E-UTRAN supports Dual Connectivity (DC) operation whereby a multiple Rx/Tx UE in RRC_CONNECTED is configured to utilise radio resources provided by two distinct schedulers, located in two eNBs connected via a non-ideal backhaul over the X2 interface

In DC, the radio protocol architecture that a particular bearer uses depends on how the bearer is setup. Three alternatives have been defined as follows: ı Master Cell Group (MCG) bearer ı Secondary Cell Group (SCG) bearer ı Split bearer. RRC is located in the MeNB and signaling (SRBs) is always of the MCG bearer and therefore only use the radio resources provided by the MeNB. The MCG bearer can be seen as the legacy bearer that transports both data and signaling. Split bearer and SCG bearer are data only bearers. The main difference between the latter two is that for split bearer the S1-U interface terminates in the MeNB whereas for SCG bearer the S1-U interface terminates at the SeNB.

Multi-Radio Dual Connectivity (MR-DC) is a generalization of the Intra-E- UTRA Dual Connectivity (DC), where a multiple Rx/Tx UE may be configured to utilise resources provided by two different nodes connected via non-ideal backhaul, one providing NR access and the other one providing either E-UTRA or NR access. One node acts as the MN and the other as the SN. The MN and SN are connected via a network interface and at least the MN is connected  to the core network.