The importance of understanding complementary technologies such as wireless local area network ( WLAN ) , free space optics ( FSO ) and digital cellular phone networks was stressed , given the fact that currently in many cases only very small aperture terminal ( VSAT ) system solutions were available and these could be very expensive . Other potential complementary or `` hybrid '' solutions , such as the digital video broadcasting ( DVB ) -return channel system or the FSO technology system , could be considered .