All,
Why section 5.1 is an author's impression:
Statement:
"SONET and SDH were defined as competing standards"
Fact:
SONET was developed first by ANSI based on the 24 channel PDH hierarchy
existing in North America and Japan. The basic rate based on DS3.
Some time later ETSI developed SDH based on the 30 channnel PDH deployed
in Europe. The basic rate based on E4 (3x DS3).
To be able to deploy SONET and SDH worldwide the regional SDO experts
came together in ITU-T to define a frame structure and a frame-rate
that would allow interconnection of SONET and SDH.
A compromise was agreed and approved in an ITU-T meeting in Seoul
in 1988.
Statement:
"Significant confusion resulted from this situation."
Fact:
The result of the compromise is documented in ITU-T recommendation
G.707 which includes the frame definition and frame-rates, and
documents how SONET and SDH can interconnect.
Statement:
"Equipment manufacturers needed to select the market segment they
intended to address. The cost of chipsets for a limited market
increased.
Fact:
Most equipment vendors did/do sell their equipment in both regions.
I was involved in chip designs for SONET/SDH in several companies,
they all support SONET and SDH in a single chip, and the selection
is a matter of provisioning, the addition cost to support both was
minimal (single chip: higher volume = lower cost)
Statement:
"Service providers needed to consider the merits of the two standards
and their long-term role in the industry when examining their
investment options.
Fact:
Because the regions or applicability of SONET and SDH are well
known SPs do not have to make this consideration.
Statement:
"Only a limited number of equipment manufactures were available
for selection."
Question:
What do you consider a limited number?
Statement:
"As SONET was considered to be the variant, interworking had to be
performed before the SDH-based segment was reached."
Fact:
SONET is *NOT* a variant it is equivalent to SDH.
The reason for placing the interconnection functionality on the SONET
side was that in a previous agreement on interconnection the
functionality was placed on the European side.
Conclusion:
There is a single frame structure used by both SONET and SDH.
This is documented in ITU-T recommendation G.707 (ANSI and ETSI
still have their SONET resp. SDH standard available in their
own documentation, but they are aligned with G.707).
It depends on the application of the frame structure in an environment
with 24 channel legacy PDH to call it SONET and in an evironment
with 30 channel legacy PDH to call it SDH.
The meeting in Seoul in 1988 shows that SDOs can compromise to
find a common frame structure that can be used in different
regions/applications.
Best regards, Huub.
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