IMS

Radio Stack Requirement for IMS/VoLTE 

Even though most of the IMS related function is mostly about application layer technology, for optimal functioning there would be several important issues related to Radio Protocol Stacks.

The topics described in this page is not the mandatory features. I put down these topics for reference. The detailed requirement would vary with the requirement from each carriers or with stages of IMS deployment.

PDCP Parameters

There can be two major factors in PDCP related to IMS, especially VoLTE. Followings are factors that should be considered for VoLTE services.

• ROHC
• 7 Bit PDCP SN

IRAT Measurements with Events B1 and B2

In case of SRVCC, LTE and some other radio technology (e.g, UTRAN, GERAN,1xRTT & HRPD). In live network, it is most likely Network would tell UE to perform a specific measurement to guarantee the operation of SRVCC.

SON/ANR

Especially for SRVCC or Handover during VoLTE, Network should have proper neighbour cell information for a specific UE. Some of the neighbour cell information would already be stored in the network during the deployment, but some of the information should be gathered by some dynamic mechanism called 'SON/ANR' and UE has to support this capability.

FGI in UE Capability Information

FGI Field in UE Capability Information for IMS/VoLTE so that Network properly perform proper configuration. (For details, refer to UE Capability Information section in RRC/NAS for IMS page)

Requirements in Specification

< 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 utilized for Evolved Packet System (EPS) bearer with Quality of Service Class Indicator (QCI) = 5 and another AM DRB for EPS bearer with QCI = 8/9. UM DRB is utilized for EPS bearer with QCI = 1. EPS bearer usage is described in Section 4.3.

4.2.2 DRX Mode of Operation

In order to maximize lifetime of the UE battery, LTE Discontinuous Reception (DRX) method as specified in 3GPP TS 36.300 and 3GPP TS 36.321 must be deployed. Support of DRX is mandatory for both UE and network.

4.2.3 RLC configurations

Radio Link Control (RLC) entity 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 delays, and is mapped to a radio bearer running the RLC protocol in unacknowledged mode (UM).

< 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 combination of radio bearers for conversational video service (see Annex B in 3GPP TS 36.331):

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

If the network uses an 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 36.523-1 >

< 36.523-1 Table 12-1: Radio Bearer Combinations >