Jul 9, 2026

Gy Interface Deep Dive: The Complete Guide to Online Charging

Master the Gy online charging interface — DCCA protocol, SCUR/ECUR/IEC modes, CCR/CCA message flows, AVP definitions, quota management, and practical troubleshooting for PGW/UGW and OCS.

Gy Interface Deep Dive: The Complete Guide to Online Charging

The Gy interface is the backbone of prepaid mobile data services. It connects the PGW/UGW (Packet/User Gateway) to the OCS (Online Charging System) using the Diameter Credit Control Application (DCCA) protocol. Every time a prepaid subscriber browses the web, streams video, or sends an MMS, the Gy interface handles the real-time credit check.

This guide covers everything you need to know — from protocol fundamentals to practical troubleshooting.

What is the Gy Interface?

The Gy reference point is defined in 3GPP TS 32.291 and operates between the PGW (in EPC) or UGW (Huawei) and the OCS. It enables online charging — meaning the network verifies subscriber credit in real-time before granting service.

UE → eNodeB → SGW → PGW/UGW ←————→ OCS
                          |
                      Gy Interface
                    (Diameter DCCA)
                    TCP/SCTP:3868
                    App-ID: 4

Key Characteristics

Property	Value
Protocol	Diameter Credit Control Application (DCCA)
RFC	RFC 4006
Transport	TCP or SCTP
Port	3868 (IANA)
Application-ID	4
Command-Code	272 (Credit-Control)
3GPP Spec	TS 32.291, TS 32.299

Protocol Stack

┌─────────────────────────────────┐
│     DCCA (RFC 4006)             │  ← Gy Application
├─────────────────────────────────┤
│     Diameter Base (RFC 6733)    │  ← Protocol framework
├─────────────────────────────────┤
│     TCP or SCTP                 │  ← Transport (port 3868)
├─────────────────────────────────┤
│     IP                          │  ← Network layer
└─────────────────────────────────┘

Diameter Message Header

Every Diameter message starts with a 20-byte header:

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|    Version    |                 Message Length                |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| command flags |                  Command-Code                 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                         Application-ID                        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                      Hop-by-Hop Identifier                    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                      End-to-End Identifier                    |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Command Flags:

Bit	Name	Description
R	Request	1 = Request message, 0 = Answer
P	Proxiable	May be relayed or proxied
E	Error	Protocol error in message
T	Retransmitted	Possible duplicate after failover

Online Charging Modes

The Gy interface supports four distinct charging modes, each suited for different service scenarios.

SCUR — Session Charging with Unit Reservation

The primary and most common mode. Used for volume/time-based data charging.

How SCUR Works:
═══════════════

1. User starts data session
2. UGW sends CCR-I → OCS (request quota)
3. OCS reserves quota from subscriber account
4. UGW grants service → user consumes data
5. When quota runs low → UGW sends CCR-U (request more)
6. Session ends → UGW sends CCR-T (final report)
7. OCS debits exact amount from account

When to use: Data sessions, internet browsing, video streaming, any continuous data service.

ECUR — Event Charging with Unit Reservation

Used for event-based charging where the outcome is a discrete event.

ECUR Flow:
══════════
1. Event detected → CCR-I to OCS
2. OCS grants quota for the event
3. Event completes → CCR-T reports usage
4. OCS debits or refunds accordingly

When to use: MMS send/receive, content downloads, app purchases.

eECUR — Enhanced ECUR

The older 3GPP definition with CC-Update interaction during the event. Supports quota updates for long-running events.

IEC — Immediate Event Charging

No reservation — OCS charges immediately. The most signaling-efficient mode.

When to use: SMS, USSD, any service with guaranteed outcome.

Mode Comparison

Mode	Session Type	Reservation	Typical Use	Signaling
SCUR	Single DCC	Yes (chunks)	Data, streaming	High
ECUR	Separate DCC	Yes (event)	MMS, downloads	Medium
eECUR	Separate DCC	Yes + Update	Long events	Medium-High
IEC	Separate DCC	No	SMS, USSD	Low

CCR/CCA Message Flows

Credit-Control-Request (CCR)

Command-Code 272, Direction PGW/UGW → OCS.

