Re: [STDS-802-3-EPOC] PLC Preamble and Initial Frequency Offset Frequency
Yeah, I was not advocating that we specify something that will work for
1588 - I pointed out that more than just tighter-tolerance oscillators
would be needed. I was just making the comment that 100 ppm oscillators
will not do for that specialized application. If we can make them work
for routine applications, I have no problem with that.
Thanks,
jim
Please ignore all the changes in font in my message
- my email client does that just to mess with my head.
Jim Farmer, K4BSE
Mobile 678-640-0860
jofarmer@xxxxxxxxxxxxxx (spam blocked)
Personal web site: http://home.mindspring.com/~jofarmer/index.html
Boss lady: http://www.kathysflute.com/
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"A people that values its privileges above its principles soon loses both."
Dwight D. Eisenhower, Inaugural Address, January 20, 1953
On 3/14/2014 2:03 AM, Duane Remein wrote:
> Re: [STDS-802-3-EPOC] PLC Preamble and Initial Frequency Offset Frequency
>
> So for EPON, we just adopted the Ethernet 100 ppm for both OLT and
> ONU. Of course it didn’t make a great deal of difference there either.
> While it might be nice to specify something better than that for the
> CLT 802.3 WG is pretty attached to the 100 ppm spec.
>
> As Marek points out most equipment vendors do much better than than.
>
> Best Regards,
>
> Duane
>
> FutureWei Technologies Inc.
>
> duane.remein@xxxxxxxxxx
>
> Director, Access R&D
>
> 919 418 4741
>
> Raleigh, NC
>
> *From:*Marek Hajduczenia [mailto:marek.hajduczenia@xxxxxxxxx]
> *Sent:* Thursday, March 13, 2014 10:29 PM
> *To:* STDS-802-3-EPOC@xxxxxxxxxxxxxxxxx
> *Subject:* Re: [STDS-802-3-EPOC] PLC Preamble and Initial Frequency
> Offset Frequency
>
> Jim,
>
> Bear in mind that any implementer is free to choose components with
> **better** precision, tolerances etc. when manufacturing their
> products -- it is called product differentiation. The standard
> specifies the minimum that has to be met. So if I were a product
> designer and wanted to develop a product (CNU) that supports
> IEEE1588v2, I would certainly use more precise oscillators. However,
> in cases of products that do not need to meet such tight requirements,
> 100ppm oscillators would do just fine.
>
> Does that make sense? I know it is a mantra we repeat over and over
> again, but we are supposed to develop a standard and not a product spec.
>
> Regards
>
> Marek Hajduczenia, PhD
> Network Architect, Principal Engineer
> Bright House Networks
> /Office +1-813-295-5644
> Cell +1-813-465-0669/
>
> *From:*Jim Farmer [mailto:jofarmer@xxxxxxxxxxxxxx]
> *Sent:* March 13, 2014 9:41 PM
> *To:* STDS-802-3-EPOC@xxxxxxxxxxxxxxxxx
> <mailto:STDS-802-3-EPOC@xxxxxxxxxxxxxxxxx>
> *Subject:* Re: [STDS-802-3-EPOC] PLC Preamble and Initial Frequency
> Offset Frequency
>
> 100 ppm seems to be about where most inexpensive oscillators are. I
> have not measured any, but my experience is that they will likely not
> be any better than that. It is reasonable to specify something
> tighter, especially at the CLT. For cable TV headend equipment, we
> used to specify 25 ppm and we beat that pretty easily. But it did take
> a somewhat more "complex" crystal and an individual adjustment, both
> of which add that-of-which-we-are-not-to-speak. And we didn't have to
> cover the outdoor temperature range. If a computer gains or looses 1
> minute in a week (between synchronization to a time server), then its
> oscillator is in error by 99 ppm. And that is at room temperature. If
> you have two independent oscillators, one on each end, you have to
> conservatively assume that they are off indifferent directions, which
> could double Steve's estimate. But you can specify a much better
> oscillator, just that you have to exchange, uh, "complexity" for
> accuracy. Probably a good thing to do at the CLT.
>
> This presupposes that we will not have to support IEEE 1588 through
> the system - that requires very tight frequency tolerances. Or we
> would have to re-specify for 1588 - will likely take different
> hardware anyway from what I know (which is not all that much).
>
> jim
>
> Please ignore all the changes in font in my message
> - my email client does that just to mess with my head.
