With BT's 21CN, communications are sent over a high quality, secure network using IP as the transport protocol. To maximise efficiency a technique called MPLS allows different services - for example, voice, audio and video calls - to be assigned their own portion of the capacity on the links between metro nodes and for that portion to be given the appropriate prioritisation. This is an essential protocol for converged IP networks since it ensures, for example, that voice is prioritised over less time-critical data.