Estimate concurrent IMS sessions, SIP transaction rate and the number of CSCF nodes a subscriber base needs.
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.
IMS Session
sessions = (subs × BHCA ÷ 3600) × AHT; SIP TPS = attempts/s × messages per session + 2 × (subs ÷ refresh interval).
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.
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.
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.