The IuR reference interface in 3G is an interface that connects two Radio Network Controllers (RNCs) in the UMTS Terrestrial Radio Access Network (UTRAN). The IuR interface enables the exchange of signaling information between two RNCs, such as mobility management, load balancing, and handover coordination. The IuR interface also supports the transmission of user data between two RNCs, such as voice, video, or packet data. The IuR interface uses the Radio Network Subsystem Application Part (RNSAP) protocol to carry the signaling information, and the AAL2-SAR SSCS protocol to carry the user data. The IuR interface is mainly needed for soft handovers involving two RNCs, though not required as the absence of IuR will cause these handovers to become hard handovers.