When to use: Use to model end-to-end network availability from individual component reliability figures. Series components multiply their availabilities; parallel (redundant) components dramatically improve uptime. Shows achievable SLA tier and expected annual downtime. Use to justify redundant ISP circuits, HSRP/VRRP gateway pairs, and dual power supplies to meet enterprise uptime requirements.
This calculator models end-to-end network availability from individual component reliability figures and determines the achievable SLA tier and annual downtime. Engineers use it during design reviews to justify redundant ISP circuits, dual-homed firewalls, and HSRP gateway pairs to meet contractual uptime obligations.
System availability is calculated by multiplying the fractional availability of each series component: A_system = A1 × A2 × A3 × ... Each component's availability is entered as a percentage (e.g., 99.99%) and represents the fraction of time the component is operational. Even a single low-availability component (such as a 99.9% ISP circuit) dominates and prevents the system from reaching higher SLA tiers.
For parallel (redundant) components, the combined availability is 1 − (1 − A1)(1 − A2), which for two identical 99.9% circuits yields 99.9999%. This dramatic improvement is why dual-ISP configurations are specified for mission-critical applications. Mean time between failures (MTBF) is approximated as 1/(1 − A) in units of years.
Network availability SLA tiers align with Uptime Institute Tier standards: Tier I (99.671%), Tier II (99.749%), Tier III (99.982%), and Tier IV (99.995%). ANSI/TIA-942 references these tiers for data center infrastructure design. ITU-T G.827 defines availability objectives for international network connections. Enterprise network SLAs are typically contractually defined at 99.9% (carrier Ethernet) to 99.99% (MPLS), with financial penalties for exceeding downtime allowances. RFC 2544 defines network benchmarking methodologies used to measure actual versus SLA availability.
The ISP circuit is typically the lowest-availability element in the chain, often rated at 99.9% (8.76 hr/yr) by carrier SLA. Adding a second ISP from a diverse physical path (different conduit, different central office) converts this from the weakest link to a near-perfect 99.9999% element. Core switches from enterprise vendors achieve 99.995–99.999% with redundant supervisors and PSUs. Always verify that diversity is genuine: many "dual-ISP" designs share the same physical last-mile fiber path and fail together during street cuts.
Enter the availability percentage for each network component in the series path from ISP to end device. Add or rename components to match your specific topology. Enable the Dual ISP checkbox to model a parallel redundant ISP circuit. The system availability, achieved SLA tier, and estimated annual downtime update in real time. Compare the result with your contractual SLA target and identify the weakest component limiting your achievable tier.
99.999% availability corresponds to five nines, allowing only 5.26 minutes of downtime per year. This is the target for critical financial and healthcare network infrastructure and requires fully redundant hardware with fast-failover protocols.
Because series components multiply: even if every other device is 99.999%, a single 99.9% ISP circuit caps the system at 99.9%. This is why dual-ISP is the most effective single investment for improving network availability.
Yes. Without a first-hop redundancy protocol, the loss of a gateway router is a hard outage. HSRP and VRRP provide sub-second gateway failover to a standby router, which the availability model treats as a parallel pair at the router tier.
Equipment vendors publish MTBF in the product datasheets. For example, a Cisco ASA firewall may specify 200,000+ hours MTBF. Convert to availability as A = MTBF / (MTBF + MTTR), where MTTR is the mean time to repair, typically 4–8 hours for enterprise equipment with next-day support contracts.
General corporate networks typically accept 99.9% (8.76 hr/yr). Financial trading, healthcare, and e-commerce platforms require 99.99% (52 min/yr) or better. VoIP and video conferencing quality degrades during any outage exceeding 150 ms, so fast-failover design is as important as annual downtime figures.
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