Synchronous Optical NETwork

<communications, standard> (SDH) An international digital telecommunications network hierarchy which standardises transmission around the bit rate of 51.84 megabits per second, which is also called STS-1. Multiples of this bit rate comprise higher bit rate streams. Thus STS-3 is 3 times STS-1, STS-12 is 12 times STS-1, and so on. STS-3 is the lowest bit rate expected to carry ATM traffic, and is also referred to as STM-1 (Synchronous Transport Module-Level 1). The SDH specifies how payload data is framed and transported synchronously across optical fibre transmission links without requiring all the links and nodes to have the same synchronized clock for data transmission and recovery (i.e. both the clock frequency and phase are allowed to have variations, or be plesiochronous). SDH offers several advantages over the current multiplexing technology, which is known as {Plesiochronous Digital Hierarchy}. Where PDH lacks built-in facilities for automatic management and routing, and locks users into proprietary methods, SDH can improve network reliability and performance, offers much greater flexibility and lower operating and maintenance costs, and provides for a faster provision of new services. Under SDH, incoming traffic is synchronized and enhanced with network management bits before being multiplexed into the STM-1 fixed rate frame. The fundamental clock frequency around which the SDH or SONET framing is done is 8 KHz or 125 microseconds. SONET (Synchronous Optical Network) is the American version of SDH. (1995-03-02)

<networking> (SONET) A broadband networking standard based on point-to-point optical fibre networks. SONET will provide a high-bandwidth "pipe" to support ATM-based services. The SONET standard will establish a digital {hierarchical network} with a consistent worldwide transport scheme. SONET has been designed to take advantage of fibre, in contrast to the plain old telephone system which was designed for copper wires. SONET carries circuit-switched data in frames at speeds in multiples of 51.84 megabits per second (Mbps) up to 48 * 51.84 Mbps = 2.488 gigabits per second. Since SONET uses multiple channels to transmit data, each SONET frame can be considered to be a two-dimensional table of bytes that is 9 rows high and 90 columns deep. For every OC-n level, SONET can transmit n number of frames at a given time. Groups of frames are called superframes. SONET is the American version of SDH. [Wulf Losee; Corporate Computing 8.92; STACKS; LAN Magazine 10.93]. (1994-11-30)