<CCR> ::= <Diameter Header: 272, REQ, PXY>
  <Session-Id>                           // Unique per session
  {Origin-Host}                           // PGW/UGW hostname
  {Origin-Realm}                          // PGW/UGW realm
  {Destination-Realm}                     // OCS realm
  {Auth-Application-Id}                   // Must be 4
  {Service-Context-Id}                    // e.g., "32260@3gpp.org"
  {CC-Request-Type}                       // 1=I, 2=U, 3=T, 4=E
  {CC-Request-Number}                     // Increments per request
  [Requested-Action]                      // For EVENT_REQUEST
  [User-Name]                             // IMSI/MSISDN
  *[Subscription-Id]                      // Subscriber identity
  [Termination-Cause]                     // Reason for session end
  [Multiple-Services-Indicator]           // MSCC support flag
  *[Multiple-Services-Credit-Control]     // Per-rating-group
  [Service-Information]                   // PS-Information

CC-Request-Type Values:

Value	Name	When Sent
1	INITIAL_REQUEST	First CCR of a new session
2	UPDATE_REQUEST	Quota threshold or exhaustion
3	TERMINATION_REQUEST	Session end
4	EVENT_REQUEST	Event-based charging

Credit-Control-Answer (CCA)

Command-Code 272, Direction OCS → PGW/UGW.

<CCA> ::= <Diameter Header: 272, PXY>
  <Session-Id>
  {Result-Code}                           // 2001=SUCCESS
  {Origin-Host}                           // OCS hostname
  {Origin-Realm}
  {Auth-Application-Id}                   // 4
  {CC-Request-Type}                       // Echoes request
  {CC-Request-Number}                     // Echoes request
  [CC-Session-Failover]                   // 0=No, 1=Yes
  *[Multiple-Services-Credit-Control]     // Granted quotas
  [Credit-Control-Failure-Handling]       // 0/1/2
  [Validity-Time]                         // Quota validity

Key AVPs (Attribute-Value Pairs)

Core Session AVPs

AVP Name	Code	Type	Description
Session-Id	263	OctetString	Unique per-session identifier
Auth-Application-Id	258	Unsigned32	Must be 4 for DCCA
Service-Context-Id	461	OctetString	Charging context identifier
CC-Request-Type	416	Unsigned32	1=I, 2=U, 3=T, 4=E
CC-Request-Number	415	Unsigned32	Sequence per session
Result-Code	268	Unsigned32	2001=SUCCESS
Origin-Host	264	OctetString	Sender hostname
Origin-State-Id	278	Unsigned32	Restart detection

Quota & Credit AVPs

AVP Name	Code	Type	Description
Granted-Service-Unit	431	Grouped	Quota granted by OCS
Used-Service-Unit	446	Grouped	Units consumed
CC-Time	420	Unsigned32	Time quota (seconds)
CC-Total-Octets	421	Unsigned64	Total volume quota
CC-Input-Octets	412	Unsigned64	Uplink volume
CC-Output-Octets	414	Unsigned64	Downlink volume
Rating-Group	432	Unsigned32	Charging classification
CC-Unit-Type	454	Unsigned32	1=TIME, 2=MONEY, 4=TOTAL_OCTETS

Threshold & Timer AVPs

AVP Name	Code	Description
Time-Quota-Threshold	868	Trigger update when time remaining below threshold
Volume-Quota-Threshold	869	Trigger update when volume remaining below threshold
Quota-Consumption-Time	881	Idle timeout before quota stops (discontinuous traffic)
Validity-Time	448	How long granted quota remains valid

Failure Handling AVPs

AVP Name	Code	Values
CC-Session-Failover	418	0=Not Supported, 1=Supported
Credit-Control-Failure-Handling	427	0=TERMINATE, 1=RETRY, 2=CONTINUE
Direct-Debiting-Failure-Handling	428	0=TERMINATE_SERVICE, 1=CONTINUE_SERVICE
Final-Unit-Action	449	1=TERMINATE, 2=REDIRECT, 3=RESTRICT

PS-Information AVP

The Service-Information AVP carries packet-switched charging data:

Service-Information ::= <AVP Header: 873>
  [PS-Information]
    [3GPP-Charging-Id]           // Bearer charging ID
    [3GPP-PDP-Type]              // IPv4/IPv6/IPv4v6
    *[PDP-Address]               // UE IP
    [Called-Station-Id]          // APN name
    [SGSN-Address]               // SGSN IP
    [GGSN-Address]               // GGSN/PGW IP
    [3GPP-IMSI-MCC-MNC]          // MCC+MNC
    [3GPP-RAT-Type]              // GERAN/UTRAN/E-UTRAN
    [3GPP-User-Location-Info]     // Cell ID, TAC
    [3GPP-MS-TimeZone]           // UTC offset
    [Charging-Rule-Base-Name]    // PCRF rule group
    [Serving-Node-Type]          // SGSN/MME/PGW
    [Start-Time]                 // Session start epoch
    [Stop-Time]                  // Session end epoch

Termination-Cause Values

Code	Name	Description
1	DIAMETER_LOGOUT	User disconnected
2	DIAMETER_SERVICE_NOT_PROVIDED	Pre-auth disconnect
3	DIAMETER_BAD_ANSWER	Malformed auth answer
4	DIAMETER_ADMINISTRATIVE	Admin disconnect (ASR)
5	DIAMETER_LINK_BROKEN	Transport failure
6	DIAMETER_AUTH_EXPIRED	Session time expired
7	DIAMETER_USER_MOVED	User moved to another node
8	DIAMETER_SESSION_TIMEOUT	Inactivity timeout

