GFBR / MFBR / Session-AMBR / UE-AMBR / ARP — Bandwidth, Priority and the "Which Priority?" Problem¶

Learning objectives¶

After reading this chapter, you will be able to:

• Explain that GFBR / MFBR are parameters of GBR QoS Flows only, and that Non-GBR bandwidth is controlled by Session-AMBR / UE-AMBR
• Explain the difference between Session-AMBR and UE-AMBR in scope (PDU Session / UE) and enforcement point (UE / NG-RAN / UPF)
• Explain that a 5QI is not a bandwidth value, and that bandwidth is carried separately by GFBR / MFBR / AMBR
• Explain that the three elements of ARP (Priority Level / Pre-emption Capability / Pre-emption Vulnerability) are for admission / pre-emption and, per 3GPP, do not represent a packet scheduling priority (treatment in transport DSCP or RAN implementations is a separate matter)
• When you see "Priority" in a trace or log, identify whether it is ARP / 5QI / Precedence / Paging / vendor-internal
• Isolate "low throughput," "a GBR Flow cannot be created" and "high ARP but still slow" per parameter and per interface

Prerequisites¶

• QoS Flow / QFI / 5QI — basics of QoS Flow, QFI and 5QI (this chapter keeps them to a minimum)
• QoS — the overall 5G QoS model (GBR / Non-GBR, QoS profile / QoS rules / QER)
• PFCP and PDR / FAR / QER / URR — the QER on the UPF side
• Supplementary: PCF / N7 / N10 / N2 / QoS Dictionary

Why — "decide the 5QI and the QoS is decided" is not true¶

If a QoS design meeting only decides "voice uses 5QI=1," the following questions remain unanswered:

• How many kbps are guaranteed for that voice Flow?
• How far beyond the guarantee may it go?
• When the radio is busy and a new voice Flow cannot be admitted, may an existing Flow be evicted?
• How much may Non-GBR Internet traffic use across the whole PDU Session, and across the whole UE?

A 5QI is a number that indicates "which QoS characteristics"; it carries neither a bandwidth value nor a priority for admission or eviction. Separate parameters (GFBR / MFBR / Session-AMBR / UE-AMBR / ARP) take care of those (TS 23.501 §5.7.2).

Analogy: The 5QI is the "seat class (reserved / unreserved)," GFBR is the "number of seats already reserved," MFBR is the "maximum number of seats that group may use," Session-AMBR / UE-AMBR is the "cap on how many people may sit in unreserved seats (per group / per family)," and ARP is the rule for "when the train is full, may someone be asked to get off to let a later group board (and is this group one that can be asked to get off)?" The role of ARP in the specifications is not to decide "how fast the train runs."

What — the overall picture of the parameters¶

Resource Types and the parameters they have¶

There are three Resource Types for a QoS Flow: GBR / Delay-critical GBR / Non-GBR (TS 23.501 §5.7.3.2). Which bandwidth parameters a Flow has depends on this classification.

Viewed as a hierarchy¶

flowchart TD
    UE["UE
UE-AMBR (cap on the Non-GBR sum)"] --> S1["PDU Session A
Session-AMBR"]
    UE --> S2["PDU Session B
Session-AMBR"]
    S1 --> F1["QoS Flow (Non-GBR)
5QI / ARP"]
    S1 --> F2["QoS Flow (Non-GBR)
5QI / ARP"]
    S1 --> F3["QoS Flow (GBR)
5QI / ARP / GFBR / MFBR"]
    S2 --> F4["QoS Flow (Non-GBR)
5QI / ARP"]

• The sum F1 + F2 is constrained by Session A's Session-AMBR, and the sum F1 + F2 + F4 by the UE-AMBR.
• F3 (GBR) is counted toward neither AMBR; it is handled by its own GFBR / MFBR.

What — GFBR / MFBR (GBR QoS Flows only)¶

TS 23.501 §5.7.2.5 defines GFBR (UL / DL) and MFBR (UL / DL) as additional parameters that only GBR QoS Flows have.

