Ensuring cable VoIP quality

09 Apr 2006

Steve Alexander of Ciena Corp

Now that telcos are more serious about providing video service to complete their residential triple-play offering, cable operators must also introduce or solidify their telephony product to bundle with video and high-speed data.

While there are myriad technical and commercial approaches to delivering telephony service, they all have to ensure that the resulting service is reliable and of high quality.

There are two myths about VoIP quality. One is that 'VoIP quality' is an oxymoron. The fact is that with a properly designed architecture using high-quality network elements, VoIP can provide quality and reliability equal to or better than the circuit-switched network.

The other is that VoIP quality is not necessary. The argument is that cellular service, with its dropped calls and sometimes static-laden reception, has conditioned users to accept low quality. But the fact is that people accept this as a trade-off for the convenience of mobility. IP cable telephony (cable VoIP) as an alternative to the telco POTS line does not offer such a glaring benefit. Cellular service has also continually improved.

Cable VoIP can increase ARPU by more than $30 a month, attract new subscribers for a service bundle, and reduce churn rate. But these rewards will be fleeting if the service is unreliable or the quality is intermittently poor. With so many providers to choose from, cable operators may only get one chance to win and keep telephony subscribers.

Factors to VoIP quality

The key measures contributing to VoIP quality include dropped calls, blocked calls and Mean Opinion Score (MOS).

Dropped calls occur either due to the mid-call failure of either operator-owned client endpoint or any of the network elements along the bearer path, or due to a fiber cut along that path.

Insufficient resources somewhere along either the signaling path or the bearer path cause blocked calls. Since it is not economical or practical to dedicate resources for 100% utilization and every caller/called combination, a blocking probability, typically

MOS is a subjective measure of voice quality and is directly affected by the codec used as well as the packet latency, packet jitter and dropped packets.

The main goals of the VoIP network architecture and transport design, therefore, are to avoid dropped and blocked calls, minimize packet latency and jitter, and minimize dropped packets while simplifying the network for easier troubleshooting.

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