EPS Fallback — The Mechanism for Completing Voice Calls in Areas Where VoNR Is Not Offered¶

Learning objectives¶

After reading this chapter, you will be able to:

• Explain that EPS Fallback is "the mechanism for moving a voice call to 4G(EPS) and completing it in areas where VoNR is not offered."
• Explain the flow in which EPS Fallback is triggered by a request to establish a voice QoS Flow (5QI=1).
• Distinguish the two transition methods: Handover-based (with N26) and Redirect-based (without N26).
• Explain the role that the N26 (AMF⇔MME) interface plays in EPS Fallback.
• Diagram the flow in which, after the transition, the UE establishes voice via VoLTE (a QCI=1 dedicated bearer).

Prerequisites¶

• VoNR (Voice over NR) — EPS Fallback is the branch of "first attempt VoNR → on no support, move to 4G," so understand VoNR's voice call establishment.
• QoS (the 5G QoS model) — understanding of 5QI / QoS Flow / GBR, and the correspondence with the 4G-side QCI / EPS bearers.
• Handover — the Handover-based transition is a kind of Inter-RAT Handover.
• Registration (initial registration) — the premise is that the UE is registered in 5GS and that 4G/5G interworking is configured.
• Supplementary: AMF / SMF / PCF

What you'll learn in this chapter¶

• Why — why 5G needs a mechanism to "deliberately drop down to 4G."
• Basic Concept — the idea of "guiding to the 4G counter only for voice."
• What N26 is (column) — the interface that underpins handover across 4G/5G.
• Architecture — the cast of 5GC, 4G(EPC), and NG-RAN involved in EPS Fallback.
• Procedure — Call Flow — detecting the voice QoS request → determining no VoNR support → transition to EPS → VoLTE completion.
• Handover-based and Redirect-based — the two transition methods depending on the presence of N26.
• Comparison with EPC / Release differences / Trouble Shooting / FAQ / Practice

Why — why 5G needs a mechanism to "deliberately drop down to 4G"¶

VoNR is the ideal form that carries voice natively on 5G, but in early 5G deployment the areas (cells) that supported VoNR were limited. Even where the radio (NR) and the core (5GC) were available, the cell often could not provide a voice QoS Flow (5QI=1, GBR).

Meanwhile, 4G(LTE)'s VoLTE was already widely in operation and could complete voice stably. So in the early stage of 5G, the following compromise was chosen.

• Data communication on 5G(NR) — to reap the benefit of high speed and large capacity.
• Only at the moment a voice call is needed, temporarily move to 4G(EPS) — to reliably complete voice via the proven VoLTE.

This mechanism of "moving the UE from 5GS→4G(EPS) at the time of voice call establishment" is EPS Fallback. It is specified in 3GPP TS 23.502 §4.13.6.1. It becomes unnecessary once VoNR is widely offered, but in early deployment it was a virtually essential function in practice.

Overview¶

EPS Fallback works in the following flow.

1. Detecting voice call establishment — the UE originates/receives an IMS voice call, and just as with VoNR, a request to establish a voice QoS Flow (5QI=1) arises from the 5GC.
2. NG-RAN determines no VoNR support — the radio side determines that this cell cannot provide a voice QoS Flow.
3. Moving the UE to 4G(EPS) — the NG-RAN triggers EPS Fallback and moves the UE to 4G(LTE) either Handover-based or Redirect-based.
4. Voice completed as VoLTE on 4G — after the transition, the UE establishes a dedicated bearer (QCI=1) on 4G/EPS and talks as VoLTE.

The key point is that this is the branch of "first attempt VoNR → the cell does not support it → move to 4G." The voice trigger itself (the 5QI=1 request) is common with VoNR; the branch from there onward is specific to EPS Fallback.

The diagram below shows how it is common with VoNR up to the 5QI=1 request for the voice call, and branches for the first time at the NG-RAN's determination of VoNR support. Only in the case of no support does the transition method further split into Handover/Redirect by the presence of N26.

flowchart TD
    START["Voice call (originate/receive)
→ 5QI=1 QoS Flow establishment request
【common with VoNR】"] --> Q1{"NG-RAN:
Does this cell support VoNR?"}
    Q1 -- "Supported" --> VONR["Completed with VoNR
Establish the 5QI=1 voice lane on NR
(does not move to 4G)"]
    Q1 -- "Not supported" --> EPSFB["EPS Fallback triggered"]
    EPSFB --> Q2{"N26 (AMF⇔MME)
present?"}
    Q2 -- "Yes" --> HO["Handover-based
Inter-RAT Handover
(state carried over via N26, smooth)"]
    Q2 -- "No" --> RD["Redirect-based
Guide to LTE via RRC Release
(UE reconnects on 4G)"]
    HO --> VOLTE["VoLTE completed on 4G/EPS
Voice on a QCI=1 dedicated bearer"]
    RD --> VOLTE

    classDef common fill:#eef,stroke:#66a;
    classDef ok fill:#efe,stroke:#3a3,stroke-width:2px;
    classDef fb fill:#fee,stroke:#c33;
    class START common;
    class VONR ok;
    class EPSFB,HO,RD,VOLTE fb;

Basic Concept — a beginner-friendly explanation¶

EPS Fallback can be likened to "we cannot handle this at this counter, so we will guide you to the proven counter next door."

