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Shedding Light On Sonet March 20, 2000 By Darrin Woods Sonet is slowly moving into everyday business life as bandwidth requirements within the enterprise increase. Sonet, or synchronous optical network, is the standard for transmitting synchronous data on optical-electrical media. It allows the entire contents of a 650-MB CD-ROM to move from coast to coast in less than one second. Businesses that can't contain their entire workforce in a single building are adding Sonet rings to interconnect offices in MANs (metropolitan area networks), and packet-over-Sonet has the potential to supplant ATM in a local area network and across a wide-area network. A Little History Sonet was born out of necessity. As the population expanded and communication demands grew, copper--once the transmission material of choice--ceased to be economical or practical to carry the huge number of calls nationwide. Copper also was highly prone to electrical spikes from storms and other electrical interference. Enter fiber optics. Now data that once required hundreds of copper cables could be directed down a glass fiber only slightly thicker than a human hair. Carriers jumped on this technology and tried to one-up each other in the amount of fiber each had. To keep up with carriers' needs, vendors created complex systems to multiplex traffic onto these tiny strands. Unfortunately, these systems were usually proprietary, and interconnection of different products even from the same vendor was difficult. Interconnection between two carriers (each using different hardware vendors) was that much more difficult. Something had to be done. In 1984, the Exchange Carriers Standards Association (ECSA) proposed a method to interconnect the fiber optic systems from multiple vendors. Bellcore extended the original ECSA idea in 1985 and proposed what we now know as Sonet. In 1988, the initial Sonet standards were approved as ANSI documents T1.105-1988, which described optical rates and data format, and T1.106-1988, which described the physical interface. Sonet signals are referenced in two ways: STS (synchronous transport signal) is the electrical portion, and OC (optical carrier level) is the optical portion. Although Sonet was designed to eliminate the electrical transmission of data, STS is used for very short distances, usually only within a switch cabinet. Until pure optical switching is available, the electrical equivalent is necessary. STS-x describes frame generation within a switch, since it is done electrically; OC-x describes transmission of the signal from point to point. Because Sonet sends 8,000 STS frames per second--or one frame every 125 microseconds, the same frame rate that has been around since the DS-1 was invented--it's easy to incorporate current transmission timings. Bandwidth ranges from 51.84 Mbps at the OC-1 level to 9953.28 Mbps at OC-192. There are specifications for higher bandwidths, with some vendors talking about OC-768 products--equivalent to seven CD-ROMs transmitted in one second--but these specifications have not been finalized. At the physical OC-x level, data travels in one of two ways--WDM (wave-division multiplexing) or DWDM (dense-wave-division multiplexing). WDM pulses a single laser to transmit data. The faster this laser can be pulsed, the more bandwidth that can be pushed through the fiber. WDM can effectively pulse a laser at OC-48 speeds. To reach higher bandwidths, however, the size of the pipe must be increased. Enter DWDM, which achieves a higher bandwidth by combining multiple OC-48 WDM lasers (each operating at a different wavelength)--essentially using the same pipe but enlarging it by transmitting more wavelengths of light. You have to take care when purchasing equipment. If you choose to buy lit fiber from a LEC (local exchange carrier), the carrier probably will specify and supply the equipment. Even though Sonet is a standard, there are subtle differences in the way some bytes are used in the overhead by different manufacturers. Therefore, it's important to match the LEC equipment. Alternatively, dark fiber is available for purchase from LECs, and is used to connect local offices in a campuswide type of environment. You can usually choose your own equipment when you purchase dark fiber.
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