Subject: Re: [eman] feedback on drafts

From: Benoit Claise <[email-masked]>
Date: Wed, 29 Jun 2011 11:19:32 +0200

Michael,

Sorry for overlooking your email
>
> All,
>
> I have been a silent observer of this group for about a month.
>
> By way of introduction, I am a principal engineer at Raritan.  Raritan 
> manufactures rack power distribution units and is an industry leader 
> in outlet level metered rack PDUs.  Our products contain a billing 
> grade AC energy metering chip for each outlet and our devices contain 
> a networked SNMP agent.  Raritan also develops and sells SNMP data 
> center power management software -- and our management software is 
> designed to work with any vendor's PDU -- not just our own.  I learned 
> of this group through our client Cisco that outfits their data centers 
> with our PDUs and power management software.
>
> As an "implementer" of AC energy management products and as someone 
> that has experience with many vendors' implementation of SNMP energy 
> management, here are two thoughts on the MIBs produced by the eman 
> ietf group.
>
>    1. Timestamps
>
> When taking power measurements, there is no indication of the time 
> when the measurement occurred.  The major vendors of metered power 
> distribution units and panel energy meters all include time stamped 
> data logs.  These data logs typically contain 100+ time stamped 
> measurements per sensor (voltage/current/power, etc.).
>
> The proposed MIBs do not take time of measurement into consideration 
> and this will be a problem.  Assume we want to get readings on an 
> hourly basis from an SNMP manager that must communicate with many 
> thousands of devices.  Without data logging, it isn't possible to get 
> accurately get timed measurements due to the variable delay times in 
> the manager, network and agents.  Our experience supporting every 
> major vendor's PDU SNMP agent indicates variable SNMP GET delays of 
> 2-45 seconds.
>
There is a difference between energy, power, and power quality.
- Energy slots needs to aggregated between different devices, so yes 
knowing the precise time is important.
In the MIB module, we have pmEnergyIntervalStartTime and the parameters 
we have the pmEnergyParametersIntervalLength. So the energy is taken 
care of, right?
- For power, The measurement will happen at the time of the SNMP 
request, and not regularly, right?
I'm wondering: aren't we trying to solve a SNMP response problem by 
inserting a timestamps in the MIB module? It's a good practice to 
request the sysUpTime when polling counters, so the measurement time (in 
this case) is tagged.
- For power quality, isn't it the same thing as for the power? I mean, 
will the power quality measurement start at the time of the SNMP request?

Bottom line: if there is a new requirement, then it should be part of 
the EMAN requirements draft, as the EMAN MIB modules take their inputs 
from the requirements and the framework.

>    2. AC Power Terminology
>
> There is a bit of a lack of understanding of AC power in the MIBs. 
>  Here are some examples:
>
> ·AC power is not an RMS measurement, it is an average reading.  AC 
> power is sinusoidal at 2x the AC line frequency, so use of the term 
> "instantaneous AC power" is misleading and power meters do not report it.
>
Agreed. This is the reason why instantaneous was written between quotes.

        pmPower OBJECT-TYPE
             SYNTAX          Integer32
             UNITS           "Watts"
             MAX-ACCESS      read-only
             STATUS          current
             DESCRIPTION
                "This object indicates the 'instantaneous' RMS
                consumption for the Power Monitor.  This value is
                specified in SI units of watts with the magnitude of
                watts (milliwatts, kilowatts, etc.) indicated separately
                in pmPowerUnitMultiplier. The accuracy of the measurement
                is specfied in pmPowerAccuracy. The direction of power
                flow is indicated by the sign on pmPower. If the Power
                Monitor is consuming power, the pmPower value will be
                positive. If the Power Monitor is producing power, the
                pmPower value will be negative.

                The pmPower MUST be less than or equal to the maximum
                power that can be consumed at the power state specified
                by pmPowerState.

                The pmPowerMeasurementCaliber object specifies how the
                usage value reported by pmPower was obtained. The pmPower
                value must report 0 if the pmPowerMeasurementCaliber is
                'unavailable'.  For devices that can not measure or
                report power, this option can be used."
             ::= { pmPowerEntry 3 }

Certainly some more clarifications are required. What do you suggest? 
Some more explanation in the description clause, or a generic paragraph 
from all the 'instantaneous'

> ·Voltage & current RMS measurements are by definition averaged values, 
> they are not "instantaneous RMS" measurements
>
Same as above.
>
> ·3-phase PDUs are not black and white either Wye or Delta.  In North 
> American 208V systems, power distribution units contain a combination 
> of delta wired circuits (208V) and wye wired circuits (120V).
>
The question is whether EMAN should cover that third case.
If the group thinks that we should, then it should be specified in the 
EMAN requirements.
Question: what's specific to this combination of delta wired circuits 
(208V) and wye wired circuits (120V)? It implies that we need to have a 
third series of tables, specifically for this model?

> ·The nameplate MIB objects use names such as "max voltage" and have 
> fixed single numeric values.  Actual device nameplate values follow 
> the UL (or IEC) 60950-1 conventions and are rated (not max) ranges 
> (not single values).  Example:  "Rated Voltage: "190-240V 3-phase".
>
You're right.
The goal behind the nameplate (and maybe the MIB variable name is 
wrong), was to get the max value in order to do power planning. So 240V 
in this case.
The requirement needs to be clearly spelled out in the EMAN requirements.
>
> ·There are many IEC and ANSI standards for AC energy meters.  The 
> attempts in the MIB to define energy metering accuracy are nonstandard.
>
Can you please elaborate, i.e. clarify which parts are not standard?
The intend was not to reinvent anything, but to refer to existing 
standards (IEC). Example: accuracy and unit multipliers
>
> ·A major concern in AC data center management is detection of tripped 
> circuit breakers (or blown fuses).  The MIB contains no modeling for 
> over current devices.
>
As far as I understood, circuit breakers were not in the EMAN scope.
Note that circuit breakers are not described in the EMAN requirements.

Note: discussing with Juergen, he plans on submitting a new version of 
the EMAN requirements very soon.

Thanks for your feedback, Mouli and Benoit.
>
> I hope the above are taken as constructive help. I mean no disrespect 
> to the members that have put in long hours developing the MIBs.  I am 
> available to help improve the group's efforts.
>
> Regards,
>
> **Michael Suchoff***
> *Principal Engineer
>
> Raritan . 801 Jones Franklin Rd . Raleigh, NC 27606 USA
> Tel: 1.919.851.3663 ext.1371
>
> [email-masked] <mailto:[email-masked]>
>
> Visit our blog: blog.raritan.com <http://blog.raritan.com/>
>
>
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