1High availability overviewCommunication interruptions can seriously affect widely-deployed value-added services such as IPTVand video conference. Therefore, the basic network infrastructures must be able to provide highavailability.The following are the 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.Table 1 Availability requirementsLevel Requirement Solution1 Decrease system software andhardware faults• Hardware—Simplifying circuit design, enhancingproduction techniques, and performing reliability tests.• Software—Reliability design and test2 Protect system functions from beingaffected if faults occurDevice and link redundancy and deployment of switchoverstrategies3 Enable the system to recover as fastas possiblePerforming fault detection, diagnosis, isolation, andrecovery technologiesThe level 1 availability requirement should be considered during the design and production process ofnetwork devices. Level 2 should be considered during network design. Level 3 should be consideredduring network deployment, according to the network infrastructure and service characteristics.Availability evaluationMean Time Between Failures (MTBF) and Mean Time to Repair (MTTR) are used to evaluate theavailability of a network.MTBFMTBF is the predicted elapsed time between inherent failures of a system during operation. It is typicallyin the unit of hours. A higher MTBF means a high availability.MTTRMTTR is the average time required to repair a failed system. MTTR in a broad sense also involves spareparts management and customer services.MTTR = fault detection time + hardware replacement time + system initialization time + link recovery time+ routing time + forwarding recovery time. A smaller value of each item means a smaller MTTR and ahigher availability.
