Question 1

What SLA level should an ISP offer their customers?

Accepted Answer

The right SLA depends on the service tier. Residential broadband typically carries a 99.5% to 99.9% SLA, allowing between 3 hours 36 minutes and 43 minutes of monthly downtime. Business-grade fiber usually requires 99.9% to 99.99%, and carrier-grade or critical infrastructure demands 99.99% or higher. Higher SLA commitments require more robust monitoring, redundancy, and faster incident response.

Question 2

How is network uptime measured for SLA purposes?

Accepted Answer

Uptime is typically measured as the percentage of time a network service is available during a given period, usually monthly. Most ISPs measure availability at the network edge — at the OLT or BNG level — rather than at every individual customer connection. Network monitoring tools track device status continuously; for example, NetSense NMS polls critical devices every 30 seconds to detect outages within a minute.

Question 3

What counts as downtime in an ISP SLA?

Accepted Answer

This is defined in the SLA contract itself, but typically includes any unplanned outage that prevents customers from using the service. Most SLAs exclude scheduled maintenance windows (if communicated in advance), customer-side equipment failures, and force majeure events. Partial outages — where some but not all customers are affected — may be prorated or measured differently depending on the agreement.

Question 4

What is an error budget and how should ISPs use it?

Accepted Answer

An error budget is the total amount of downtime allowed within an SLA period before you breach the agreement. For example, a 99.9% monthly SLA gives you an error budget of 43 minutes and 12 seconds. ISPs use error budgets to balance reliability with operational changes: when the budget is healthy, teams can proceed with upgrades and deployments; when it is running low, all efforts shift to stability and incident prevention.

Question 5

How can ISPs proactively protect their SLA targets?

Accepted Answer

Proactive monitoring is the most effective way to protect SLA commitments. This means real-time monitoring with fast polling intervals (30 seconds or less), smart alert correlation to reduce noise and surface genuine issues, proactive fault detection for fiber-specific problems like optical signal degradation and partial PON failures, and automated escalation policies that ensure the right team responds quickly. Purpose-built ISP monitoring tools detect issues before they impact customer uptime.

Question 6

Does planned maintenance count against SLA uptime?

Accepted Answer

It depends on the contract. Many ISP SLAs explicitly exclude pre-announced maintenance windows from downtime calculations, but this must be clearly stated in the agreement. Best practices include: communicating maintenance windows at least 48 hours in advance, scheduling during low-traffic periods, minimizing duration, and tracking maintenance windows separately from unplanned outages in your monitoring system.

SLA Level	Common Name	Monthly Downtime	Yearly Downtime
99%	Two nines	7h 12m	3d 15h 36m
99.5%	—	3h 36m	1d 19h 48m
99.9%	Three nines	43m 12s	8h 45m 36s
99.95%	—	21m 36s	4h 22m 48s
99.99%	Four nines	4m 19s	52m 34s
99.999%	Five nines	26s	5m 15s

SLA Uptime Calculator

Common SLA tiers

99% — Two Nines

99.9% — Three Nines

99.99% — Four Nines

99.999% — Five Nines

99.9999% — Six Nines

SLA Calculation Cheatsheet

Availability Calculation

Reliability Metrics

Service Credit Calculation

Availability from MTBF & MTTR

Frequently Asked Questions