Subject: RE: [ipcdn] docsIfUpChannelTxTimingOffset in RFC2670

From: Rich Woundy <[email-masked]>
Date: Mon, 26 Feb 2001 16:47:56 -0500

Victor,

Thanks for the clarifications and corrections. Looks like I erred on the 
following:
(a) the polarity of the timing offsets (always non-negative),
(b) the accumulation of the timing adjustments in multiple Ranging Response 
messages (hence, individual adjustments could be negative or positive), and
(c) the round-trip property of the timing offset (not one-way).

Based on your analysis I'll assume that the MIB object description in RFC 
2670 should have the following minor correction:

    docsIfUpChannelTxTimingOffset OBJECT-TYPE
            SYNTAX      Unsigned32
            MAX-ACCESS  read-only
            STATUS      current
            DESCRIPTION
                "A measure of the current round trip time at the CM, or the
                 maximum propagation round trip time seen by the CMTS.  Used
                 for timing of CM upstream transmissions to ensure synchronized
                 arrivals at the CMTS. Units are in terms of
                 (6.25 microseconds/64)."
            REFERENCE
                "DOCSIS Radio Frequency Interface Specification,
                 Section 6.5."
            ::= { docsIfUpstreamChannelEntry 6 }

Comments?

-- Rich

At 11:27 AM 2/23/2001 -0800, Victor Hou wrote:
>A few points of clarification:
>
>1.  The ranging offset is a timing correction employed by the CM in
>transmitting a burst.  The ranging offset maintained by the CM should be a
>positive number signifying the amount of time that the CM should transmit
>earlier relative to its local clock that is a time-delayed replicate of the
>CMTS master timebase communicated via the timestamp messages.
>
>2.  The timing adjustment in a RNG-RSP, in contradistinction to the ranging
>offset, can be positive or negative and is an adjustment to the ranging
>offset used by the CM.  As stated in the RFI, the convention is that a
>negative adjustment decreases the amount of time of the overall ranging
>offset used by the CM, thus, resulting in a later time of transmission.
>
>3.  The RFI specification does say that the CM, even from the very first
>time it sends a RNG-REQ, must apply an "initial ranging offset."  The spec
>says that this is to include downstream PHY interleaving delay (depending on
>the interleaver mode) and "internal fixed delay."  The purpose of applying
>this initial offset is to allow the subsequent ranging procedure to
>compensate mainly for the variable and unknown propagation delay.  This
>"initial ranging offset" in effect is always incorporated as part of the
>ranging offset of the CM.
>
>4.  The ranging offset, defined as the sum of the "initial ranging offset"
>and subsequent ranging adjustments from the CMTS, reflects a round-trip
>delay between CMTS and CM, not just one direction.  The ranging offset minus
>the "initial ranging offset" is roughly the round-trip propagation delay,
>not just one direction.
>
>Now, with all that said, the MIB object in question for the CM should be the
>ranging offset ("initial ranging offset" plus subsequent adjustments).
>
>The MIB object for the CMTS is trickier.  It currently says "maximum round
>trip as seen by the CMTS."  As discussed earlier, the ranging adjustments
>made by the CMTS do not include what the CM already adjusted for--the
>"initial ranging offset."  Thus, my suggestion is that the wording be
>changed to "maximum propagation round trip as seen by the CMTS."  Now, there
>is the question of whether the CMTS is expected to sum all ongoing
>adjustments of ranging offset for all CMs in order to properly determine and
>report this "maximum propagation round trip."   Alternatively, it may be
>sufficient for the CMTS to just use the cumulative timing adjustments when
>it sends the first RNG-RSP to a CM with the "success" indicator.  That, to
>me, should be left as an item of vendor-implementation choice.
>
>Final note:  it is possible that the CM's "initial ranging offset" is not
>very accurate.  Thus, it must be understood that the CMTS's perception of
>propagation round trip may not be highly accurate--it depends on "how" the
