Quality of Service requirements

Last Updated : Sep 16, 2015 |
Prolog information
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To achieve good voice quality, the enterprise network must meet certain requirements. The terms used to describe acceptable voice quality are toll quality and business communication quality. Optimal voice quality is toll quality, but business communication quality is well suited for most enterprises. Business communication quality is not as high as toll quality, but is still much better than cell phone quality.
The following table summarizes the network delay, jitter, and packet loss requirements that the network must meet. Even if these requirements are met, other factors might still prevent you from achieving optimal voice quality.
Quality of Service (QoS) requirements
Requirements
Description
Network delay
Voice quality:
  • To obtain toll quality, the delay cannot exceed 80 millseconds (ms).
  • To obtain business communication quality, the delay must be between 80 to 180 ms. Business communication quality is suitable for most enterprises.
  • Delays exceeding 180 ms provide a lower quality than business communication quality, but this might still be acceptable for some enterprises.
Network jitter
To achieve optimal voice quality, the average jitter must be less than half the network packet payload. This value can vary depending on the type of service the jitter buffer has in relation to other buffers and to the packet size used.
Assuming the packet size is 20 ms, to prevent problems with voice quality, the network jitter must not exceed 20 ms.
Network packet loss
Voice quality:
  • To obtain toll quality, the packet loss cannot exceed 1%.
  • To obtain business communication quality, the packet loss cannot exceed 3%.
  • Packet losses exceeding 3% might result in signalling interferences.
When transporting voice over low speed links, normal data packets can prevent or delay voice packets from getting across the link. The voice transportation can result in unacceptable speech quality. To ensure low speech latency and help maintain sufficient voice quality, implement another Quality of Service (QoS) mechanism, such as a QoS router, on the traffic routers and switches in your network. QoS mechanisms must be implemented on the WAN routers that connect the branch with the rest of the enterprise network.
QoS must be based on DSCP tagging. Use the DSCPs to determine the appropriate priority queuing and handling to implement on the network routers. DSCP tagging should be then be configured consistently on the IP Office and on the Avaya Aura® Communication Manager for the telephones connected to the Avaya Aura® system.