WinlabMadwifi -> MIT's !SampleRate Algorithm

!SampleRate Algorithm:

implemented by
 * sample.c
 * sample.h

This algorithm conducts a per-MAC destination rate control. For each different destination, link statistics are stored separately.

Description of functions in sample.c:

{{{
   I. static __inline int best_rate_ndx(struct sample_node *sn, int size_bin, int require_acked_before)
}}}

The objective of this function is to return the bit-rate with the lowest avg. pkt. tx. time.
{{{
   Process information starting from the lowest rate:
   1. if ((avg. pkt. tx. time for this pkt. size and bit-rate <= 0) or 
          ((packets have not been acked for this pkt. size and bit-rate) and 
          (multi-rate retry is disabled)))
         process next higher bit-rate;
   2. if (processing 9Mbps)
         process next higher bit-rate; // 9Mbps is never better than 12Mbps
   3. if (successive failures at this rate > 3)
         process next higher bit-rate;
   4. if (avg. pkt. tx. time for this rate is lowest so far)
         set best_rate_index to that of current rate;

   if (best rate index found)
      return best_rate_index;     
   else
      return -1;
}}}

{{{
   II. static __inline int pick_sample_ndx(struct sample_node *sn, int size_bin)
}}}

The objective of this function is to return the index of a "suitable" bit-rate that can be sampled.
{{{
   1. if (no packets have been sent successfully at any of the higher bit-rates)
         return (index of lowest bit-rate); 
   2. current_tx_time = avg. tx. time for the bit-rate at which the card was operating in the previous interval;
   3. Process information starting from the index of the lowest rate (for (i = 0); i < max_rate_index; i++)):
      1. Choose 1 + the index of the last rate that was sampled + i;
      2. if (i == current_rate_index)
             proceed to next higher bit-rate;
      3. if (perfect_tx_time(rate(i)) > avg. tx. time (current rate))
             proceed to next higher bit-rate;
      4. if ((last tx. at rate(i) was less than 10 seconds ago) and 
             (successive failures at rate(i) > 3)) 
             proceed to next higher bit-rate;
      5. if ((rate(i) > 11Mbps) and (rate(i) > current rate + 2))
             proceed to next higher bit-rate;
      6. if (rate(i) == 9Mbps)
             process next higher bit-rate; // 9Mbps is never better than 12Mbps
      7. if ((current rate == 11Mbps) and (rate(i) > 12Mbps))
             proceed to next higher bit-rate;
      8. return i;
}}}