LTE standards are developed by 3GPP. 3GPP is a partnership project between different standardization organizations ( known as organizational partners) in the field of telecommunications, which was established in 1998 to formulate technical specifications and technical reports for a 3G mobile system based on evolved GSM core networks and the radio access technologies that support (Universal Terrestrial Radio Access (UTRA) both FDD and TDD modes. Subsequently, the work of 3GPP was extended to include the maintenance and development of the Global System for Mobile communication (GSM) technical specifications and technical reports including evolved radio access technologies namely General Packet Radio Service (GPRS) and Enhanced Data rates for GSM Evolution (EDGE).
The organizational partners (OG) of 3GPP are the following:
• The Association of Radio Industries and Businesses (ARIB)-Japan
• Alliance for telecommunications Industry Solutions ( ATIS) –North America
• China Communications Standards Association ( CCSA) –China
• The European Telecommunications Standards Institute ( ETSI) Europe
• Telecommunications Technology Association (TTA) Korea
• Telecommunication Technology Committee ( Japan)
The main role of the OGs is to determine the general policy and strategy of 3GPP. The OGs may invite Market Representation Partners( MPR) as they deem fit to take part in the work of 3GPP and offer market advice and to bring to the latter a consensus view of market requirements.
The technical specifications within 3GPP are developed by Technical Specification Groups (TSGs). Each TSG is responsible for preparing, approving and maintaining specifications in terms of its terms of reference. The TSG may organize their work in Working Groups(WG) and liaise with other groups as appropriate. The TSGs report to the Project Coordination Group ( PCG), which is the highest decision making body in 3GPP.
When LTE was first conceived , 3GPP made a decision that LTE would be the first technology in the GSM family not to support circuit-switched connections
The reason for this decision was that a single IP-based transport system is key to ensuring that LTE networks are efficient enough to carry large volumes of traffic cost-effectively. Unfortunately this decision came with a price that LTE would not support voice services. As such there was there was an assumption that operators would provide voice services using an IP Multimedia Subsystem (IMS) and the related Multimedia Telephony Service (MMtel) standard.
However, when IMS/MMtel was conceived in 2006, many operators were not enthusiastic in their support for IMS and there was some pressure to define an alternative means of implementing voice services on LTE networks. Consequently, 3GGP began with the process of defining CSFB approach as an interim solution, that could be used by operators until they were ready to adopt IMS. This opened the door to other approaches and 3GPP began to define another interim approach namely circuit-switched over packet-switched system (CS over PS). The emergence of these different solution raised some questions as to whether these interim approaches would end up being used for a long time. Another issue that was raised was that the GSM principle of interoperability might be put at risk. Would it be possible for devices supporting these solution to roam on each other networks. As a result, 3GPP decided to halt the CS over PS work. 3GPP decision to halt the CS over PS work was based on the grounds that they did not want to be perceived as endorsing the third option.
Consequently, in March 2009 the VoLGA Forum was established to continue with the definition of CS over PS outside of 3GPP. The VoLGA Forum has 19 member companies, including Alcatel-Lucent, Cisco, Deutsche Telekom, Ditech Networks, GENBAND, HTC, Huawei, Kineto Wireless, LG Electronics, Mavenir, Motorola, Samsung, Sonus, SPA, Tecore Networks, Ulticom, Vitendo, WiChorus and ZTE. Ericsson was originally a member of the VOLGA Forum but revoked their membership in December 2009 citing that when VOLGA was established they were under the impression that VOLGA would become a global solution however it seemed that VOLGA would not be supported by mobile operators. In September 2009, the VoLGA Forum published the first complete set of approved specifications for delivering mobile voice and SMS messaging services over LTE access networks.
VoLGA will enable mobile operators to do the following:
• Bring voice and SMS services over LTE radio access networks and femtocells.
• Support new revenue-generating applications over LTE, such as IMS combinational services and the Rich Communications Suite (RCS).
• Provide an improved overall user experience capitalizing on the performance advantages of LTE.
• Navigate a smooth transition to IMS telephony.
• Launch data-only LTE devices leveraging the existing SMS-based customer care applications such as remote device management, configuration updates and SMS-based service announcements.
