How to Calculate ITIL Network Availability

Manish Rathi

Last updated 21/07/2021

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Consider below Scenario and let me know is it fairly simple or tricky to calculate Network Availability SLA ?

• NovelVista has several remote offices with their own Internet connection.
• Each office connects back to the headquarters-HQ’s via a VPN tunnel.
• Users at each remote office need to access a mix of internal applications across the VPN tunnel and external services via the local Internet connection.

Here is my Question

• What will be network availability value formula if the VPN tunnel is down, but not the Internet connection?
• Will this availability value calculation formula change if the Internet connection goes down?

Solution

We discussed this, brainstormed and got amazing answers… Here is summary of discussion

In normal situation we will calculate network availability based on the uptime of the Internet connection, because if the Internet goes down, it will affect both internal and external applications. But on other side in reality - a failure of the VPN tunnel would still impact the access to internal applications but not external applications (partial outage).

• Both VPN and Internet will have its own availability measures different technology even if they are network related. Best would be to divide and measure them.
• E2E business KPI's is what customers are looking at.. as we have business processes.. we have to link infrastructure and software components associated and measure business KPI's rather than SLA.
• One of approach to calculate network availability can be weighted average between the availability of the Internet connection and the availability of the VPN tunnel: “Network Network Availability = Weight external_apps x Network Availability Internet + Weight Internal_apps x Network Availability VPN"
• The two weights values should be set and discussed with the business architect considering business criticalness.
• For example, let’s assume that 80% of the business applications used by the users are external (Internet) and that the remaining 20% are internal (VPN). If in one-year time interval, the Internet connection never failed, while the VPN tunnel was unavailable for 1 day, then the value of the overall network availability for that location is:

• Network Availability = 80% * 100% + 20% * (99.726%) = 99.945%

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