13High availability overviewCommunication interruptions can seriously affect widely-deployed value-added services such asIPTV and video conference. Therefore, the basic network infrastructures must be able to providehigh availability.There are three effective ways to improve availability:• Increasing fault tolerance• Speeding up fault recovery• Reducing impact of faults on servicesAvailability requirementsAvailability requirements fall into three levels based on purpose and implementation, as shown inTable 1.Table 1 Availability requirementsLevel Requirement Solution1 Decrease system software andhardware faults• Hardware: Simplified circuit design, enhancedproduction techniques, and reliability tests.• Software: Reliability design and tests2 Protect system functions from beingaffected by failures Device and link redundancy and switchover3 Enable the system to recover as fastas possibleFault detection, diagnosis, isolation, and recoverytechnologiesThe level 1 availability requirement should be considered during the design and productionprocess of network devices. The level 2 availability requirement should be considered duringnetwork design. The level 3 availability requirement should be considered during networkdeployment, according to the network infrastructure and service characteristics.Availability evaluationTypically, MTBF and MTTR are used to evaluate the availability of a network.MTBFMTBF is the predicted elapsed time between inherent failures of a system during operation. It istypically expressed in hours. A higher MTBF means a higher availability.
