The main thing that i28 had that i13 did not, is 56k modulation support for
K56Flex. i13 supported the physical modem cards, but not beyond V.34
modulation. The incremental releases (13, 28) had some pretty nice
features that we used to used prior to switching to 56k. Up until now, the
only way to get 56k modulation on the Maxes was by using patch releases.