What Do Service Levels Really Mean?

Availability is defined as the level of redundancy applied to a system for ensuring an absolute degree of operational continuity during planned and unplanned outages. Many organizations will require that Lync Server systems are available without service interruption at all times. However, many people forget to consider the impact that regular maintenance on the Lync systems, as well as contributing components such as the network and directory services, can have on the availability of Lync. When an organization states that they want to have “five 9’s” of availability for Lync, it would mean that the Lync service can have no more than 5.256 minutes of downtime for the entire year! SLA percentages are often misunderstood, and they can have a major impact on the design of the infrastructure a well as the surrounding operations. Table 15.2 outlines the most common SLA definitions and the associated downtime.

Table 15.2. SLA Definitions and Associated Downtime

It is important to be familiar with common SLA definitions when you are defining business requirements for any organization. Because planned outages such as maintenance and upgrades are often counted toward the overall downtime of a system, it is critical to design the Lync system to accommodate that requirement. A Lync system with the SLA of four nines would allow only 52.56 minutes of downtime for the entire year. This will require maintenance to happen without any service interruption to end users.