John English

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How do you know that your Voice Over Long-Term Evolution (VoLTE) network is in trouble or delivering a poor next-gen voice network experience? If you are a mobile service provider you care about the quality of voice delivered by your VoLTE network and you want to make sure that it works! Your company is likely advertising that it delivers the best voice quality ever! So what metrics do Network Operations and Engineering need to pay attention to in order to assure that their VoLTE service is high quality?

VoLTE is essentially a Voice over Internet Protocol (VoIP) service utilizing SIP signaling to establish and operate the VoIP sessions and utilizes RTP for the media (or the actual voice conversation) that runs over-the-top of the LTE user plane and relies on the IMS network for seamless service connection. VoLTE also employs new, high-definition voice codecs, called Adaptive Multi-Rate Narrow Band/Wide Band (AMR-NB and AMR-WB) that promise better than toll, tandem voice quality. These high-definition codecs take digitized voice and optimize it for transport on mobile networks by adapting to the available bandwidth.

For starters, the VoLTE service assurance solution must provide key metrics on SIP signaling and media metrics on the RTP streams, and render voice quality specifically for high-definition codecs. As VoLTE is a SIP-based voice session, the key signaling metrics for VoLTE are similar to those for VoIP calls to measure network effectiveness to establish VoLTE sessions – SER (ASR), SEER (NER), and SCR (CCR), the time it takes to set up a VoLTE call – Call Setup Delay and Post Dial Delay, the length of call KPIs, Average and Maximum Call Duration, as well as the basic success/failure measurements.

The three primary media metrics, MOS, Jitter and Packet Loss involve measuring packet impairments that most impact voice quality. (There are also deeper measurements of the payload to render metrics on Echo, SNR (Noise) and Level but these come with a steep price in terms of cost and performance to monitor.) Such media metrics must measure by codec and represent the deviation from the expected norm for each one. These media metrics must also be complemented with an expanded set of metrics that track additional network impairments that can plague early and ongoing VoLTE deployments. These key metrics include QoS mismatch, 1-way audio and out-of-sequence packets.

Each of these metrics answer a question around a particular aspect of VoLTE call quality, and it is collectively that they answer the question…“Is my network going to operate the way we expect it to work, and will my subscribers receive the VoLTE voice quality of service our customers expect?

Not surprisingly, there is interrelationship with some of these key signaling and media metrics. For example, a decrease in the average call duration (also known as Average Length of Call – ALOC) or the increase in the number or percentage of RTP streams with 1-way audio may be indicators of poor voice quality because callers are hanging up and redialing in hopes of securing a better voice quality. It is therefore important to cross reference signaling KPIs with the corresponding media KPIs for the same subscriber sessions.

Together, a service assurance solution employing both signaling and media metrics provides a holistic view of the quality of VoLTE sessions. Indeed, these metrics provide the foundation for a proactive or early warning system to assure the successful delivery of a VoLTE service.

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