Configuration Overview

Simple Network Management Protocol (SNMP) is a network management standard widely used in TCP/IP networks to monitor and control computers, routers, printers, and other devices connected to a network.

SNMP uses distributed architecture consisting of managers and agents. A manager is an SNMP application that generates queries to SNMP agent applications. An agent is an SNMP application that responds to queries from SNMP manager applications. The SNMP agent is responsible for retrieving and updating local management information based on the requests of the SNMP manager. The agent also notifies registered managers when a significant event or a trap occurs.

The SNMP manager is realized in Plant SCADA through the SNMPII driver. On computers running Windows 2000, the SNMP agent is implemented by the SNMP service (SNMP.exe). Run the SNMP services on the machine that the I/O server is running on.

The Plant SCADA driver uses the Microsoft SNMP services and thus supports SNMPv1 and SNMPv2C.

Every SNMP-manageable device has a set of "objects" that can be read or modified via SNMP. This set of managed objects is known as a Management Information Base (MIB).

Note:
• The Plant SCADA driver does not support the GetNext request.

• SNMPII driver supports SNMP GETS and PUTS (read and write), trap receiving, forwarding, and trap generation. Each device runs in a separate NT thread for maximum speed.

• If an SNMP device only supports a fixed IP reply address, Plant SCADA I/O server redundancy will not be supported for that device.

• SNMPII driver has partial support for SNMP protocol version V2C and currently it supports only the SNMP GETS and PUTS (read and write) operation. The other operations will be supported in the future release version of this driver.

Each device to be monitored or controlled by Plant SCADA has to be connected to a TCP/IP network as illustrated in the following diagram:

All SNMP-manageable devices that reside on a TCP/IP network are required to support a minimum standard set of managed objects, collectively known as MIB-II. Industry vendors can define additional MIBs that allow unique hardware or software services developed by the vendor to be monitored and managed by an SNMP server.

Every object in a MIB is identified by a universally unique label referred to as an object-identifier (OID). The identifier includes the object's type (such as counter, string, gauge or address), the object's access level (such as read/write), size restrictions and range information. (For more information about how OIDs work in Plant SCADA, see Tag-based Driver Considerations.)

The object name space is implemented as a multi-part, hierarchical, naming scheme. A hierarchical naming scheme can be viewed as an inverted tree with the branches pointing downward. Each point where a new branch is added is referred to as a node. This OID is internationally accepted and allows developers and vendors to create new components and resources and assign a unique OID to each new component or resource.

The OID naming scheme is governed by the Internet Engineering Task Force (IETF). The IETF grants authority for parts of the name space to individual organizations such as Microsoft. For example, Microsoft has the authority to assign the OIDs that can be derived from branching downward from the node in the MIB name tree that starts at .1.3.6.1.4.1.311, as illustrated here:

SNMP programs use the OID to identify the objects on each device that can be managed by using SNMP. For example, when Plant SCADA needs information about managed objects from a computer on the network, the SNMPII driver sends a message over the network that requests information about the object as identified by the OID. The computer that receives the message can use the OID to retrieve information from the specific object on the computer and send the information back to Plant SCADA.

The OID in the hierarchy is written as a sequence of sub-identifiers beginning at the root and ending at the object. Sub-identifiers are separated with a period. For example, the OID for "sysDescr", which is a textual description of the SNMP manageable device, is .1.3.6.1.2.1.1.1.0.