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.
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
Thomas Dineen wrote:
> 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
> 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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