5G NR Interfaces: X2&Xn, S1&NG, F1&E1 

What are differences between X2 and Xn interfaces, S1 and NG interfaces, F1 and E1 interfaces ?

In LTE networks, X2 and S1 interface are defined as an interfaces between RAN nodes and between RAN and Core Network.

As you know 5G is expected to operate in two modes NSA and SA modes.

For NSA mode we are using the same X2 and S1 interfaces than 4G with some extensions where as for SA mode new interfaces are defined (Xn and NG)

These new intefaces are listed below:

• Interface between RAN Node as X2/Xn
• Interface between RAN and Core Network as S1/NG
• Interface for Function Split and Open Interface  as F1/E1 within RAN Node
• Interface between PHY and Radio as eCPRI

Interface between RAN Nodes (X2/Xn)

The X2 interface used between eNBs in LTE is reused between RAN nodes in non-standalone operation between eNB and en‒gNB and the Xn interface is newly specified between RAN nodes in standalone operation between ng‒eNB/gNB and ng‒eNB/gNB

The extensions of X2 include functions adopting EN-DC and flow control for split bearers for non-standalone operation. The flow control function, which was defined for LTE-DC split bearers in Release 12, is used for appropriately split downlink data when using the radio resources of multi-ple RAN nodes. Although functions and interfaces for basic flow control were specified for LTE-DC, the information exchanged between RAN nodes is further enhanced to optimize the flow control for non-standalone operation.

Xn is based on the X2 function, the UE context management function is enhanced for adopting new QoS flow framework and network slice.

Interface between RAN Node and Core Network (S1/NG)

Similar to interface between RAN nodes, the interfaces between RAN nodes and Core network also differ for non-standalone and standalone operation. In non-standalone operation, the S1 interface is reused for between RAN node and EPC. On the other hand a new interface with name NG is specified between RAN nodes (ng‒eNB/gNB and 5GCs in standalone operation.

The extensions of S1 include a function that reports data volume for a specific RAT in non-standalone operation. Thus, this function was introduced for calculating the amount of data volume via NR. In non-standalone operation, since the S1‒C interface is only established between the Master Node and Core Network, the data volume through Master Node terminated bearers is counted by the Master Node itself and reported directly to the Core Network via S1, while the data volume from Secondary Node terminated bearers is counted by Secondary Node and reported to Master Node via X2, and then reported by Master Node to Core Node via S1.

Similar with Xn for X2, although NG is based on the S1 function, the bearer/session management functions and UE context management functions have been enhanced for adopting a new QoS framework and network slices.

Functional Split and Open Interfaces (F1, E1) within RAN Nodes

In 3GPP standardization, the functional split within a RAN node which places part of the functions in separate logical nodes, and an open interfacing between logical nodes has been discussed

To address the issue of explosive in-creases of the bandwidth required for the transport between the Central Unit (CU) and Distributed Unit (DU) by the introduction of massive MIMO and extending the frequency bandwidth, the new functional split between CU (gNB-CU) and DU (gNB-DU) within gNB and the corresponding open interface between these nodes were defined. Specifically, a functional split was adopted where the PDCP layer and above can be located in the gNB-CU, and the RLC layer and below can be located in the gNB-DU. The standard interface between them is specified as F1.

Also, with 3GPP standardization, an open interface between the C-plane termination parts and U-plane termination parts of gNB-CU has been specified so that this sort of functional separation can be achieved even between different vendors. A node that terminates the C-plane of gNB-CU is called gNB-CU-CP, and a node that terminates the U-plane of the gNB-CU is called gNB-CU-UP. The standard interface between these nodes is specified as E1.