When someone asks how many concurrent calls a server can handle, the answer depends on CPS, ASR and ACD. These variables describe call attempts, answered calls and average duration; they should not be reduced to one fixed capacity claim.
Quick definitions
- CPS (Calls Per Second): call attempts started each second.
- ASR (Answer Seizure Ratio): the percentage of attempts that are answered.
- ACD (Average Call Duration): the average duration of answered calls, in seconds.
- CC (Concurrent Calls): connected calls active at the same time.
The estimate
Using Little’s Law, the average number of connected calls can be estimated with:
- 40 CPS × 10% ASR × 15 seconds ACD = 60 concurrent calls.
- 40 CPS × 40% ASR × 60 seconds ACD = 960 concurrent calls.
CC = CPS × (ASR / 100) × ACDWhat the numbers mean
CPS is affected by server sizing, database performance, network conditions and architecture. ASR depends mainly on routes, destination quality and calling policy. ACD reflects the duration of answered calls. Measure each value in your own operation before planning capacity.