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 were 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.

    The figure shows an example of CUPS as applied in 4G/LTE EPC (Evolved Packet Core). SGW and PGW are split into their respective control plane and user plane parts. These parts communicate via the PFCP protocol. Each plane has a well-defined functionality. For example, packet forwarding and transport level packet marking are exclusively user plane features.

    When a PDU session is established, the CP function gets involved. CP associates with a UP function and configures the latter to process user plane traffic. Subsequently, when user plane traffic flows, UP function is actively involved. UP function occasionally updates CP function as necessary. UP bandwidth per session is in the order of Gbps whereas CP bandwidth requirement is perhaps only a few Mbps. UP latency for URLLC is 1ms whereas for CP its 10ms. With these differing performance requirements, it makes sense to separate CP and UP functions.

    In 5G CUPS, SMF takes care of the signalling while UPF is the user plane counterpart. SMF and UPF communicate using PFCP over the N4 interface.

  • 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.

    In 5G's Next-Generation Radio Access Network (NG-RAN), gNB is no longer a monolith. It's disaggregated into Central Unit (CU), Distributed Unit (DU) and Radio Unit (RU). Furthermore, the CU is split into CU-CP and CU-UP for the control plane and user plane respectively. A gNB consists of one CU-CP and one or more CU-UP/DU/RU each.

    Due to CUPS applied to CU, we have these new interfaces: E1 interface between CU-CP and CU-UP, F1-c interface between CU-CP and DU, and F1-u interface between CU-UP and DU. Moreover, old X2, Xn and NG interfaces are disaggregated into X2-C/X2-U, Xn-C/Xn-U and NG-C/NG-U respectively. The E1 interface is purely a control plane interface.

    CU/DU split can be done in many ways. For standardization work, 3GPP adopted what's called Option 2 in which RRC/PDCP-C are in CU-CP, SDAP/PDCP-U are in CU-UP, and RLC/MAC/PHY are in DU.

    In 4G/LTE, ng-eNB is disaggregated into ng-eNB-CU and ng-eNB-DU. Between these two, W1-C and W1-U interfaces are defined. However, unlike in 5G, ng-eNB-CU is not split into separate control and user plane entities.

  • What are the key benefits of CUPS?
    Benefits of CUPS. Source: Cisco 2021, table 2.
    Benefits of CUPS. Source: Cisco 2021, table 2.

    Due to CUPS, CP and UP functions can be scaled independently. During peak periods, more UP nodes can be added without increasing the CP nodes. UP functions can be located closer to UEs that they serve, thus reducing latency. A UE may be served by a single CP function and many UP functions, one for each PDU session.

    For operators, CUPS can result in lower operating cost. By processing data at the edge, backhaul costs can be reduced. Each UP function can be specialized for the application use case it serves. CUPS allows operators deliver new services and handle traffic growth without sacrificing customer experience.

    CP and UP functions can evolve independently. For example, CP nodes may be upgraded without upgrading UP nodes, provided the evolution is backward compatible.

    Likewise, CUPS in NG-RAN gives operators flexibility, and optimize for cost and/or performance.

  • What are the possible deployments due to CUPS?
    Three deployments due to CUPS. Source: Miyazaki et al. 2022, fig. 3.
    Three deployments due to CUPS. Source: Miyazaki et al. 2022, fig. 3.

    The figure shows three possible deployments. For low latencies, SGW-U/PGW-U can be deployed closer to cell sites. In urban areas with a large subscriber base, high performance and high capacity SGW-U/PGW-U may be deployed. If we need resilience to disasters and congestion, SGW-U/PGW-U may be deployed with redundancy. In other words, if one SGW-U/PGW-U goes down, another in hot standby mode will take over. For that matter, we can have similar redundancy in the control plane as well.

    In the RAN, we can co-locate CU-CP and CU-UP in the cloud. An alternative is to co-locate CU-UP and DU at the edge while CU-CP remains centralized in the cloud. This can reduce the midhaul latency but at the expense of control plane signalling overhead.

  • How does a CP function select a UP function?
    UP function selection problem during 5G-to-4G handover. Source: Pandey et al. 2023, fig. 4.
    UP function selection problem during 5G-to-4G handover. Source: Pandey et al. 2023, fig. 4.

