5/7/2023 0 Comments Telephone company![]() Each node on the ring uses a device known as an add/drop multiplexer (ADM). If the primary ring is broken, the hardware detects the failure and uses the counter-rotating ring to reconnect. ![]() Similar to the fiber distributed data interface (FDDI) ring, a SONET ring has a dual-ring structure and is self-healing (i.e., automatically restores the traffic in case of link or node failures). SONET can work as the physical layer protocol for a single point-to-point circuit as well as for a ring network. Virtually all long-distance telephone traffic in the United States now uses trunks running SONET. The advance in WDM technology has already reached a point where a single fiber can carry several hundreds of thousands of simultaneous voice and Internet connections. Today WDM equipment exists that can support 100 or more channels per fiber, 10–40 Gbps per channel. By installing WDM and SONET equipment at each end of a fiber, a LEC can gain the equivalent of many new fibers without installing a new cable. Instead of using a single laser, WDM uses multiple lasers operating at different wavelengths on the same fiber. Since the early 1990s a different multiplexing technique called wavelength-division multiplexing (WDM) has been used increasingly. Multiple channels are created using TDM, i.e., by allocating a specific time slot to an individual channel. In conventional fiber networks, the light signal transmitted on the fiber strand is of a single wavelength. SONET trunks as fast as OC-192 are available today that offer a bit rate of 10 Gbps. OC-3 trunks have a rate of 155 Mbps, and ATM to be discussed in Section III runs at this rate. All SONET trunks are a multiple of STS-1. The basic SONET channel, called STS-1 (Synchronous Transport Signal-1) or OC-1 (Optical Carrier-1), therefore has a bit rate of 51.84 Mbps (= 810 × 8 × 8000). SONET transmits 810-byte frames, 8000 times per second, matching the sampling rate of the PCM channels. SONET provides higher bit rates than T3 to carry mixed types of information such as data, audio, and video. Like the traditional PSTN, SONET is a synchronous system using TDM. Three different versions of SDH exist: SDH-Europe, SDH-Japan, and SDH-SONET (Synchronous Optical Network) for North America. In 1989 Bellcore (the RBOCs research arm at the time) and CCITT (renamed to ITU-T in 1993) established a fiber optics communication standard, a physical layer protocol, called Synchronous Digital Hierarchy (SDH). Telephone companies originally deployed fiber optics using their own proprietary TDM systems. Park, in Encyclopedia of Information Systems, 2003 II.B.
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