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The Synchronous Optical Network (SONET) and Synchronous Digital Hierarchy (SDH) are a set of related standards for synchronous data transmission over fiber optic networks that are often used for framing and synchronization at the physical layer. SONET is the United States version of the standard published by the American National Standards Institute (ANSI). SDH is the international version of the standard published by the International Telecommunications Union (ITU).

SONET/SDH can be used in an ATM or non-ATM environment. Packet Over SONET/SDH (POS) maps IP datagrams into the SONET frame payload using Point-to-Point Protocol (PPP). In the ATM environment, connections to SONET/SDH lines may be via multi-mode, single-mode or UTP.