Relationship between Link quality and signal level

[David Goodenough]

Why would I get:- 

         Link Quality:0/92  Signal level:-69 dBm  Noise level:-90 dBm
 
from iwconfig?  Normally the link quality first number is roughly the
difference between the signal level and the noise level, but here is
reports 0.

The other end of this link reports a more healthy link quality:-

          Link Quality:30/92  Signal level:-79 dBm  Noise level:-100 dBm
 
but (I presume because the first one says 0) the current rate is 1Mb/s

This link is running ad-hoc, these are the only two stations on this 
network, and neither end can detect (using iwlist scan) any other users
on this channel.  There is a clear line of site, and there are virtually
no buildings in between to generate any non-802.11 interference.

Both ends are using Senao PCMCIA 802.11b cards, prism chipset, both
at 1.8.0 firmware.

Any ideas welcome.

David