Contents

Introduction

Prerequisites

Requirements

Components Used

Overview

T-ADS Functionality

MME's Role in T-ADS Functionality

Configure

Network Diagram

Configurations

Verify

Introduction

This document describes how the Mobility Management Entity handles Terminating Access Domain Selection functionality and its implementation in MME.

Prerequisites

Requirements

Cisco recommends that you have knowledge of the StarOS-Mobility Management Entity (MME) admin guide.

3GPP Technical Specifications - 29.272, 23.292

Components Used

This document is not restricted to specific software and hardware versions.

The information in this document was created from the devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If your network is live, ensure that you understand the potential impact of any command.

Overview

T-ADS Functionality

Terminating Access Domain Selection (T-ADS) is a functionality in the IP Multimedia Subsystem (IMS) network that determines where a call is terminated for a Voice over Long-Term Evolution (VoLTE) user and ensures that the IMS routes the call to the User Equipment (UE) when it is in 2nd Generation/3rd Generation (2G/3G) coverage. If LTE coverage is lost, the UE can still be able to use Circuit-Switched (CS) voice services, even if Single Radio Voice Call Continuity (SR-VCC ) is not available. To route Mobile-Originating (MO) and Mobile-Terminating (MT) calls appropriately, the IMS must know whether the UE is in LTE or Universal Terrestrial Radio Access Network (UTRAN) or GSM EDGE Radio Access Network (GERAN) CS coverage. T-ADS is the function that provides this support.

This call flow describes how the Home Subscriber Server (HSS), MME and IMS network interact to determine the best access network for terminating a voice call.

1. IMS Network Receives an Incoming Call

- A terminating voice call arrives at the Serving Call Session Control Function (S-CSCF) in the IMS core.

- The S-CSCF queries the HSS to determine the best way to route the call based on the UE's current access network.

2. S-CSCF → HSS: Send User-Data-Request (UDR).

- The S-CSCF sends a UDR to HSS to fetch the latest subscriber information.

- The request asks for:

- IMS-VoPS (IMS Voice over PS Sessions support)

- Last known Radio Access Technology (RAT) type

- UE’s last seen location Tracking Area Identity (TAI) or Routing Area Identity (RAI) information.

3. HSS → MME: Insert-Subscriber-Data-Request (ISDR)

- To get real-time UE location and network support details, HSS sends ISDR to MME.

- ISDR contains:

- T-ADS Data Request Flag (T-ADS Data Request:1)

- Request for RAT-Type

- Request for Last-UE-Activity-Time

4. MME → HSS: Insert-Subscriber-Data-Answer (ISDA)

- MME retrieves:

- Current UE registration details

- Latest RAT-Type (LTE, 3G, 5G)

- Last-UE-Activity-Time

- MME responds to HSS with an ISDA message, providing the requested information.

5. HSS Determines the Best Terminating Access

- HSS evaluates:

- If UE is active in LTE, and VoLTE is supported → IMS call via LTE.

- If UE is in GERAN/UTRAN (2G/3G), and Circuit Switched FallBack (CSFB) is available → Redirect call via CSFB.

- If UE is in New Radio (NR) = 5G Non-StandAlone/StandAlone (NSA/SA), determine Evolved Packet System (EPS) fallback or Voice over New Radio (VoNR) options.

6. HSS → S-CSCF: Send User-Data-Answer (UDA)

- HSS responds with a User-Data-Answer (UDA), which includes:

- IMS-Voice-Over-PS-Sessions-Supported AVP

- Homogeneous-Support-of-IMS-Voice-Over-PS-Sessions AVP

- Last known RAT Type (LTE, GERAN, UTRAN, NR)

- Last-UE-Activity-Time (if available)

7. HSS → S-CSCF: Update with Call Routing Decision

- HSS informs S-CSCF about the selected access network:

- If LTE IMS VoPS is supported, continue call setup over LTE.

- If LTE IMS VoPS is NOT supported, initiate CSFB to 2G/3G for circuit-switched fallback.

- If 5G NSA, decide whether to use EPS fallback or VoNR.

8. IMS Call Setup Over the Selected Access

- If VoLTE is supported → Call proceeds over LTE IMS.

- If CSFB is required → Call is redirected to 2G/3G via SGs interface.

- If EPS fallback is triggered → Call is handed over to LTE from 5G.

T-ADS Functionality Call Flow

MME's Role in T-ADS Functionality

MME plays a crucial role in the Terminating Access Domain Selection (T-ADS) process by providing real-time network and subscriber-related information to the HSS and IMS Core. Its primary functions in T-ADS include:

1. Providing UE's Last Known Access Information

- When HSS requests the last known Radio Access Technology (RAT) Type and Last-UE-Activity-Time, the MME retrieves and sends this data.

- This helps the HSS and IMS core determine whether the UE is on an LTE/5G network (for VoPS) or a legacy network (for CS fallback).

2. Handling Insert-Subscriber-Data Requests (ISDR) from HSS

- Upon receiving an ISDR message from the HSS, the MME extracts:

- Current RAT Type (LTE, NR, GERAN, UTRAN).

- Timestamp of the last radio contact with the UE.

- IMS Voice over PS Sessions Support status in the UE’s registered tracking area.

- The MME then sends an Insert-Subscriber-Data Answer (ISDA) message back to the HSS, containing the requested details.

3. Supporting VoLTE or Circuit-Switched Fallback (CSFB) Decisions

- If the IMS Voice over PS Sessions is supported, the IMS Core can continue the call over LTE VoLTE.

- If the UE is not on an IMS VoPS-supported network, the MME can facilitate CSFB to 2G/3G networks via SGs interface.

4. Assisting IMS Core in Call Routing Decisions

- The IMS Core and SCC AS rely on the MME’s response to determine whether:

- The call can proceed over VoLTE.

- EPS Fallback to LTE from 5G is needed.

CSFB to a 2G/3G network is required for call delivery.

Configure

Network Diagram

Network Diagram of EPS and IMS System

Configurations

Certain configurations need to be done in MME to facilitate T-ADS support:

associate

Associates various MME -specific lists and databases with this call control profile

Mode

Exec > Global Configuration > Call Control Profile Configuration

configure > call-control-profile profile_name

Entering the command sequence results in the prompt:

[local]host_name(config-call-control-profile-profile_name)#

Syntax

associate hss-peer-service service_name s6a-interface

network-feature-support-ie

Configures support for the IMS Voice over Packet-Switched indication and Homogeneous Support of IMS Voice over PS indication.

Product

MME

Privilege

Administrator

Mode

Exec > Global Configuration > Call Control Profile Configuration

configure > call-control-profile profile_name

Entering the command sequence results in the prompt:

[local]host_name(config-call-control-profile-profile_name)#

Syntax

network-feature-support-ie ims-voice-over-ps supported

diameter update-dictionary-avps

Specifies which release of 3GPP TS 29.272 is to be used for the HSS peer service.

Mode

Exec > Global Configuration > Context Configuration > HSS Peer Service Configuration

configure > context context_name > hss-peer-service service_name

Entering the command sequence results in the prompt:

[context_name]host_name(config-hss-peer-service)#

Syntax

diameter update-dictionary-avps 3gpp-r11

Verify

Using the mentioned configurations, the call flow was tested in our internal TAC lab and could see the proper parameters from StarOS MME.

Here is the successful pcap collected using mentioned reference configurations.

MME PCAP Highlighting ULA Diameter Message

MME PCAP Highlighting ISDR Diameter Message

MME PCAP Highlighting ISDA Diameter Message