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OmniCSCF

OmniCSCF is Omnitouch's IMS Call Session Control Function - the P-CSCF, I-CSCF, and S-CSCF (with integrated E-CSCF) that register subscribers and control every IMS voice and RCS session. This is the operations and architecture guide; per-component references live in docs/.

Table of Contents​

  1. Overview
  2. Understanding IMS Architecture
  3. Call Session Flows
  4. CSCF Components
  5. Common Operations
  6. Additional Documentation

Overview​

OmniCSCF is the IMS (IP Multimedia Subsystem) Call Session Control Function core for mobile and fixed-line networks. It delivers the session control required for VoLTE (Voice over LTE / 4G), VoNR (Voice over New Radio / 5G), VoWiFi (Voice over Wi-Fi), and fixed-line VoIP, and routes RCS and other application-layer signalling to Application Servers via IFC. Management and monitoring are provided by OmniWeb.

What is IMS?​

The IP Multimedia Subsystem (IMS) is the 3GPP-standardized architecture for delivering IP-based multimedia services. It provides:

  • Session control for voice, video, and messaging services
  • Quality of Service (QoS) management for real-time communications
  • Service convergence across mobile, fixed, and WiFi networks
  • Standards-based interoperability with other carriers and networks
  • Rich Communication Services (RCS) capabilities
  • Fixed-Mobile Convergence (FMC) for unified service delivery

OmniCSCF implements all core CSCF functions defined in 3GPP TS 23.228, providing a complete, production-ready IMS core network solution.

OmniCSCF Components​

OmniCSCF provides complete management of all CSCF network elements:

  • P-CSCF (Proxy-CSCF) - User-facing edge proxy and security anchor
  • E-CSCF (Emergency-CSCF) - Emergency services routing (integrated with P-CSCF)
  • I-CSCF (Interrogating-CSCF) - Network entry point and topology hiding
  • S-CSCF (Serving-CSCF) - Core session control, registration, and service triggering

Key Capabilities​

Capabilities:

  • Full 3GPP-compliant IMS session control, real-time across all sessions
  • GSMA IR.92/IR.94 compliant - Works with any standards-compliant device, no custom carrier bundles required
  • VoLTE, VoNR (5G), and VoWiFi support, plus routing of RCS and other application signalling to Application Servers via IFC
  • Fixed-line SIP service integration
  • Emergency services (E911/E112) support with location services
  • Service triggering via Initial Filter Criteria (IFC) and Application Server integration over the ISC interface
  • QoS policy enforcement for both VoLTE and VoNR - Diameter Rx to the PCRF (4G/LTE) and the N5 SBI to the PCF (5G NR)
  • Charging integration (online and offline)
  • Topology hiding, network security, and IPsec-based security associations
  • Diameter-based AAA and policy integration
  • Multi-tenancy support for MVNO scenarios

Management & Operations:

  • Real-time monitoring via OmniWeb, the unified management portal
  • Prometheus metrics integration (see Metrics Reference)
  • RESTful API for automation
  • Distributed clustering for high availability
  • Live troubleshooting and diagnostics

Integrated Components:

  • OmniePDG: Evolved Packet Data Gateway for VoWiFi (IR.94 compliant)
  • OmniTAS: Telephony Application Server for supplementary services
  • OmniMessage: SMS/MMS Application Server (3GPP TS 24.341)

Monitoring and administration are handled through OmniWeb, Omnitouch's unified management portal. Its P/I/S-CSCF pages show registrations, dialogs, Diameter peers, NRF status, logs and dashboards.

Fixed-Line Services​

OmniCSCF serves fixed-line subscribers over the same IMS core as mobile VoLTE/VoNR, so residential and business fixed services share one registration, routing, and application layer.

Authentication. Fixed-line devices have no UICC/ISIM, so they authenticate with password-based SIP Digest rather than IMS-AKA. OmniCSCF supports the full range of digest schemes - plain SIP Digest (MD5), 3GPP SIP Digest (TS 33.203 Annex N), ETSI TISPAN HTTP-Digest, and CableLabs/PacketCable Digest - alongside IMS-AKA (AKAv1/AKAv2) for UICC-based devices. With the default HSS-Selected mode the S-CSCF challenges each subscriber with whatever scheme the HSS provisioned, so mobile (AKA) and fixed (Digest) subscribers coexist on one core. See S-CSCF → Authentication.

