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Part 2: How SNMP Handles Alarm Messages

Previous Page: Part 1: An Introduction to SNMP
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SNMP uses five basic messages (Get, GetNext, GetResponse, Set and Trap) to communicate between the master station (also called manager) and the agent.

The SNMP Get Request and SNMP Get Next are messages that allow the manager to request information for a specific variable. The agent, upon receiving a Get or SNMP GetNext requests, will issue a GetResponse message to the manager with either the information requested or an error indication as to why the request cannot be processed.

A Set message allows the manager to request a change be made to the value of a specific variable in the case of an alarm remote that will operate a relay. The agent will then respond with an SNMP Get Response message indicating the change has been made or an error indication as to why the change cannot be made.

The Trap message allows the agent to spontaneously inform the manager of an "important" event.

As you can see, most of the messages (Get, GetNext, and Set) are only issued by the SNMP manager. Because the Trap message is the only message capable of being initiated by an agent, it is the message used by DPS Telecom remote telemetry units (RTUs) to report alarms. This notifies the SNMP manager as soon as an alarm condition occurs, instead of waiting for the SNMP manager to ask.

The small number of commands used is only one of the reasons why SNMP operations are a simple way to monitor networks and managed systems. The other simplifying factor is its reliance on an unsupervised or connectionless communication link.

This simplicity has led directly to its widespread use, specifically in the Internet Network Management Framework. Within this framework, it is considered robust because of the independence of the managers from the agents; that is, if an agent fails, the manager will continue to function, or vice versa.

Essential SNMP: What is a Trap?

The SNMP trap is a message sent without a request from the agent to the master station. With traps, managed devices at your remote sites are able to notify your manager about events. They send alerts even if the master station has not poll them yet.

An SNMP Trap is a change-of-state (COS) message - it could mean an alarm, a clear or simply a status message. These SNMP messages are only sent when something within the agent changes state. You often have to parse variable bindings to decode a Trap. The Variable Bindings Style (VBS) is used to identify every single trap.

To make sure the meaning of a Trap is understood, all DPS Telecom SNMP equipment transmits a unique Trap ID for both alarm and clear for each alarm point. Unlike a classic telemetry master, basic SNMP managers don't keep a standing alarm list, so it's difficult to tell what's happening in your network by looking at a list of Traps.

Reality Check: What Features Do I Need in an SNMP RTU?

How do you find the right SNMP RTU? Look for more features than just SNMP support. Many managed objects can output SNMP Traps - when you're evaluating an RTU, look instead at how many alarm monitoring functions it can perform.

NetGuardian 832 G5
The NetGuardian 832A monitors 32 discrete alarms and 8 analog alarms, pings 32 network elements, controls 8 relays, provides LAN reach through access to 8 serial ports, and reports via SNMP or DCPX, e-mail, or pagers.

Here are 5 essential features that your SNMP RTU must have:

  1. Discrete alarm inputs (also called digital inputs or contact closures)

    These are typically used to monitor equipment failures, intrusion alarms, beacons, and flood and fire detectors.

  2. Analog alarm inputs

    While discrete alarms monitor on/off conditions, analog alarms measure continuously variable levels of voltage or current. Analog alarms monitor temperature, humidity and pressure, all of which can critically affect equipment performance.

  3. Ping alarms

    An RTU that supports ping alarms will ping managed devices on your network at regular intervals. If a device fails to respond, the RTU will send an alarm as an SNMP Trap, providing immediate notification that the device has failed or gone offline.

  4. Control relays

    Don't waste time and money sending a technician to a remote site miles away simply to turn a switch. An RTU with control relay outputs will let you operate remote site equipment directly from your NOC.

  5. Terminal server function

    Your RTU can also serve as a terminal server to remote-site serial devices. Your devices connect to the RTU's serial ports, giving you immediate Telnet access via LAN from your NOC at any time.

DPS Telecom offers SNMP RTUs that meet all these requirements - and offer stand-alone local visibility through any web browser, expandable alarm capacity, LAN access via dial-up connection and more.

To learn more about DPS RTUs, request a live Web Demo.

Next Page: Part 3: Understanding the MIB
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