Control and User Plane Separation
In traditional 4G LTE networks, control plane and user plane functionalities were combined into the same network elements. The Serving Gateway (SGW), the PDN Gateway (PGW) and the Traffic Detection Function (TDF) are examples. In Release 14, 3GPP published a modified architecture is which the two planes are separated. Called Control and User Plane Separation (CUPS), SGW got split into SGW-C and SGW-U. Likewise, we obtained PGW-C, PGW-U, TDF-C and TDF-U. CUPS was subsequently adopted in the design of the 5G System, both in the 5G Core and in the NG-RAN.
CUPS leads to better scalability and performance. For the more demanding use cases of 5G, CUPS may play a significant role. Control and user plane specifications can now evolve independently. However, implementing CUPS has its challenges.
At the SA WG2 Meeting #S2-110, a study item titled Feasibility Study on Control and User Plane Separation of EPC nodes is proposed. This is subsequently approved in September at the 3GPP TSG SA Meeting #69. The document notes that such a separation shall not affect the functionality of the nodes being split. It shall not affect UE or RAN. New reference points shall be introduced only among the split parts of SGW, PGW and TDF.
3GPP publishes TR 23.714 titled Study on control and user plane separation of EPC nodes as part of Release 14. The document identifies the different issues and possible solutions. The selected solution is a functional split of user plane and control plane functions, including the case of combined SGW/PGW. User plane functions shall be selected by the respective control plane functions.
The O-RAN Alliance publishes documents detailing the O-RAN architecture. These documents show CUPS applied to the RAN. In particular, the Open Distributed Unit (O-DU) and the Open Central Unit (O-CU) are split. E1 interface connects O-CU-CP and O-CU-UP. F1-c and F1-u interfaces connect O-CU-CP and O-CU-UP to the O-DU. These interfaces are in fact specified by 3GPP and adopted by O-RAN.
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- 3GPP. 2016. "TR 23.714: Study on control and user plane separation of EPC nodes." V14.0.0, June. Accessed 2023-03-09.
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