3Com® VCX V7111 VoIP Gateway User Guide 355 Table MIB Objects: Contain multiple sections of management data. These objects aredistinguished from ‘Discrete’ items (above) by requiring a "." (dot) extension to theirnames that uniquely distinguishes the particular value being referenced. The "." (dot)extension is the "instance" number of an SNMP object. For "Discrete" objects, thisinstance number is zero. For "Table" objects, this instance number is the index into theSNMP table. SNMP tables are special types of SNMP objects which allow parallel arraysof information to be supported. Tables are distinguished from scalar objects, so thattables can grow without bounds. For example, SNMP defines the "ifDescr" object (as astandard SNMP object) that indicates the text description of each interface supported bya particular device. Since network devices can be configured with more than oneinterface, this object can only be represented as anBy convention, SNMP objects are always grouped in an Entry directory, within an object witha Table suffix. (The ifDescr object described above resides in the ifEntry directory containedin the ifTable directory).SNMP Extensibility FeatureOne of the principal components of an SNMP manager is a MIB Compiler which allows newMIB objects to be added to the management system. When a MIB is compiled into an SNMPmanager, the manager is made “aware” of new objects that are supported by agents on thenetwork. The concept is similar to adding a new schema to a database.Typically, when a MIB is compiled into the system, the manager creates new folders ordirectories that correspond to the objects. These folders or directories can typically beviewed with a MIB Browser, which is a traditional SNMP management tool incorporated intovirtually all Network Management Systems.The act of compiling the MIB allows the manager to know about the special objectssupported by the agent and access these objects as part of the standard object set.Carrier Grade Alarm SystemThe basic alarm system has been extended to a carrier-grade alarm system. A carrier-gradealarm system provides a reliable alarm reporting mechanism that takes into account EMSoutages, network outages, and transport mechanism such as SNMP over UDP.A carrier-grade alarm system is characterized by the following: The device has a mechanism that allows a manager to determine which alarms arecurrently active in the device. That is, the device maintains an active alarm table. The device has a mechanism to allow a manager to detect lost alarm raise and clearnotifications [sequence number in trap, current sequence number MIB object]. The device has a mechanism to allow a manager to recover lost alarm raise and clearnotifications [maintains a log history]. The device sends a cold start trap to indicate that it is starting. This allows the EMS tosynchronize its view of the device's active alarms.
