ASR measures how many call attempts were answered; NER (network effectiveness ratio) measures how many reached the called party's phone, whether answered, busy or not answered. ASR mixes route quality with user behaviour, while NER isolates the network, so watch NER to judge a route and ASR to judge the business result.
The two formulas
| Metric | Formula | What it tells you |
|---|---|---|
| ASR (Answer Seizure Ratio) | Answered calls / total call attempts x 100 | How many attempts became billable calls |
| NER (Network Effectiveness Ratio) | (Answered + user busy + no answer + user reject) / total call attempts x 100 | How often the network delivered the call to the user |
NER is defined by the ITU (Recommendation E.411) and counts every attempt where the network did its job, even if the person did not pick up. The difference between the two is therefore user behaviour: people who were busy, did not answer or rejected the call. Both metrics, together with ACD and PDD, are introduced in ASR, ACD, PDD and NER explained.
Why ASR alone can mislead you
ASR depends heavily on who you are calling and when. The same route can show 55% ASR for conversational traffic and 15% for a dialer campaign calling cold lists. Typical reasons ASR changes without any change in the route:
- a new number list with many invalid or inactive numbers;
- calls placed at times when people do not answer;
- a caller ID that people do not recognise or that is flagged as spam, see how to check CLI delivery;
- a change in traffic mix, for example more calls to fixed lines.
If you compare two carriers on ASR alone, the one that received the better number list wins. NER is less affected because busy and unanswered calls count as successes for the network.
Why NER alone is not enough
NER can be high on a route that is still bad for you. A route with false answer supervision shows excellent NER and ASR because it answers everything. A route that drops calls after a few seconds also keeps a good NER. That is why NER must be read together with ACD and the share of short duration calls.
How to use them together
| What you see | Likely reading |
|---|---|
| NER stable, ASR falls | Change on your side: numbers, timing, CLI reputation |
| NER and ASR fall together | Network problem: congestion, blocking, routing errors |
| NER and ASR very high, ACD very low | Possible false answer or dropped calls |
| NER high, many 486 and 480 responses | Network fine, users busy or unreachable |
To compute NER you need the release cause of every failed call, so your switch must log SIP response codes or Q.850 causes in the CDRs. Map user causes (for example 486 Busy Here, 480 Temporarily Unavailable after ringing, 603 Decline) separately from network causes (503 Service Unavailable, 502, 408 Request Timeout, 404 for valid numbers). Mapping rules differ slightly between switches, so keep the same rules when you compare carriers. More on this in monitoring route quality with CDRs.
Which one to put in an SLA
Carriers rarely accept an ASR commitment, because they do not control your number lists. NER, PDD and a cap on short calls are fairer targets for a quality agreement. Internally, keep ASR on your dashboard: it is the number closest to revenue and to cost per contact. When ASR falls, the guide why ASR drops on a route walks through the checks.
If a route's NER is consistently below what other routes achieve on the same traffic, it may be time to compare options. VoIPMatcher sources wholesale VoIP termination for your destination and traffic type: request a VoIP route.
Frequently asked questions
What is a good NER in VoIP?
There is no fixed industry figure, because NER depends on the destination and on how each switch maps release causes. On a healthy route NER sits well above ASR, since busy and unanswered calls count as network successes. The useful test is comparison: the same traffic, the same hours and the same cause mapping across carriers.
Which SIP codes count against NER?
Network failures count against NER, for example 503 Service Unavailable, 502 Bad Gateway, 408 Request Timeout and 404 for numbers that are valid. User results such as 486 Busy Here, 603 Decline and 480 after ringing count as network successes. Mapping rules differ by switch, so keep them identical when you compare carriers.
What does ASR stand for in telecom?
ASR stands for answer seizure ratio: answered calls divided by total call attempts, as a percentage. A seizure is a call attempt that occupies a circuit or channel. ASR is the number closest to revenue, but it mixes route quality with user behaviour, so read it together with NER and ACD.
Why is ASR lower than NER?
Because ASR counts only answered calls as successes, while NER also counts calls that reached the phone but were busy, not answered or rejected. The gap between the two is user behaviour. A widening gap with stable NER usually means a change in your number lists, calling hours or caller ID reputation, not in the route.
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