Control and User Plane Separation

CUPS in 4G splits SGW and PGW. Source: Adapted from Cisco 2021, fig. 1.
CUPS in 4G splits SGW and PGW. Source: Adapted from Cisco 2021, fig. 1.

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.

Discussion

  • Could you give an overview of CUPS?
    LTE EPC architecture illustrating CUPS. Source: Miyazaki et al. 2022, fig. 1.
    LTE EPC architecture illustrating CUPS. Source: Miyazaki et al. 2022, fig. 1.
  • Is CUPS applicable for the RAN?
    CUPS is adopted in O-RAN architecture. Source: Polese et al. 2022, fig. 4.
    CUPS is adopted in O-RAN architecture. Source: Polese et al. 2022, fig. 4.
  • What are the key benefits of CUPS?
  • How does CUPS affect existing 4G/5G protocols?
  • Which are the main documents that specify CUPS?
  • What are some challenges in implementing CUPS?
  • How is the industry adoption of CUPS?

    TBD.

Milestones

Jul
2015

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.

Jun
2016

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.

Sep
2016

3GPP publishes TS 23.214 that specifies the architectural changes to support CUPS in LTE EPC. This is for Release 14. This evolves for other releases: V15.0.0 (Sep 2017), V16.0.0 (Jun 2019) and V17.0.0 (Jun 2021).

Dec
2017
In 5G Core, control plane is clearly separated from the user plane. Source: Leyva-Pupo et al. 2019, fig. 1.
In 5G Core, control plane is clearly separated from the user plane. Source: Leyva-Pupo et al. 2019, fig. 1.

3GPP approves the first specifications for 5G, called "early drop" of Release 15. This release specifies CUPS in 5G Core in TS 29.244 (Jun 2017) and CUPS in NG-RAN in TS 38.401 (Dec 2017). Thus, unlike in 4G, 5G has adopted CUPS from the outset.

Jan
2020

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.

References

  1. 3GPP. 2015. "New SID on Feasibility Study on Control and User Plane Separation of EPC nodes (FS_CUPS)." TD SP-150519, 3GPP TSG SA Meeting #69, September 15-17. Accessed 2023-03-09.
  2. 3GPP. 2016. "TR 23.714: Study on control and user plane separation of EPC nodes." V14.0.0, June. Accessed 2023-03-09.
  3. Cheung, Derek. 2020. "5G Core Part 3 —User Plane Traffic and GTP-U Tunnels." On Medium, July 6. Accessed 2023-03-09.
  4. Cisco. 2021. "Cisco Control/User Plane Separation (CUPS)." Data sheet, Cisco, July 19. Accessed 2023-03-09.
  5. Craven, C. 2021. "What Are the Open RAN Standards?" SDxCentral, February 18. Accessed 2023-03-11.
  6. Dano, Mike. 2019. "Another set of 5G standards was just released, but no one really cares." LightReading, April 5. Accessed 2023-03-09.
  7. ETSI. 2018. "TS 138 401: 5G; NG-RAN; Architecture description." V15.2.0, July. Accessed 2023-03-09.
  8. ETSI. 2022a. "TS 123 214: Universal Mobile Telecommunications System (UMTS); LTE; Architecture enhancements for control and user plane separation of EPC nodes." V17.0.0, May. Accessed 2023-03-09.
  9. ETSI. 2023a. "TS 123 501: 5G; System architecture for the 5G System (5GS)." V17.7.0, January. Accessed 2023-03-09.
  10. ETSI. 2023b. "TS 123 502: 5G; Procedures for the 5G System (5GS)." V17.7.0, January. Accessed 2023-03-09.
  11. ETSI. 2023c. "TS 129 244: LTE; 5G; Interface between the Control Plane and the User Plane nodes." V17.7.1, January. Accessed 2023-03-09.
  12. Hrustic, Adi. 2022. "Real Time 5G Simulator." Master's thesis, Department of Computer Science and Engineering, Chalmers University of Technology. Accessed 2023-03-09.
  13. Huawei. 2019. "CUPS White Paper." White paper, Huawei, February 20. Accessed 2023-03-09.
  14. Jordan, E. 2021. "Open RAN functional splits, explained." 5G Technology World, February 24. Accessed 2023-03-11.
  15. Leyva-Pupo, I., A. Santoyo-González, and C. Cervelló-Pastor. 2019. "A Framework for the Joint Placement of Edge Service Infrastructure and User Plane Functions for 5G." Sensors, MDPI, vol. 19, no. 18, September 14. Accessed 2023-03-09.
  16. Mitra, R. N., M. M. Kassem, J. Larrea, and M. K. Marina. 2021. "CUPS Hijacking in Mobile RAN Slicing: Modeling, Prototyping, and Analysis." 2021 IEEE Conference on Communications and Network Security (CNS), Tempe, AZ, USA, pp. 38-46. doi: 10.1109/CNS53000.2021.9705046. Accessed 2023-03-09.
  17. Miyazaki, Y., K. Okuda, K. Kunitomo, and T. Kaida. 2022. "CUPS for Flexible U-Plane Processing Based on Traffic Characteristics." NTT DOCOMO Technical Journal, vol. 23, no. 3. Accessed 2023-03-09.
  18. Nick. 2020. "CUPS – Control and User Plane Separation in LTE & NR with PFCP (Sx & N4)." Blog, Nick vs Networking, September 13. Accessed 2023-03-09.
  19. O-RAN Alliance. 2022. "O-RAN Architecture Description 8.0." XRAN-FH.CUS.0-v01.00, November 17. Accessed 2023-03-11.
  20. O-RAN SC Docs. 2023. "O-RAN Architecture Overview." Docs, O-RAN SC, revision 998000d3. Accessed 2023-03-09.
  21. ONF. 2020. "SD-RAN: ONF's Software-Defined RAN Platform Consistent with the O-RAN Architecture." White paper, Open Networking Foundation, August. Accessed 2023-03-11.
  22. Polese, M., L. Bonati, S. D’Oro, S. Basagni, and T. Melodia. 2022. "Understanding O-RAN: Architecture, Interfaces, Algorithms, Security, and Research Challenges." arXiv, v2, August 1. Accessed 2023-03-09.
  23. Schmitt, P., B. Landais, and F. Yong Yang. 2017. "Control and User Plane Separation of EPC nodes (CUPS)." News, 3GPP, July 3. Accessed 2023-03-09.
  24. Srinivas, Darshan. 2022. "Optimised user plane routing in a 5G mobile communications network." Master's thesis, Delft University of Technology, September 30. Accessed 2023-03-09.
  25. Tang, Q., O. Ermis, C. D. Nguyen, A. D. Oliveira, and A. Hirtzig. 2022. "A Systematic Analysis of 5G Networks With a Focus on 5G Core Security." IEEE Access, vol. 10, pp. 18298-18319. doi: 10.1109/ACCESS.2022.3151000. Accessed 2023-03-09.

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Devopedia. 2023. "Control and User Plane Separation." Version 3, March 11. Accessed 2023-05-02. https://devopedia.org/control-and-user-plane-separation
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Last updated on
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