The term downtime is used to refer to periods when a system is unavailable. Downtime or outage duration refers to a period of time that a system fails to provide or perform is primary function. Reliability, Availability, Recovery, and Unavailability are related concepts. The expected Unavailability is the percentage of a timespan that a system is unavailable or offline. This is usually a result of the system failing to function because of an unplanned event, or because of routine maintenance.

The term is commonly applied to networks and servers. The common reasons for unplanned outages are system failures (such as a crash) or communications failures (commonly known as network outage).

The opposite of downtime is uptime.

Characteristics

Unplanned downtime may be the result of a software bug, human error, equipment failure, malfunction, high bit error rate, power failure, overload due to exceeding the channel capacity, a cascading failure, etc.

See also: Planned downtime]]

Telecommunication Outage Classifications

Downtime can be caused by failure in hardware (physical equipment), software (logic controlling equipment), interconnecting equipment (such as cables, facilities, routers,...), Wireless transmission (wireless, microwave, satellite), and/or capacity (system limits).

The failures can occur because of damage, failure, design, procedural (improper use by humans), engineering (how too use and deployment), overload (traffic or system resources stressed beyond designed limits), environment (support systems like power and HVAC), scheduled downtime (outages designed into the system for a purpose such as software upgrades and equipment growth), other (none of the above but known), or unknown.

The failures can be the responsibility of customer/service provider, vender/supplier, utility, government, contractor, end customer, public individual, act of nature, other (none of the above but known), or unknown.

Impact

Outages caused by system failures can have a serious impact on the users of computer/network systems, in particular those industries that rely on a nearly 24-hour service:

• medical informatics
• nuclear power and other infrastructure
• banks and other financial institutions
• aeronautics, [[airlines
• news reporting
• e-commerce and online transaction processing
• persistent online games

Also affected can be the users of an ISP and other customers of a telecommunication network.

Corporations can lose business due to network outage or they may default on a contract, resulting in financial losses.

Those people or organizations that are affected by downtime can be more sensitive to particular aspects:

• some are more affected by the length of an outage - it matters to them how much time it takes to recover from a problem
• others are sensitive to the timing of an outage - outages during peak hours affect them the most