Supported devices. Any standards-compliant SIP device works. The models below are the ones we recommend and have deployed in large numbers, but they are examples - not a restricted list:

  • Analogue Telephone Adapters (ATAs) - Grandstream and Cisco, with zero-touch provisioning when paired with OmniCRM.
  • IP phones - Grandstream and Yealink.
  • DECT IP handsets.

Hosted PBX / IP-Centrex. Combined with OmniTAS, OmniCSCF offers IP-Centrex features - hunt groups, call queues, IVRs, and other business call handling - so you can deliver both residential lines and hosted PBX services over the IMS.

Understanding IMS Architecture​

IMS Network Architecture​

The OmniCSCF solution sits at the heart of the IMS architecture, providing the session control layer that connects user equipment to services and manages all call sessions.

How CSCFs Work Together​

The CSCF functions work as a coordinated system to handle IMS sessions:

  1. P-CSCF - First Point of Contact

    • User equipment (mobile, WiFi, or fixed-line devices) establish secure connections to P-CSCF
    • Provides IPsec security associations for mobile devices
    • Acts as the QoS policy enforcement point - via Diameter Rx to the PCRF for VoLTE (4G/LTE), and via the N5 SBI to the PCF for VoNR (5G NR)
    • Handles NAT traversal and media anchoring
    • Routes emergency calls to E-CSCF functionality
    • Maintains user location information

  2. I-CSCF - Network Gateway & Load Balancer

    • Hides internal network topology from external networks
    • Queries HSS to select appropriate S-CSCF for users
    • Performs S-CSCF load balancing based on capabilities
    • Acts as the entry/exit point for roaming scenarios
    • Enforces Network Domain Security (NDS/TLS)

  3. S-CSCF - Core Session Controller

    • Performs user registration and authentication
    • Maintains session state for all active calls
    • Enforces routing policies and service logic
    • Triggers Application Servers based on IFC (Initial Filter Criteria)
    • Integrates with charging systems (online and offline)
    • Manages supplementary services

Integration with Supporting Systems​

OmniCSCF integrates with IMS support functions via standard 3GPP Diameter interfaces (4G/EPC) and, for VoNR, via 5G service-based (SBI) interfaces:

InterfaceTransportFrom → ToPurpose3GPP Spec
CxDiameterI-CSCF/S-CSCF ↔ HSSUser authentication, profile retrieval, S-CSCF assignmentTS 29.228
DxDiameterI-CSCF ↔ SLFSubscription locator for multi-HSS environmentsTS 29.229
RxDiameterP-CSCF ↔ PCRFQoS policy authorization, media flow control (4G/VoLTE)TS 29.214
RoDiameterS-CSCF → OCSOnline charging (credit control)TS 32.299
RfDiameterS-CSCF → CDFOffline charging (CDR generation)TS 32.299
ISCSIPS-CSCF ↔ ASService triggering and application server invocationTS 23.228
ShDiameterAS ↔ HSSApplication server access to user dataTS 29.328
N5HTTP/2 SBIP-CSCF (AF) → PCFQoS/media authorization for VoNR (Npcf_PolicyAuthorization)TS 29.514
NnrfHTTP/2 SBIP-CSCF ↔ NRFNF registration (Nnrf_NFManagement) and optional PCF discovery (Nnrf_NFDiscovery)TS 29.510

The N5 and Nnrf interfaces are HTTP/2 service-based interfaces (SBI, JSON payloads), in contrast to the Diameter interfaces used for the 4G/EPC-anchored services. See the 5G / VoNR note below.

For Diameter peer management, see Diameter Operations.

5G / VoNR Policy and QoS​

For VoNR (Voice over New Radio) sessions anchored in the 5G core, the P-CSCF acts as an Application Function (AF) and authorizes media QoS over the N5 SBI interface to the PCF (Npcf_PolicyAuthorization), rather than over Diameter Rx to the PCRF as it does for VoLTE. The rest of the IMS session control (I-CSCF, S-CSCF, IFC, charging, application servers) is unchanged - only the policy/QoS leg differs by access type.

