ASR and ACD: definition and a practical example
Short answer: ASR (Answer Seizure Ratio) is the percentage of call attempts that were answered, and ACD (Average Call Duration) is the average length of the answered calls. Both come from the carrier's or PBX's call detail records (CDR).
In Portuguese: ASR e ACD
What are ASR and ACD?
ASR and ACD are the two oldest quality indicators of a voice route, calculated from call detail records, the CDRs.
ASR (Answer Seizure Ratio, from ITU-T Recommendation E.411) is the percentage of call attempts that were answered: answered calls ÷ attempted calls × 100. ACD (Average Call Duration) is the average length of the answered calls: total talk time ÷ number of answered calls.
Carriers define ASR in slightly different ways. Some leave out of the calculation the calls the destination rejected, such as busy, and the difference changes the number. Check the definition in the contract before comparing routes.
What a change indicates
- ASR dropping sharply: the route may have stopped completing calls, wholly or in part. It is the cue to look at the response codes, such as SIP 503.
- A very short ACD: calls are answered and drop soon after, which usually points to silent audio, SIP ALG or a codec problem.
- A very long ACD: stuck calls, for example a call that did not hang up at the far end.
Both look at the whole, not at a single call. They hide the failure of the 0800 that nobody called overnight, and they do not say what the person who called heard.
In practice
In one day, a SIP trunk made 1,000 call attempts and 600 were answered, with 30,000 seconds of talk time in total (illustrative numbers).
| Indicator | Calculation | Result |
|---|---|---|
| ASR | 600 ÷ 1,000 × 100 | 60% |
| ACD | 30,000 s ÷ 600 | 50 s |
If tomorrow the ASR falls to 20% with no change in traffic, the route has a problem. If it falls to 20% because a campaign dialed a poor list, the trunk is fine.
How Monitor uses this
Monitor does not calculate ASR or ACD, because it does not analyze your customers’ traffic. It works the other way round: it calls a number you control, from the outside, and reports whether the call connected, with the SIP code when it does not. Use the ASR in your CDRs to see that something changed and the test call to know whether the circuit still completes calls. See synthetic monitoring, post dial delay and SIP trunk and VoIP monitoring.
Frequently asked questions
What is a good ASR?
There is no good number for everyone. ASR depends on the kind of traffic: calls to customers who do not pick up lower the figure with no network fault at all. Compare a route's ASR with its own on similar days, and investigate when it drops suddenly, because the sharp drop is the useful signal.
Does a low ASR mean the trunk has a problem?
Not always. A low ASR can come from a trunk fault, but also from a busy called number, a campaign to a poor list, or a cheap and unstable route. ASR tells you something changed; a test call to a number you control shows whether the trunk still completes calls.