Page 98 - Optical Switching And Networking Handbook

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Synchronous Digital Hierarchy 83

interoperability and improvements in network management because
of proliferation of the number of carriers providing long-distance
services.
The International Telecommunications Union-Telecommunications
Standardization Sector (ITU-TS) agreed that something had to be
done to improve and standardize the multiplexing and interoper-
ability while at the same time taking advantage of the higher capac-
ity of optical fiber. The older bit interleaving of multiplexers should
be replaced by byte interleaving to provide for better network man-
agement.The new standard appeared as SONET in the North Amer-
1
icas, drafted by BellCore. Later this same standard developed into
the SDH/SONET standard as approved by the ITU. Although
SONET and SDH were drafted initially in support of fiber, radio-
based systems supporting the same multiplexing rates also became
available.

Why SDH/SONET

Many reasons explain why SONET and SDH were necessary. The
primary reason was that previous technology (PDH) was limited in
many ways, such as
U.S. and European systems had little in common in their
mapping and multiplexing systems. Therefore, expensive
translators were required for trans-Atlantic traffic on leased
lines.
“Standard” equipment from different vendors in the same
country was incompatible. Everyone produced a proprietary
solution that worked with his or her own equipment.
Systems did not offer self-checking of equipment and network
components. Expensive manual checks were required, and
extraordinary repair systems were the norm.

No standard for high-bandwidth links existed. Everything
maintained the proprietary approach. This created havoc in the
industry and needed to be improved.

1 The BellCore name has since been changed to TelCordia Technologies.