Different techniques to support the provision of voice over LTE

LTE is the new cellular system developed by the 3GPP. The systems is composed of a radio part , called evolved UTRAN (eUTRAN) and the core network part called the Evolved Packet Core. The system composed of these two parts is called the Evolved packet System (EPS). The core network supports GERAN and UTRAN radio access and it also supports interworking with the legacy circuit switched domain. The non 3GPP networks are also supported. LTE is a 4G technology which precedes UMTS and HSPA, and it is adheres to the requirements of the IMT advanced standard as prescribed by the ITU. However, EPS does not have a circuit switched domain to handle voice calls in the traditional way as in 2G And 3G. In 2G and 3G voice has been separated from the data by utilizing circuit switched and packet switched domain respectively. According to GSMA, voice revenue will still account for 61 % of all mobile service revenue in 2013. So it critical for the mobile industry had to come up with a solution for LTE to be able to handle voice calls. There are different techniques that can be used to support voice over LTE and they all will be based on voice over IP (VOIP). This blog does cover all the proposed alternative techniques. It will only cover the four main techniques that have been identified.

• Circuit Switched Fallback ( CSFB) to 2G/3G Circuit Switched-3GPP
• Circuit Switched over packet Switched-voice over LTE via Generic Access( VOLGA)
• Over the top (OTT)
• IMS based 3GPP Multimedia Subsystem (IMS) Multimedia Telephony(MMTel)


Circuit Switched Callback
CSFB is a 3GPP interim standard solution for early deployments stages of LTE where IMS is not available. During the early stages of LTE deployment it may take some time before IMS and VOIP services can be provided. 3GPP decided on a phased approach for the provision on voice services on LTE. Since CSFB only provides support for voice and sms , it is considered as an interim solution in the evolution towards into a fully fledged multimedia communication service.


3GPP CSFB standards work specifies the architecture enhancements for functionality to enable Fallback from E-UTRAN access to 2G/3G CS domain access. Using this functionality enables the provision voice and other CS-domain services (e.g. CS UDI video / SMS/ LCS / USSD) by reuse of CS infrastructure. The specifications documents overall CS Fallback architecture and stage 2 level specifications for the 3GPP components. Roaming aspects are documented as well. CT1 and CT4 specifies Stage3 capabilities and functionalities according to SA2 Stage 2 in TS 23.272 as follows:

• combined EPS/IMSI attach and tracking area update procedures based on similar mechanisms as those used by the combined GPRS attach and routing area update procedures from 3GPP TS 24.008, in order to register the UE to the CS domain for future Fallback from E-UTRAN access to 2G/3G CS domain access;
• IMSI or combined EPS/IMSI detach procedure, both UE and network initiated, based on similar mechanisms as those used by the GPRS detach procedure from 3GPP TS 24.008;
• CS fallback related capability for the UE to receive the CS paging request via EPS and respond to this paging via the GERAN/UTRAN, or to request a CS call in E-UTRAN and establish the call in GERAN/UTRAN;
• procedures for supporting Mobile originating and Mobile terminating SMS in EPS;
• procedures required for the support of CS fallback to 1xRTT;
• procedures on the SGs interface between the MME and the MSC, based on the Gs interface procedures.

Figure 1:CS fallback in EPS architecture

The CS fallback in EPS function is realized by reusing the Gs interface mechanism between the MSC Server and the MME. Gs is an interface for exchanging information between MSC/VLR and SGSN. MME is a logical node accommodating a base station (eNodeB) and providing mobility management and other functions.
If you have a mobile LTE terminal, a call arrives at the terminal from the existing CS domain via EPC. The mobile terminal receives a paging message and recognises that the network is calling for a CS based voice and therefore switches to 2G/3G . The response confirming the call acceptance will be sent to from the mobile terminal to the 2G/3G Cs system and from that point on all call control for the voice service will be performed on the 2G/3G side.
The CS callback consists of a function that notifies a mobile terminal about a call request from the CS domain and combined mobility management functions between CS domain and the EPC for that purpose.

Circuit Switched over packet Switched-voice over LTE via Generic Access ( VOLGA)

VOLGA is positioned as the interim alternative to address the provision of voice services on LTE prior to IMS LTE deployments. VOLGA is not standardized by 3GPP. The main idea of the VOLGA solution is to adapt Universal Mobile Access/Generic Access Network (UMA/GAN) for LTE and reuse 3GPP Single Radio Voice Call Continuity (SRVCC) mechanism for handover from LTE to 2G/3G CS. Using the 3GPP GAN standard as the basis for voice and SMS service delivery over LTE has a number of advantages.
This solution is recommended and developed by the VOLGA Forum. The forum is working to develop VoLGA specifications, based on the existing 3GPP GAN standard, which are designed to provide voice, SMS (and other circuit-switched) services as they transition between GSM, UMTS and LTE access networks. It is expected that the VoLGA specifications will be presented to the 3GPP for consideration as a recognized standard. The concept is to connect the already existing Mobile Switching Centers ( MSC) to the LTE network via a gateway. As no fallback to a legacy network is required, call setup times are not increased and the user's quality of experience is similar to that of the 2G or 3G voice environment.

Over the top (OTT)
Some network operators could decide to offer voice services over LTE with external partners such as Skype and Google talk. However, the challenge to this solution is that external voice service providers have no control over quality of service in the wireless network, as such they cannot ensure a good quality of experience to the user. This could be solved by installing logic in the network to ensure quality of service for data streams that are recognized to belong to an external voice service the user has subscribed to. Another challenge with OTT is that calls cannot be handed over to a circuit switched 2G or 3G network when a user leaves the LTE coverage area.

IMS based 3GPP Multimedia Subsystem (IMS) Multimedia Telephony(MMTel)
3GPP IMS Multimedia Telephony based on IMS is the standardized long term solution to offer voice services over LTE. IMS is an access system that works towards different access types including fixed networks, UMTS, GSM, LTE and WLAN. It is composed of a service layer with interfaces towards application servers or service platforms and a session control layer dealing with integration towards underlying access networks. IMS Centralized Services (ICS) and Single Radio Voice Call continuity (SRVCC) are essential features for the use of IMS for VOIP. SRVCC handles handover from packet based access to Circuit switched for calls that are anchored in the IMS. The SRVCC uses IMS for call control and requires support of ICS. ICS enables connections to be incorporated in IMS sessions in a consistent way. ICS is also needed to support supplementary services such as call hold and call wait.
IMS based solution has many advantages, it can support all standard voice call service features such as call waiting, call hold and call barring, and is capable of serving large numbers of subscribers. IMS can also be used to integrate voice calls with enhanced, rich features such as presence, instant messaging and video content, across networks run by different mobile operators. The One Voice group was formed to work on an IMS-based solution. The work of One Voice group is used as the basis of the GSMA’s Voice over LTE initiative (VoLTE), which is aiming to accelerate the launch of IMS-based voice services as the long term solution for voice over LTE.

References
Ericsson white paper .December 2010. Voice over LTE.

Kjuus, Hakon, E. and Nordvik, AnneMari. 2010. Handling of Voice and Data Services in the Evolved Packet Core.

Tanaka Itsuma, Koshimizi Takashi, Nishida Katsutoshi. CS Fallback Function for Combined LTE and 3G Circuit Switched Services. NTT Docomo Technical Journal Vol.11 No.3

Martin Sauter. August 2009. VOLGA – A Whitepaper.

http://www.3gpp.org