The P-CSCF locates the serving PCF by registering with and querying the NRF (Nnrf_NFDiscovery), which resolves the correct PCF for the subscriber via the NRF→BSF chain. See P-CSCF Documentation for detail.

Call Session Flows​

Understanding how CSCFs process different types of sessions is essential for operations and troubleshooting.

IMS Registration Flow​

When a device registers to the IMS network, the CSCFs coordinate to authenticate and authorize the user:

Key Points:

  • P-CSCF maintains IPsec security association with UE
  • I-CSCF queries HSS to find/assign S-CSCF
  • S-CSCF performs authentication and stores user profile
  • User's service profile (IFC) determines which Application Servers will be triggered

Mobile Originated Call Flow​

When a registered user initiates a call:

Key Points:

  • P-CSCF coordinates with PCRF for QoS bearer establishment
  • S-CSCF evaluates IFC to determine service triggering
  • OmniTAS provides telephony application services (call forwarding, screening, etc.)
  • OmniMessage handles SMS/MMS traffic when triggered by IFC
  • For monitoring active calls, see S-CSCF Dialog Management

Emergency Call Flow (E-CSCF)​

Emergency calls receive special handling to ensure connectivity even without full IMS registration, and to guarantee the PSAP receives a usable calling-line identity (CLI) / callback number per ETSI TS 23.167 (Annex K):

Key Points:

  • Emergency is detected from the Request-URI urn:service:sos (plus the dialled numbers 112/911)
  • E-CSCF functionality is integrated into P-CSCF
  • At registration the P-CSCF caches an IMEI→MSISDN binding (from the Contact +sip.instance IMEI and the P-Associated-URI) in a cluster-replicated store with a 4-hour TTL, then uses it to supply the caller's MSISDN on a later emergency call
  • For an unknown/anonymous caller with no cached identity, the P-CSCF asynchronously queries the PCRF via Rx (Rx_AAR_Subscription), bound by the UE source IP, to retrieve EPC-level identities with precedence MSISDN → IMSI → IMEISV (TS 23.167 Annex K / TS 29.214 §4.4.6a)
  • The resolved identity is asserted as From and P-Asserted-Identity on the sos.apn domain so the PSAP receives a usable callback/CLI
  • Works even for unregistered or roaming users
  • For the deep detail, see P-CSCF Emergency Services

SMS over IMS - Mobile Originated (3GPP TS 24.341)​

When a user sends an SMS via IMS, OmniMessage handles the message delivery:

Key Points:

  • SMS encoded in SIP MESSAGE method per 3GPP TS 24.341
  • Content-Type: application/vnd.3gpp.sms identifies SMS payload
  • S-CSCF IFC triggers OmniMessage for SMS traffic
  • OmniMessage interfaces with traditional SMSC infrastructure
  • Supports both GSM-7, UCS-2 character sets and concatenated messages

SMS over IMS - Mobile Terminated (3GPP TS 24.341)​

When an SMS arrives for an IMS-registered user, OmniMessage routes it via IMS:

Key Points:

  • SMSC forwards SMS to OmniMessage via traditional protocols (MAP/SMPP)
  • OmniMessage converts to SIP MESSAGE method
  • S-CSCF routes based on registered IMPU
  • Supports delivery reports and status notifications
  • Fallback to traditional SMS if user not IMS-registered

For SMS operations and monitoring, see S-CSCF IFC Management.

Roaming Scenarios​

VoLTE (and VoNR) roaming uses S8 Home Routed (S8HR) - the model GSMA mandated for IMS voice roaming (IR.65, IR.88; NG.114 for 5G):

S8 Home Routed (S8HR): The roaming subscriber's IMS APN PDN connection is anchored at the home network's P-GW (5G: home UPF) over the S8 interface - the visited network's S-GW simply tunnels the bearer back to the home P-GW via GTP. The UE therefore reaches the home P-CSCF directly and registers with the home S-CSCF exactly as if it were on the home network. Consequences:

  • No IMS in the visited network - there is no VPLMN P-CSCF/I-CSCF/S-CSCF in the path. The visited network provides only EPC transport (the S8 bearer).
  • Home network keeps full control over authentication, service triggering (IFC → OmniTAS, OmniMessage, etc.), charging and lawful interception.
  • Simplified roaming agreements - operators need only an S8/EPC roaming relationship, not IMS interconnect.
  • GSMA IR.92 / IR.94 compliant.

