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.
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