N26-Based Interworking

Non-roaming 5GS-EPS architecture showing N26 interface. Source: ETSI 2023a, fig. 4.3.1-1.
Non-roaming 5GS-EPS architecture showing N26 interface. Source: ETSI 2023a, fig. 4.3.1-1.

N26 is an interface that connects two Core Network (CN) nodes: 4G/LTE's Mobility Management Entity (MME) and 5G's Access and Mobility Management Function (AMF). These nodes are responsible for the mobility of the User Equipment (UE) in both idle mode and connected mode. When a UE moves from Evolved Packet System (EPS) to 5G System (5GS), or vice versa, the N26 interface is used so that MME and AMF can coordinate the UE mobility.

The N26 interface enables seamless session continuity during inter-system mobility. Via this interface, contexts relevant to the UE are transferred. In the data plane, data packets are suitably forwarded during the mobility procedures.

N26 interface is optional. If it's not implemented, alternative procedures are available. However, UE mobility takes longer and there may be longer service interruption during handover.

Discussion

  • Could you explain a typical N26-based handover procedure?
    5GS-to-EPS handover with N26 interface. Source: Adapted from ETSI 2023b, fig. 4.11.1.2.1-1.
    5GS-to-EPS handover with N26 interface. Source: Adapted from ETSI 2023b, fig. 4.11.1.2.1-1.

    The figure shows 5GS-to-EPS handover scenario with N26 messages highlighted. When NG-RAN informs AMF that handover is required, AMF obtains PDU session contexts and sends the same to MME (3). MME sets up the sessions at SGW, triggers the handover with E-UTRAN and informs AMF that EPS is ready to receive the UE (9). AMF then commands the UE via NG-RAN to handover.

    When UE connects to E-UTRAN, the latter informs MME. MME notifies AMF that handover is successful (12c). This triggers AMF to release contexts for those sessions that couldn't be transferred to EPS. Later when a timer expires, AMF asks NG-RAN to release resources pertaining to the handed over UE (21c).

    For the data plane, AMF sends NG-RAN data forwarding tunnel info (11a). For direct forwarding, this is E-UTRAN tunnel info. For indirect forwarding, this is CN tunnel info and forwarding is done via PGW-U+UPF, SGW, and target eNB. MME modifies the EPS bearers after UE has handed over successfully (13-17).

    While details differ, EPS-to-5GS handover using N26 is similar.

  • What's the protocol stack on the N26 interface?

    GPRS Tunnelling Protocol for Control plane is used on the N26 interface. Specifically, GTPv2-C is used. It's lower layers are UDP, IP, L2 and L1. Destination UDP port number for GTPv2-C initial message is 2123. For every UE, there's only one pair of TEID-Cs, for Tunnel Endpoint Identifier (TEID).

    During the handover preparation phase, source AMF/MME sends target MME/AMF Forward Relocation Request. The latter replies with Forward Relocation Response. During the handover completion phase, target MME/AMF sends source AMF/MME Forward Relocation Complete Notification. The latter replies with Forward Relocation Complete Acknowledge.

    Relocation Cancel Request/Response are also relevant to N26-based handovers. Source AMF/MME triggers the request so that resources in the target system can be released.

    N26 messages for idle mode mobility are Identification Request/Response and Context Request/Response.

  • For N26-based handover, how's the target AMF or MME determined?
    5GS TAI FQDN and TAI FQDN. Source: Adapted from ETSI 2023g, sec. 19.4.2.3, 28.3.2.6.
    5GS TAI FQDN and TAI FQDN. Source: Adapted from ETSI 2023g, sec. 19.4.2.3, 28.3.2.6.

    Consider EPS-to-5GS N26-based handover. Based on the UE's measurement reports, the source eNB selects a target gNB. Every gNB belongs to a tracking area. Hence, the target Tracking Area Identity (TAI) is known. This consists of MCC, MNC and TAC. From TAI, the 5GS TAI FQDN is determined by convention, as shown in the figure.

    MME applies the S-NAPTR procedure to find the candidate list of AMFs. This includes host name, service, port, IPv4 and IPv6 addresses. The S-NAPTR procedure is started at the 5GS TAI FQDN. For this to work, the operator must have provisioned NAPTR records for all enabled interfaces of the node indicated by the 5GS TAI FQDN.

    3GPP has defined service and protocol names in conformance with IETF RFC 3958. For the AMF N26 interface service, the app-service name "x-3gpp-amf" and app-protocol name "x-n26" are used. The S-NAPTR procedure is started with these service parameters.

