Re: A Minor NIT
Rich & Tom,
I had thought I understood pretty much everything, but now on the
issue of pointer adjustment, there may be a problem. As I understand
this, if there is a positive adjustment, there is need to to
byte stuff (or is it 192 * bytes?). But since the SPE is composed of
66-bit code words, how can we stuff the SPE by the 192 bytes. Pls
correct me if my understanding is wrong.
-Wesley Lee
----------------------------
Tom,
Very well said. You echo my understanding also. This implies that SONET/SDH SPE
pointer adjustment, while operating on byte boundaries, is 100% compatible with
an SPE payload that is on 66-bit boundaries. Does anyone out there disagree with
this statement?
Best Regards,
Rich
--
Thomas Dineen wrote:
>
> Gentlepeople:
>
> The SPE content is considered bytes when being passed through
> the SONET network. When the SONET network needs to "adjust"
> 1 SPE with respect to another SPE in order to account for
> clock jitter and tolerances, it does so on byte boundaries.
> This could potentially further separate a 66-bit word. The
> SONET network also uses byte boundaries to find the OH bytes
> within a frame.
>
> While I generally agree with the above statement I would
> like to point out one possible NIT regarding SONET pointer adjustments.
> I prefer to view, and please feel free to jump in if you disagree, the SONET
> STS-N Frame structure, and especially its encapsulated SPE structure
> as a continuous stream. Now pointer adjustments do cause short term
> instantaneous variances in the data rate, but in the end the average rate
> will be that of the associated Stratum Clock reference.
>
> A positive pointer adjustment will add one stuff byte, and thus
> delete
> one payload byte from this SPE. A negative pointer adjustment will
> delete one stuff byte, and thus add one payload byte to this SPE,
> actually the H3. Please note that in either case all of the payload bytes
> get there, eventually.
>
> But in the end the SPE stream is concatenated together logically
> to form a continuous stream of bytes. The parsing of 66b encoded frames
> will occur on this stream of bytes and thus should be un affected by
> activities at the SONET physical layer including pointer adjustments
> and frame structure.
>
> Thomas Dineen
--
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