dispersion can be postive or nagative based on that type of window we are using;
e.at 1550nm: dispersion is -20ps/km.nm (NAGATIVE)
at 850nm the dispersion is 80 ps ps/km.nm(positive)
NOW WHAT DOES THIS NAGATIVE OR POSITIVE SIGN INDICATES;
+VE DISPERSIONS MEANS AS THE WAVELENGTH INCRESES THE SPEED DECRESESS9i.e longer wavelength components will take longer time to reach the same destination)
-vedispersion means as the value increases the speed increases(i.e shorter wavlenght will take longer time to reach the same destination)
this time delay between the wavelength components(whether the dispersion is positive or nagative) leads to ISI.
with regards