    For 5GS-to-EPS N26-based handover, MME is selected based on target eNB cell ID and TAI. The S-NAPTR procedure is started from TAI FQDN with service parameters "x-3gpp-mme:x-s10".

  • Could you explain a typical N26-based idle mode mobility procedure?
    5GS-to-EPS idle mode mobility with N26 interface. Source: Adapted from ETSI 2023b, fig. 4.11.1.3.2-1.
    5GS-to-EPS idle mode mobility with N26 interface. Source: Adapted from ETSI 2023b, fig. 4.11.1.3.2-1.

    For 5GS-to-EPS idle mode mobility, UE does Tracking Area Update (TAU) or Initial Attach procedures via E-UTRAN/EPS. During such procedures, MME uses the N26 interface to contact AMF and obtain context information. Relevant messages are Context Request/Response/Acknowledge.

    To select a suitable AMF, MME uses the UE's GUTI (mapped from 5G-GUTI) to determine the AMF Instance FQDN. Via S-NAPTR (Straightforward-Naming Authority Pointer) procedure, MME then finds all N26 interfaces of that AMF.

    In the reverse direction, there's EPS-to-5GS Mobility Registration procedure. This is with or without AMF reallocation. NG-RAN selects an AMF upon receiving registration request from the UE. This AMF may decide to select another AMF based on NSSAIs associated with established PDU sessions. Context Request/Response involve the initial AMF and Context Acknowledge is sent by the new AMF.

  • How is N26 relevant to EPS Fallback and 5G SRVCC?
    Comparing three EPS Fallback modes. Source: ZTE 2022, p. 13.
    Comparing three EPS Fallback modes. Source: ZTE 2022, p. 13.

    EPS Fallback is a feature that operators can use during initial 5G deployments. 5G is used mainly to increase capacity in hotspots. For a Voice over LTE (VoLTE) call, 5GS-to-EPS handover is executed. N26 interface becomes useful. With N26, service interruption is minimal and user experience is better. UE will do single registration rather than dual registration. Single-registration mode with N26 support is most preferred.

    EPS Fallback is a stopgap towards Voice over NR (VoNR). When 5G is fully rolled out, VoNR calls becomes possible. Even in this case, N26 interface limits the service interruption to less than 100ms. Without N26, this could be more than a second.

    5G SRVCC (Single Radio Voice Call Continuity) is a Release 16 feature. A VoNR call can get dropped at the edge of a 5G NR cell. To prevent this, the call is handed over to 3G UTRAN CS bearers, that is, circuit switching is used. N26 interface is not relevant to 5G SRVCC.

  • What happens if N26 interface is not supported by a core network?

    Support for the N26 interface is optional. Thus, target MME/AMF can't obtain context from source AMF/MME via N26. Handover command can't be sent to the target RAN or UE. Instead, Initial Attach, TAU or Registration procedures are performed. HSS+UDM is updated once UE location is updated in the target system. Specifically, HSS+UDM stores the FQDN for the S5/S8 interface of the SMF+PGW-C.

    Consider 5GS-to-EPS mobility. UE in single-registration mode does TAU or Initial Attach with EPS. UE in dual-registration mode does Initial Attach. After UDM is updated, UE performs PDN Connectivity procedure to transfer PDU sessions from 5GS to EPS. Request Type is set to "handover" for IP address preservation. For EPS-to-5GS mobility, UE does the Registration procedure. Registration type is "mobility registration update" (single mode) or "initial registration" (dual mode).

  • What other technical details are relevant to N26-based interworking?

    UE operates in single-registration mode when N26 interface is used. Single-registration mode is when UE has only one MM state in either 5GS or EPS. A single coordinated registration is done. EPS-GUTI and 5GS-GUTI are mapped to each other. In dual-registration mode, UE registers separately to both systems and has two GUTIs. Single-registration mode is mandatory for a UE that supports both 5GC NAS and EPC NAS.

    The N26 interface may be intra-PLMN or inter-PLMN. For 5GS-to-EPS handover, the Forward Relocation Request message may include Return Preferred indicator. MME stores the last used 5GS PLMN ID. If the UE should return to 5GS at a later point, the last used 5GS PLMN will be preferred. Likewise, for EPS-to-5GS handover, AMF stores the last used EPS PLMN ID.

    For EPS-to-5GS mobility, SSC Mode 1 is used by default. In this case, IP address of the UE is preserved.

