Voice over LTE (VoLTE)

Voice over LTE (VoLTE)

According to the latest ‘Evolution to LTE’ report released by Global mobile Suppliers Associations (GSA), 113 commercial LTE networks are in operation in 51 countries around the world. By the end of next year, 209 LTE networks are expected to be deployed in 75 countries. LTE has been designed as a data-only IP technology. There is no inherent support for circuit switched voice. Since eventually, most operators will transform to LTE networks, the technology must incorporate voice. There are three popular options for providing voice services over LTE networks

1. Circuit-Switched Fallback (CSFB) – Presently, CSFB is the most widely used solution for carrying voice over LTE networks but it is not a true voice over LTE implementation. It involves switching to a 2G or 3G connection before initiating and receiving a circuit switched voice call. Although the technology requires certain hardware infrastructure modifications and utilizes multiple network elements to accomplish the transition from data to voice and vice-versa, it remains a relatively cost-effective solution to provide voice in LTE. It also has the ability to carry text messages. CSFB will be prevalent in the industry for at least next few years but it cannot be a long-term strategy. It has certain disadvantages. Sudden data session suspension and call setup delays can lead to poor user experience. Femtocells are not supported. CSFB can be extended to the reasoning behind lack of support of simultaneous data and voice sessions on Verizon, Sprint and other CDMA networks. Verizon has widespread LTE coverage in USA. But when it comes to voice, the phones on their network switch to the legacy 2G or 3G CDMA connection which is not designed to handle simultaneous voice and data. An extra radio in the phone solves this problem and most Verizon LTE phones incorporate the required hardware to carry two connections. But some smartphone manufacturers like Apple are reluctant to incorporate that extra radio to sustain battery life and lower costs. AT&T’s legacy GSM and UMTS networks have no such issues. Voice and data sessions can co-exist on phones subscribed to their network. This is also one of the factors contributing to better iPhone sales on AT&T as compared to Verizon despite Verizon having a better overall network than AT&T.

2. Voice over LTE Generic Access (VoLGA) - VoLGA provides voice over LTE by taking advantage of the network operator’s incumbent 2G or 3G infrastructure. A Generic Access Network (GAN) adds WiFi as an access network to a 3G UMTS system. Through GAN, a dual mode device can access network services by using WiFi. VoLGA uses this idea of GAN and replaces WiFi with LTE. There are no modifications required to the 3G and LTE radio network or core. VoLGA Access Network Controller (VANC) acts as a gateway between the LTE and 3G circuit switched domain. Signalling and data packets are transported between the device and circuit-switched network by VANC in a transparent flow. VoLGA offers better call setup times as compared to CSFB and supports femtocells. VoLGA gained traction when Deutsche Telekom (DT) announced its support for the technology in 2010. But DT later dropped VoLGA in favor of CSFB and it will eventually migrate to voice over IMS in LTE networks. VoLGA is now considered dead despite being a less expensive means for providing voice services on LTE networks.

3. Voice over IMS – The IP Multimedia Subsystem (IMS) is the chosen long-term strategy for deploying VoLTE. CSFB and VoLGA are not actual VoLTE technologies, but they are considered as two technologies used to implement voice on LTE networks in the short-term. However, an IMS based LTE network is the real VoLTE. In 2010, the global association of mobile operators, GSMA, announced the ‘One Voice’ initiative which defined a minimum mandatory set of standards to achieve interoperable high quality IMS-based voice and SMS service over LTE networks. The goal of this initiative was to harmonize the implementation of voice and SMS services on LTE for leading operators and handset manufacturers. IMS is an IP-based network which is deployed by operators to provide a range of applications such as text, media and video on a single IP platform. IMS is based on the Session Initiation Protocol (SIP). The SIP server in IMS is also known as the Call Session Control Function (CSCF) server. It is used for voice call control and service delivery. In simple terms, the outcome of this VoLTE architecture is that the voice is transported as data flows inside the LTE data.  Thus IMS enables LTE to administer VoIP and high speed data service simultaneously. One important feature of VoLTE is implementation of Single Radio Voice Call Continuity (SRVCC). The concept of SRVCC requires that when a user moves outside the LTE coverage area, it should experience a smooth handover to another technology such as 3G/UMTS or 2G/GSM. IMS achieves this by switching the session to circuit-switched domain. Another vital aspect of VoLTE will be seamless roaming and interconnection with another providers’ network. Note that IMS is not a new technology. It was first developed around 2006 but lost momentum once the focus started shifting to LTE. Now LTE itself needs IMS’ support. Over the period of last year, IMS based VoLTE technology has emerged as the industry wide standard for providing voice services through LTE. Ericsson is the leader in VoLTE solutions closely followed by Alcatel-Lucent and Nokia Siemens Networks. But IMS based VoLTE is a complex system and large scale deployments are not expected anytime soon. However, early deployments have already commenced in some parts of the world. SK Telecom and LG U+ in South Korea launched VoLTE services earlier this year. MetroPCS, which was recently acquired by T-Mobile, is the first player in US to offer VoLTE services. Verizon and AT&T are expected to join the VoLTE bandwagon in the later half of 2013.

Financial instability and wide scale utilization of HSPA+ in different parts of the world has resulted in slow LTE deployment and adoption. VoLTE comes beyond that and for now CSFB seems sufficient. Over the top (OTT) services like Skype and Facetime (for Apple devices) have somewhat obviated the need to launch VoLTE urgently. If I can talk and see my family and friends in US and overseas by using the data plan or WiFi on my device, I would not care much about plain voice service. Having said that, we also understand that voice is still a major chunk of operators’ revenue. So with time, as most operators migrate to LTE, they will have to use IMS for voice unless some better alternative comes along. The IMS based architecture is broadly recognized as the permanent solution for carry voice over LTE networks.