SNMP Around the Station

September 1, 2012


In the series of Trends in Technology articles I've written about SNMP (simple network management protocol) several times, but this time we'll look at specific examples of what can be done with commonly available equipment. We'll also look at a common (and free) MIB browser that you can download and experiment with.

SNMP is an IP protocol that provides network elements a means by which they can communicate with and/or control one another. There are various parts that make up the entire picture: the SNMP agent, the SNMP manager and the MIB. The SNMP agent is the network element to manage; the SNMP Manager does the management; and the MIB (Management Information Base) is essentially a set of instructions that tells the Manager what information is actually available from the agent, and where to find it.

The information available to the manager from the agent takes a couple of forms. The first of those forms are known as traps. Traps are simply pieces of information, put in place by the designer of the equipment upon which the SNMP agent is running, that can tell you something important about that piece of equipment. (Think of a trap as a red flag. Raising the red flag prompts the trap message to be generated and sent.) The transmission of traps happens in an outbound manner though; so, in a sense it's kind of a passive way of keeping track of problems in an agent. It's kind of a no-news-is-good-news methodology. As the user, you would configure the equipment to tell it what traps you were interested in (in some cases it may be all or nothing at all) and where to send them -- in other words, the IP address of the SNMP manager. That manager receives the traps and either logs them or lets you know immediately.

Another way to use SNMP is to have the manager query the agent on a regular basis. In this case, you will use SNMP Get commands to retrieve various pieces of information from the agent. You can also make use of SNMP Set commands to have the manager control the agent. This is one of the most useful facets of SNMP -- having the manager tell the agent to do something via set commands, based on information gathered via get commands. When combined with scripting, the manager can essentially be a proxy for you to take actions on your behalf.

Now you may be thinking, Why bother with this when I can see all that I need via a browser? and that's a good question. The answer is simply this: SNMP works in the background while you are busy doing something else. Opening a browser uses your time and attention, and you pretty much only concentrate on one site at a time. On the other hand, one SNMP manager can look at multiple sites, all the time, 24/7. While someone's Web interface may look pretty, that isn't much use while you are sleeping, is it?

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SNMP in action

Let's look at a MIB browser. An MIB is basically a small text file (you can paste it in to notepad if you want to see what one looks like) that the manager reads to find information in the agent. Conceptually, the MIB is configured like a tree; as you go up the trunk and out the branches, the information becomes more specific, until you reach a leaf. The leaf is the end and represents a very specific piece of information that you want the manager to be able to see. The leaf is found by the manager by its reading of the OIDs (object identifiers) that are present in the MIB.

The browser I use is made by iReasoning Network. Go to ireasoning.com/mibbrowser.shtml and note in the right hand column a link to download free personal edition. Download the MIB browser from there.

For this demonstration we're going to use a Broadcast Devices DPS-100D in-line wattmeter. (I've chosen this device because the MIB is simple and illustrates the point well.) After you launched the MIB browser, download the MIB for this device. Go to broadcast-devices.com/images/BDI%20MIB%20FILES.zip, unzip the files and save them somewhere convenient. The final step in setting this up is to run the MIB browser, then select File > Load MIBs. Then, with the open window, find the Broadcast Devices MIB files you just unzipped. First click BDI DPS-100D MIBS\BDI-Enterprise-MIB-SMIv2.mib and click open, then click BDI DPS-100D MIBS\BDI-DPS100D-Product-MIB-SMIv2.mib and click open. You need both.

With all this done, you can type in an IP address in the window in the upper lefthand corner of the MIB browser. The address for our demonstration unit is 108.170.120.155. Then select Operations > Walk. You should then see the result table populated.

This remote device has been left open for purposes of this demonstration; note however that there are basic security features with SNMP (version 1) namely the Read Community and the Write Community. (If you click on Advanced Features you'll see where to enter those.) These are basically just passwords, and typically the Read Community is the word and the Write Community is the word . Obviously you can change these in your own SNMP configurations.

Notice the results table has four columns: name/OID, value, type, and IP:port. As I browse this MIB I noted a couple of things I wanted to be able to read: the forward and reflected power (for example) and the internal temperature along with the external temperature. Looking in the next column, I see their values (what they were when the walk was executed); and I see the OID type. Here is where this gets useful: Notice that if you highlight any one of the lines in the results table that its OID (Object Identifier) shows up in the OID window, which is near the top of the page, above the results table. The OID (as you can see) is a series of numbers separated by decimal points: One example would be the forward power OID: <1.3.6.1.4.1.37691.2.2.1.1.1.3.7.0>. Note there is a one-to-one correspondence between this OID and the information leaf you want to read.

When you configure your SNMP manager, the OID and the type are the information the browser needs to be able to read. The value will be read every time the agent is queried by the manager (Get). You can do this manually by highlighting the OID you want (forward power in this case) and then using the operations window (upper righthand corner) and choosing Get, you'll see the MIB browser return the value again. Of course your SNMP manager will automate this function.

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More data

When you look at more complex MIBs you may see all kinds of things you want to read. This is a simple example, and more complex equipment (with SNMP support) will usually have all kinds of information for you.

If a new piece of equipment going in at the transmitter site has an Ethernet port for Web browsing support, it may very well have SNMP support as well. If so, your wiring is complete the moment you plug in that RJ-45 cable. You can read the information via your IP network wherever there is an SNMP manager; and conversely, if you have an SNMP manager, it can read agents all over your IP network.