Bit Rate
  ▲
  │  ─────────────  MFBR  … anything above this is subject to rate control
  │       ↑
  │       │  GFBR to MFBR: flows if there is room
  │       │  (sharing among QoS Flows is decided by the 5QI Priority Level)
  │  ─────────────  GFBR  … guaranteed by the network
  │       ↑ guaranteed range
  └───────────────────────────►

• "GFBR = 5 Mbps" does not mean "no more than 5 Mbps." The cap is the MFBR.
• Conversely, "GFBR = 5 Mbps" does not mean "the user can always measure 5 Mbps." The guarantee is an average over the Averaging Window, and nothing flows if the application sends nothing. A separate mechanism (Notification Control, TS 23.501 §5.7.2.4) exists for notifying when the GFBR can no longer be maintained, e.g. because radio conditions deteriorate.
• UL and DL can be set independently.

What — Session-AMBR and UE-AMBR (caps for Non-GBR groups)¶

Session-AMBR¶

Session-AMBR is the cap on the bit rate that can be expected for the sum of all Non-GBR QoS Flows of a PDU Session (TS 23.501 §5.7.2.6).

PDU Session 10 (Session-AMBR DL = 100 Mbps)
├─ QFI 1  Non-GBR   40 Mbps ┐
├─ QFI 2  Non-GBR   30 Mbps ├─ the cap is reached at 100 Mbps in total
├─ QFI 3  Non-GBR   30 Mbps ┘
└─ QFI 4  GBR       managed separately by GFBR/MFBR (not counted toward Session-AMBR)

• Each QFI does not get 100 Mbps. It is a cap on the aggregate.
• Where the value comes from: the SMF either uses the subscribed Session-AMBR from the UDM (N10, SM subscription data via Nudm_SDM) as is, modifies it based on local policy, or uses the authorized Session-AMBR from the PCF (N7) (TS 23.501 §5.7.2.6).
• The SMF signals the decided Session-AMBR to the UPF, the UE and the (R)AN. It is signalled to the (R)AN so that the (R)AN can calculate the UE-AMBR, described below (same clause).

UE-AMBR¶

UE-AMBR is the cap on the bit rate that can be expected for the sum of all Non-GBR QoS Flows of all PDU Sessions of a UE (TS 23.501 §5.7.2.6).

• Where the value comes from: the AMF provides it to the (R)AN based on the subscribed UE-AMBR from the UDM (N8, Access and Mobility Subscription data) or a serving-network UE-AMBR obtained from the PCF (e.g. for roaming subscribers; the Release in which it was introduced is to be confirmed). The UE-AMBR is provided by the AMF to the (R)AN, and the SMF does not handle it (TS 23.501 §5.7.2.6).
• The (R)AN sets its UE-AMBR to the smaller of "the sum of the Session-AMBRs of all PDU Sessions with an active user plane at that (R)AN" and "the received UE-AMBR" (TS 23.501 §5.7.2.6).

Example: received UE-AMBR = 300 Mbps
         active PDU Sessions: Session-AMBR 100 Mbps + 50 Mbps = 150 Mbps
         → UE-AMBR actually used by the (R)AN = min(150, 300) = 150 Mbps

What — differences in enforcement point (UE / RAN / UPF)¶

"Who actually limits the rate" differs per parameter. The following summarizes TS 23.501 §5.7.1.8 (AMBR / MFBR enforcement and rate limitation; the sub-clause number is to be confirmed) and §5.7.2.6.

flowchart LR
    UE["UE
UL: rate limitation
with Session-AMBR"] -- "Uu" --> RAN["NG-RAN
UL/DL: UE-AMBR
GFBR assurance and MFBR for GBR Flows"]
    RAN -- "N3" --> UPF["UPF
Session-AMBR (QER)
MFBR / per-SDF MBR (QER)"]
    UPF -- "N6" --> DN["DN"]

The MFBR enforcement points in the diagram are to be confirmed, as shown in the table below.

What — a 5QI is not a bandwidth value¶

A 5QI points to QoS characteristics such as Resource Type / Priority Level / Packet Delay Budget / Packet Error Rate / Averaging Window / Maximum Data Burst Volume (TS 23.501 §5.7.3; standardized 5QIs are in the table in §5.7.4). No bit rate value is included anywhere.

