Thinking Differently About VoLTE and Next-Generation Wireless Voice Services

As LTE networks start to mature across the world, more and more carriers are looking introduce voice services on LTE networks using a technology called Voice over LTE (VoLTE). What is not always understood is that voice services on LTE are very different from those on 2G and 3G networks. An appreciation of the differences leads us to the conclusion that we must think and act differently for 4G. This article addresses the uniqueness of VoLTE and briefly explains the technical features required to make VoLTE work. Most importantly, we discuss the implications this has on making sure VoLTE and next-generation wireless voice services work as expected.

Voice Quality and Call Performance Can't Go Backward

Wireless subscribers are accustomed to a certain experience when making voice calls on 2G and 3G wireless networks, and those expectations do not change when carriers offer VoLTE on their 4G networks. In fact, they probably don’t know, or care, that instead of getting a dedicated voice bearer, their call is being funneled through a giant data pipe along with all the other IP traffic generated by wireless devices in the area. They don’t care that delivering the same quality of service they are used to in this all-data world is very, very hard to do on wireless networks. It just has to work—as well, or better, than it did in the past. If not, users of 4G smartphones will either turn off 4G on their phone or return their devices in favor of something that works better.

Making sure this does not happen means focusing on the aspects of VoLTE call performance captured in Table 1.

Table 1. VoLTE Call Performance
End User Experience Key Performance Indicators (KPIs)
Ability to make and maintain calls Call Initiation Rate (%), Call Drop Rate (%)
Time it takes for a phone to start ringing Call Setup Time (s)
Speech quality during a call Mean Opinion Score (MOS)
Mouth-to-ear delay or latency variations during a call Mouth-to-ear delay (s), Latency, jitter

Voice Services on LTE Are Unlike Anything in the [Wireless] Past

All that matters for 2G/3G voice today is the robustness of layers 1-3 in the mobile device and radio access network (RAN). The dedicated voice bearer needs to be reliably setup and maintained—not always an easy task, but performance issues almost always come down to problems with the physical and access layers. However, LTE voice services are based on data bearers and VoLTE is delivered via IP Multimedia Subsystem (IMS) networks. For this to work, the entire stack (all seven layers) must function and interoperate correctly in devices and in the network. The dependency on interworking between the application layers of devices and core network elements is new in the wireless world. In the wired world, landline VoIP took a while to mature and be viable, even though it had the advantage of being able to solve many problems just by increasing bandwidth. In wireless, the spectrum is limited, bandwidth is more expensive, and radio interface performance is very unpredictable. This presents challenges never before faced by wireless network operators, mobile device vendors, or network equipment manufacturers.

Overcoming the Challenge

Fortunately, networks and devices are being upgraded with new features that will help ensure that high quality LTE voice services become a reality. They can be broken down into the following areas: RAN, IMS Core, Codecs, and QoS (see Table 2).

Table 2. New Features Needed to Make VoLTE Work
RAN IMS Core Codec QoS
Semi-persistent scheduling resource allocation Session Initiation Protocol (SIP) Adaptive Multi-Rate (AMR) Dedicated vs. Non-Dedicated Bearers
Transmission Time Interval (TTI) Bundling Session Description Protocol (SDP) HD Voice (Wideband AMR) Quality of Service Class Identifier (QCI)
Robust Header Compression (RoHC) Real-time Transport Protocol (RTP) Dynamic scheduler in eNodeB
Real-time Streaming Protocol (RTSP)

Making Sure VoLTE and Next Generation Wireless Voice Services Work as Expected

Because voice services such as VoLTE can encounter issues on all-IP networks at so many layers, the place to start when looking to ensure a good end-user experience is at the endpoints. Measuring the KPIs in Table 1 in an end-to-end system ensures that QoE can be completely characterized, and it also provides a basis for measuring improvements and benchmarking different components.

A measurement system that can accurately measure speech quality, call setup time, call initiation rate, call drop rate, and mouth-to-ear latency at the endpoints (example reports shown in Figure 3) is very valuable in evaluating performance, both on live networks as well as in a lab environment. Because VoLTE is so new and deployed on very few commercial networks to date, much of the testing needs to be done in the lab. Ideally, the same call performance measurement system can be used for lab and live network testing to ensure that all KPIs are measured the same way and to make data analysis more valuable.

Once end-to-end measurement solutions are in place for voice services, strategies can be developed for diving down into the lower layers of mobile device and network element implementation to help debug and diagnose problems when they occur. This requires a way to capture and analyze data at various layers so that root causes can be isolated and fixed. This “top down” approach of verifying voice service performance is very different from what is typically done for 2G and 3G, but it is absolutely essential for VoLTE and next generation LTE voice services. All-IP wireless data networks are a new territory for voice services, and require us to think differently in order to make sure the mobile experience remains the same or [hopefully] improves.

Read the extended version of this blog on Wireless Design & Development.

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