SLA: definition and a practical example

Short answer: An SLA (Service Level Agreement) is the contractual commitment that defines the minimum level of a service, such as 99.9% availability, and how it is measured and calculated.

In Portuguese: SLA

What is an SLA?

An SLA (Service Level Agreement) is the part of the contract that says how much the service has to work and what happens when it does not. In telephony, the central number is availability: the percentage of time the line, the trunk or the 0800 (toll-free) number received and completed calls.

The formula is simple: availability = (total time − unavailable time) ÷ total time × 100. The hard part is the rest of the agreement. A well-written SLA defines the measurement period (month or year), what counts as unavailable (no dial tone, no audio, call rejected), who measures, what is excluded (such as a scheduled maintenance window) and the compensation, usually a credit on the bill.

Other indicators often appear alongside availability, such as response and repair time, measured by MTTA and MTTR. Without independent measurement, the company depends on the carrier’s own report to know whether the SLA was met.

In practice

How much downtime each target allows:

SLA	Maximum per month (30 days)	Maximum per year (365 days)
99%	7 h 12 min	3 days 15 h 36 min
99.5%	3 h 36 min	43 h 48 min
99.9%	43 min 12 s	8 h 45 min 36 s
99.99%	4 min 19 s	52 min 34 s

The difference between yearly and monthly matters: with a 99.9% yearly SLA, a 6-hour outage in March does not breach the contract, but it would breach a 99.9% monthly SLA. Run the numbers in the SLA calculator and see the full method in the telephony SLA guide.

How Monitor uses this

The SLA reports are monthly and show the figure for the last 12 months, which decides a yearly SLA, and the month’s figure as a reference. Periods when monitoring had no visibility because of 2N5 are left out of the calculation, so they count neither against the customer’s line nor in its favor.

Frequently asked questions

How do you calculate a trunk's availability?

Divide the time the trunk was working by the total time in the period and multiply by 100. In a 30-day month (43,200 minutes) with 90 minutes of downtime, availability is (43,200 − 90) ÷ 43,200 × 100 = 99.79%. The deciding point is how downtime is measured: by real calls, by opened tickets or by the carrier's own report.

Yearly or monthly SLA: which is better for the customer?

Monthly usually protects more. At 99.9%, a monthly SLA tolerates about 43 minutes per month, while a yearly one tolerates about 8 hours and 45 minutes across the whole year. A single 6-hour outage fits within the yearly SLA but breaks the monthly one by a wide margin. With a yearly SLA, the calculation has to track the last 12 months to know whether the target still holds.