Which is the mandatory-to-implement voice codec in the IP-NNI publication?

The mandatory-to-implement voice codec in the IP-NNI (Internet Protocol Network-to-Network Interface) publication is G.711 mu-Law. This codec is widely recognized for its efficiency in voice transmission over IP networks, primarily due to its high-quality audio output and low latency, making it suitable for real-time communications.

G.711 mu-Law encodes audio signals at a bitrate of 64 kbps, which is compatible with various telephony systems, ensuring seamless interoperability between different networks and equipment. This characteristic is essential in a standardized interface like IP-NNI, where different service providers must be able to communicate effectively without degradation in call quality.

In contrast, the other codecs listed have specific characteristics or usage scenarios that do not meet the mandatory requirement for all implementations within the IP-NNI framework. For instance, while G.723 and G.729 offer compression benefits and reduced bandwidth usage, they are not mandated as the default codec across all systems. G.726 serves a specific niche in applications where ADPCM encoding is preferred but is not universally applicable to all voice transmission scenarios laid out in the IP-NNI guidelines.