The new standard appeared first as SONET, drafted by Bellcore in the United States, and then went through revisions before it emerged in a new form compatible with the international SDH. Both SDH and SONET emerged between 1988 and 1992.

SONET is an ANSI standard; it can carry as payloads the North American PDH hierarchy of bit rates: 1.5/6/45 Mbps, plus 2 Mbps (known in the United States as E-1). SDH embraces most of SONET and is an international standard, but it is often regarded as a European standard because its suppliers—with one or two exceptions—carry only the ETSI–defined European PDH bit rates of 2/34/140 Mbps (8 Mbps is omitted from SDH). Both ETSI and ANSI have defined, detailed SDH/SONET feature options for use within their geographical spheres of influence.

The original SDH standard defined the transport of 1.5/2/6/34/45/140 Mbps within a transmission rate of 155.52 Mbps and is being developed to carry other types of traffic, such as asynchronous transfer mode (ATM) and Internet protocol (IP), within rates that are integer multiples of 155.52 Mbps. The basic unit of transmission in SONET is at 51.84 Mbps, but in order to carry 140 Mbps, SDH is based on three times this (i.e., 155.52 Mbps [155 Mbps]). Through an appropriate choice of options, a subset of SDH is compatible with a subset of SONET; therefore, traffic interworking is possible. Interworking for alarms and performance management is generally not possible between SDH and SONET systems. It is only possible in a few cases for some features between vendors of SDH and slightly more between vendors of SONET.

Although SONET and SDH were conceived originally for optical fiber transmission, SDH radio systems exist at rates compatible with both SONET and SDH.

In summary, the following are true:

• SONET is a digital hierarchy interface conceived by Bellcore and defined by ANSI for use in North America.
• SDH is (a) a network node interface (NNI) defined by CCITT/ITU–TS for worldwide use and partly compatible with SONET; and (b) one of two options for the user-network interface (UNI) (i.e., the customer connection), and formally the U reference-point interface for support of BISDN.

Future of SDH

Almost all new fiber-transmission systems now being installed in public networks use SDH or SONET. They are expected to dominate transmission for decades to come, just as their predecessor PDH has dominated transmission for more than 20 years (and still does in terms of total systems installed). Bit rates in long-haul systems are expected to rise to 40 Gbps soon after the year 2000, at the same time as systems of 155 Mbps and below penetrate more deeply into access networks.