5QI = X
 ├─ Resource Type        … GBR / Delay-critical GBR / Non-GBR
 ├─ Priority Level       … relative scheduling priority (not ARP)
 ├─ Packet Delay Budget
 ├─ Packet Error Rate
 └─ Averaging Window etc. … GBR types only
      ✕ no bandwidth value → GFBR / MFBR / AMBR carry it separately

For the concrete characteristic values of each 5QI, refer to the table in TS 23.501 §5.7.4 (this chapter does not list the values). The basics of 5QI and QFI are covered in QoS Flow / QFI / 5QI.

What — ARP (Allocation and Retention Priority)¶

The three elements of ARP (TS 23.501 §5.7.2.2)¶

The same clause states that Priority Levels 1 to 8 should only be assigned to services authorized to receive prioritized treatment within the serving network, and 9 to 15 to resources authorized by the home network and thus applicable when the UE is roaming.

flowchart TD
    N["Request for a new QoS Flow
(ARP Priority Level = 2, Capability = enabled)"] --> A{"Does the (R)AN
have spare resources?"}
    A -- "Yes" --> OK["Admitted (ARP plays almost no role)"]
    A -- "No" --> B{"Is there an existing Flow
with a lower Priority Level
and Vulnerability = enabled?"}
    B -- "Yes" --> P["Pre-empt the existing Flow and admit"]
    B -- "No" --> R["Reject the new Flow
(admission failure)"]

The selection algorithm (which existing Flow to choose, what counts as "resource shortage") is (R)AN implementation dependent.

ARP is not a scheduling priority (its purpose in the specifications)¶

In 3GPP, the purpose of ARP is the decision to admit / retain / evict a Flow (admission / pre-emption) (TS 23.501 §5.7.2.2). The scheduling priority among QoS Flows is carried by the 5QI Priority Level (§5.7.3.3), and the treatment of packets after admission is decided mainly by the 5QI QoS characteristics (Priority Level, Packet Delay Budget, etc.) and GFBR / MFBR, together with the (R)AN scheduler.

Two caveats apply, however:

• DSCP marking on the N3 transport network may be based on the 5QI and ARP (TS 23.501 §5.7.1.6 / §5.7.1.7, to be confirmed), so in deployments that use an ARP → DSCP mapping, the treatment in the transport network can change with ARP. The UPF does not receive ARP itself; the SMF decides the DL DSCP value and passes it to the UPF in the PFCP FAR (Transport Level Marking in the Forwarding Parameters) (TS 29.244 §8.2.12, to be confirmed).
• Whether the (R)AN scheduler refers to ARP is implementation dependent.

ARP is not sent to the UE¶

ARP is used mainly for (R)AN admission / pre-emption and as a binding parameter when the SMF binds PCC Rules to a QoS Flow (TS 23.501 §5.7.1.5 / TS 23.503 §6.1.3.2.4, to be confirmed). It is included in the NGAP QoS Flow Level QoS Parameters, but not in the NAS QoS flow description (to the UE) nor in the PFCP QER (to the UPF) (TS 24.501 §9.11.4.12 / TS 29.244 §7.5.2.5, to be confirmed). "There is no ARP in the UE log" is normal.

How — when you see "Priority," which priority is it?¶

"Priority = 1" alone is not enough information. Before discussing anything, identify at least which of the following it is.

Analogy: ARP is the membership rank that decides "can you get into a full restaurant, or will someone give up their seat," the 5QI Priority Level is "the order in which dishes are served after you sit down," and Precedence is "which reception desk's rule is applied first." A high membership rank does not mean your food arrives sooner.

How — the flow PCF → SMF → RAN / UPF / UE¶

Call Flow (QoS decision and distribution during PDU Session Establishment)¶

sequenceDiagram
    participant UDM
    participant AMF
    participant SMF
    participant PCF
    participant UPF
    participant gNB
    participant UE
    Note over AMF,UDM: At Registration: the AMF obtains the subscribed UE-AMBR (N8)
    SMF->>UDM: Nudm_SDM_Get (SM subscription data)
    UDM-->>SMF: subscribed Session-AMBR, default 5QI / ARP
    opt Dynamic PCC
        SMF->>PCF: Npcf_SMPolicyControl_Create
        PCF-->>SMF: SM Policy Decision
(authorized Session-AMBR, default QoS, PCC Rules, QoS data)
    end
    Note over SMF: Decide 5QI / ARP / GFBR / MFBR per QoS Flow
Decide the Session-AMBR
    SMF->>UPF: PFCP Session Establishment
Create QER (Session-AMBR / MFBR / GFBR / QFI)
    UPF-->>SMF: Response
    SMF->>AMF: Namf_Communication_N1N2MessageTransfer
N2 SM (PDU Session AMBR, QoS Flow Level QoS Parameters)
N1 SM (Session-AMBR, QoS rules, QoS flow descriptions)
    AMF->>gNB: PDU Session Resource Setup Request
(UE Aggregate Maximum Bit Rate if needed)
    Note over gNB: Admission decision using ARP
Calculate the UE-AMBR
    gNB->>UE: DRB setup via RRC + NAS (PDU Session Establishment Accept)
    gNB-->>AMF: PDU Session Resource Setup Response

