hi alistu,
look dispersion is all about comparative velocity of different frequency components of pulse.
longer wavelength (higher frequency component of pulse)travel slower than shoter wavlength (lower frequency component of pulseat 1550)(i guess, i may be wrong her but i am talking abouty logic how chirping migitate dispersion)
if a pulse has centre frequency f0 and fh as hiher frequency and fl as lower frequency.
then positive means order will be fl fo fh.
and negative chirp means fh fo fl.
now we have look how these frequency components will travel inside the fiber.
the explanation it would be very long, i suggest you want through chapter ( dispersion compensation method) the book by o p aggrerwal.
there is full fleged explanation and equations in that book.
with regards