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Voice quality network requirements
248 Avaya Application Solutions IP Telephony Deployment Guide
The effects of packet loss on VoIP service are multifold:
● Problems caused by occasional packet loss are difficult to detect because each codec has
its own packet loss concealment method (PLC). Therefore, it is possible that voice quality
would be better using a compression codec (G.729A), which includes its own PLC,
compared to a full bandwidth G.711 codec without PLC.
● Packet loss is more noticeable for tones (other than DTMF) than for voice. The human ear
is less able to detect packet loss during speech (variable-pitch), than during a tone
(consistent pitch).
● Packet loss is more noticeable for contiguous packet loss than for random packet loss
over time. For example, the effect of losing ten contiguous packets is worse than losing ten
packets evenly spaced over an hour time span.
● Packet loss is generally more noticeable with larger voice payloads per packet than with
smaller packets, because more voice samples are lost in a larger payload.
● In the presence of packet loss, the time for a codec to return to normal operation depends
on the type of codec.
● Even small amounts of packet loss can greatly affect a TTY/TDD device's (for
hearing-impaired people) ability to work properly.
● Packet loss for signaling traffic increases network traffic substantially when the loss
exceeds 3%, possibly impacting voice quality.
Network packet loss
Like packet delay, Avaya offers customers a tiered approach of recommendations to deal with
network packet loss to balance new network costs and the constraints of business directives.
The maximum loss of IP packets (or frames) between endpoints should be:
● 1% or less for best quality.
● 3% or less for Business Communications quality. Again, this quality is much better than
cell-phone quality.
● More than 3% may be acceptable for voice but may negatively impact signaling, which
might degrade voice quality due to increased traffic. More information on signaling
bandwidth requirements can be found in white papers on the Avaya Support website.
The Converged Network Analyzer (CNA) system is capable of providing continuous
measurements of network packet loss (see C
NA Application Performance Rating on page 254).
CNA can also generate alarms when network packet loss rises to levels that are detrimental for
voice quality. For more information on CNA, see The Converged Network Analyzer
on
page 344.
Avaya’s VoIP Monitoring Manager (VMM) can measure packet loss for ongoing calls. The
Agilent (HP) Internet Advisor, Finisar's Surveyor Explorer, Radcom's Prism, NAI's Sniffer, and
others also measure packet loss.