For mobile network operators, voice and short message service (SMS) have
consistently been major sources of revenue. Optimum support of these conventionally
circuit switched services in data-oriented LTE is essential. This white paper
summarizes the technology options for supporting voice and SMS in LTE, including
circuit switched fallback (CSFB), SMS over SGs, and voice over LTE (VoLTE). It
includes background information on the standardization process, and the commercial
implications for the different options. The white paper also addresses test and
measurement requirements resulting from the support of voice and SMS in LTE. This
includes terminal protocol testing and voice quality testing.
Basic understanding of the LTE access stratum and non-access stratum protocol
architecture is assumed. For a detailed introduction to LTE technology, please refer to
LTE was specified by the 3rd Generation Partnership Project (3GPP) in release 8. LTE
is designed as a pure packet switched system. Legacy circuit switched services are no
longer supported. This implies that support of voice within LTE has to be done with
voice over IP.
Supporting voice over IP in a cellular communication system brings a lot of new
challenges. Subscribers expect the same quality of service they know from circuit
switched voice services, in GSM networks for example. Network operators worldwide
have invested heavily in their established circuit switched networks, so moving the
voice services to a new technology must have clear commercial and technical
advantages for them. Additionally, LTE is complex and designing the LTE technical
specifications has taken time. 3GPP release 8 was just the first release of the LTE
standard, with many enhancements added in the release 9 specifications.
For these reasons, many network operators have decided to deploy their first
commercial LTE networks focusing on support for data services, by providing data
dongles as terminals for high-speed internet connectivity. Voice service support is
provided by the legacy networks in this first phase of LTE deployment.
Voice support in LTE requires the right mechanisms and architecture in radio and core
networks, to guarantee quality of service and a good user experience. But it is not only
the LTE network that is affected: nationwide LTE coverage can hardly be achieved
from day one onwards. Thus, subscriber mobility between LTE and legacy GSM,
UMTS HSPA/HSPA+, and CDMA2000® 1xRTT/EV-DO1) networks is a key
requirement. This is important, to achieve a seamless service experience for all
services, including voice as soon as it will be provided within LTE.