When evaluating international voice route quality, there are four core metrics: ASR, ACD, PDD, and MOS. Understanding what these metrics mean and their benchmark ranges is essential for selecting and managing a voice service provider.
ASR — Answer Seizure Ratio
ASR = Answer Seizure Ratio
Formula: ASR = Answered Calls / Total Call Attempts x 100%
Meaning: Out of every 100 call attempts, how many are actually answered by the receiving party. This is the most important metric for measuring route connect efficiency.
ASR Benchmarks
| Rating | ASR Range | Description |
|---|---|---|
| Excellent | > 65% | Premium direct routes, typically limited to landline calls in developed countries |
| Good | 50% - 65% | Acceptable commercial route quality |
| Fair | 40% - 50% | Possible routing quality issues or poor destination number quality |
| Poor | < 40% | Route quality issues that require investigation |
Factors that affect ASR:
- Telecommunications infrastructure of the destination country
- Time of day (business hours vs. off-hours)
- Number type (landline ASR is typically higher than mobile)
- Routing hops (more intermediate carriers = lower ASR)
- Destination number quality (invalid numbers, powered-off phones, rejected calls, etc.)
ACD — Average Call Duration
ACD = Average Call Duration
Formula: ACD = Total Call Duration / Answered Calls
Meaning: The average length of answered calls. An unusually low ACD may indicate frequent call drops.
ACD Benchmarks
| Scenario | Normal ACD | Warning Sign |
|---|---|---|
| Customer Service Center | 3 - 8 minutes | < 2 minutes may indicate call drops |
| Outbound Telemarketing | 2 - 5 minutes | < 1 minute may indicate quick hang-ups by recipients |
| Voice Verification (OTP) | 0.3 - 1 minute | Normal range — verification calls are inherently short |
| Notification Calls | 0.5 - 2 minutes | Normal range |
PDD — Post Dial Delay
PDD = Post Dial Delay
Meaning: The time from dialing the last digit to hearing ringback tone (or the called party answering). PDD directly impacts user experience.
PDD Benchmarks
| Rating | PDD Range | Description |
|---|---|---|
| Excellent | < 2 seconds | Negligible delay — users barely notice |
| Good | 2 - 4 seconds | Acceptable commercial-grade experience |
| Fair | 4 - 6 seconds | Users begin to perceive the wait |
| Poor | > 6 seconds | Poor user experience that may cause callers to hang up |
Common causes of high PDD:
- Routing through multiple intermediate carriers
- Slow cross-carrier signaling processing
- Codec transcoding overhead
- Excessive network latency
MOS — Mean Opinion Score
MOS = Mean Opinion Score
Scale:1 (Poor) to 5 (Excellent), originating from ITU-T P.800 standard. Objective MOS can be calculated using the ITU-T G.107 E-Model algorithm.
Codec MOS Reference Values
| Codec | Theoretical MOS | Real-World Network MOS |
|---|---|---|
| G.711 (PCMA/PCMU) | 4.1 | 3.8 - 4.2 |
| G.729 (A/B) | 3.7 | 3.4 - 3.9 |
| G.722 | 4.1 | 3.8 - 4.2 |
| Opus (wideband) | 4.5 | 4.0 - 4.5 |
MOS Rating Scale
| MOS Range | Rating | User Experience |
|---|---|---|
| 4.0 - 5.0 | Excellent | Clear and smooth, close to face-to-face conversation |
| 3.5 - 4.0 | Good | Occasional slight noise, no impact on comprehension |
| 3.0 - 3.5 | Fair | Perceptible audio degradation, but still understandable |
| 2.0 - 3.0 | Poor | Frequent choppy audio, repetition needed to understand |
| 1.0 - 2.0 | Very Poor | Nearly impossible to hold a conversation |
Comprehensive Benchmark Summary
| Metric | Premium Route | Standard Route | Economy Route |
|---|---|---|---|
| ASR | > 60% | > 50% | > 40% |
| ACD | > 4 minutes | > 3 minutes | > 2 minutes |
| PDD | < 2 seconds | < 4 seconds | < 6 seconds |
| MOS | > 4.0 | > 3.8 | > 3.5 |
How to Measure These Metrics
- CDR Analysis: Extract ASR and ACD data from SIP trunk CDRs (Call Detail Records)
- RTP Monitoring: Use RTP stream analysis tools (e.g., Wireshark, rtcp pairs) for real-time MOS measurement
- SIPp Load Testing: Use SIPp to simulate calls and batch-measure ASR and PDD
- Provider Dashboards: Most premium SIP providers offer real-time quality monitoring panels
Reference Standards:
- ITU-T G.107 — E-Model (MOS calculation standard)
- ITU-T P.800 — MOS subjective assessment method
- ITU-T P.862 — PESQ (objective voice quality assessment)
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