transmission error messages

["James Moore" <jim_(_dot_)_moore_(_at_)_firelinedsl_(_dot_)_com>]

Got several of these error messages the other day at the console 
(different numbers, but the text of the messages were the same):

xl0: transmission error: 90
xl0: tx underrun, increasing tx start threshold to 120

I'd appreciate it if someone could provide a brief explanation of what 
these messages mean, and point me toward any available resources that 
might shed more light.

Thanks,
James Moore