• Everyday procedures (data communication) can be done quickly at the newest counter (5G).
• But when you ask for the special procedure of a "voice call," this counter (a VoNR-unsupported cell) answers "we cannot handle it here."
• So the UE is guided to the proven counter next door (4G/VoLTE). There are two ways to guide it.
  • Take it by the hand and lead it over (Handover-based) — carry over the state and move it smoothly so it is not cut off en route.
  • Tell it the address and let it go on its own (Redirect-based) — redirect it with "please go over there," and the UE reconnects on 4G.

Once the guidance is done, the UE establishes voice (VoLTE) at the 4G counter. Whether it returns to 5G after the call ends depends on the radio-side policy (implementation-dependent).

Architecture¶

graph LR
    UE["UE"] -- "N1(NAS)" --> AMF["AMF (5GC)"]
    UE -- "Radio (NR→LTE)" --> RAN["NG-RAN"]
    AMF -- "N11" --> SMF["SMF (5GC)"]
    PCF["PCF (5GC)"] -- "N7" --> SMF
    RAN -- "N2" --> AMF
    AMF -- "N26" --> MME["MME (4G/EPC)"]
    RAN -. "Inter-RAT transition" .-> ENB["eNB (4G/LTE)"]
    MME --> ENB

How to read the diagram: for a voice call, when PCF→N7→SMF requests a voice QoS Flow (5QI=1), it is conveyed to SMF→N2(via AMF)→NG-RAN. When the NG-RAN determines "this cell does not support VoNR," it triggers EPS Fallback. If N26 (AMF⇔MME) is present, the state is carried over and it Handovers to LTE; if not, it Redirects to eNB(LTE) via RRC Release. After the transition, the UE establishes VoLTE (QCI=1) voice on 4G/EPS. For the details of the VoNR establishment flow, see the VoNR chapter (they are not repeated in this chapter).

Network Function¶

Interface¶

Procedure — Call Flow¶

EPS Fallback is understood in four stages: (A) a voice QoS Flow request arises → (B) the NG-RAN determines no VoNR support → (C) transition to 4G(EPS) → (D) VoLTE completion.

sequenceDiagram
    participant UE
    participant RAN as NG-RAN
    participant AMF
    participant SMF
    participant PCF
    participant MME as MME(4G)
    participant ENB as eNB(4G/LTE)

    Note over UE,PCF: (A) Voice call → voice QoS Flow (5QI=1) request (common with VoNR)
    UE->>SMF: Voice call establishment triggered by SIP INVITE (via IMS)
    PCF->>SMF: N7 SM Policy (5QI=1 GBR)
    SMF->>AMF: N11 QoS Flow addition request
    AMF->>RAN: N2 voice QoS profile notification

    Note over RAN: (B) Determines this cell does not support VoNR → triggers EPS Fallback

    alt Handover-based (with N26)
        RAN->>AMF: N2 Handover Required
        AMF->>MME: Carry over context via N26
        MME->>ENB: Prepare 4G-side resources
        RAN-->>UE: Inter-RAT Handover Command (→LTE)
    else Redirect-based (without N26)
        RAN-->>UE: RRC Release (redirect to LTE)
        UE->>ENB: Reconnect on LTE, (if needed) TAU/Attach
    end

    Note over UE,ENB: (C)/(D) VoLTE voice established on 4G/EPS
    UE->>ENB: Establish voice (RTP) on a dedicated bearer (QCI=1)
    ENB-->>UE: VoLTE call completed

Explanation of the procedure:

1. (A) A voice QoS Flow request arises — when the UE originates/receives an IMS voice call, just as with VoNR, the establishment of a 5QI=1 (conversational voice, GBR) QoS Flow is requested via the path PCF→N7→SMF→N11→AMF→N2→NG-RAN. Up to here it is common with VoNR (details in the VoNR chapter).
2. (B) Determination of no VoNR support — the NG-RAN determines "this cell cannot provide a voice QoS Flow (no VoNR support)" and triggers EPS Fallback.
3. (C) Transition to 4G(EPS) — there are two transition methods.
   • Handover-based (with N26) — the NG-RAN sends an N2 Handover Required, the AMF carries over the UE context to the MME via N26, and moves the UE smoothly to LTE via an Inter-RAT Handover (see the Handover chapter).
   • Redirect-based (without N26) — the NG-RAN redirects the UE to LTE via RRC Release, and the UE reconnects on 4G (TAU/Attach depending on the situation).
4. (D) VoLTE completion — on 4G/EPS the UE establishes a dedicated bearer (QCI=1) and passes voice (RTP) as VoLTE. Call control stays on IMS (on both 4G and 5G, call control is IMS).