Because S8HR anchors the IMS APN in the home network, roaming is transparent to the CSCF: there is no roaming-specific CSCF configuration - the routing that keeps the IMS APN home-anchored is provisioned in the EPC (P-GW/APN selection), not here.

CSCF Components​

P-CSCF/E-CSCF - Edge Proxy and Security Anchor​

The Proxy-CSCF is the first IMS element that user equipment contacts. It serves as the security boundary and policy enforcement point.

Core Functions:

  • Security Association Management: Establishes and maintains IPsec tunnels with mobile devices for signaling and media protection
  • QoS Policy Enforcement: Authorizes and enforces media QoS by access type - Diameter Rx to the PCRF for VoLTE (4G/LTE bearers), and the N5 SBI (Npcf_PolicyAuthorization) to the PCF for VoNR (5G NR)
  • NAT Traversal: Handles far-end NAT traversal for devices behind NAT/firewalls
  • Compression: SigComp support for bandwidth-constrained networks
  • Service Route: Maintains service route for subsequent requests

Emergency Services (E-CSCF):

  • Integrated emergency call routing without requiring full IMS registration
  • Emergency detection via Request-URI urn:service:sos (plus 112/911)
  • Location information handling for E911/E112
  • Calling-line identity (CLI) resolution for anonymous/unknown callers per TS 23.167 Annex K: cached IMEI→MSISDN binding, with fallback to an Rx subscription query to the PCRF (MSISDN → IMSI → IMEISV precedence)
  • Integration with LRF (Location Retrieval Function)

Supported Access Types:

  • LTE (VoLTE) via IPsec
  • 5G NR (VoNR) via IPsec - QoS authorized via the N5 AF interface to the PCF
  • WiFi (VoWiFi) via IPsec
  • Fixed broadband via SIP
  • Cable/DSL residential gateways

For detailed operations, see P-CSCF Documentation.

I-CSCF - Topology Hiding and Load Balancing​

The Interrogating-CSCF acts as the contact point within an operator's network for connections from other networks or from the same network.

Core Functions:

  • Topology Hiding: Shields internal network structure from external networks
  • S-CSCF Assignment: Queries HSS via Cx interface to assign S-CSCF to new users
  • S-CSCF Selection: Selects appropriate S-CSCF based on capabilities and load
  • Routing Proxy: Routes incoming requests to the assigned S-CSCF
  • Network Domain Security: Enforces NDS/TLS for inter-operator security

Key Features:

  • Multi-S-CSCF Support: Distributes users across multiple S-CSCF instances
  • Capability Matching: Matches user requirements to S-CSCF capabilities
  • Roaming Support: Handles both home-routed and local breakout scenarios
  • Subscription Locator: Dx interface support for multi-HSS environments

Use Cases:

  • Interconnection point for roaming partners
  • Load distribution across S-CSCF cluster
  • Geographic routing for disaster recovery
  • MVNO traffic segregation

For detailed operations, see I-CSCF Documentation.

S-CSCF - Core Session Controller​

The Serving-CSCF is the central component of the IMS network, providing session control and service intelligence.