Common Result Codes

Code	Name	Meaning
2001	DIAMETER_SUCCESS	Request succeeded
3002	DIAMETER_UNABLE_TO_DELIVER	Cannot reach home server
4012	DIAMETER_CREDIT_LIMIT_EXCEEDED	No credit / quota exhausted
4013	DIAMETER_USER_UNKNOWN	Subscriber not found
5012	UNABLE_TO_COMPLY	OCS processing error
5014	DIAMETER_MISSING_AVP	Required AVP missing

Complete SCUR Call Flow

UE → eNodeB → SGW → UGW/PGW ←————→ OCS

Step 1: [CCR-I] UGW → OCS
  CC-Request-Type = 1 (INITIAL)
  Session-Id = "ugw.operator.com;123456;1"
  Service-Context-Id = "32260@3gpp.org"
  Subscription-Id: IMSI = 001012345678901
  Rating-Group = 1
  PS-Information: APN="internet", RAT=E-UTRAN

Step 2: [CCA-I] OCS → UGW
  Result-Code = 2001 (SUCCESS)
  Granted-Service-Unit: CC-Total-Octets = 100MB
  CC-Session-Failover = 1 (SUPPORTED)
  Volume-Quota-Threshold = 10MB

Step 3: [CCR-U] UGW → OCS  (after 90MB consumed)
  CC-Request-Type = 2 (UPDATE)
  CC-Request-Number = 1
  Used-Service-Unit: CC-Total-Octets = 90MB
  Requested-Service-Unit: CC-Total-Octets = 50MB

Step 4: [CCA-U] OCS → UGW
  Result-Code = 2001
  Granted-Service-Unit: CC-Total-Octets = 50MB

Step 5: [CCR-T] UGW → OCS  (user disconnects)
  CC-Request-Type = 3 (TERMINATION)
  CC-Request-Number = 2
  Used-Service-Unit: CC-Total-Octets = 35MB
  Termination-Cause = 1 (LOGOUT)

Step 6: [CCA-T] OCS → UGW
  Result-Code = 2001
  Total charged: 125MB

Troubleshooting Guide

Common Issues and Fixes

1. CCR-I Timeout (no CCA response)

Check TCP/SCTP connectivity on port 3868
Verify OCS is reachable and Diameter peer is up
Check firewall rules between UGW and OCS

2. Result-Code 4012 (CREDIT_LIMIT_EXCEEDED)

Subscriber has no credit or quota
Normal for prepaid subscribers with zero balance
UGW should deny service access

3. Result-Code 5012 (UNABLE_TO_COMPLY)

OCS cannot process the request
Check Service-Context-Id configuration
Verify rating group exists in OCS

4. Session Failover Not Working

Verify CC-Session-Failover = 1 in CCA
Check secondary OCS is configured and reachable
Verify Origin-State-Id increments on restart

5. Quota Exhaustion During Active Session

Volume-Quota-Threshold too high — lower it (e.g., 10% of quota)
CCR-U not being sent — check threshold configuration
OCS not granting sufficient quota in CCA-U

Debug Commands

# View active Gy sessions
display diameter session all

# Check Gy peer status  
display diameter peer all

# Monitor CCR/CCA messages
debugging diameter credit-control all

# View quota statistics
display charging quota statistics

# Check OCS connectivity
display diameter connection

Wireshark Filters

# All Diameter messages
diameter

# Only CCR/CCA (Gy traffic)
diameter.cmd.ccr

# Filter by Result-Code
diameter.result_code != 2001

# Filter by Application-Id (DCCA = 4)
diameter.application_id == 4

# CCR with specific IMSI
diameter and diameter.imsi == "001012345678901"

# Quota exhaustion (Result-Code 4012)
diameter.result_code == 4012

Best Practices

Set appropriate quota thresholds — Request new quota at 80-90% consumption to avoid service interruption
Enable session failover — Configure CC-Session-Failover = 1 for high availability
Use Multiple-Services-Credit-Control — Per-rating-group charging for granular billing
Monitor Quota-Consumption-Time — Set for discontinuous traffic (e.g., social media, messaging)
Configure Credit-Control-Failure-Handling = 2 (CONTINUE) — Don’t drop sessions on transient OCS failures
Regular DWR/DWA health checks — Ensure Diameter peer liveness monitoring is active

References

3GPP TS 32.291 — Telecommunication management; Charging management; Diameter charging application
3GPP TS 32.299 — Diameter charging applications
RFC 4006 — Diameter Credit-Control Application
RFC 6733 — Diameter Base Protocol
RFC 3588 — Diameter Base Protocol (obsoleted by 6733)

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