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OmniCSCF

OmniCSCF is Omnitouch's IMS Call Session Control Function. It contains the P-CSCF, the I-CSCF, and the S-CSCF (with an integrated E-CSCF). These functions register subscribers. They also control every IMS voice and RCS session. This document is the operations and architecture guide. The per-component references are 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 that VoLTE (Voice over LTE / 4G), VoNR (Voice over New Radio / 5G), VoWiFi (Voice over Wi-Fi), and fixed-line VoIP need. It also routes RCS and other application-layer signalling to Application Servers through IFC. OmniWeb does the management and the monitoring.

What is IMS?​

The IP Multimedia Subsystem (IMS) is the 3GPP-standardized architecture that delivers IP-based multimedia services. It provides these functions:

  • 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 that 3GPP TS 23.228 defines. It is a complete, production-ready IMS core network solution.

OmniCSCF Components​

OmniCSCF provides complete management of all CSCF network elements. The elements are:

  • 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. It works with any standards-compliant device. It needs no custom carrier bundles.
  • Support for VoLTE, VoNR (5G), and VoWiFi. It also routes RCS and other application signalling to Application Servers through IFC.
  • Fixed-line SIP service integration
  • Emergency services (E911/E112) support with location services
  • Service triggering through Initial Filter Criteria (IFC) and Application Server integration over the ISC interface
  • QoS policy enforcement for both VoLTE and VoNR. It uses 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)

OmniWeb, Omnitouch's unified management portal, does the monitoring and the administration. 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. Thus residential and business fixed services share one registration, routing, and application layer.

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

Supported devices. Any standards-compliant SIP device works. Omnitouch recommends the models below. Omnitouch has also deployed them in large numbers. These models are examples. They are not a restricted list.

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

Hosted PBX / IP-Centrex. OmniCSCF works together with OmniTAS to offer IP-Centrex features. These features are hunt groups, call queues, IVRs, and other business call handling. Thus you can deliver both residential lines and hosted PBX services over the IMS.

Understanding IMS Architecture​

IMS Network Architecture​

The OmniCSCF solution is at the center of the IMS architecture. It is the session control layer. This layer connects user equipment to services. It also controls all call sessions.

How CSCFs Work Together​

The CSCF functions work together as one system to process IMS sessions:

  1. P-CSCF - First Point of Contact

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

  2. I-CSCF - Network Gateway & Load Balancer

    • Hides the internal network topology from external networks
    • Queries the HSS to select the correct S-CSCF for each user
    • Does S-CSCF load balancing by capability
    • Acts as the entry point and exit point for roaming
    • Enforces Network Domain Security (NDS/TLS)

  3. S-CSCF - Core Session Controller

    • Does user registration and authentication
    • Keeps the session state for all active calls
    • Enforces routing policies and service logic
    • Triggers Application Servers by IFC (Initial Filter Criteria)
    • Integrates with charging systems (online and offline)
    • Controls supplementary services

Integration with Supporting Systems​

OmniCSCF integrates with IMS support functions through standard 3GPP Diameter interfaces (4G/EPC). For VoNR, it uses 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). The 4G/EPC-anchored services use Diameter interfaces instead. 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 that are anchored in the 5G core, the P-CSCF acts as an Application Function (AF). It authorizes media QoS over the N5 SBI interface to the PCF (Npcf_PolicyAuthorization). For VoLTE it uses Diameter Rx to the PCRF instead. The rest of the IMS session control (I-CSCF, S-CSCF, IFC, charging, application servers) does not change. Only the policy/QoS leg differs by access type.

To locate the serving PCF, the P-CSCF registers with the NRF and queries it (Nnrf_NFDiscovery). The NRF resolves the correct PCF for the subscriber through the NRF→BSF chain. See P-CSCF Documentation for detail.

Call Session Flows​

You must understand how the CSCFs process different types of sessions. This knowledge is essential for operations and troubleshooting.

