QoS & Bearer Management

Overview

The PGW-C implements a policy-driven bearer and QoS management system that coordinates three key interfaces:

Gx (Diameter) - Receives policy decisions and QoS parameters from PCRF
S5/S8 (GTP-C) - Manages bearer contexts with SGW-C
Sxb (PFCP) - Programs QoS enforcement rules into PGW-U

Architecture Flow

Key Concepts

Session: Contains UE information, bearer map, PDR/FAR/QER/BAR maps, and AMBR
Bearer Context: Links EBI (EPS Bearer ID) to specific PDRs, FARs, and QERs
QER (QoS Enforcement Rule): Enforces MBR/GBR limits and gate status in the user plane
Default Bearer: Always created with PDN session, provides basic connectivity
Dedicated Bearer: Created dynamically based on PCRF policy, provides specific QoS guarantees

Configuration

Important: Dynamic QoS Policy

All QoS parameters are dynamically received from the PCRF via Diameter Gx interface and defined in the PCRF (See OmniHSS for more info).

Operators configure the PCRF connection in config/runtime.exs:

config :pgw_c,
  diameter: %{
    listen_ip: "0.0.0.0",
    host: "omni-pgw_c.epc.mnc999.mcc999.3gppnetwork.org",
    realm: "epc.mnc999.mcc999.3gppnetwork.org",
    peer_list: [
      %{
        host: "pcrf.epc.mnc999.mcc999.3gppnetwork.org",
        realm: "epc.mnc999.mcc999.3gppnetwork.org",
        ip: "192.168.1.100",
        initiate_connection: true
      }
    ]
  }

QoS policies, charging rules, and bandwidth limits are configured on the PCRF, not in PGW-C configuration files.

Bearer Lifecycle

Default Bearer Creation

The default bearer is created during PDN session establishment:

Workflow:

SGW-C sends Create Session Request
PGW-C allocates UE IP address from configured pool
PGW-C sends CCR-Initial to PCRF with IMSI, APN, IP address
PCRF responds with CCA-Initial containing QoS parameters:
- Default-EPS-Bearer-QoS (QCI, ARP)
- QoS-Information (AMBR adjustments)
PGW-C creates bearer context with:
- Fixed IDs: Downlink PDR=1, Uplink PDR=2, Downlink FAR=1, Uplink FAR=2, QER=1, BAR=1
- QER programmed with MBR from bearer QoS
PGW-C sends PFCP Session Establishment Request to PGW-U
PGW-C sends Create Session Response to SGW-C

Default bearer characteristics:

Always exists for the lifetime of the PDN session
Typically uses QCI 5 or QCI 9 (non-GBR)
EBI tracked in session state
Cannot be deleted independently (deleting it terminates the session)

Dedicated Bearer Creation

Dedicated bearers are created dynamically based on PCRF policy:

Trigger: Re-Auth Request (RAR) from PCRF with Charging-Rule-Install

Workflow:

PCRF sends RAR with Charging-Rule-Definition containing:
- Charging-Rule-Name (policy rule identifier)
- Flow-Information (packet filters)
- QoS-Information (QCI, MBR, GBR, ARP)
- Precedence (rule matching priority)
PGW-C translates dynamic rule to PFCP entities:
- Each Flow-Information entry → new PDR with SDF Filter
- QoS-Information → new QER with MBR/GBR enforcement
- Flow-Description → IP 5-tuple matching rules
PGW-C sends PFCP Session Modification Request to add PDRs/FARs/QERs
PGW-C initiates Create Bearer Request to SGW-C
SGW-C responds with Create Bearer Response confirming establishment

Example Charging-Rule-Definition:

Charging-Rule-Name: "video_streaming"
Flow-Information:
  - Flow-Description: "permit in ip from any to 10.0.0.1 5000-6000"
    Flow-Direction: 1 (downlink)
QoS-Information:
  QoS-Class-Identifier: 7
  Max-Requested-Bandwidth-UL: 5000000 (5 Mbps)
  Max-Requested-Bandwidth-DL: 10000000 (10 Mbps)
  Guaranteed-Bitrate-UL: 1000000 (1 Mbps)
  Guaranteed-Bitrate-DL: 2000000 (2 Mbps)
Precedence: 100
Flow-Status: 2 (ENABLED)

Bearer Modification

Bearer QoS can be modified via:

