Mike's PBX Cookbook

Key VoIP Tests



Excessive end-to-end delay makes conversation inconveient and unnatural. Each component in the transmission path - sender, network, and receiver - adds delay. ITU-TG.114 (One-Way Transmission Time) recommends 150 mSec as the maximum desired one-way latency to achieve high-quality voice.

Sample Delay Budget Table

ParameterFixed delayVariable delay
CODEC (G.729)25 mSec
PacketizationIncluded in CODEC
Queuing delayDepends on uplink. In the order of a few mSec.
Network delay50 mSecDepends on network load.
Jitter buffer50 MSec
Total 125 mSec 

End-to-end Delay

end-to-end delay


Quantifies the effects of network delays on packet arrivals at the receiver. Packets transmitted at equal intervals from the left gateway arrive at the right gatway at irregular intervals. Excessive jitter makes speech choppy and difficult to understand. Jitter is calculated based on the inter-arrival time of successive packets. For high-quality voice, the average inter-arrival time at the receiver should be nearly equal to the inter-packet gaps at the transmitter and the standard deviation should be low. Jitter buffers (packet buffers that hold incoming packets for a specified amount of time) are used to counteract the effects of network fluctuations and create a smooth packet flow at the receiving end.


Packet loss

Typically occurs either in bursts or periodically due to a consistantly congested network. Periodic loss in excess of 5-10% of all voice packets transmitted can degrade voice quality significantly. Occassional bursts of packet loss can also make conversation difficult.

packet loss

Sequence Errors

Congestion in packet switched networks can cause packets to take different routes to reach the same destination. Packets may arrive out of order resulting in garbled speech.

sequence errors

Recommendations for Measuring Voice Quality

ITU-T Recommendations P.800 - Subjective quality test based on Mean Opinion Scores (MOS). Preselected voice samples recorded according to recommendation P.50 are played back to a mixed group of men and women under controlled conditions. The scores given by the group are weighed to give a single MOS score ranging from 1 (worst) to 5 (best). A MOS of 4 is considered "toll-quality" voice.

Mean Opinion Scores (MOS) for Various Voice Quality Tests

ScoreOpinion Scale: Conversation TestDifficulty
Opinion Scale: Listening TestListening: Effort Scale Loudness: Preference Scales
5Excellent----ExcellentComplete relaxation possible, no effort requiredMuch louder than preferred
4Good----GoodAttention necessary; no appreciable effort requiredLouder than preferred
3Fair----FairModerate effort requiredPreferred
2Poor----PoorConsiderable effort requiredQuieter than preferred
1Bad yesBadNo meaning understood with any reasible effortMuch quieter than preferred

Objective Voice Quality Measurements

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