The whole procedure is TS 23.502 §4.3.2.2.1 (UE-requested PDU Session Establishment, non-roaming). For how N1 / N2 / N4 connect, see PDU Session Establishment Deep Dive. GBR Flows are often added after PDU Session establishment by PDU Session Modification (TS 23.502 §4.3.3.2) (e.g. the voice Flow in VoNR). What the PCF decides and how is covered in the next chapter, PCF / SM Policy Association / PCC Rule.

Signal Flow — which IE on which segment carries each parameter¶

IE and attribute names are conceptual correspondences; exact names and structures are to be confirmed per TS version. The N7 QoS data types are defined under TS 29.512 §5.6.2, and the common data types (ARP, BitRate, etc.) under TS 29.571 §5 (sub-clause numbers to be confirmed).

In the UPF, QERs express both the AMBR and the MFBR¶

As seen in PFCP and PDR / FAR / QER / URR, one PDR can reference multiple QERs. The SMF typically uses this to layer a per-QoS-Flow QER and a per-PDU-Session QER. That the MBR of a QER can represent the Session-AMBR / the QoS Flow MBR / the SDF MBR is shown in TS 23.501 §5.8.2.11.4 (QoS Enforcement Rule, to be confirmed).

DL PDR (Non-GBR traffic of QFI 1)
 ├─ QER #10  QFI=1                   ← DL QFI marking
 └─ QER #1   MBR = Session-AMBR      ← shared by all Non-GBR PDRs in the Session

DL PDR (GBR traffic of QFI 4)
 └─ QER #20  QFI=4, MBR=MFBR, GBR=GFBR   ← does not reference the Session-AMBR QER

How QERs are allocated (how shared QERs are built, how per-SDF MBRs are layered) is SMF implementation dependent.

Value representations differ per segment¶

The same 100 Mbps is displayed as 100000 on N4 and 100000000 on N2. Align the units before comparing.

Packet Analysis (Wireshark)¶

Display filters¶

Each of N7 / N2 / N1 / N4 is only one cross-section of "the moment QoS was decided." The trick is to line up the four segments for the same PDU Session in the same time window.

Decode example (conceptual)¶

The following is a conceptual representation of the display structure; the values are fictitious (capture needed).

NGAP PDUSessionResourceSetupRequest
 └─ PDUSessionResourceSetupRequestTransfer
     ├─ PDUSessionAggregateMaximumBitRate
     │   ├─ DL: 100000000 bits/s
     │   └─ UL:  50000000 bits/s
     └─ QosFlowSetupRequestList
         ├─ QosFlowSetupRequestItem
         │   ├─ qosFlowIdentifier: 1
         │   └─ qosFlowLevelQosParameters
         │       ├─ qosCharacteristics: nonDynamic5QI (5QI=9)
         │       └─ allocationAndRetentionPriority
         │           ├─ priorityLevelARP: 8
         │           ├─ pre-emptionCapability: shall-not-trigger-pre-emption
         │           └─ pre-emptionVulnerability: pre-emptable
         └─ QosFlowSetupRequestItem
             ├─ qosFlowIdentifier: 4
             └─ qosFlowLevelQosParameters
                 ├─ qosCharacteristics: nonDynamic5QI (5QI=1)
                 ├─ allocationAndRetentionPriority ...
                 └─ gBR-QosInformation
                     ├─ maximumFlowBitRateDL / UL
                     └─ guaranteedFlowBitRateDL / UL

