Making Sure VoLTE Delivers High Def!
VoLTE Voice Performance Including: MOS and Transcoding Issues
VoLTE utilizes a family of codecs (G.722) that adapt to the available bandwidth and offer the ability to deliver an enhanced voice quality that may actually sound better to the listener that any landline conversation! Mobile service providers are heralding their new, next-gen VoLTE voice networks as providing the best mobile voice anywhere, anytime, with a new high-definition sound quality. But, how do you know that your mobile network is delivering that new high-definition voice quality?
Codecs are used to convert analog voice signals (or the words that we speak aloud) into digitally encoded versions for efficient transport over IP networks.
Codecs vary in the sound quality they can achieve, the bandwidth required to transport them, and the amount of processing needed to handle them. Each handset, media gateway, service, etc., typically supports several different codecs, and when talking to each other, negotiate which codec they will use.
To assure that VoLTE networks are providing superior voice quality mobile service provider’s performance monitoring solution must recognize and support these new Adaptive Multi-Rate Narrow Band and Wide Band codecs (AMR-NB, AMR-WB). This means tracking the codec used for each subscriber session, by each handset type, measuring its packet impairments, and rendering a voice quality score measured by Mean Opinion Score (MOS). Rolling up these key performance indicators for all monitored network segments provides the essential data to assure the VoLTE service and more specifically, to track the voice quality for these new high-definition codecs.
As mentioned above, AMR-NB and AMR-WB are not the only codecs seen in the network. Service provider networks must cope with a myriad of VoIP codecs (G.711, G.729, etc.) appearing in their network. Border switches and media gateways must manage transcoding or mediation (or the negotiation) of various codecs in their network. Codec mismatches can wreak havoc on voice communications, and so mobile operators must instrument these media gateways handling transcoding to proactively protect the network as well as to monitor for a plethora of packet impairments that can hurt voice quality.
A common method for mitigating transcoding issues is Transcoder Free Operation (TrFO), where the network transports compressed speech, eliminating unnecessary coding and decoding of the signals when both ends use the same. Nevertheless, it is necessary to monitor these operations as TrFO does not eliminate all of the packet impairment and other network impairments.
Instrumenting the natural aggregation points for RTP streams at media gateways, SBC/NBS, and other traffic handover points is the usual recommended approach to monitoring VoIP traffic. However, with VoLTE, mobile service providers must consider expanding their network monitoring coverage to the user plane in their LTE network as both the SIP signaling and RTP streams of VoLTE calls run over-the-top of the LTE network. End-to-end and hop-by-hop coverage of both the LTE network and IMS network will provide mobile service providers with the requisite visibility to proactively monitor voice quality and perform swift service triage to protect their investment in their next-gen voice network.