The lecture provides an introduction to V5 interface. V5 interface is a kind of open, standard Service Node Interface (SNI) between Access Network (AN) and Service Node (SN).v It provides interoperability, compatibility for equipment supplied by different vendors both at switch side and at access network side. In this lecture, we will introduce V5 related basic concepts, access model, protocol architecture, LapV5, AN frame relay and various L3 protocol layers. Several applications such as optical access network and wireless access network are included. We will also mention the enhancement in the second edition of V5 recommendation.