In July 2010, the VoLGA Forum released an updated set of specifications for delivering mobile voice and SMS services over LTE. This latest release adds key new features and capabilities to facilitate rapid deployment, including the following:
• The use of VOLGA over HSPA
• Support for SIM-less emergency calling
• Optimized voicee-bearer routing
The One Voice Initiative was a collaboration between AT&T, Orange, Telefónica, TeliaSonera, Verizon Wireless, Vodafone, Alcatel-Lucent, Ericsson, Nokia Siemens Networks, Nokia, Samsung and Sony Ericsson that used current open standards to define the minimum mandatory set of functionality for interoperable IMS-based voice and SMS over LTE. A powerful group of more than 40 organizations including major operators and equipment manufacturers behind this initiative. T-Mobile was not a member of One Voice as they are a member of VOLGA. The goals of One Vice Initiative included the following:
• Define a common approach to implement voice and SMS in LTE
• Speed up commercial deployment of IMS voice over LTE
• Pave the way to true multimedia architecture by building an LTE voice ecosystem based on IMS
• Avoid fragmentation caused by alternative market options
The intention of this initiative was to convince the GSMA and 3GPP to formally recommend its adoption and it succeeded in doing that. Version 1 of One Voice, Voice over IMS profile was published in November 2009 and this was announced in a press release at LTE Americas in Texas, USA.
The GSMA adopted the technical profile developed by the One Voice Initiative and established the Voice over LTE ( VOLTE) initiative. The GSMA VoLTE initiative was formally announced on 15 February 2010 in a press release stating that: “In establishing the VoLTE initiative, GSMA has adopted the work of the One Voice Initiative* as the basis of the work to lead the global mobile industry towards a standard way of delivering voice and messaging services for Long-Term Evolution (LTE). Using IP Multimedia Subsystem specifications developed by 3GPP as its basis, GSMA have expanded upon the original scope of One Voice work to address the entire end-to-end voice and SMS ecosystem by also focusing on Roaming and Interconnect interfaces, in addition the interface between customer and network”.
In September 2010, the GSMA published the IREG Permanent Reference Document IR.92, which outlines the specifications for migrating 2G & 3G mobile voice, video and messaging services to 4G mobile broadband networks including LTE. The requirements in the specification are compliant with 3GPP Release 8 and there are some additional requirements from 3GPP Release 9 in support of packet switched emergency calls.
The requirements in this specification comprise IMS features, media requirements, bearer management features, LTE radio requirements.
The VoLTE profile uses a subset of general IMS features and of the MMTel service. The subset is selected to provide an IP telephony service with a similar user experience as the CS service of 2G and 3G networks. The aim of the profile is to provide a minimal set of features that can serve as a starting point. The profile must still be open to functional growth in networks and devices to reach the full 3GPP specifications for IMS and MMTel, for example, using multimedia, supporting users with multiple devices. Considerable efforts have therefore been made to ensure that user equipment(UE) that supports the VoLTE feature set can be used even in networks that provide all 3GPP IMS and MMTel features. The baseline profile mandates the use of different options for the IMS feature. For example, SIP registration; authentication; addressing; basic call handling; forking; and signaling compression. With regard to authentication, the baseline profile supports both MSISDN and SIP-URI based addressing. The baseline profile supports a single handheld device scenario.
The media handling in the VoLTE baseline profile have a set of requirements that provide voice services with the same quality as that of legacy circuit-switched service. In spite of the additional delay introduced by de-jittering, the end-to-end delay of MMTel terminals and LTE networks is on par with or even better than that of 2G/3G circuit-switched telephony.
When there are frame error rates, LTE voice bearers add erroneous transmission repair. As a result, most packets are sent successfully over the LTE air interface, and the residual frame error rate for voice media is low. The addition of solutions for codec rate adaptation, which is specified in the MMTel specification, can further enhance media transport.
In terms of this profile, LTE EPC systems must be equipped with policy and charging control (PCC).PCC has a set of Rx and Gx interfaces that connect EPC with the IMS domain. In addition, the PCC architecture introduces a policy and charging rules function (PCRF) that encompasses policy decision and flow-based charging control functions. With PCC implemented, IMS can start a network initiated process to set up suitable 3GPP EPS bearers for telephony service.
To support roaming services, the profile uses a roaming model where the IMS P-CSCF is located in the visited network and uses local breakout from the packet core network . As a result IMS voice services for roaming users require new IMS-based roaming agreements between the network service providers. If there are no such agreements in place, the UE is required to use CSFB for voice calls while roaming if LTE is used in the visited network.
The VOLTE profile, also has a specification for IMS emergency calls. This is according to the 3GPP Rel-9 which, added the functions in EPC to enable IMS emergency calls over packet switched access. Release 9 also includes support for SIM-less emergency calls , user and control plane location services, and enhancements to the air interface to improve over the air time difference of arrival (OTDOA) accuracy.
References
http://www.3gpp.org
http://www.volga-forum.com
http://www.cellular-news.com
http://www.gsa.com
Lennart Norell, Eric Parsons and Per Synnergren. 2010. Telephony services over LTE and to end. Ericsson review.
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