ows critical alarms and NEs represented by symbols. Different severity levels for alarms issued by telecom equipment in the network are indicated through colour and symbols. An alarm is a signal issued by a NE that some interven- tion is needed for the NE to return to normal functioning. The severity of an alarm depends on the risk that it poses to service. For example, failure of the primary power supply to a switch might have a lower severity level than a multiplexer failure. Alarm or fault management include receiving, processing and displaying alarms as they are received from exchanges and other network equipment. The Tele- communications Management Network (TMN) model of the telecom standards section of the International Telecommunications Union (ITU-T) defines alarm levels (Table 2.1). Table 2.1 Alarm severity levels. Severity level Description Critical Immediate action needed Major Act as soon as possible Minor Watch situation carefully Warning Investigate during next scheduled maintenance 2 Network Management OSS Overview Colour-coding of alarms allows the severity level of an alarm to be determined immediately. An OSS should allow network operations staff to drill down to more detailed information about a particular alarm. All alarms are logged and stored for later reference or printing. Alarms can be: Unacknowledged or acknowledged by network operations staff: an acknowledged alarm has been recognised and some corrective action has been taken, such as the issue of a "trouble ticket" which causes a technician to visit the site of the fault in the network; or Resolved or inactive: an alarm with resolved status has been corrected while an inactive alarm has not deliberately been resolved but the alarm has ceased to indicate the original fault. The staff can control and supervise individual network elements by sending com- mands to them and interpreting their responses. Management commands can be sent for immediate execution, or they can be timed for execution at a particular future time. The response of th