Gx RAR with updated Charging-Rule-Definition
PFCP Session Modification to update existing QERs (change bitrates), FARs (change forwarding), or PDRs (change packet filters)

Bearer Deletion

Triggers:

Delete Session Request (SGW-initiated) - Deletes default bearer and terminates session
Re-Auth Request with Charging-Rule-Remove (PCRF-initiated) - Deletes dedicated bearer

Workflow:

Remove bearer from session state
Remove associated PDRs/FARs/QERs
Send Delete Bearer Request to SGW-C (if PCRF-initiated)
Send PFCP Session Modification (remove rules) or Session Deletion (if default bearer)

QoS Parameters

QCI (QoS Class Identifier)

Source: PCRF via Gx QoS-Class-Identifier AVP

Standard Values:

QCI 1: Conversational Voice (GBR, 100ms delay budget)
QCI 2: Conversational Video (GBR, 150ms delay budget)
QCI 3: Real Time Gaming (GBR, 50ms delay budget)
QCI 4: Non-Conversational Video (GBR, 300ms delay budget)
QCI 5: IMS Signaling (non-GBR, 100ms delay budget) - Default for default bearer
QCI 6: Video (TCP-based), Live Streaming (non-GBR, 300ms delay budget)
QCI 7: Voice, Interactive Gaming (non-GBR, 100ms delay budget)
QCI 8: Video (TCP-based), e.g., YouTube (non-GBR, 300ms delay budget)
QCI 9: Default Internet (non-GBR, 300ms delay budget)

Operator Note:

QCI is received from PCRF and signaled to SGW-C in Bearer-Level-QoS IE
PGW-C does not directly enforce QCI behavior - actual enforcement is via MBR/GBR in QERs
Lower QCI values typically indicate higher priority
QCI determines packet forwarding treatment and scheduling priority

ARP (Allocation and Retention Priority)

Source: PCRF via Allocation-Retention-Priority grouped AVP

Components:

Priority-Level: 1 (highest priority) to 15 (lowest priority)
Pre-emption-Capability: Can this bearer pre-empt lower-priority bearers?
- 0 = ENABLED (can pre-empt others)
- 1 = DISABLED (cannot pre-empt)
Pre-emption-Vulnerability: Can this bearer be pre-empted by higher-priority bearers?
- 0 = ENABLED (can be pre-empted)
- 1 = DISABLED (cannot be pre-empted)

Default Values:

Priority-Level: 1
Pre-emption-Capability: ENABLED (0)
Pre-emption-Vulnerability: DISABLED (1)

Operator Note:

ARP is signaled to SGW-C and ultimately to eNodeB
Not enforced by PGW-C - enforcement is typically at eNodeB during radio admission control
Used during network congestion to determine which bearers to admit or drop
Critical for emergency services (priority-level 1) and high-value services

MBR (Maximum Bit Rate)

Source: PCRF via Max-Requested-Bandwidth-UL and Max-Requested-Bandwidth-DL AVPs

Format: Bytes per second (converted to kbps internally: bytes / 1000)

Applied to: All bearers (default and dedicated)

How it works:

PGW-C creates QER with mbr: %Bitrate{ul: kbps_ul, dl: kbps_dl}
QER sent to PGW-U via PFCP
PGW-U enforces rate limiting (traffic policing)
Excess traffic above MBR is dropped

Example:

Max-Requested-Bandwidth-UL: 5000000 (5 Mbps)
Max-Requested-Bandwidth-DL: 10000000 (10 Mbps)

→ QER created with mbr: {ul: 5000, dl: 10000} kbps
→ PGW-U drops uplink packets exceeding 5 Mbps
→ PGW-U drops downlink packets exceeding 10 Mbps

GBR (Guaranteed Bit Rate)

Source: PCRF via Guaranteed-Bitrate-UL and Guaranteed-Bitrate-DL AVPs

Format: Bytes per second (converted to kbps)

Applied to: Dedicated bearers only (GBR bearers)

How it works:

If GBR is specified in Charging-Rule-Definition, bearer is GBR type
PGW-U enforces minimum bitrate guarantee via QER
Requires proper scheduling at eNodeB to reserve radio resources
GBR bearers have admission control - can be rejected if resources unavailable

Example:

Guaranteed-Bitrate-UL: 1000000 (1 Mbps)
Guaranteed-Bitrate-DL: 2000000 (2 Mbps)

