Voice Quality Metrics: "ASR" and "ACD" Explained

Introduction to Voice Call Quality Metrics - ASR and ACD

Among the metrics you will find reported on a regular basis, for almost any voice calling service, are the "Answer-Seizure Ratio" or "ASR" and the "Average Call Duration", also known as "ACD."

These are the two most commonly used metrics to indicate Voice over Internet Protocol (VoIP) route quality. They are the first metrics most of us look at when we encounter what we suspect there may be problems with a particular route for calls.

ASR (Answer-Seizure Ratio)

ASR is the ratio of the number of successfully connected calls to the number of attempted calls, expressed in a percentage. The ASR is also sometimes called the "Call Completion Rate":

ASR % = (total number of answered calls / total number of calls) × 100

Here's an example, if your team made 200 outbound calls to customers, and 158 of them were successfully connected then you could calculate the ASR % as follows:

ASR (%) = (158 [successful calls] / 200 [dialed calls]) x 100 = 79%

ACD (Average Call Duration)

ACD is a quality metric used by telephony providers.

ACD = duration of all answered calls/number of answered calls

For example, if there were 158 answered calls with a total duration of 250 minutes, then:

ACD = 250 [duration of all answered calls] / 158 [total number of answered calls] = 1.58 minutes

What is considered "normal"?

Service providers like Toku aim to have ASR's of 60% or more. An ASR or 40–50% is considered "okay" but definitely something to improve upon. A high ASR indicates a reliable network since most calls that are attempted (dialed) are answered.

Low ASR may be caused by:

Busy destination lines
Far-end switch congestion (capacity problems at the carrier)
User behavior - busy signals, rejected calls, all count as call failures.

When it comes to ACD, the assessment gets to be a bit more subjective, though there are still some basic rules to consider.

From a service provider's perspective, an ACD of 4–5 minutes is considered okay. Anything above 6 minutes is excellent. This is because, at these durations, one can infer that few calls are being dropped or abandoned due to line quality issues.

However, ACD is often evaluated within businesses to assess operational effectiveness, not just the quality of once's voice service provider, and the metric could be low, or high, and be either good or bad, depending on the type of business.

In any case, a very low ACD, one that cannot be explained by valid behavior due to the nature of one's business, could indicate one or more of the following issues:

Congestion
Long PDD (Post Dial Delay)
Looping
FAS (False Answer Supervision – dead air)
Echo
Jitter (variance in delay)
Delay (delays of more than 300 ms decrease ACD)
Packet loss
Low-quality codecs
The capacity (measured in packets per second or PPS) of an Ethernet network can be a bottleneck.
Too many hops (Session Border Controller passing media through them) will also decrease ACD.
Traffic profiles can also contribute toward a low ACD - again, how your business uses the service may play a part in determining ACD.

Important Considerations

Differences by Destination

These metrics can vary dramatically by destination and routes. It is important to look at these metrics in the context of the environmental factors related to the destination. While ASR for a country like the United States might be well above 70%, the ASR for countries will poorer telecommunications infrastructure will have dramatically lower ASRs, by default.

Your Mileage, like your requirements, may vary.

Because user behavior can play a part in driving ASR and ACD these metrics are used as relative measures of quality. It is important to consider your own usage patterns and business requirements when evaluating these metrics for your own situation.