Standard TCP performs poorly in networks with a large bandwidth-delay product.
HSTCP makes minor modifications to standard TCP's congestion control mechanism to overcome this limitation.
When the congestion window is small, HSTCP behaves exactly like standard TCP so a(w) is 1 and b(w) is 0.5.
This means that HSTCP's window will grow faster than standard TCP and also recover from losses more quickly.
Since only the congestion control mechanism is modified, HSTCP can be used with other TCP options like SACK.