ndovers/reselection to/from LTE) as covered later in this chapter. The data routing can be done in different ways depending where to split the user plane between LTE and 5G. The different alternatives (option 3 variants) are as follows: Option 3 with Data split in the LTE eNB. The user plane data received from the EPC is divided by the LTE eNB between LTE radio and the 5G gNB (data over the X2 interface). This allows a single service (with bearer split) to be transmitted from both the gNB and eNB and respectively received from both sides in the uplink. 5.2 5G Architecture Options Option 3 with data split Option 3a with data Option 3x with data in LTE eNB split in EPC split in 5G gNB LTE eNB 5G gNB LTE eNB 5G gNB LTE eNB 5G gNB Control Figure 5.2 Different data split options for the option 3 family. Option 3A with data split in the EPC. This does not allow data rate aggregation for a single service, but a single service is only serviced either by an LTE eNB or a 5G gNB. In this case, the X2-interface is not used for user plane transmission. It can be antici- pated that for some low-data-rate service like VoLTE, this could be used while mobile broadband would use one of the other solutions enabling data rate aggregation. Option 3X with data split in the 5G gNB. Data received from the EPC is divided by the 5G gNB between 5G radio and LTE eNB (data over the X2 interface). This is usually favored as it minimizes the investment on the LTE side since typically the LTE eNB has not been dimensioned originally to handle such high data rates, including the backhaul transmission part. Also, the latency is reduced if most of the packets go via 5G in any case (Figure 5.2). There exist also further architecture options which were finalized in June 2019, addressing the additional dual connectivity options as follows. These are not foreseen to be supported by the first phase UEs to be used with 5G network openings as these are finalized only as part of "late drop" in Release 15: Option 4, with the 5G gNB being the connection anch