IMS Registration Flow​

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

Key Points:

  • The P-CSCF keeps the IPsec security association with the UE
  • The I-CSCF queries the HSS to find and assign the S-CSCF
  • The S-CSCF does the authentication and stores the user profile
  • The user's service profile (IFC) determines which Application Servers the S-CSCF triggers

Mobile Originated Call Flow​

When a registered user starts a call:

Key Points:

  • The P-CSCF works with the PCRF to establish the QoS bearer
  • The S-CSCF evaluates the IFC to determine the service triggering
  • OmniTAS provides telephony application services (call forwarding, screening, etc.)
  • OmniMessage processes SMS/MMS traffic when the IFC triggers it
  • To monitor active calls, see S-CSCF Dialog Management

Emergency Call Flow (E-CSCF)​

Emergency calls receive special handling. This handling ensures connectivity even without full IMS registration. It also ensures that the PSAP receives a usable calling-line identity (CLI) / callback number per ETSI TS 23.167 (Annex K):

Key Points:

  • The P-CSCF detects emergency from the Request-URI urn:service:sos (plus the dialled numbers 112/911)
  • The E-CSCF function is integrated into the 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. On a later emergency call it uses this binding to supply the caller's MSISDN.
  • For an unknown/anonymous caller with no cached identity, the P-CSCF queries the PCRF asynchronously through Rx (Rx_AAR_Subscription), bound by the UE source IP. This query retrieves EPC-level identities with precedence MSISDN → IMSI → IMEISV (TS 23.167 Annex K / TS 29.214 §4.4.6a).
  • The P-CSCF asserts the resolved identity as From and P-Asserted-Identity on the sos.apn domain. Thus the PSAP receives a usable callback/CLI.
  • The handling 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 through IMS, OmniMessage does the message delivery:

Key Points:

  • The SIP MESSAGE method encodes the SMS per 3GPP TS 24.341
  • The Content-Type application/vnd.3gpp.sms identifies the SMS payload
  • The S-CSCF IFC triggers OmniMessage for SMS traffic
  • OmniMessage connects to traditional SMSC infrastructure
  • OmniMessage supports the GSM-7 and 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 through IMS:

Key Points:

  • The SMSC forwards the SMS to OmniMessage through traditional protocols (MAP/SMPP)
  • OmniMessage converts the SMS to the SIP MESSAGE method
  • The S-CSCF routes the SMS by the registered IMPU
  • OmniMessage supports delivery reports and status notifications
  • If the user is not IMS-registered, OmniMessage falls back to traditional SMS

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

Roaming Scenarios​

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

S8 Home Routed (S8HR): The home network's P-GW (5G: home UPF) anchors the roaming subscriber's IMS APN PDN connection over the S8 interface. The visited network's S-GW tunnels the bearer back to the home P-GW through GTP. Thus the UE reaches the home P-CSCF directly. The UE then registers with the home S-CSCF exactly as if it were on the home network. The results are:

  • 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).
  • The 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. They do not need IMS interconnect.
  • GSMA IR.92 / IR.94 compliant.

S8HR anchors the IMS APN in the home network. Therefore roaming is transparent to the CSCF. There is no roaming-specific CSCF configuration. The EPC (P-GW/APN selection) provisions the routing that keeps the IMS APN home-anchored. This document does not cover that routing.

CSCF Components​

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

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

Core Functions:

  • Security Association Management: Establishes and keeps IPsec tunnels with mobile devices to protect signaling and media
  • QoS Policy Enforcement: Authorizes and enforces media QoS by access type. It uses Diameter Rx to the PCRF for VoLTE (4G/LTE bearers). It uses the N5 SBI (Npcf_PolicyAuthorization) to the PCF for VoNR (5G NR).
  • NAT Traversal: Does far-end NAT traversal for devices behind NAT/firewalls
  • Compression: Supports SigComp for bandwidth-constrained networks
  • Service Route: Keeps the service route for the next requests

Emergency Services (E-CSCF):

