Voice and SMS in LTE

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[资料名称】：Voice and SMS in LTE

【资料作者】：C. Gessner, O. Gerlach

【资料日期】：2011-05

【资料语言】：英文

【资料格式】：PDF

【资料目录和简介】：

1 Introduction ............................................................................ 3

2 Overview ................................................................................. 3

3 Circuit switched fallback (CSFB) .......................................... 5

3.1  Circuit switched fallback to GERAN or UTRAN ........................................5

3.2  Circuit switched fallback to 1xRTT...........................................................10

4 SMS over SGs....................................................................... 15

5 Support of voice and SMS via IMS ..................................... 18

5.1  Overview of the IMS framework ................................................................19

5.2  EPS attach and P-CSCF discovery ...........................................................23

5.3  IMS registration, authentication and key agreement..............................24

5.4  Obtaining voice services via IMS..............................................................27

5.5  Radio aspects .............................................................................................29

5.6  Obtaining SMS services via IMS ...............................................................31

5.7  Single radio voice call continuity .............................................................34

6 Test solutions for voice and SMS in LTE ........................... 35

6.1  Verifying the terminal protocol stack .......................................................35

6.2  Voice quality testing ..................................................................................39

7 Conclusion............................................................................ 40

8 Literature............................................................................... 40

9 Additional information ......................................................... 42

10  Abbreviations ....................................................................... 42

部分内容：

2 Overview
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.

When it comes to voice support in LTE, the IP Multimedia Subsystem (IMS) is a key
technology. IMS provides a framework for supporting IP based services and requires
new IMS-specific network elements as part of the dedicated core network architecture.
The first version of IMS was standardized in 3GPP release 5, with many
enhancements specified in subsequent releases.
In the early days of LTE standardization, it had been assumed that IMS would be
commercially available when the first LTE networks were deployed. Voice support was
also considered as being addressed by IMS. However, rollout of IMS was slower than
expected, with the result that voice support in LTE had become a real challenge for
many network operators.
Alternatives and intermediate solutions for supporting voice services in LTE were
investigated more deeply. The most important and commercially relevant one is the
circuit switched fallback which basically provides subscribers with voice services via
the legacy networks GSM, UMTS, or CDMA2000® 1xRTT. The users “fall back” to one
of these technologies as soon as they initiate a voice call within LTE coverage or if
they accept a terminating voice call. CSFB has become the recommended solution for
network operators who are looking for an intermediate non-IMS voice solution. A
related recommendation from the Next Generation Mobile Networks (NGMN)
organization was launched [2]. The first LTE networks are already using CSFB for
voice support. CSFB is also supporting a variety of roaming scenarios, ensuring that
existing circuit switched roaming agreements can be met.
A closely related topic to support of voice is the support of SMS as another key circuit
switched service. SMS is a huge revenue driver for network operators worldwide. A
non-IMS based solution for SMS was specified by 3GPP, the “SMS over SGs” solution
for GSM and UMTS networks, where SGs is the name of a core network internal
interface. SMS over SGs allows network operators to support SMS as a circuit
switched service within LTE. In October 2009, the industry initiative NGMN announced
the recommendation to implement “SMS only over SGs” as a minimal requirement to
enable roaming [2].
In the long run, support of voice and messaging in LTE over IMS remains the major
target. A related industry initiative is VoLTE (Voice over LTE). It was formally
announced in February 2010 [3] by the network operator organization GSMA (Global
System for Mobile Communications Association). VoLTE has developed the framework
for optimum support of voice and SMS over IMS in LTE, including roaming and
interconnect issues. VoLTE is based on existing IMS multimedia telephony (MMTel)
concepts [4].
Currently there is quite a variety of scenarios, technology options, and deployment
scenarios. In the following sections, the different techniques to support voice and SMS
in LTE will be explained in more detail. Support of these techniques, CSFB, SMS over
SGs, and VoLTE, also implies new test and measurement requirements in the
development and verification of terminals and networks. This white paper highlights
possibilities to test the functionality and the performance of voice and SMS services.
The focus is on terminal testing. Especially when it comes to voice over IP, evaluating
voice quality and voice call performance are key issues for both manufacturers and
network operators. For testing voice quality, reliable and industry-established
mechanisms are already available which will be introduced in this paper.