Core Functions:

  • Registration: Authenticates users and maintains registration bindings
  • Session Control: Manages all call states (dialog establishment, modification, termination)
  • Service Triggering: Evaluates Initial Filter Criteria (IFC) to invoke Application Servers
  • Routing: Routes SIP requests based on service logic and user preferences
  • Charging Integration: Coordinates with online (OCS) and offline (CDF) charging systems

Service Triggering via IFC: The S-CSCF uses XML-based Initial Filter Criteria downloaded from HSS to determine when to route calls through Application Servers (such as OmniTAS for telephony services and OmniMessage for SMS/MMS):

  • Trigger Points: Match on SIP method, Request-URI, Session-Case (originating/terminating)
  • Priority-based: IFC processed in priority order
  • Service Chaining: Multiple AS can be invoked in sequence (e.g., OmniTAS → OmniMessage)
  • Default Handling: Configurable behavior when AS is unreachable

Supported Services:

  • Call forwarding (busy, no answer, unconditional)
  • Call barring (outgoing, incoming, roaming)
  • Call screening and filtering
  • Number translation and routing
  • Prepaid/postpaid charging
  • Usage tracking and quota enforcement
  • Supplementary services (call waiting, hold, transfer)

Scalability Features:

  • Distributed dialog storage
  • Stateful session handling
  • Database-backed user profiles
  • Horizontal scaling via I-CSCF distribution

For detailed operations, see S-CSCF Documentation.

Diameter Interface Management​

OmniCSCF provides comprehensive Diameter peer management across all CSCF components.

Supported Diameter Applications:

ApplicationInterfaceApp IDUsed ByPurpose
3GPP CxCx16777216I-CSCF, S-CSCFUser authentication, profile retrieval
3GPP DxDx16777216I-CSCFSubscription location in multi-HSS
3GPP RxRx16777236P-CSCFPolicy authorization, QoS control
3GPP RoRo4 (CC)S-CSCFOnline charging (credit control)
3GPP RfRf3 (Accounting)S-CSCFOffline charging (CDR)
3GPP ShSh16777217ASUser data access from AS

Diameter Capabilities:

  • Automatic peer discovery via DNS
  • Failover and redundancy support
  • Watchdog and connection management
  • Per-peer statistics and monitoring
  • Dynamic peer enable/disable

For Diameter operations and troubleshooting, see Diameter Management Guide.

Common Operations​

OmniCSCF provides comprehensive operational capabilities through OmniWeb, the unified management portal. This section covers common operational tasks and their significance.

Service Triggering and IFC Management​

Initial Filter Criteria (IFC) determines when and how the S-CSCF routes sessions to application servers like OmniTAS and OmniMessage.

IFC Operations:

  • Dump user's IFC to view configured service profile from HSS
  • Test IFC matching with simulated call scenarios
  • Verify AS routing to ensure proper service invocation
  • Debug service failures by examining trigger point evaluation

Example IFC Structure:

<InitialFilterCriteria>
  <Priority>10</Priority>
  <TriggerPoint>
    <SPT><Method>INVITE</Method></SPT>
    <SPT><SessionCase>0</SessionCase><!-- Originating --></SPT>
  </TriggerPoint>
  <ApplicationServer>
    <ServerName>sip:omnitas.ims.example.com</ServerName>
    <DefaultHandling>0</DefaultHandling><!-- Must invoke -->
  </ApplicationServer>
</InitialFilterCriteria>

For IFC testing and troubleshooting, see S-CSCF IFC Operations.

Diameter Peer Management​

Monitoring Diameter Connectivity:

OmniCSCF relies on Diameter interfaces for HSS, PCRF, and charging integration. Operators can:

  • Monitor peer status (I_Open = connected, Closed = disconnected)
  • View peer capabilities (supported Diameter applications)
  • Enable/disable peers for maintenance or failover testing
  • Track peer statistics (requests, failures, timeouts)

Critical Diameter Connections:

  • Cx to HSS (I-CSCF, S-CSCF): User authentication and profiles
  • Rx to PCRF (P-CSCF): QoS policy and bearer control
  • Ro to OCS (S-CSCF): Online charging and credit control

For Diameter troubleshooting, see Diameter Operations Guide.

NRF Registration (5G Core)​

Monitoring 5G Service Discovery:

In a 5G deployment the P-CSCF registers with the NRF as an Application Function (AF) and, per VoNR session, discovers the PCF serving each UE's PDU session (NRF→BSF). Operators can:

  • Monitor NRF registration status - whether the P-CSCF's AF profile is registered and its heartbeat current
  • View discovered PCF bindings - the per-subscriber PCF resolved for active VoNR sessions
  • Confirm the AF instance identity is stable across restarts, so the NRF does not accumulate stale registrations

NRF registration state and discovered PCF bindings are shown on the OmniWeb P-CSCF page (NRF tab). See P-CSCF Access Detection & QoS and the 5G SBI Operations Guide.

