Communication by the SNMP protocol

SNMP is a protocol that allows the SNMP manager (admin, PROMOTIC application) to control SNMP agent by transmitting the SNMP messages. SNMP message is a packet sent via Ethernet UDP/IP on port 161. This message can be used to read or set parameter values from/into the agent.
See: Wikipedia: A list of TCP and UDP port numbers used by protocols to run network applications.
The SNMP protocol exists in three versions 1, 2, 3. The version 2 contains additional authorization and the 3 version encryption. The most commonly used version of the SNMP protocol is the version 2. Unfortuantely this version is not in any way standardized and therefore the devices using such communication are often incompatible.

For this communication in the PROMOTIC system can be used:


OID (Object Identifier):
Each value in the SNMP is identified by numeric identifier OID. OID consists of a sequence of whole numbers separated by full stops (e.g. "").
Each valid OID must begin with numbers "1.3...".
Each device supporting the SNMP protocol should return at least the values of following objects:
- "" = Time period since device power on.
- "" = Text description of the device.

MIB database (Management Information Base):
Each SNMP agent has the list of all his parameters saved in so-called the MIB database that contains the name, OID, data type, read/write capability and short description for each parameter. However this database is not stored in the device itself, but it is available in the form of (single or multiple) text file(s) with the .mib extension. In order to work with the SNMP the MIB database is not necessarily needed, in the SNMP packet there is only the OID, but if we do not know the right OID, then the database can help us find it.
Note: The PmSNMP communication driver does not use the MIB database. The MIB can be read by external programs in order to find the addresses (OID) of desired parameters for the PROMOTIC driver.

There is a possibility to configure the monitored side in a way that the agent sends the admin the information (SNMP MESSAGE) automatically without any request. This can happen usually after a defined condition is met (breakdown, colision, reaching some limit value, ...). This kind of agent configuration is called SNMP TRAP (i.e. "trap for events").
Note: The PmSNMP communication driver does not support SNMP TRAP.
PROMOTIC 9.0.27 SCADA system documentation MICROSYS, spol. s r.o.

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