Alpha Technologies Cordex Controller Ver.3.1x - Manual (Page 115)

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10. Simple Network Management Protocol (SNMP)
10.1 Overview
SNMP was developed in 1988 as an operating system for the management of the data flow from a series
of remote information generators, or Agents, connected to a central computer, or Manager, by way of a
network. The software installed in the Agent (whether it is supplied by Alpha or customer supplied) uses
SNMP to translate the data stream into a format that can be readily downloaded by the client's network
to the Manager computer located at the customer's Central Office used as a hub for the network. Each
Agent collects data from a source, translates that data into the SNMP format and then transmits the trans-
lated data, when requested, to the Manager computer.
In the case of a network of Alpha Power Systems, the data stream from an individual power plant is de-
veloped through the CXC System Controller, which directly interfaces with that power system. This data
stream is referred to as the Management Information Base or MIB (see 10.2).
An SNMP Agent may have a number of simple messages that can be sent directly towards the retrieval
of single or multiple object variables or to establish the value of a single variable. The Alpha SNMP Agent
software employs an event notification called a “Trap” to the management system to identify the occur-
rence of conditions that exceed a predetermined value such as an alarm event. Trap Protocol Data Units
(PDUs) have the following format:
Enterprise
– identifies the type of object causing the trap
Agent Address – IP address of agent that sent the trap
Generic Trap ID – the common standard traps
Specific Trap ID – proprietary or enterprise trap
Time Stamp – when trap occurred in time ticks
10.1.1 Outline of the SNMP Protocol
• Each SNMP managed object belongs to a community.
• Network Management Station (NMS) may belong to multiple communities.
• A community is defined by a community name, which is an Octet String with 0 to 255 octets in
length.
• Each SNMP message consists of three components:
1. Version number
2. Community name
3. Data - a sequence of PDUs associated with the request.
10.1.2 Underlying Communication Protocols
SNMP assumes that the communication path is a connectionless communication sub-network. In other
words, no prearranged communication path is established prior to the transmission of data. As a result,
SNMP makes no guarantees about the reliable delivery of the data. Although in practice most messages
get through, and those that don't can be retransmitted. The primary protocols that SNMP implements are
the User Datagram Protocol (UDP) and the Internet Protocol (IP). SNMP also requires Data Link Layer
protocols such as Ethernet or Token Ring to implement the communication channel from the manage-
ment to the managed agent.
SNMP's simplicity and connectionless communication also produce a degree of robustness. Neither the
manager nor the agent relies on the other for its operation. Thus, a manager may continue to function
even if a remote agent fails. When the agent resumes functioning, it can send a trap to the manager, no-
tifying it of its change in operational status. The connectionless nature of SNMP leaves the recovery and
error detection up to the Manager computer and even up to the Agent.