Guide

Voice quality, in twelve words

The measures a carrier and a call centre exchange to tell whether a route works: what they count, and how to read them.

Updated on 5 October 2026

Route measures

ASR: Answer-Seizure Ratio: answered calls divided by attempted calls, as a percentage. It says how many calls reach an answer.
NER: Network Effectiveness Ratio: like ASR, but it also counts as successful the calls that fail because the user is busy or does not answer. It measures the network, not the behaviour of the people called.
ACD: Average Call Duration: the average length of answered calls. A very low ACD is often the sign of dropped calls or a quality problem.
PDD: Post-Dial Delay: the time between the end of dialling and the first ringing signal. The longer it is, the more calls are abandoned before they start.
FAS: False Answer Supervision: a call marked as answered, and billed, when nobody actually answered.

MOS: Mean Opinion Score: the perceived quality of the voice, from 1 to 5. It is estimated from network measurements.
Latency: The delay between the person speaking and the person listening. Above 150 milliseconds one way, the conversation starts to stumble.
Jitter: The variation in delay from one voice packet to the next. It is absorbed by a small buffer, at the cost of some latency.
Packet loss: The share of voice packets that do not arrive. Even a few percentage points are heard as swallowed words.
Codec: How the voice is compressed: G.711 is landline quality, G.722 and Opus are high definition, G.729 saves bandwidth.

CLI: Calling Line Identification: the number the called person sees. A CLI route delivers it intact; a non-CLI route does not guarantee it.
CDR: Call Detail Record: the line describing each call, with start, duration and outcome. The two parties compare their CDRs to reconcile the invoice.

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