  • Routes emergency calls without full IMS registration
  • Detects emergency through the Request-URI urn:service:sos (plus 112/911)
  • Processes location information for E911/E112
  • Resolves the calling-line identity (CLI) for anonymous/unknown callers per TS 23.167 Annex K. It uses the cached IMEI→MSISDN binding. If that fails, it falls back to an Rx subscription query to the PCRF (MSISDN → IMSI → IMEISV precedence).
  • Integrates with the LRF (Location Retrieval Function)

Supported Access Types:

  • LTE (VoLTE) through IPsec
  • 5G NR (VoNR) through IPsec. The N5 AF interface to the PCF authorizes the QoS.
  • WiFi (VoWiFi) through IPsec
  • Fixed broadband through SIP
  • Cable/DSL residential gateways

For detailed operations, see P-CSCF Documentation.

I-CSCF - Topology Hiding and Load Balancing​

The Interrogating-CSCF is the contact point in an operator's network. It accepts connections from other networks and from the same network.

Core Functions:

  • Topology Hiding: Hides the internal network structure from external networks
  • S-CSCF Assignment: Queries the HSS through the Cx interface to assign an S-CSCF to a new user
  • S-CSCF Selection: Selects the correct S-CSCF by capability 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 many S-CSCF instances
  • Capability Matching: Matches user requirements to S-CSCF capabilities
  • Roaming Support: Handles both home-routed and local breakout scenarios
  • Subscription Locator: Supports the Dx interface 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. It provides session control and service intelligence.

Core Functions:

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

Service Triggering via IFC: The S-CSCF downloads XML-based Initial Filter Criteria from the HSS. It uses the IFC 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: The S-CSCF processes the IFC in priority order
  • Service Chaining: The S-CSCF can invoke many AS in sequence (e.g., OmniTAS → OmniMessage)
  • Default Handling: Configurable behavior when an 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 complete 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 complete operational capabilities through OmniWeb, the unified management portal. This section covers common operational tasks and their significance.

Service Triggering and IFC Management​

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

IFC Operations:

  • Dump the user's IFC to view the configured service profile from the HSS
  • Test IFC matching with simulated call scenarios
  • Verify AS routing to confirm correct service invocation
  • Debug service failures. Examine the 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 uses Diameter interfaces for HSS, PCRF, and charging integration. Operators can do these tasks:

  • 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). For each VoNR session, the P-CSCF discovers the PCF that serves the UE's PDU session (NRF→BSF). Operators can do these tasks:

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

The OmniWeb P-CSCF page (NRF tab) shows the NRF registration state and the discovered PCF bindings. 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 guide has its own Troubleshooting section. The P-CSCF guide covers registration/IPsec and emergency-call issues. The S-CSCF guide covers registration/call-setup/SMS issues. The I-CSCF guide covers S-CSCF selection and Cx issues. The Diameter guide covers Diameter peer issues.

Regulatory Compliance​

3GPP Standards Reference​

OmniCSCF implements these 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. This compliance ensures 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. This profile ensures that commercial devices work 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. It needs no custom carrier bundles, proprietary APNs, or special provisioning.

✓ 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 gives users the same VoLTE experience when they visit other IR.92-compliant networks

What This Means: Operators can launch VoLTE services immediately with existing consumer devices (iPhone, Samsung, Google Pixel, etc.). They do not need to wait 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. It enables VoLTE services over untrusted WiFi networks.

VoWiFi Architecture with OmniCSCF:

VoWiFi Components:

  • OmniePDG: Evolved Packet Data Gateway. It terminates the IPsec tunnel for untrusted WiFi access.
  • OmniCSCF P-CSCF: Processes VoWiFi registrations in the same way as VoLTE (same service routes, same IFC triggering)
  • Seamless Handover: Devices can move between LTE and WiFi without call interruption

IR.94 Benefits:

  • The same IR.92 benefits apply to VoWiFi
  • Devices discover the ePDG automatically through DNS (no manual configuration)
  • A single IMS registration covers both VoLTE and VoWiFi
  • Indoor coverage extension without femtocells or DAS

For ePDG operations and VoWiFi troubleshooting, see the OmniePDG documentation.

Other GSMA Standards​

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