Signal Flow¶

Handover-based and Redirect-based¶

The EPS Fallback transition method is divided into two by the presence of the N26 interface.

Comparison with EPC¶

• In 4G: voice on LTE is completed from the start with VoLTE (IMS + a QCI=1 dedicated bearer). As a mechanism to drop to another RAT, there historically existed CS Fallback, which dropped to circuit-switched 3G, etc., when VoLTE was not supported (early 4G). EPS Fallback can be understood as inheriting this CS Fallback idea of "moving to the proven RAT only for voice," in the 5G→4G context.
• In 5G: when a voice call arises in a VoNR-unsupported cell, EPS Fallback moves the UE to 4G(EPS) and completes it via VoLTE (QCI=1). The identifier correspondence is 5QI=1 ↔ QCI=1 (conversational voice, GBR) and 5QI=5 ↔ QCI=5 (IMS signaling, Non-GBR). That call control is IMS is common to 4G/5G.

Release differences¶

• Rel-15: in early 5G deployment, VoNR-unsupported areas were numerous, and EPS Fallback was positioned as virtually essential in practice. The procedure is specified in TS 23.502 §4.13.6.1.
• Rel-16 onward: with the expansion of VoNR support, the importance of EPS Fallback trends downward going forward. However, the individual feature additions and revisions are to be confirmed against each TS (this chapter does not assert them).

Trouble Shooting¶

3GPP Specification¶

• 3GPP TS 23.502 §4.13.6.1 — the procedure of EPS Fallback for IMS voice (the main basis for this chapter)
• 3GPP TS 23.501 §5.7.4 — the standardized 5QI table (5QI=1 conversational voice/GBR, 5QI=5 IMS signaling/Non-GBR)
• 3GPP TS 23.501 / 23.502 — the specification of 4G/5G Interworking and N26 (AMF⇔MME) (detailed section numbers to be confirmed)
• 3GPP TS 23.228 — IMS (voice call control; call control common to 4G/5G. This chapter gives only an outline; to be referenced)

FAQ¶

Q. What kind of mechanism is EPS Fallback, in the end? A. It is the mechanism that, when a voice call arises in a VoNR-unsupported cell, temporarily moves the UE from 5GS(NR) to 4G/EPS(LTE) and completes voice via the proven VoLTE (QCI=1). It is specified in 3GPP TS 23.502 §4.13.6.1.

Q. What triggers EPS Fallback? A. It is triggered when the establishment of an IMS voice call causes a request to establish a voice QoS Flow (5QI=1, GBR), and the NG-RAN that receives it determines "this cell does not support VoNR." The trigger itself is common with VoNR.

Q. What is the difference between Handover-based and Redirect-based? A. It is the presence of N26 (AMF⇔MME). With N26, the state is carried over and it transitions smoothly via an Inter-RAT Handover (Handover-based); without it, the UE is redirected to LTE via RRC Release and made to reconnect (Redirect-based). Generally the Handover-based method is faster and smoother.

Q. Is N26 mandatory? A. No, it is not mandatory. With N26, a smooth Handover-based transition is possible, but even without N26, EPS Fallback can be realized in a Redirect-based manner. It is a design choice.

Q. Does it return to 5G after the voice ends? A. It is not uniformly decided in the spec; whether it returns depends on the radio-side mobility policy (implementation-dependent).

Summary¶

• EPS Fallback is the mechanism that, at the time of voice call establishment in areas where VoNR is not offered, moves the UE from 5GS(NR)→4G/EPS(LTE) and completes voice via VoLTE (QCI=1) (3GPP TS 23.502 §4.13.6.1).
• The trigger is a request to establish a voice QoS Flow (5QI=1) (common with VoNR); the branch that triggers it through the NG-RAN's determination of no VoNR support is specific to EPS Fallback.
• There are two transition methods: Handover-based (with N26, state carried over) and Redirect-based (without N26, guided via RRC Release).
• N26 (AMF⇔MME) carries over the mobility context between 4G/5G and governs the transition method.
• The identifiers correspond as 5QI=1↔QCI=1 and 5QI=5↔QCI=5, and call control stays on IMS, common to 4G/5G.

Practice — comprehension check and exercises¶

Comprehension check¶

Exercises¶

• For a VoNR-supported cell and a VoNR-unsupported cell, try drawing a diagram of how far the Call Flow at voice origination is common and where it branches (hint: common up to the 5QI=1 request, branching at the NG-RAN's support determination).
• For the two cases of with/without N26, lay the EPS Fallback sequences side by side, compare them, and summarize the difference in the number of steps until voice is established.

Next Step¶

• 4G-5G Interworking (N26 / interworking) — the general framework of 4G⇔5G interworking that contains EPS Fallback
• VoNR (Voice over NR) — the ideal form that makes EPS Fallback unnecessary. Deepen the voice call establishment flow before the branch
• Handover — the details of the Inter-RAT Handover used by the Handover-based transition
• QoS (the 5G QoS model) — return to and deepen the correspondence of 5QI↔QCI and QoS Flow↔EPS bearer