Subscriber Capacity Calculator
Work out how many subscribers a cluster of core nodes supports, with redundancy reserved and headroom against today's load.
Inputs
Subscribers Supported
1,071,428
Nodes Counted Toward Capacity
3
Headroom Above Today's Load
16.0%
Spare Subscriber Capacity
171,428
Step by step
Values used
Deployed nodes = 4 nodes; Session capacity per node = 500,000 sessions; Average sessions per subscriber = 1.40 sessions; Reserve one node for failover (N+1) = Yes; Subscribers today = 900,000 subscribers
Subscriber Capacity
supported subscribers = (effective nodes × sessions per node) ÷ sessions per subscriber, where N+1 removes one node from the total.
Subscribers Supported
= 1,071,428
Nodes Counted Toward Capacity
= 3
Headroom Above Today's Load
= 16.0
Spare Subscriber Capacity
= 171,428
How it works
Core nodes are licensed in sessions, not subscribers, and a modern handset holds more than one session at a time. Reserving a node for failover means the cluster's advertised capacity is deliberately lower than the sum of its parts. Sizing to the full cluster total is the classic mistake: the day one node fails, the remaining nodes reject sessions and the outage becomes a capacity incident rather than a redundancy event.
Formula
Subscriber Capacity
supported subscribers = (effective nodes × sessions per node) ÷ sessions per subscriber, where N+1 removes one node from the total.
- effective nodes
- Nodes available once the failover spare is set aside
- sessions per subscriber
- Concurrent PDN/PDU sessions the average subscriber holds
Frequently Asked Questions
How is Subscriber Capacity calculated?
supported subscribers = (effective nodes × sessions per node) ÷ sessions per subscriber, where N+1 removes one node from the total. Core nodes are licensed in sessions, not subscribers, and a modern handset holds more than one session at a time. Reserving a node for failover means the cluster's advertised capacity is deliberately lower than the sum of its parts.
Why does Subscriber Capacity matter?
Sizing to the full cluster total is the classic mistake: the day one node fails, the remaining nodes reject sessions and the outage becomes a capacity incident rather than a redundancy event.
What values do I need to enter?
This calculator takes 5 inputs: Deployed nodes, Session capacity per node, Average sessions per subscriber, Reserve one node for failover (N+1), Subscribers today. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.
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