Now let's take a look at some real world examples of SNMP usage.

A year ago, Clear Channel New York began a new transmitter facility build for WLTW and WWPR. For this project we acquired two Nautel NV20s (one for each station) and we decided to repurpose two BE FM10Bs (which had been WWPRs transmitters) so that one became an alternate for WWPR, and the other became an alternate for WLTW. The NV20s provide complete metering and control via the Advanced User Interface that is accessed via Ethernet. We also have nine Harris Z10s, which have complete metering and control available via a serial connection (which we use). However, the BE transmitters date from the early 1990s, and only had legacy GPIO connections on their internal terminal blocks.

Figure 1: The BE transmitter before

Figure 1: The BE transmitter before


For this project, I really wanted to try to update the BE transmitters so that they were accessible via the same network as the Nautel transmitters. We chose the Mini Control Silver from Audemat to accomplish this. To simplify the installation, I decided to install the MCS for each station inside the transmitter itself. (See Figure 1, the before shot, and Figure 2, the after shot).

Figure 2: The BE transmitter after

Figure 2: The BE transmitter after


The Mini Control Silver is kind of the baby brother of the Relio (which is built by SeaLevel, and programmed by Audemat). Our new transmitter facility uses Relio for remote control, and each of these devices acts as an SNMP manager, communicating with the MCS (inside the transmitter), which acts as an SNMP agent in this case.

The MCS has eight analog inputs, eight status inputs, and eight relays, so I was able to connect to everything I needed inside the FM10B -- the standard meters, on/off controls, and overload reset. It's powered from a source inside the transmitter.

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The next thing to do was connect the MCS up to our LAN. When I did that, the FM10B made its appearance on our network.

Figure 3. Click to enlarge.

Figure 3. Click to enlarge.


Control of the FM10B could be done via a direct connection between a computer and the MCS (via Audemat's program called Master View) but that isn't what we do; all the normal user controls for the FM10B are viewed on the GUI of the WWPR Relio (or WLTW Relio). In fact, one entire tab in the GUI is dedicated to the FM10B. If you look at Figure 3 you'll see a column of buttons on the lefthand side that correspond to the BE controls: In the middle column there are two grey boxes for status (they turn green when ON) and in the righthand column, the typical transmitter meters. These controls (except the bottom two) and the meters (except the bottom one) are carried out via SNMP -- no extra wiring involved. The pushbutton controls are done via SNMP Set and the metering is done via SNMP Get (with continual querying).

A script for changing transmitters exists on Relio; controls for that function are actually done via real wire. (Those are the bottom two buttons, and the bottom meter.) The scripting is done with Audemat's program called ScriptEasy. Once you decide that you are going to use SNMP, you'll load the particular MIB you need in to the mini-control-silver; afterward you can use its own MIB browser to pick out what you want it to read. SNMP-accessible objects are then available to use in the script, in addition to the eight relays, eight status inputs, and eight meter inputs.

Another method

Another very well known remote control that takes advantage of SNMP is the Burk Arc Plus. Burk has a custom interface for the NV series of transmitters. Bill Eisenhamer of KIFM in San Diego (Lincoln Financial Media) was kind enough to send me a description of how the system works in practice.

Figure 4. Click to enlarge.

Figure 4. Click to enlarge.


The PlusConnect-NV talks to the transmitters via the local LAN, and continually queries the AUI of the transmitters. The information gathered feeds to the ARCPlus for display, command and status. (AutoPilot gets information that it needs from the ArcPlus.) The user configuration is done using drop-down menus to map pre-defined channels to ArcPlus using AutoPilot Plus (Figure 4).

Figure 5. Click to enlarge.

Figure 5. Click to enlarge.


The PlusConnect-NV is configured via telnet; this includes its IP address, port numbers and polling rate (Figure 5).

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A piece of software that goes along with ArcPlus is called AutoLoad Plus, and the most recent version allows you to instruct ArcPlus to automatically issue a command or to run a macro in the event that a predefined limit or status changes. Macros are created in the AutoLoad Plus software, using predefined statements selected via drop-down lists, on a per-channel basis. You can also use a visual format for creating scripts, known as Jet Flowcharts. These run on the computer that runs AutoPilot.

Figure 6. Click to enlarge.

Figure 6. Click to enlarge.


Like the Mini Control Silver, the Burk system can also speak via SNMP with other agents. For example, Eisenhamer uses AutoPilot to communicate over the LAN with multiple PowerWare UPS units and the Broadcast Tools TempSentinel. After retrieving and loading the appropriate MIB in to AutoPilot, you can use its MIB browser to pick the OIDs that correspond to parameters you wish to monitor (Figure 6). After that, you configure the channel type (meter or status) and label, units and limits (Figure 7).

Figure 7

Figure 7


The SNMP protocol has been around for many years and is now really finding its way into the broadcast engineering vernacular. (Better late than never!) I'm seeing more and more equipment that supports SNMP: transmitters, remote controls, audio codecs, microwave receivers and transmitters, and ancillary equipment such as air conditioners, UPSs and of course networking equipment like switches and routers. The SBE now offers an online course about SNMP as well.


Irwin is transmission systems supervisor for Clear Channel NYC and chief engineer of WKTU, New York. Contact him at doug@dougirwin.net.



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