s to recover from a trouble condition or failure. Mean time to respond: The average time the support or maintenance team takes to respond to a request to fix a network issue is calculated in mean time to respond. The MTTR value is usually mentioned as part of the support agreement the service provider has with a network or equipment support vendor. OSS/BSS Functions Failuer start Uptime after Downtime caused by repair next failure Normal operation Failure Failure Operational period Figure 20.9 Attributes monitored for service quality. Mean time to replace/repair/resolve: A system or equipment trouble might sometime require replacement of the system/equipment or repairing the same. Some other methods like installing a patch or upgrading a compo- nent can also resolve the trouble condition. This leads to different acro- nyms like mean time to replace/repair/resolve corresponding to whatever may be the case the service provider wants in the support agreement. Maximum time to recover/resolve/repair: Mean time is mostly an unreli- able parameter to enforce. For example, mean time to resolve in 12 hours does not necessarily mean that when a trouble condition occurs the issue will be fixed during 12 hours. The support personal might solve one minor bug in one hour and later take 20 hours to solve the next issue, which might have been a critical issue that required a quick resolution. Even when the support team takes 20 hours, mean time to resolve within 12 hours is not violated as the average time taken to resolve is less than 12. To avoid this kind of issue, maximum time to resolve an issue is specified in the agreement. Maximum time to resolve/repair/recover is a more reliable parameter than can be easily monitored and enforced on all trouble or failure scenarios. MTBF: Mean time between failures is the average time between system fail- ures. MTBF is usually measured at the system level and is a part of the con- tractual agreement. MTBF is a measure of the reliability of the system. For computing the value of MTBF, th