1. a certain level of guaranteed bandwidth
2. certain maximum jitter and latency levels
3. a guaranteed level of "up time"
Items 1 and 3 are provided by choosing the right equipment with the necessary redundancy options to achieve the five 9s reliability. It is also very common to configure multiple alternative routes for the traffic.
For item 1, the wide area network (WAN) link at the edge is typically one of the most difficult things to handle. A device that can perform bandwidth management functions and provide prioritization of traffic is typically used. External test solutions for providers exist to help them ensure that they meet the required SLA. An example is found on this Web site.
This was first published in March 2006