Defining good voice quality varies with business needs, cultural differences, customer expectations, and the hardware and software used. The requirements set forth are based on the ITU-T and EIA/TIA guidelines and extensive testing. Avaya requirements meet or exceed most customer expectations. However, the final determination of acceptable voice quality lies with the customer definition of quality and the design, implementation, and monitoring of the end-to-end data network.
Quality is not a discrete value where the low side is good and the high side is bad. A trade-off exists between real-world limits and acceptable voice quality. Lower delay, jitter, and packet loss values can produce the best voice quality, but might also come with a cost to upgrade the network infrastructure to get to the low values. Another real-world limit is the inherent WAN delay. An IP trunk that links the west coast of the United States to India could add a fixed delay of 150 ms into the overall delay budget.
Perfectly acceptable voice quality is attainable but will not be toll quality. Therefore, Avaya presents a tiered choice of elements that make up the requirements.
The critical objective factors in assessing IP Telephony quality are delay, jitter, and packet loss. To ensure good and consistent levels of voice quality, Factors that affect voice quality lists Avaya’s suggested network requirements. These requirements are valid for both LAN only and for LAN and WAN connections. Note that all measurement values are between endpoints and therefore reflect the performance of the network without endpoint consideration.
Table 1: Factors that affect voice quality
Network factor |
Measurement |
Delay (one-way between endpoints) |
A delay of 80 ms or less can, but might not, yield the best quality. A delay of 80 ms to 180 ms can yield business-communication quality. Business-communication quality is much better than cell phone quality, and is well-suited for a majority of businesses. Also, business-communication quality is defined as less than toll quality but much better than cell phone quality. Delays that exceed 180 ms might still be acceptable depending on customer expectations, analog trunks used, and the codec type. |
Jitter (variability of the delay between endpoints) |
Note:
This value has some latitude, depending on the type of service that the jitter buffer has in relationship to other router buffers and the packet size used.
|
Packet loss (maximum packet/frame loss between endpoints) |
< 1% can yield the best quality, depending on several factors. < 3% gives business communications quality, which is much better than cell phone quality. > 3% might be acceptable for voice but might interfere with signaling. |
For more information, see Voice quality network requirements.
