High availability and reliability are closely related topics and are measured as a percentage relative to the total required service time (100%). Therefore, in Hosted IP/PBX and cloud voice services, vendors will very often state a availability or reliability measure of five nines (99.999%). This is not just an arbitrary figure they should be able to justify this claim, for what it relates to is a loss of service of – if the service is expected to run 24/7 every day for a year – of 5.39 minutes of total downtime in a year.
This is not an easy level to achieve as it includes both planned and unplanned loss of service to the end customer. So to be able to reach these stellar reliability measures vendors have to implement high-availability designs and configurations. These measures include duplicating devices and applications so there is always automatic failover and no single-point-of-failure in the system. Adding hardware and software redundancy is expensive but necessary to achieve 99.999% availability. Therefore, any reputable service provider will have to invest heavily in infrastructure, server clustering and automatic fail-over of servers and databases.
In an enterprise data center this is not difficult to achieve and measure as the equipment is always being managed and monitored via network management systems. However when customers are considering a cloud service providers claims of 99.999% service availability they must be aware that this is what service providers are referring too – the equipment running in their data center. This is not the same as the service available to the end-customer, as the Internet is an unmanaged and an inherently unreliable medium, which is outside the control of the service provider.
Service providers will strive to meet these lofty goals for service availability but a customer using a cloud provider or a hosted IP/PBX is unlikely to achieve anything close to 99.999’s due to internet outages. There is away to remediate this by have dual internet connections through different internet service providers that preferably are in diverse geographical locations. This method however requires complex routing configurations on the internet routers and adds to the cost of the VoIP services.