5G Radio Access Network Architecture Based on Flexible Functional Control / User Plane Splits

5G networks are supposed to offer a high flexibility in a several ways. In this regard, a twofold split of the processing in the radio access network is under discussion: A control plane / user plane split to support the software defined networking principle and a radio protocol stack layer based split to allow a flexible placement of processing functions between a central and distributed units. In this work, the motivation and state of the art for both splits are described including a discussion of the advantages and disadvantages. It is followed by a description of a network architecture allowing a flexible implementation of these splits. This especially focuses on the required interfaces between control and user plane.