**6. Limitations and improvements**

Architecture based on single-fibre bidirectional link seems the most interesting and cost efficient approach. ONU becomes a key element for the network evolution. Transparent and flexible architecture based on WDM technology is necessary thus colourless ONU need to be available. High gain should be provided by the transmitter to reach the necessary optical budget and high modulation speed is needed. Bit rates up to 10 Gbit/s (per wavelength) are required to follow the evolution of the 10 Gbit/s GPON. RSOA could be the missing building block to reach this ideal network. However the modulation bandwidth and the chirp are still issues that need to be solved.