    In traditional 4G EPC, MME selects SGW and PGW using DNS. This is based on UE's location and Access Point Name (APN). With CUPS, MME selects SGW-C/PGW-C and the latter select SGW-U/PGW-U. SGW-C/PGW-C will use UE's location, APN, UP performance and load status. Thus, with CUPS, SGW-C/PGW-C uses dynamic information in the selection of SGW-U/PGW-U.

    Suppose a UE has an IMS voice session and a data session. Before CUPS, an EPC wishing to avoid the S5 hop would select the same node for SGW and PGW of both sessions. With CUPS, a combined SGW-C/PGW-C plus two specialized combined SGW-U/PGW-U nodes can be selected.

    In some scenarios, selecting the right SGW-U/PGW-U can be a problem. For example, SGW-C and PGW-C may select UP functions that are not co-located. This results in two hops in the data path. The figure shows a UE in 5G-to-4G handover. SGW-C doesn't know which UP function is being currently used.

  • What's the migration path from traditional deployment to CUPS-based deployment?

    A traditional 4G transport network may not get the full benefits of CUPS. Migrating EPC towards CUPS enables smooth evolution towards 5G Core. Cisco has proposed a three-stage migration: inline CUPS where CP and UP functions are logically separated; co-located CUPS where CP and UP functions are separate nodes but co-located in the same data center; remote CUPS where the functions are in different data centers.

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. 3GPP. 2021. "NG-RAN Architecture." 3GPP, News, February 19. Accessed 2023-08-27.
  4. Bertenyi, B., R. Burbidge, G. Masini, S. Sirotkin, and Y. Gao. 2018. "NG Radio Access Network (NG-RAN)." Journal of ICT, River Publishers, vol. 6, no. 1&2, pp. 59–76. doi: 10.13052/jicts2245-800X.614. Accessed 2023-08-26.
  5. Cisco. 2021. "Cisco Control/User Plane Separation (CUPS)." Data sheet, Cisco, July 19. Accessed 2023-03-09.
  6. Craven, C. 2021. "What Are the Open RAN Standards?" SDxCentral, February 18. Accessed 2023-03-11.
  7. Dano, Mike. 2019. "Another set of 5G standards was just released, but no one really cares." LightReading, April 5. 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. 2022b. "TS 137 470: LTE; 5G; W1 interface; General aspects and principles." V17.0.0, April. Accessed 2023-08-26.
  10. ETSI. 2023a. "TS 123 501: 5G; System architecture for the 5G System (5GS)." V17.7.0, January. Accessed 2023-03-09.
  11. ETSI. 2023b. "TS 138 401: 5G; NG-RAN; Architecture description." V17.5.0, July. Accessed 2023-08-26.
  12. 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.
  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. Khaturia, M., P. Jha, and A. Karandikar. 2020. "5G-Flow: Flexible and Efficient 5G RAN Architecture Using OpenFlow." arXiv, v1, October 15. Accessed 2023-08-26.
  16. 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.
  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. O-RAN Alliance. 2022. "O-RAN Architecture Description 8.0." XRAN-FH.CUS.0-v01.00, November 17. Accessed 2023-03-11.
  19. O-RAN SC Docs. 2023. "O-RAN Architecture Overview." Docs, O-RAN SC, revision 998000d3. Accessed 2023-03-09.
  20. Oracle. 2018. "5G Core: How to Get There." White paper, Oracle, September 21. Accessed 2023-08-27.
  21. Osseiran, Afif, Stefan Parkvall, Patrik Persson, Ali Zaidi, Sverker Magnusson, and Kumar Balachandran. 2020. "5G wireless access: an overview." White Paper, Ericsson, April 21. Accessed 2023-08-26.
  22. Pandey, S., S. Anagani, S. Vasanthkumar, R. Singh, and R. Jain. 2023. "Enhanced User Plane Selection Functionalities in 5G Deployments." Technical Disclosure Commons, January 17. Accessed 2023-08-26.
  23. 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.
  24. 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.
  25. 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.

Further Reading

  1. 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.
  2. 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.
  3. 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.
  4. 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.

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Devopedia. 2023. "Control and User Plane Separation." Version 4, August 27. Accessed 2023-11-12. https://devopedia.org/control-and-user-plane-separation
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Last updated on
2023-08-27 11:45:51