→ QER created with gbr: {ul: 1000, dl: 2000} kbps
→ Network guarantees at least 1 Mbps uplink and 2 Mbps downlink
→ Used for VoIP, video calls, live streaming

Operator Note:

GBR requires sufficient network capacity planning
Oversubscribing GBR resources leads to admission failures
Monitor GBR usage via session counts and bearer metrics

AMBR (Aggregate Maximum Bit Rate)

Source: PCRF via APN-Aggregate-Max-Bitrate-UL and APN-Aggregate-Max-Bitrate-DL AVPs

Scope: Applies to all non-GBR bearers for the APN (not per-bearer)

How it works:

AMBR is an aggregate limit across all non-GBR bearers in a session
Sent to SGW-C in Create Session Response
Enforcement typically at eNodeB/SGW
PGW-C stores AMBR in session state and signals it to SGW-C

Example:

APN-Aggregate-Max-Bitrate-UL: 50000000 (50 Mbps)
APN-Aggregate-Max-Bitrate-DL: 100000000 (100 Mbps)

→ All non-GBR bearers combined cannot exceed 50 Mbps uplink / 100 Mbps downlink
→ Individual bearers limited by their own MBR
→ AMBR provides additional overall cap per UE/APN

Operator Note:

Set via subscriber profile in HSS/PCRF
Used to enforce subscription tiers (e.g., 10 Mbps plan vs 100 Mbps plan)
Does not affect GBR bearers

Flow Status and Gating

Flow Status (Gx) to Gate Status (PFCP) Mapping

The PCRF controls whether traffic is allowed via the Flow-Status AVP in Charging-Rule-Definition:

Flow-Status (Gx)	Gate-Status (PFCP QER)	Meaning
0 = ENABLED-UPLINK	ul: OPEN, dl: CLOSED	Only uplink traffic allowed
1 = ENABLED-DOWNLINK	ul: CLOSED, dl: OPEN	Only downlink traffic allowed
2 = ENABLED	ul: OPEN, dl: OPEN	Both directions allowed
3 = DISABLED	ul: CLOSED, dl: CLOSED	No traffic allowed
4 = REMOVED	ul: CLOSED, dl: CLOSED	Bearer being deleted

Use cases:

DISABLED: Used for parked services or credit exhaustion (packets dropped but bearer retained)
ENABLED-UPLINK: Unusual, but could be used for upload-only services
ENABLED-DOWNLINK: Download-only services or credit-limited scenarios
ENABLED: Normal operation

Monitoring & Observability

Prometheus Metrics

Session-level metrics:

session_registry_count        # Active bearers (IMSI, EBI pairs)
address_registry_count        # Allocated UE IPs
charging_id_registry_count    # Active charging sessions

Gx interface metrics:

gx_inbound_messages_total{message_type="gx_RAR"}     # Policy updates from PCRF
gx_outbound_messages_total{message_type="gx_CCR"}    # Policy requests to PCRF
gx_outbound_transaction_duration_bucket              # Latency to PCRF

PFCP interface metrics:

sxb_outbound_messages_total{message_type="pfcp_session_establishment_request"}
sxb_outbound_messages_total{message_type="pfcp_session_modification_request"}
sxb_outbound_transaction_duration_bucket

Bearer creation metrics:

s5s8_inbound_messages_total{message_type="create_session_request"}   # Default bearers
s5s8_outbound_messages_total{message_type="create_bearer_request"}   # Dedicated bearers

Web UI Monitoring

PGW Sessions Page (/pgw_sessions):

Search by IMSI, IP address, MSISDN, or APN
View active bearers per session
Inspect bearer QoS parameters (QCI, MBR, GBR, AMBR)
Real-time auto-refresh (2 seconds)

Diameter Page (/diameter):

PCRF peer connectivity status
Gx session count
Peer state (connected/disconnected)

Logs Page (/logs):

Real-time log streaming
Filter by "Credit Control" for CCR/CCA exchanges
Filter by "Re-Auth" for RAR events (policy changes)
Filter by "PFCP" for user plane programming events

Key Log Messages

[debug] Sending Credit Control Request: ...          # CCR to PCRF
[debug] Handling Credit Control Answer: ...          # CCA from PCRF (contains QoS)
[debug] Handling Re-Auth Request                     # RAR from PCRF (policy change)
[debug] Sending Session Establishment Request        # PFCP to PGW-U (program QERs)
[debug] Sending Session Modification Request         # PFCP to PGW-U (update QERs)