PFCP Session Establishment Request
 ├─ Create QER
 │   ├─ QER ID: 1
 │   ├─ Gate Status: UL=OPEN, DL=OPEN
 │   └─ MBR: UL=50000 kbps, DL=100000 kbps     ← Session-AMBR
 └─ Create QER
     ├─ QER ID: 20
     ├─ Gate Status: UL=OPEN, DL=OPEN
     ├─ MBR: UL=…, DL=…                        ← MFBR
     ├─ GBR: UL=…, DL=…                        ← GFBR
     └─ QFI: 4

Order of checks¶

flowchart LR
    A["① N7
values authorized by the PCF"] --> B["② N4
QER MBR / GBR"]
    A --> C["③ N2
AMBR / ARP / GFBR / MFBR"]
    C --> D["④ N2 Response
any Failed to Setup?"]
    A --> E["⑤ N1
Session-AMBR / QoS flow description to the UE"]

Check whether the values decided by the PCF (①) reach the UPF (②), the RAN (③) and the UE (⑤) with the same meaning. If a Flow sent in ③ appears in the QoS Flow Failed to Setup List in ④, it was not admitted on the RAN side.

Trouble Shooting¶

Isolation order for "throughput is low"¶

flowchart TD
    S["Throughput lower than expected"] --> T{"Is the traffic on a GBR Flow
or a Non-GBR Flow?"}
    T -- "GBR" --> G["MFBR / per-SDF MBR
(N2 and QER)"]
    T -- "Non-GBR" --> U["UE-AMBR
(N2 and the RAN's effective value)"]
    U --> SA["Session-AMBR
(N1 / N2 / QER)"]
    G --> Q["QER Gate Status / MBR"]
    SA --> Q
    Q --> R["RAN QoS and radio quality"]
    R --> D["Limits in transport and on the DN side"]

The key is to first establish "which QoS Flow carries this traffic (GBR or Non-GBR)." Looking for the Session-AMBR for traffic on a GBR Flow will not give you an answer. For checking PDR matching and the QFI, see PDR / PDI / F-TEID / SDF Filter / Precedence.

Boundary between 3GPP specifications and vendor implementations¶

For example, "which hardware queue in the UPF polices the MFBR, and how" is outside the scope of 3GPP.

Comparison with EPC (4G)¶

The biggest difference is APN-AMBR → Session-AMBR: 4G caps "per APN (multiple PDN connections to the same APN together)," whereas 5G caps "per PDU Session" (the 4G provision is TS 23.401 §4.7.3, to be confirmed).

3GPP Specification¶

Summary¶

• GFBR / MFBR are parameters of GBR QoS Flows only (including Delay-critical GBR). GFBR is the guarantee and MFBR the cap; sharing between them is decided by the 5QI Priority Level
• Non-GBR bandwidth is capped by Session-AMBR (per PDU Session) and UE-AMBR (per UE). Traffic on GBR Flows is not counted toward the AMBR
• Enforcement: Session-AMBR = UE (UL) and UPF, UE-AMBR = (R)AN. The AMF passes the UE-AMBR to the (R)AN, and it never appears in PFCP
• A 5QI is not a bandwidth value. Bandwidth is carried separately by GFBR / MFBR / AMBR
• ARP has three elements — Priority Level (1 to 15), Pre-emption Capability and Pre-emption Vulnerability — and its purpose in the specifications is admission / pre-emption. Scheduling priority among QoS Flows is carried by the 5QI Priority Level. ARP is not sent to the UE or the UPF (with caveats: it may be used for N3 DSCP marking, and its treatment in RAN implementations varies)
• When you see "Priority," identify whether it is ARP / 5QI / Precedence / Paging / vendor-internal. The units of values (kbps / bps) also differ per segment

Check your understanding¶

Next Step¶

• Previous chapter: basics of QoS Flow, QFI and 5QI → QoS Flow / QFI / 5QI
• Next chapter: how the PCF decides these values and passes them to the SMF → PCF / SM Policy Association / PCC Rule
• The QER that executes the values in the UPF → PFCP and PDR / FAR / QER / URR / PDR / PDI / F-TEID / SDF Filter / Precedence
• Where the values come from (subscription data) → N10 / N8 / UDM
• Procedures that add or modify GBR Flows → PDU Session Modification / VoNR
• Overview and terminology → QoS / QoS Dictionary / PCF / N2 / N4 / N7