The standard, ITU G.9903, shows a theoretical data rate in table 7.8 (see below for a replication of this table), assuming that PHY frames are transmitted back to back without any gap . Data rate for various modulations (excluding FCH) (from ITU G.9903):
FCC Number of symbols Data rate (bit/s) D8PSK (Note1) Data rate (bit/s) DQPSK (Note1) Data rate (bit/s) DBPSK (Note1) Data rate (bit/s) Robust (Note2) 12 152,899 95,957 39,015 4,217 20 N/A 138,135 61,864 11,016 28 N/A 166,469 72,213 15,584 NOTE 1 -- Reed-Solomon with a 16 byte parity. NOTE 2 -- Reed-Solomon with an 8 byte parity.
The simple MAC GUI can transmit PHY frames with minimum inter-frame interval of 2ms. For FCC band it’s about 8.6 data symbols (231us). This is the reason why simple MAC data rate is lower than theoretical shown in ITU spec. For longer PHY frames (higher number of symbols) the difference between theoretical and actual data rate will be minor. Also according to ITU spec minimum IFS has to be 10 symbols or 2.3ms.