Operational Tasks

Verify QoS Applied to Session

Access Web UI → PGW Sessions page
Search for IMSI (e.g., 999000123456789)
Expand session details

Check qer_map section:

qer_id: 1
gate_status: {ul: OPEN, dl: OPEN}
mbr: {ul: 50000, dl: 100000}  # kbps
gbr: {ul: 10000, dl: 20000}   # kbps (or nil for non-GBR)

Verify values match expected PCRF policy

Troubleshoot Missing QoS

Symptom: Session created but QoS not applied

Steps:

Check PCRF connectivity:
- Access Web UI → Diameter page
- Verify PCRF peer status = "connected"
- If disconnected, check network connectivity and Diameter configuration
Verify CCR/CCA exchange:
- Access Web UI → Logs page
- Search for "Credit Control Answer"
- Verify QoS-Information AVP present in CCA log
- Check for errors in CCA (Result-Code should be 2001 = SUCCESS)
Verify PFCP programming:
- Search logs for "PFCP Session Establishment Request"
- Verify QER included in message
- Check PGW-U logs for PFCP processing errors
Check PCRF policy configuration:
- Verify subscriber profile in PCRF
- Confirm APN-specific policy rules exist
- Check PCRF logs for policy evaluation errors

Monitor Bearer Creation Rate

Prometheus queries:

# Default bearer creation rate (sessions/second)
rate(s5s8_inbound_messages_total{message_type="create_session_request"}[5m])

# Dedicated bearer creation rate
rate(s5s8_outbound_messages_total{message_type="create_bearer_request"}[5m])

# Policy update rate from PCRF
rate(gx_inbound_messages_total{message_type="gx_RAR"}[5m])

Capacity Planning

Key metrics to monitor:

# UE IP address utilization (percentage)
(address_registry_count / <configured_pool_size>) * 100

# Active bearer count
session_registry_count

# PCRF query latency (P95)
histogram_quantile(0.95, gx_outbound_transaction_duration_bucket)

Capacity limits:

Address pool size: configured in config/runtime.exs under ue.subnet_map
TEID space: 32-bit (4 billion unique identifiers, auto-managed)
Concurrent sessions: typically limited by address pool size

Planning guidelines:

Monitor IP address utilization - scale pool before exceeding 80%
Monitor PCRF latency - high latency impacts session setup time
Monitor dedicated bearer creation rate - indicates policy complexity

Session Management - PDN session lifecycle
Diameter Gx Interface - PCRF policy protocol details
PFCP Interface - User plane programming
Configuration Guide - System configuration
Monitoring Guide - Metrics and observability

Overview​

Architecture Flow​

Key Concepts​

Configuration​

Important: Dynamic QoS Policy​

Bearer Lifecycle​

Default Bearer Creation​

Dedicated Bearer Creation​

Bearer Modification​

Bearer Deletion​

QoS Parameters​

QCI (QoS Class Identifier)​

ARP (Allocation and Retention Priority)​

MBR (Maximum Bit Rate)​

GBR (Guaranteed Bit Rate)​

AMBR (Aggregate Maximum Bit Rate)​

Flow Status and Gating​

Flow Status (Gx) to Gate Status (PFCP) Mapping​

Monitoring & Observability​

Prometheus Metrics​

Web UI Monitoring​

Key Log Messages​

Operational Tasks​

Verify QoS Applied to Session​

Troubleshoot Missing QoS​

Monitor Bearer Creation Rate​

Capacity Planning​

Related Documentation​

Overview

Architecture Flow

Key Concepts

Configuration

Important: Dynamic QoS Policy

Bearer Lifecycle

Default Bearer Creation

Dedicated Bearer Creation

Bearer Modification

Bearer Deletion

QoS Parameters

QCI (QoS Class Identifier)

ARP (Allocation and Retention Priority)

MBR (Maximum Bit Rate)

GBR (Guaranteed Bit Rate)

AMBR (Aggregate Maximum Bit Rate)

Flow Status and Gating

Flow Status (Gx) to Gate Status (PFCP) Mapping

Monitoring & Observability

Prometheus Metrics

Web UI Monitoring

Key Log Messages

Operational Tasks

Verify QoS Applied to Session

Troubleshoot Missing QoS

Monitor Bearer Creation Rate

Capacity Planning

Related Documentation