>CM calculated its "inital ranging offset."  If the CM overestimates its
>"initial ranging offset" and happens to be very close to the CMTS, it is
>possible that there will be no adjustment or an initial negative adjustment
>by the CMTS, provided that the burst did not arrive so early that it
>collided with the previous burst.  So it is possible for the CMTS to
>perceive that a CM needs a negative ranging offset (again, from CMTS
>perspective, this does not include the "initial ranging offst").  That a
>negative ranging offset would be the maximum seen by the CMTS is not
>probable.  Thus, it may not be necessary to have a signed value for this MIB
>object.
>
>Victor Hou
>Pacific Broadband Communications
>
>-----Original Message-----
>From: Rich Woundy [mailto:[email-masked]]
>Sent: Friday, February 23, 2001 5:42 AM
>To: [email-masked]; [email-masked]
>Cc: [email-masked]
>Subject: Re: [ipcdn] docsIfUpChannelTxTimingOffset in RFC2670
>
>
>James,
>
>This is a good question, which I am now looking at, as part of the RFC2670
>update.
>
>The timing adjustment is defined in the RFI to be "the time by which to
>offset frame transmissions so that frames arrive at the expected mini-slot
>time at the CMTS." I believe that this value is always non-positive,
>because it takes a non-negative amount of time for a frame to be
>transmitted from a CM to a CMTS. (Note: I may misunderstand the polarity of
>this value -- it may be always non-negative. I definitely don't understand
>how this value can be both positive and negative.)
>
>The timing offset is defined in the MIB to be "A measure of the current
>round trip time at the CM, or the maximum round trip time seen by the
>CMTS." I believe that this value is always non-negative. The MIB timing
>offset appears to be (approximately, but not necessarily) double the
>absolute value of the RFI timing adjustment, because it measures round-trip
>rather than one-way time. Perhaps the MIB object is misnamed.
>
>Is this correct?
>
>And is this how folks have implemented the MIB object value?
>
>-- Rich
>
>At 02:57 PM 8/4/2000 -0700, James Yee wrote:
> >This was brought up on the side during the last IPCDN meeting in
> >Pittsburgh....
> >
> >In RFC2670 it is defined:
> >
> >         docsIfUpChannelTxTimingOffset         Unsigned32
> >
> >whereas in the DOCSIS RFI specs the description for the
> >timing adjust TLV has consistently been:
> >
> >         TX timing offset adjustment (signed 32-bit, units of (6.25
> > microsec/64))
> >
> >See Table 6-21 of DOCSIS 1.1 RFI.
> >
> >Is this signed/unsigned inconsistency intentional or
> >should the MIB object SYNTAX be changed to be consistent
> >with the RFI spec?
> >
> >Thanks,
> >
> >James
> >
> >_______________________________________________
> >IPCDN mailing list
> >[email-masked]
> >http://www1.ietf.org/mailman/listinfo/ipcdn

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---

From: Andre Lejeune <[email-masked]>
Date: Tue, 27 Feb 2001 09:52:25 -0500

Rich,

Thanks to the very clear explanation from Victor, it is easier
to understand the issues in computing the docsIfUpChannelTxTimingOffset.
However, I don't see how adding the word "propagation" clarifies anything,
specially when the Mib text is not accompanied by Victor's email...

In my opinion, it would be better to state how the values are calculated
for each of the CM and CMTS in the Mib text and to change the "or" in
the sentence to an "and", because it makes one feel that the two result
in the same value.

So, what about this text instead:

             DESCRIPTION
                 "At the CM, a measure of the current round trip time obtained from the
                  ranging offset (initial ranging offset + ranging offset adjustments).
                  At the CMTS, the maximum propagation round trip time seen by the CMTS
                  obtained from the largest cummulative sum of the ranging offset adjustments.
                  Generally, these measurements are positive. If negative, report 0. Used
                  for timing of CM upstream transmissions to ensure synchronized
                  arrivals at the CMTS. Units are in terms of
                  (6.25 microseconds/64)."

My 2 cents!

André