Additional Documentation​

Component-Specific Operations Guides​

For detailed operations and troubleshooting for each CSCF component, see the component guides below. Each carries its own Troubleshooting section - registration/IPsec and emergency-call issues in the P-CSCF guide, registration/call-setup/SMS issues in the S-CSCF guide, S-CSCF selection and Cx issues in the I-CSCF guide, and Diameter peer issues in the Diameter guide:

Regulatory Compliance​

3GPP Standards Reference​

OmniCSCF implements the following 3GPP specifications:

SpecificationTitleRelevance
TS 23.228IP Multimedia Subsystem (IMS) - Stage 2Core IMS architecture
TS 24.229IP multimedia call control protocol (SIP)IMS SIP profile
TS 29.228Cx and Dx interfaces (CSCF-HSS)User data and authentication
TS 29.214Rx interface (P-CSCF-PCRF)QoS policy control
TS 32.299Charging - Diameter applicationsOnline/offline charging
TS 24.341SMS over IP networksSMS over IMS
TS 23.167Emergency servicesE-CSCF, emergency calls, and CLI resolution (Annex K)
TS 29.514Npcf_PolicyAuthorization (N5)VoNR QoS/media authorization (P-CSCF → PCF)
TS 29.510NRF services (Nnrf)NF registration and PCF discovery

GSMA Standards Compliance​

OmniCSCF is fully compliant with GSMA IMS profiles, ensuring interoperability with off-the-shelf devices:

IR.92 - IMS Profile for Voice and SMS (VoLTE)​

GSMA PRD IR.92 defines the mandatory IMS profile for VoLTE services, ensuring that commercial devices work seamlessly without carrier-specific configuration or custom device bundles.

Key IR.92 Benefits for OmniCSCF:

✓ Open Market Device Support: Any IR.92-compliant smartphone works immediately-no custom carrier bundles, proprietary APNs, or special provisioning required

✓ Standardized SIP Profile: Devices use standard SIP headers, authentication, and registration flows as defined in 3GPP TS 24.229

✓ Codec Interoperability: Mandatory codec support (AMR-WB for HD Voice) ensures consistent voice quality across all devices

✓ SMS over IMS: Integration with OmniMessage provides standards-based SMS delivery (TS 24.341) to any IR.92 device

✓ Emergency Services: E.164 emergency number handling (911, 112, etc.) works on all compliant devices without special configuration

✓ Roaming Consistency: Home-routed roaming ensures users get the same VoLTE experience when visiting other IR.92-compliant networks

What This Means: Operators can launch VoLTE services immediately with existing consumer devices (iPhone, Samsung, Google Pixel, etc.) without waiting for custom device certification or carrier bundle updates.

IR.94 - IMS Profile for Voice, Video and SMS (VoWiFi)​

GSMA PRD IR.94 extends IR.92 to include Voice over WiFi, enabling VoLTE services over untrusted WiFi networks.

VoWiFi Architecture with OmniCSCF:

VoWiFi Components:

  • OmniePDG: Evolved Packet Data Gateway - Provides IPsec tunnel termination for untrusted WiFi access
  • OmniCSCF P-CSCF: Handles VoWiFi registrations identically to VoLTE (same service routes, same IFC triggering)
  • Seamless Handover: Devices can move between LTE and WiFi without call interruption

IR.94 Benefits:

  • Same IR.92 benefits apply to VoWiFi
  • Devices automatically discover ePDG via DNS (no manual configuration)
  • Single IMS registration covers both VoLTE and VoWiFi
  • Indoor coverage extension without femtocells or DAS

For ePDG operations and VoWiFi troubleshooting, refer to the OmniePDG documentation.

Other GSMA Standards​

  • IR.51 - GSMA Roaming Database Structure
  • IR.88 - LTE Roaming Guidelines
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