Re: Simple solution to terminate the discussion about SONET versus SDH

[Stephen Trowbridge <sjtrowbridge@lucent.com>]

Eric,
I don't think you ask quite the right question.
The right question is:
Can all frame structures which are common to SONET and SDH be interconnected,
and do they interoperate? This is what we care about from the viewpoint of
establishing switched connections.
The answer to this is YES.

Are they fully identical from all points of view? As Juergen says, this is the
tricky question, as the answer of "No" might mislead some into thinking that
these signals cannot be interconnected between SONET and SDH. This is not
the case.

For those who care, some of the key differences are as follows. Note that these
do NOT affect the ability to interconnect these signals (although sometimes there
are rules about HOW to interconnect them).

SS bits - In the past, this was an issue in the high order pointers. SDH sent
and expected "10", for SONET these bits were unspecified. This problem was corrected
a few years ago by requiring that ALL equipment send "10" and ignore the incoming
bits. Note that even prior to aligning the standards, a great deal of the older
equipment followed this strategy as VC-4s and STS-3cs were interconnected long
before the standards alignment.

Trace identifier - For J1, within SONET, a 64 byte format is normally used. For SDH,
a 16 byte format is normally used. SONET specifies that in the case that the far end
is SDH, a 16 byte format should be used to allow interworking. For J2, this is normally
used in SDH and not normally in SONET. Interworking is acheived by having the SDH end
ignore the incoming trace identifier (a required capability in the standards).

RDI - SONET uses some extra bits that are reserved in SDH to further classify far end
alarms (called ENHANCED RDI). This provides no obstacle to interworking: The SONET
side will not be able to provide a more detailed classification of SDH end alarms (it
does know there is a far end alarm). The SDH end ignores the extra bits. The Enhanced
RDI feature only operates if both ends are SONET.

BIP - The coding for BIP is identical. The way degrade is detected is different.
SONET generally uses a Poisson algorithm to declare degrade and SDH normally uses
a bursty error detection algorithm. This provides no obstacle to interconnect - the
criteria for declaring dDEG is just slightly different. Also, SONET provides an
EXC alarm for BER > 10^-3 which does not appear in SDH. This also does not prevent
interconnect - you just have an alarm which can be declared at one end and not the
other.

These are the major differences (besides the fact that SONET and SDH tend to have
different names for identical things- This is just a US/Europe language issue).
Regards,
Steve

"Mannie, Eric" wrote:
> 
> Dear All,
> 
> There is an easy way to stop definitively this discussion based on technical
> facts:
> 
> Stephen, Juergen and Maarten, please tell us: today, are the frame
> structures and all the bytes in the SDH and SONET overhead completely
> identical, used and interpreted in the same way, is the monitoring exactly
> the same ? In particular, if I provision and operate an SDH circuit/LSP is
> this fully identical to a SONET circuit/LSP from *all* point of views ?
> 
> PLEASE ANSWER BY YES OR NO ONLY. Other explanations are not needed at this
> stage.
> 
> If the answer is yes: SONET is totally identical to SDH.
> If the answer is no: SONET is not the same as SDH.
> 
> I think that without that answer we cannot take any *technical* decision on
> this mailing list and at the IETF.
> 
> Thanks to answer.
> 
> Kind regards,
> 
> Eric
> 
> ps: feel free to forward this e-mail to any ITU-T mailing list if a
> confirmation is needed.