RBS 2308 have jumper and feeder cables beetween base station and antennas.
RBS 2311 have optical cables beetween MU (main unit) and RRU (radio remote unit). So MU can be on the ground, and RRU can be on the top of the pole together with antennas. So the line is MU—optical cabel—RRU—jumper cable—antennas.
RBS 2311 is practical when you can not apply feeder cables
regards