When someone asks “how many concurrent calls can my server handle?”, the answer depends on a combination of CPS, ASR and ACD. This guide explains each variable, shows how to calculate CC (concurrent calls), compares real operating scenarios and provides an interactive simulator.
Quick definitions
- CPS (Calls Per Second): how many call attempts per second your system can originate.
- ASR (Answer Seizure Ratio): the percentage of attempts that become answered calls.
- ACD (Average Call Duration): the average duration of answered calls, in seconds.
- CC (Concurrent Calls): how many connected calls are active at the same time.
Formula: Little’s Law applied to telephony
CPS multiplied by ASR gives the number of answered calls per second. Multiplying that rate by ACD in seconds gives the average number of calls connected at the same time.
CC = CPS × (ASR / 100) × ACDPractical examples
| Scenario | CPS | ASR | ACD (s) | Estimated CC |
|---|---|---|---|---|
| Robocall on a weak route | 40 | 10% | 15 | 60 (40 × 0.10 × 15) |
| Human agents, good route and long calls | 40 | 40% | 60 | 960 (40 × 0.40 × 60) |
| Reported test with a likely measurement error | 40 | 70% | 6 | 168 expected. If 24 was reported, review the data collection. |
How to increase CPS (server capacity)
To raise CPS, scale the architecture with a SIP Proxy Master and Slaves. The proxy distributes attempts across Slaves, increasing the total origination rate without overloading one node. Size the database, network and every SIP layer together; adding nodes does not correct poor routes or user behavior.
- ASR improves with better routes, valid CLI presentation, anti-fraud controls and reasonable redial policies.
- ACD increases when calls are useful, with clear IVRs, low latency and stable audio.
- CPS depends on MagnusBilling sizing, database performance, network capacity and SIP Proxy architecture.
Interactive simulator: CPS × ASR × ACD → CC
Enter the values and add a scenario. You can compare several traffic profiles and see the combined answered-call rate, concurrent calls and approximate traffic in erlangs.
Current estimate960 CC
Formula: CC = CPS × (ASR / 100) × ACD. This estimates the average number of calls connected at the same time.
| # | Inputs | Answered/s | CC (connected) | Traffic (erlangs ≈ CC) | Action |
|---|---|---|---|---|---|
| Total | 0 | 0 | 0 | ||
If measured peak CC differs greatly, confirm that ACD is in seconds and that ASR and CPS use the same attempts and observation window.
Frequently asked questions
1. Can I assign one fixed CC number to my server?
No. CC changes with the combination of CPS, ASR and ACD. The server directly affects CPS, while the other inputs depend on the traffic being processed.
2. What influences ASR the most?
Route quality, valid CLI presentation, anti-fraud controls, calling times, DNC compliance and reasonable redial policies.
3. How should ACD be measured?
Use answered calls only and measure duration in seconds. Do not include unanswered attempts in ACD.
4. What if measured peak CC differs greatly from the estimate?
Confirm that ACD is in seconds and that ASR and CPS came from the same observation window and the same set of attempts. Then check whether short traffic bursts are being compared with long-term averages.
Original article
This edition restores the complete scope of the archived article, including its third example, scaling guidance, operating rules, FAQ and the interactive simulator.