Transmission over single-mode fiber (SMF) optic transmission systems (FOTS), SONET grew out of the SYNTRAN (SYNchronous TRANsmission) standard developed at Bellcore. SONET was initially developed in 1984 and finally standardized by the American National Standards Institute (ANSI) in 1988. Also in 1988, SONET was internationalized by the ITU-T as Synchronous Digital Hierarchy (SDH), which differs primarily with respect to low-level line rates and some terminology. SONET/SDH essentially is a broadband optical version of T-carrier, defined in multiples of T3 bandwidth plus overhead for additional signaling and control functions. Primarily intended for carrier backbone implementation, SONET/SDH specifies a network-to-network interface (NNI), also known as network node interface, that supports the interconnection of national and regional networks into a cohesive global network. The user network interface (UNI) provides a standard basis for connection from the user premises to a SONET/SDH local loop. T3 signal to which additional signaling and control overhead is added to for the Synchronous Transport Signal level-N (STS-N). The term Optical Carrier-N (OC-N) applies once the electrical signal is converted to optical format. The SONET hierarchy begins with is OC-1 at a signaling rate of 51.84 Mbps, which is the equivalent of a T3 (44.736 Mbps) plus overhead, and currently tops out at the OC-768 nominal rate of 40 Gbps, which is the equivalent of 768 T3s plus overhead.Table S-2 details the SONET/SDH hierarchy. 1 OC-2, OC-9, OC-18, OC-36, and OC-96 are considered to be orphaned rates. 2 OC-3 was defined by the CCITT as the basic transport rate for B-ISDN. 3 This level is not fully defined. ment for voice encoded using the standard pulse code modulation (PCM) technique employed in the public switched telephone network (PSTN). The STS-1 frame, as illustrated in Figure S-5, comprises a Synchronous Payload Envelope (SPE) of 783 octets, plus 27 octets of overhead comprising Section Overhead (SOH), Line Overhead (LOH), and Path Overhead (POH).The payload in each frame comprises one or more Virtual Tributaries (VTs) organized into Virtual Tributary Groups (VTGs).A process known as concatenation allows multiple STS-1 frames to be multiplexed, switched, and transported over the network as a single entity. SDH and SONET standards specify two physical configuration options.A path-switched ring employs a dual counter-rotating ring, with both fibers active. One ring transmits in the clockwise direction and the other in the counter-clockwise direction.This approach offers zero downtime in the event of the failure of a fiber. A line-switched ring features one active and one inactive fiber. In the event of the catastrophic failure of a node or fiber, a line-switched ring offers sub-50 millisecond restoral times for rings up to 1,200 kilometers in route distance. Figure S-6 illustrates a hierarchy of local, metropolitan, and backbone rings operating at example OC-N rates. loop, LOH, overhead, path-switched ring, PCM, POH, PSTN, signaling and control, signaling rate, SMF, SOH, SPE, T3, T-carrier, VT, and VTG.