IMS and VoLTE are usually discussed as SIP and application-layer features, but the service quality depends heavily on the LTE radio stack below IMS. Voice, video, signaling, SRVCC, and bearer setup all depend on PDCP, RLC, measurement, UE capability, and radio bearer configuration.

This page summarizes radio-stack items that are commonly checked during IMS/VoLTE testing. These are reference items rather than a single universal mandatory list; the exact requirement can vary by operator profile, deployment stage, device category, and supported services.

Contents

• PDCP Parameters
• IRAT Measurements with Events B1 and B2
• SON/ANR
• FGI in UE Capability Information
• Requirements in Specification
  • GSMA IR.92
  • GSMA IR.94
  • 3GPP TS 36.523-1

PDCP Parameters

PDCP behavior affects header overhead and sequence-number handling for IMS bearers. In VoLTE testing, the most visible PDCP-related items are ROHC support and the PDCP sequence-number configuration used for relevant radio bearers.

• ROHC
• 7 Bit PDCP SN

IRAT Measurements with Events B1 and B2

For SRVCC, the UE may need to measure other radio technologies such as UTRAN, GERAN, 1xRTT, or HRPD while it is still served by LTE. The network can configure inter-RAT measurements such as events B1 and B2 so that it can decide whether and when to trigger SRVCC.

SON/ANR

SRVCC and VoLTE handover depend on correct neighbor-cell knowledge. Some neighbor information is provisioned during deployment, but some can be learned dynamically through SON/ANR. UE support for the related measurement and reporting behavior can therefore affect mobility robustness.

FGI in UE Capability Information

Feature Group Indicator (FGI) information in UE Capability Information tells the network which LTE radio features the UE supports. For IMS/VoLTE, this helps the network choose suitable bearer, mobility, and radio configuration. For details, see UE Capability Information in RRC/NAS for IMS.

Requirements in Specification

The following items summarize specification-level radio bearer and radio capability requirements that commonly appear in VoLTE and IMS video call testing.

GSMA IR.92

4.2.1 Radio Bearers

The UE must support the following combination of radio bearers for Voice over IMS profile (see Annex B in 3GPP TS 36.331):

SRB1 + SRB2 + 4 x AM DRB + 1 x UM DRB

The network must support the following combination of radio bearers:

SRB1 + SRB2 + 2 x AM DRB + 1 x UM DRB

One AM Data Radio Bearer (DRB) is used for the EPS bearer with QCI = 5 and another AM DRB is used for the EPS bearer with QCI = 8/9. A UM DRB is used for the EPS bearer with QCI = 1.

4.2.2 DRX Mode of Operation

To maximize UE battery lifetime, LTE Discontinuous Reception (DRX) as specified in 3GPP TS 36.300 and 3GPP TS 36.321 must be deployed. DRX support is mandatory for both UE and network.

4.2.3 RLC configurations

Radio Link Control (RLC) must be configured to perform data transfer in the following modes as specified in 3GPP TS 36.322:

• Unacknowledged Mode (UM) for EPS bearers with QCI = 1
• Acknowledged Mode (AM) for EPS bearers with QCI = 5
• Acknowledged Mode (AM) for EPS bearers with QCI = 8/9

Voice service can tolerate error rates on the order of 1%, while benefiting from reduced delay, and is mapped to a radio bearer running RLC Unacknowledged Mode.

GSMA IR.94

4.3 LTE Radio Capabilities

The UE must be able to use two UM Radio Data Bearers for two GBR bearers that may be used for a conversational video call. The UE must therefore support feature group indicator 20 indicating support for the following radio bearer combination for conversational video service:

SRB1 + SRB2 + 5 x AM DRB + 3 x UM DRB

If the network uses a UM DRB for the video media stream, it must support a radio bearer combination with at least:

SRB1 + SRB2 + 2 x AM DRB + 2 x UM DRB

This corresponds to Radio Bearer combination identifier 5 in table 12.1 in 3GPP TS 36.523-1.

In addition to the bearer usage specified in section 4.2.1 of GSMA PRD IR.92, one DRB is used for an EPS bearer for video.

3GPP TS 36.523-1

36.523-1 Table 12-1: Radio Bearer Combinations