> Jim Farmer, K4BSE
> Mobile 678-640-0860
> jofarmer@xxxxxxxxxxxxxx <mailto:jofarmer@xxxxxxxxxxxxxx> (spam blocked)
> Personal web site: http://home.mindspring.com/~jofarmer/index.html <http://home.mindspring.com/%7Ejofarmer/index.html>
> Boss lady: http://www.kathysflute.com/
> Youngest daughter: http://www.joyfarmerclary.com/Sites/Joy_Farmer-Clarys_Welcome.html
>
> "A people that values its privileges above its principles soon loses both."
> Dwight D. Eisenhower, Inaugural Address, January 20, 1953
>
> On 3/13/2014 6:48 PM, Shellhammer, Steve wrote:
>
> Marek,
>
> In that case the frequency error would be around +/- 100 kHz (for
> 1 GHz RF) and so there would be even larger frequency uncertainty.
> So it would be even more important for the PLC preamble to have
> some short training fields that can be used to disambiguate the
> correct subcarrier.
>
> Is 100 PPM what you would expect for the CNU? I do not think we
> have specified the oscillator accuracy yet for the CLT or the CNU.
> Maybe something we should figure out. My preference is for a
> low-cost oscillator in the CNU and maybe a more accurate one in
> the CLT, where the cost may be less of an issue.
>
> Regards,
>
> Steve
>
> *From:*Marek Hajduczenia [mailto:marek.hajduczenia@xxxxxxxxx]
> *Sent:* Wednesday, March 12, 2014 6:00 PM
> *To:* Shellhammer, Steve; STDS-802-3-EPOC@xxxxxxxxxxxxxxxxx
> <mailto:STDS-802-3-EPOC@xxxxxxxxxxxxxxxxx>
> *Subject:* RE: [STDS-802-3-EPOC] PLC Preamble and Initial
> Frequency Offset Frequency
>
> Steve,
>
> Once you consider that we might have to work with 100ppm
> oscillators in CNUs, the resulting value gets 5 times larger. Is
> that a big problem ?
>
> Regards
>
> Marek Hajduczenia, PhD
> Network Architect, Principal Engineer
> Bright House Networks
> /Office +1-813-295-5644
> Cell +1-813-465-0669/
>
> *From:*Shellhammer, Steve [mailto:sshellha@xxxxxxxxxxxxxxxx]
> *Sent:* March 12, 2014 8:02 PM
> *To:* STDS-802-3-EPOC@xxxxxxxxxxxxxxxxx
> <mailto:STDS-802-3-EPOC@xxxxxxxxxxxxxxxxx>
> *Subject:* [STDS-802-3-EPOC] PLC Preamble and Initial Frequency
> Offset Frequency
>
> EPoC Group,
>
> I was thinking about the PLC preamble and the initial frequency
> error due to use of a low-cost and low-accuracy oscillator in the
> CNU. In 802.11 they have a way of dealing with initial frequency
> offset due to low-accuracy oscillators. I was wondering if this
> make sense in EPoC. I did a few calculations below. I also
> attached the Word document since I was not sure what the email
> reflector would do to the equations.
>
> I would be interested in knowing if this approach used in 802.11
> would be useful in EPoC.
>
> Any comments would be appreciated.
>
> Thanks,
>
> Steve
>
> -----------
>
> *PLC Preamble Frequency Calculations*
>
> Assume low-cost crystal with 20 ppm oscillator in CNU. Assume
> accuracy of oscillator in CLT is much better so we will ignore
> that oscillator error.
>
> Assume the maximum carrier frequency around 1 GHz. Could be a
> little higher but for these calculations this is good enough.
>
> *Frequency Error*
>
> Δ/f=±fc×20×10-6//=±//109×20×10-6=±20×/< m:e>/103=±20 kHz/
>
> Initial frequency error at CNU can be up to 20 kHz.
>
> PHY supports subcarrier spacing of 25 kHz and 50 kHz. The worst
> case situation from a frequency error perspective is the 25 kHz PHY.
>
> For a 25 kHz PHY the initial frequency error is up to one
> subcarrier on each side. So there are three possible tones that
> represent the middle tone when first acquiring the PLC. This would
> triple the acquisition time, since the CNU would need to search
> over three times as many cases.
>
> In the 802.11 OFDM PHY the preamble includes several short
> training fields (STFs) where only one out of every four tones is
> used, which disambiguates the subcarrier selection due to
> frequency offset.
>
> Does this make sense for EPoC?
>
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