: iv) The maximum input signal level (see 126.96.36.199) is -20 dBm.