A UE ATTACH Request contains an optional IE: Voice domain preference and UE’s usage setting, refer to TS 24.301 [46], TS 24.008 [45], using which a UE can provide additional information on its corresponding mode of operation to the core network. Using the optional IE: Voice domain preference and UE’s usage setting, an E‐UTRAN UE indicates its CSFB (to the legacy 2G or 3G network for a voice call) capability in case the network or the UE does not support VoLTE call or the UE fails to registers with the IMS.
Note: Till the TS 24.008 [45] 9.1.0 version, the CSFB capability of a UTRAN UE was indicated within the MS Network Capability mandatory IE in the UE ATTACH Request message to the GPRS core network. However, from TS 24.008 9.2.0 version onwards, the CSFB capability information is conveyed to the GPRS core network through the Voice domain preference and UE’s usage setting optional IE only. This IE is also used by an E‐UTRAN UE to convey its CSFB capability information in the ATTACH Request message to the LTE/EPC. CSFB capability information is no longer part of the MS Network Capability IE.
E‐UTRAN UE Mode of Operations and Their Transitions
In this paragraph, the mode of operation of an E‐UTRAN UE, as mentioned in Table 6.2, is further described. An E‐UTRAN UE registered with an LTE/EPS network for EPS only services is said to be operating in PS mode. An E‐UTRAN UE registered with LTE/EPS and legacy GSM/UMTS network for EPS and non‐EPS, i.e. voice, SMS, services are said to be operating in CS/PS mode. In addition to this, an E‐UTRAN UE may be required to switch between the PS and CS/PS modes of operation, and vice versa, because of various reasons that take place at the network level at runtime without the user’s notice. Some of the typical reasons are as follows:
Unavailability of CSFB feature,
Unavailability of IMS,
Changes in preferences, network operations,
Roaming, and
Failure to register with an IMS.
An E‐UTRAN UE may be also required to switch between the PS and CS/PS mode of operations, and vice versa, because of a user action. For example, consider that a VoLTE‐capable UE is registered with the LTE/EPC and IMS. The user has turned off the VoLTE preference in the handset, making the UE unable to facilitate voice‐over IMS to the user. In such a case, the UE is required to register with the CS domain core network so that it can fall back to the legacy network to facilitate voice calls to the user. As a result of the transition from the PS to CS/PS mode of operations, and vice versa, a UE requires to perform protocol‐related procedures so that it can stay updated with the core network with its current location information and continue enabling seamless voice and data services to a user.
Examples 6.4 and 6.5 below highlight the information passed by a UE as part of its signaling messages to the LTE/EPC network based on the UE mode of operations described above.
Example 6.4 UE usage setting and voice domain preference during LTE/EPS Attach Procedure
During the initial rollout of an LTE/EPS network, an operator may not have the IMS to offer its VoLTE services to subscribers. In such a scenario, the operator will be required to provide voice services to subscribers through the existing legacy GSM/UMTS network. The arrangement of providing voice service to an E‐UTRAN UE and its user is known as the CSFB method, as described earlier in Section 6.2.3, creating a UE context between the MME and the MSC over SGs interface. In this interworking scenario, the E‐UTRAN UE will send the following information during its initial registration, through the ATTACH Request procedure, with the LTE/EPC.
UE to EPC (ATTACH Request)Attach Type = Combined EPS/IMSI AttachUE Usage Setting = Voice CentricVoice domain preference = CS Voice Only
The above UE Usage Setting and the voice domain preference information may also be sent by a UE if it is not VoLTE capable or fails to register with the IMS. The LTE/EPC will send the following information as part of the ATTACH ACCEPT message to the UE:
EPC to UE (ATTACH Accept)EPS Attach Result = combined EPS/IMSI attachAdditional Update Result = CS fallback not preferred
Example 6.5 UE usage setting and voice domain preference during LTE/EPS Attach Procedure with IMS
For a VoLTE only capable E‐UTRAN UE and LTE/EPC with IMS capability, the UE will send the following information during its initial registration, through the ATTACH Request message, procedure with the EPC:
UE to EPC (ATTACH Request)UE Usage Setting = Data‐CentricVoice domain preference = IMS Voice Only
The LTE/EPC will send the following information as part of the ATTACH ACCEPT message to the UE:
EPC to UE (ATTACH Accept)EPS Attach Result = EPS OnlyEPS network feature support = IMS voice‐over PS session in S1 mode supported
For more details on the UE mode of operations, its usages, voice domain preference, and the tasks performed in each transition, refer to TS 24.301 [46].
6.7 Function of E‐UTRAN in a VoLTE Call
The nature of VoLTE traffic is different from normal IP/data traffic. The IP packets size of a VoLTE traffic is small and is predictable, whereas the IP packets of normal data services are larger than voice packets, busty in nature, and may be downloaded or uploaded quickly. After normal IP packets transfer, radio resources allocated to the UE may be released by the eNodeB. The allocation and releasing of radio resources between UEs and the eNodeB for normal IP traffic is dynamic in nature and involves signaling overhead over the LTE air interface. Unlike normal IP traffic, the data rate of voice packets of a VoLTE call is low and remains the same throughout the conversation. Thus, the same radio resources may be allocated semi‐statically for the entire duration of the session, and no additional radio resources are required to be allocated dynamically by the eNodeB. Because of this, a semi‐static allocation of radio resources reduces the signaling overhead between the UE and eNodeB. In addition to this, the UE is also not required to monitor the downlink Physical Downlink Control Channel (PDCCH) for additional resources. During a voice call, only one party talks at a time while the other party listens.
To support all the above aspects of a VoLTE call over an IMS, the following functionalities are required to be made available and enabled in the E‐UTRAN (UE and eNodeB) for rich user experience and an optimum allocation and usages of radio resources over the LTE air interface.
Semi‐Persistent Scheduling, for allocation of semi‐static radio resources to a UE, thus reducing signaling and scheduling
