Re: A Minor NIT
The WIS is responsible for extracting successive SPEs from the
SONET stream. The WIS is responsible for separating the SPE from
the OH bytes. It is also responsible for using its pointer
processor logic to find the beginning of each SPE and from that
point it knows where the end is (usually in the next SONET frame).
It then uses its pointer processor logic to find the start of
the next SPE. If it must discard bytes from the SONET frame
between the end of 1 SPE and the start of the next SPE, it does
The data it provides to the 64b/66b PCS may have been extracted
from the SPE as bytes but those bytes are concatenated into a
virtual bit stream and this bit stream is observed by the PCS
as a set of concatenated 66-bit frames, just like the 66-bit
frames used to fill the SPE at the tranmitter.
When looked at individually, the bytes inside the SPE don't
make much sense. The pointer processor logic treats the SPE
as if it consists of bytes so that is how the WIS must treat
the data stream. From another point of view, the stream is
simply bits viewed as bytes by the WIS and 66-bit frames by
Hope this helps,
Wesley Lee wrote:
> 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
> 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,
> 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
> Lucent Microelectronics Enterprise LAN Division - West
> 1381 McCarthy Blvd, Miltpitas, CA 95035
> Work: 408-952-8822 FAX : 408-952-8887 wlee@xxxxxxxxxx
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