IMS Session Calculator
Estimate concurrent IMS sessions, SIP transaction rate and the number of CSCF nodes a subscriber base needs.
Inputs
Concurrent Sessions
20,000
Session Attempts
166.7calls/second
SIP Transaction Rate
2,278TPS
CSCF Nodes Required (N+1)
3
Step by step
Values used
Registered subscribers = 500,000 subscribers; Busy-hour call attempts per subscriber = 1.20 calls; Average session duration = 120 seconds; Registration refresh interval = 3,600 seconds; SIP messages per session = 12 messages; Capacity per CSCF node = 2,000 TPS
IMS Session
sessions = (subs × BHCA ÷ 3600) × AHT; SIP TPS = attempts/s × messages per session + 2 × (subs ÷ refresh interval).
Concurrent Sessions
= 20,000
Session Attempts
= 166.7 calls/second
SIP Transaction Rate
= 2,278 TPS
CSCF Nodes Required (N+1)
= 3
How it works
Little's law turns an attempt rate and a duration into a concurrent session count. The signalling load adds the per-session message exchange to the steady background of re-REGISTER transactions, which for a large base often dominates. P-CSCF and S-CSCF licences are sold by session and TPS, and registration storms after an outage are what actually melt an IMS core.
Formula
IMS Session
sessions = (subs × BHCA ÷ 3600) × AHT; SIP TPS = attempts/s × messages per session + 2 × (subs ÷ refresh interval).
- BHCA
- Busy-hour call attempts per subscriber
- AHT
- Average session duration
- refresh interval
- SIP re-REGISTER period
Frequently Asked Questions
How is IMS Session calculated?
sessions = (subs × BHCA ÷ 3600) × AHT; SIP TPS = attempts/s × messages per session + 2 × (subs ÷ refresh interval). Little's law turns an attempt rate and a duration into a concurrent session count. The signalling load adds the per-session message exchange to the steady background of re-REGISTER transactions, which for a large base often dominates.
Why does IMS Session matter?
P-CSCF and S-CSCF licences are sold by session and TPS, and registration storms after an outage are what actually melt an IMS core.
What values do I need to enter?
This calculator takes 6 inputs: Registered subscribers, Busy-hour call attempts per subscriber, Average session duration, Registration refresh interval, SIP messages per session, Capacity per CSCF node. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.