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VoIP quality guideASR, ACD and PDD explained: the VoIP quality metrics that matter
Every wholesale voice business talks about ASR, ACD and PDD. Together they tell you whether a route really works or just looks cheap on the rate sheet. This guide explains what each metric means, how to calculate it, what can push it up or down and how to use the numbers to manage your routes.
ASR: answer-seizure ratio
ASR is the share of call attempts that are answered.
ASR = answered calls ÷ total call attempts × 100
If a route receives 1,000 calls and 450 are answered, its ASR is 45%. ASR depends heavily on the destination and the type of traffic: calls to mobiles, call-centre campaigns and retail calls all behave differently. That is why you should compare a route with other routes to the same destination and the same traffic, not with a single "good" number.
ASR falls when calls fail inside the route (congestion, wrong routing, blocked CLI) but also when people simply do not answer, for example because the caller number is hidden or unknown.
ACD: average call duration
ACD is how long answered calls last on average.
ACD = total billed minutes ÷ answered calls
If 450 answered calls add up to 1,350 minutes, the ACD is 3 minutes. A healthy ACD means real conversations are taking place. A very short ACD on a route that used to be normal can point to poor audio, dropped calls or calls answered by something other than a person.
Watch out for an unusually high ASR with a very short ACD: it can be a sign of false answer supervision (FAS), where a route reports calls as answered when they were not, so you are billed for minutes that never reached the destination.
PDD: post-dial delay
PDD is the time between sending a call and hearing it ring (or getting a response such as busy). In SIP terms it is measured from the INVITE to the first ringing or progress response. Long PDD makes callers think the call failed, and many hang up before it rings, which lowers ASR as well.
PDD grows when a call passes through many networks before it reaches the destination, or when a route is overloaded.
NER and other useful numbers
- NER (network effectiveness ratio) counts a call as successful when the network did its job, even if the person did not answer (for example busy or no answer). It separates route problems from user behaviour.
- SIP response codes show why calls fail: 404 (not found), 486 (busy), 503 (service unavailable) and so on. A rise in one code usually points straight to the problem.
- Concurrent calls and CPS (calls per second) tell you whether a route or trunk is reaching its limit.
- Margin per route turns quality into money: a route with good ASR but negative margin still loses you money.
The metrics at a glance
| Metric | What it measures | Formula | A sudden drop or rise may mean |
|---|---|---|---|
| ASR | Share of calls answered | Answered ÷ attempts × 100 | Route failing, blocked CLI, congestion |
| ACD | Length of answered calls | Billed minutes ÷ answered calls | Bad audio, dropped calls, FAS |
| PDD | Wait before ringing | INVITE → first ringing response | Too many hops, overloaded route |
| NER | Network success, ignoring user behaviour | Network-successful ÷ attempts × 100 | Technical problem in the route |
How to read the numbers together
- Low ASR, normal ACD: many calls fail, but those that connect are real. Check failure codes and CLI.
- High ASR, very short ACD: suspicious. Check for false answer supervision and listen to test calls.
- Normal ASR, high PDD: the route works but is slow. Expect complaints and hang-ups as volume grows.
- Good quality, negative margin: the route is fine technically but priced wrong. Re-check rates or move the traffic.
Monitoring routes day to day
- Watch live dashboards for ASR, ACD and concurrent calls per customer and vendor.
- Compare by destination, not just by vendor, because each country behaves differently.
- Run test calls on routes before and after changes instead of waiting for customer complaints.
- Keep the CDRs with SIP detail so you can prove a problem to a vendor or customer.
- Check margin reports daily so quality and price are reviewed together.
Track ASR, ACD and margin live with Route-Setu Voice
Route-Setu Voice, the LCR ONE Class 4 softswitch, shows live concurrent calls, ASR, ACD and today's margin on its dashboard, with a real-time calls monitor filtered by customer, vendor and response code. Call-loop test campaigns check route quality without live traffic, full call-event logs keep SIP-level detail for disputes, and six margin report types break results down by customer, vendor and destination.
FAQ
What is a good ASR in VoIP?
There is no single good number. ASR depends on the destination and the traffic type, so compare each route with other routes to the same destination carrying the same kind of calls.
How is ACD calculated?
ACD is the total billed minutes divided by the number of answered calls. For example, 1,350 minutes over 450 answered calls gives an ACD of 3 minutes.
What causes high PDD?
High post-dial delay usually comes from calls passing through too many networks before reaching the destination, or from a route that is overloaded.
What is false answer supervision?
False answer supervision (FAS) is when a route signals that a call was answered when it was not, so billing starts too early. A high ASR combined with a very short ACD is a common warning sign.
What is the difference between ASR and NER?
ASR counts only answered calls as successful. NER also counts calls where the network worked but the person did not answer, such as busy or no answer, so it shows route health more clearly.
Last updated: 26 October 2026 · LCR ONE