    When UE registers/attaches to the core network, AMF/MME informs UE if N26 interface is supported.

    Inter-system handover from NR to E-UTRA with EN-DC configuration is supported by the N26 interface.

Milestones

Dec
2017

3GPP publishes Release 15 "early drop". This release specifies the N26 interface.

Sep
2018

Jain et al. observe that operators may not deploy the N26 interface initially. This could be due to cost or longer adoption times. Hence they propose a method to reduce service interruption time during an inter-system handover. They introduce a new entity named SDN-enabled Mobility Management unit (SeMMu). This basically enhances the PDU Connectivity procedure. They show that latency drops by 50% for 5GS-to-EPS HO and by 25% for EPS-to-5GS HO.

Mar
2019

As part of Release 16, PDN type Ethernet is supported for 5GS-to-EPS mobility using N26.

References

  1. Cisco. 2023. "5GS Interworking using N26 Interface Support." MME Administration Guide, StarOS Release 21.28, Cisco, September 28. Accessed 2024-01-19.
  2. ETSI. 2020a. "TS 129 274: Universal Mobile Telecommunications System (UMTS); LTE; 5G; 3GPP Evolved Packet System (EPS); Evolved General Packet Radio Service (GPRS) Tunnelling Protocol for Control plane (GTPv2-C); Stage 3 ." V16.4.0, November. Accessed 2024-01-19.
  3. ETSI. 2023a. "TS 123 501: 5G; System architecture for the 5G System (5GS)." V17.10.0, September. Accessed 2024-01-17.
  4. ETSI. 2023b. "TS 123 502: 5G; Procedures for the 5G System (5GS)." V17.10.0, September. Accessed 2024-01-17.
  5. ETSI. 2023c. "TS 137 340: Universal Mobile Telecommunications System (UMTS); LTE; 5G; NR; Multi-connectivity; Overall description; Stage-2." V17.6.0, October. Accessed 2024-01-17.
  6. ETSI. 2023g. "TS 123 003: Digital cellular telecommunications system (Phase 2+) (GSM); Universal Mobile Telecommunications System (UMTS); LTE; 5G; Numbering, addressing and identification." V17.10.0, July. Accessed 2024-01-19.
  7. ETSI. 2023h. "TS 129 303: Universal Mobile Telecommunications System (UMTS); LTE; 5G; Domain Name System Procedures; Stage 3." V17.4.0, July. Accessed 2024-01-19.
  8. Jain, A., E. Lopez-Aguilera, and I. Demirkol, 2018. "Improved Handover Signaling for 5G Networks." 2018 IEEE 29th Annual International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC), Bologna, Italy, pp. 164-170, September 9-12. doi: 10.1109/PIMRC.2018.8580757. Accessed 2024-01-19.
  9. MediaTek. 2021. "5G NR Voice Solutions: Overview and Deployment Guidelines, Network Performance Considerations." MediaTek. Accessed 2024-01-19.
  10. Monieum-Tech. 2021. "Voice Service in 5G Network (VoNR, EPSFB)." Monieum-Tech, November 10. Accessed 2024-01-19.
  11. Nokia. 2018. "Start 5G deployment with an eye on the future." White paper, SR1808027862EN, Nokia, September. Accessed 2024-01-21.
  12. Wirelessbrew. 2021. "What is N26 Interface in 5G and what's its importance?" Wirelessbrew, November 8. Updated 2022-01-02. Accessed 2024-01-19.
  13. ZTE. 2022. "5G Voice." White paper, ZTE. Accessed 2024-01-19.

Further Reading

  1. ETSI. 2023a. "TS 123 501: 5G; System architecture for the 5G System (5GS)." V17.10.0, September. Accessed 2024-01-17.
  2. ETSI. 2023b. "TS 123 502: 5G; Procedures for the 5G System (5GS)." V17.10.0, September. Accessed 2024-01-17.
  3. Cisco. 2023. "5GS Interworking using N26 Interface Support." MME Administration Guide, StarOS Release 21.28, Cisco, September 28. Accessed 2024-01-19.
  4. Peltonen, A., R. Sasse, and D. Basin. 2021. "A comprehensive formal analysis of 5G handover." WiSec '21: Proceedings of the 14th ACM Conference on Security and Privacy in Wireless and Mobile Networks, pp. 1-12, June. Accessed 2024-01-19.

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Devopedia. 2024. "N26-Based Interworking." Version 4, January 21. Accessed 2024-06-25. https://devopedia.org/n26-based-interworking
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