Service level agreement

An SLA is a written commitment with numbers attached and a consequence if they are missed. It is the reason a leased line costs what it does, and the reason a procurement team asks for one before it asks for a speed.

The remedy is defined in your agreement rather than negotiated afterwards, and the NOC issues a root-cause summary for any incident that breaches the commitment.

99.99%

53 minutes a year, or about 4 minutes a month

99.9%

8.8 hours a year, or roughly 44 minutes a month

99.5% (our commitment)

43.8 hours a year, or about 3 hours 40 minutes a month

99%

87.6 hours a year, which is most of a working week

Figures that look alike can be far apart: 99.9% allows a fifth of the downtime that 99.5% does. We publish 99.5% because it is the figure in our agreements. When comparing quotes, check which figure each one commits to in writing, and what the credit is when it is missed.

Response is not restoration. A four-hour response target means an engineer is working the fault within four hours; it is not a promise that the line is back by then. Any supplier quoting a restoration time should be asked what happens when they miss it, because that is the part with a cost attached.

Uptime is also measured against the circuit, not against your building. A power cut at your site, a failed router you own, or a cable pulled out of a wall are outside it. That is standard across the industry and worth knowing before an incident rather than during one.