ISP Bandwidth Oversubscription Calculator
Check an access node's contention ratio: sold capacity versus uplink, busy-hour demand and the throughput each subscriber really gets.
About ISP Bandwidth Oversubscription Calculator
Oversubscription is normal and necessary: subscribers are neither all online nor all saturating their line at once. The contention ratio describes the worst case, while multiplying by concurrency and average active utilisation gives the load the uplink actually has to carry. A 50:1 ratio can be perfectly good service or a nightly disaster depending on concurrency, so the busy-hour figure — not the ratio alone — is what should trigger an uplink upgrade.
Formula
ISP Bandwidth Oversubscription
contention ratio = Σ sold speeds ÷ uplink capacity; busy-hour load = Σ sold × concurrency × average active utilisation.
- concurrency
- Share of subscribers online in the busy hour
- active utilisation
- Average fraction of their sold speed an online subscriber uses
Worked example
With the default values below, ISP Bandwidth Oversubscription Calculator returns Contention Ratio of 10.0. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Subscribers on the access nodeaccepts 1–1,000,000 subscribers
- 500 subscribers
- Speed sold per subscriberaccepts 1–100,000 Mbps
- 200 Mbps
- Uplink capacityaccepts 0.01–1,000 Gbps
- 10 Gbps
- Subscribers active in the busy houraccepts 1–100 %
- 25 %
- Average utilisation of an active subscriberaccepts 1–100 %
- 30 %
Results
- Contention Ratio
- 10.0 :1
- Busy-Hour Offered Load
- 7.50 Gbps
- Uplink Utilisation at Busy Hour
- 75.0%
- Guaranteed Throughput per Subscriber
- 20.00 Mbps
- Assessment
- Acceptable — plan the next uplink upgrade
Step by step
- 1Values used
- 2ISP Bandwidth Oversubscription
- 3Contention Ratio= 10.0 :1
- 4Busy-Hour Offered Load= 7.50 Gbps
- 5Uplink Utilisation at Busy Hour= 75.0
- 6Guaranteed Throughput per Subscriber= 20.00 Mbps
- 7Assessment= Acceptable — plan the next uplink upgrade
Assumptions and limits
- The result is derived only from the 5 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 2 decimal places, 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 ISP Bandwidth Oversubscription calculated?
contention ratio = Σ sold speeds ÷ uplink capacity; busy-hour load = Σ sold × concurrency × average active utilisation. Oversubscription is normal and necessary: subscribers are neither all online nor all saturating their line at once. The contention ratio describes the worst case, while multiplying by concurrency and average active utilisation gives the load the uplink actually has to carry.
Why does ISP Bandwidth Oversubscription matter?
A 50:1 ratio can be perfectly good service or a nightly disaster depending on concurrency, so the busy-hour figure — not the ratio alone — is what should trigger an uplink upgrade.
What values do I need to enter?
This calculator takes 5 inputs: Subscribers on the access node, Speed sold per subscriber, Uplink capacity, Subscribers active in the busy hour, Average utilisation of an active subscriber. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.