IMS Session Calculator
Estimate concurrent IMS sessions, SIP transaction rate and the number of CSCF nodes a subscriber base needs.
About IMS Session Calculator
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
Worked example
With the default values below, IMS Session Calculator returns Concurrent Sessions of 20,000. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Registered subscribersaccepts 1–100,000,000 subscribers
- 500,000 subscribers
- Busy-hour call attempts per subscriberaccepts 0.01–20 calls
- 1.2 calls
- Average session durationaccepts 1–7,200 seconds
- 120 seconds
- Registration refresh intervalaccepts 60–86,400 seconds
- 3,600 seconds
- SIP messages per sessionaccepts 4–60 messages
- 12 messages
- Capacity per CSCF nodeaccepts 100–200,000 TPS
- 2,000 TPS
Results
- Concurrent Sessions
- 20,000
- Session Attempts
- 166.7 calls/second
- SIP Transaction Rate
- 2,278 TPS
- CSCF Nodes Required (N+1)
- 3
Step by step
- 1Values used
- 2IMS Session
- 3Concurrent Sessions= 20,000
- 4Session Attempts= 166.7 calls/second
- 5SIP Transaction Rate= 2,278 TPS
- 6CSCF Nodes Required (N+1)= 3
Assumptions and limits
- The result is derived only from the 6 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 1 decimal place, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
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.