By JULIE BORT
Network World, 09/27/99

Omissions are deadly to an SLA. So here 's a checklist of items to negotiate, along with some suggested targets (when practical). Of course, all suggested targets are simply guidelines. Your SLA should reflect your specific needs.

Outage: A change in performance that keeps user groups from being able to use the network. Can apply to any or all user groups.

Outage duration: Time in minutes that service is not available.

Degraded service: Not an outage, but service that is slower than performance contracted for. Impacted user minutes (IUM): Outage duration multiplied by the number of affected users. Total user minutes (TUM): Minutes in a set period multiplied by the number of users being served.

Total service availability: The total IUMs caused by all outages divided by the TUM.

Defects per million: Minutes of downtime per million minutes of service.

Mean time between failures (MTBF): Average amount of time, typically in minutes or days, between outages. (The target figure depends on your negotiated total-service-availability rate.)

Mean time to restore (MTTR): Average amount of time, typically in minutes, it takes to restore service.

Maximum time between failures/maximum time to restore: A cap on the number of minutes in total, during a specified time frame.

Trouble rate: How often tech support is called.

Repeat trouble rate: How many times the same problem is reported. (Limit to five before escalation is mandated.) Peak hours troubleshooting (such as resolution within one hour). Off-hours troubleshooting (such as resolution within four hours).

Average round-trip latency: The time it takes the first transmission to complete. (The target should be less than 100 msec on the backbone and up to 130 msec longer end to end, depending on the type of service being contracted for and the application requirements.)

Average round-trip delay: The time it takes for routine transmission, after the first transmission establishes the connection. (Target the same as latency.) Related links

SLA savvy
Five secrets for making sure you get the most from your service-level agreements.