Voice over LTE and RCS technology Archive


Voice over LTE (VoLTE) helps operators build a next generation service architecture based on an IMS core to provide carrier-grade services over mobile and fixed broadband. This helps ensure high quality voice, video and multimedia services for subscribers, while protecting operator profitability.

VoLTE Network Architecture

VolTE IP Multimedia Subsystem (IMS) consists of four modules for HD voice session control as mentioned below:
—Proxy Call/Session Control Function (P-CSCF)
—Serving Call/Session Control Function (S-CSCF)
—Interrogating Call/Session Control Function (I-CSCF)
—the Application Server (AS)

An illustration of VoLTE Network architecture is shown below: 

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Rich Communication Services (RCS)

Similar to VoLTE, Rich Communications Services (RCS) takes advantage of IP Multimedia Subsystem (IMS) control capabilities, encompassing a rich multimedia service portfolio, including multimedia messaging, chat, file transfer, privacy, buddy lists and presence.

VoLTE enables High Definition (HD) voice quality, helping operators compete against over-the-top (OTT) VoIP providers. In addition, VoLTE offers lower delay and higher capacity compared to OTT VoIP services, which tend to generate a greater load on operator networks and adversely affect service quality.

The figure shown below illustrates how RCS and VoLTE drive the evolution of telephony, messaging and contact management services, enabling telecom operators to offer innovative and enriched communication experiences to consumers, government users and enterprises.

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The business benefits

  • Competitive, high-quality voice and video service over LTE
  • Fast time-to-market: quick, easy service development on top of LTE/ Wi-Fi
  • Full mobile voice service continuity
  • Full integration with existing network back office services
  • Core elements run on the same platform or in telco cloud, reducing total cost of ownership
  • Professional services for design, planning and integration

By providing VoLTE, an operator can enhance indoor coverage using Wi-Fi and extend its communication services with WebRTC. Voice over Wi-Fi (VoWiFi) and WebRTC use the same IMS core as VoLTE, enabling the same communication and multimedia services.

VoLTE Call Flow

An illustration of VoLTE call flow is shown below:

volte call flow

VoLTE Evolution

South Korea launched the world’s first commercial VoLTE services in 2012 (as well as MetroPCS in 2012 in the U.S. in select markets), followed by some national North American operators in 2014. From both a standards and a network vendor implementation point of view, VoLTE as a technology is ready for prime time with more evolution to follow. With key Radio Access Network (RAN) features such as Transmission Time Interval (TTI) bundling and Robust Header Compression (RoHC), VoLTE is able to match the needed link budget with the existing CS voice services.

Vendors such as Qualcomm, Samsung and LG have their own IMS stack implementations and their own designs and architectures on the network side. These differences mean portability and roaming between operators might prove to be challenging. Though most of the LTE handsets launched in 2014 will have VoLTE capability, operators will typically disable the feature until their networks are fully optimized and ready to support that service. The device ecosystem will grow substantially in the latter half of 2014 and throughout 2015.

For VoLTE calls in the roaming scenario with local break-out with the P-CSCF in the visited network, the VPMN is service aware, and there is scope for service-based charging to be deployed between the visited and home networks and to apply flow-based charging mechanisms. However, this topic is still under study in GSMA technical working groups.

While wireless networks have evolved to IP-based communications such as VoLTE and RCS, the emergency services community has also been working on an evolution to an IP-based network. The Next Generation 9-1-1 (NG9-1-1) network is their vision of the IP-based network and the core of NG9-1-1 network is the Emergency Services IP Network (ESInet). The standards for ESInet are defined primarily within IETF and NENA. Also, there are joint ATIS/TIA work efforts that are defining standards on the interconnection and interoperation of the ESInet with IMS-based services such as VoLTE

The vast majority of the requests for emergency services are voice calls to “9-1-1.” There are FCC regulations for location determination and location accuracy for voice calls to 9-1-1, and these regulations would be applicable to VoLTE and RCS calls to 9-1-1. The FCC has recently issued a Further Notice of Proposed Rulemaking (FNPRM) with proposed regulations for indoor location accuracy.
By May 15 2014, in compliance with FCC Report & Order FCC-13-64A1, the major U.S. wireless operators must support text to 9-1-1 services for PSAPs that are text capable and that request to receive text to 9-1-1 messages. The initial implementation only supports SMS messages. There are currently no FCC regulations for providing location information with the text to 9-1-1 messages but the FCC has recently issued a FNPRM for location determination of text to 9-1-1 messages.