QUESTION:
What is MIB?

ANSWER:  
A MIB (Management Information Base) is a standardized data structure from various SNMP variables that are written in a language independent from the target system.

Figure 1: Overview of MIB

With the manufacturer spanning standardization of MIBs and access mechanisms, a heterogenous network with components made by various manufacturers can be monitored and controlled. Today, the following MIB standards are valid:

• MIB2
• Bridge-MIB
• RMON-MIB
• MAU-MIB

If component specific, non-standardized data is required for network monitoring, they can be written into so-called private MIBs by the manufacturers. Thus, also values that are not included in the in the standard MIBs can be queried.
The information available for each private MIB can be obtained from the manufacturers.
The RFC1213 determines the MIB II which is supported by all SNMP-compliant devices. The RFC1213 is available for download through this link:

http://www.faqs.org/rfcs/rfc1213.html

The OSM/ESM with network management supports the following MIBs:

• RFC1213: MIB II
• RFC 2233: Interface MIB
• RFC1286, RFC1493: Bridge–MIB
• RFC1757: RMON–MIB

The private MIB for SIMATIC NET network components is available on the delivered CD. The private MIB file can also be loaded directly with a Web browser via OSM/ESM.

http://<IP–adress of OSM/ESM>/snOSM.mib

A MIB describes the entirety of all SNMP objects (SNMP variables) which are available in the network. The MIB information is structured similar to the Windows registry.
The OID (Object Indentifier) describes the address of the MIB object. For standardized MIB ojects, the address is fixed. Private MIB objects are always stored in the "Enterprise" directory. The addresses within this structure are set by the manufacturer. Only the manufacturer number has to be registered. The following internet page lists the registered manufacturers that provide privateSNMP/MIB objects.

http://www.iana.org/assignments/enterprise-numbers

Figure 2: Structure overview

The standard object "sysdescr" (address of object 0.1.3.6.1.2.1.1.1) contains a description of SNMP components. For the OSM TP62, the string "OSM TP62" is issued. To make private, non-standardized information readable, these private MIBs have to registered with a management station (e.g. SNMP OPC Server), otherwise they cannot be addressed. To make MIB objects directly readable and writable, so-called MIB browsers are used. MIB browsers are often included in the standard delivery package of network management applications.
To enable MIBs for integration in the system, so-called MIB compilers have to be used. STEP 7 / NCM PC includes a MIB compiler for configuring the SNMP/OPC server.
With the MIB Compiler from SIMATIC NET, any external device as well as new SIMATIC NET devices can be linked.
However, this requires a MIB description for this device. 

While the MIB administers various system aspects - like information regarding nods or statistical information about the throughput of packages, connections established, error messages etc. - , the SMI represents the framework for the MIB which describes exactly the kind of representation of objects and exchange through the protocol with the help of "Structure of Managed Information". Today, MIBs according to SMIv1 and SMIv2 are available. For MIB Compilers for SIMATIC NET, those MIBs have to conform to SMI v1. The SMI v2 format is not supported, but for most SNMP components, both MIB formats are available.

Keywords:
MIB, SNMP, network management, OSM, ESM