“The synchronous optical network (SONET) is the American standard (ANSI) for high speed dedicated-circuit services. The ITU-T recently standardized an almost identical service that easily interconnects with SONET, under the name synchronous digital hierarchy (SDH).” (Fitzgerald & Dennis, 2001, page 10) SONET has different optical carrier levels that start with a minimum of 51.84 Mbps. The OC level keeps on increasing and goes up to OC-192. The SONET’s lowest connection speed is faster than the T3’s highest connection speed. (Fitzgerald & Dennis, 2001, page 10)
“The digital hierarchy (DS-0, DS-1, DS-3, and so on) was created to provide cost-effective multiplexed transport for voice and data traffic from one location in a network to another. SONET has the same responsibility, albeit on a larger scale: indeed, it is sometimes described as T-1 on steroids…” (Shepard, 2001, page 38) and is “expected to provide the transport infrastructure for worldwide telecommunications for at least the next two or three decades.” (Synchronous Optical Network, 2007) The SONET network connected in a linear manner and has ADMs. These Add drop multiplexers are connected to each other via fibers. There is usually more than one string of fiber connecting these nodes to each other incase one of them stops working. Information comes and goes out of the Add Drop Multiplexers. (Prakash)
The Synchronous Transport Signal has 810 bytes and the information us distributed in a vertical manner. “The two dimensional figure is just for convenience. Actual transmission takes place serially i.e. the left most byte in the top row is transmitted, then the second byte in the first row and so on. After the 90th byte in the first row the left most byte in the second row is transmitted and it goes on.” (Prakash)
Digital signals have to be synchronized to be sent out. Just like a neurological surgery when the doctor enters a virus into the human brain to kill the tumor, he has to be synchronized with the other surgeons while injecting for the virus to be effective. This kind of surgical procedure is based on a synchronous signal: two doctors working in sync at the same rate for the same purpose. In reality however it is very difficult to be completely synchronized. “There may, however, be a phase difference between the transitions of the two signals, and this would lie within specified limits. These phase differences may be due to propagation time delays or jitter introduced into the transmission network. In a synchronous network, all the clocks are traceable to one primary reference clock (PRC). The accuracy of the PRC is better than ±1 in 1011 and is derived from a cesium atomic standard.”(Synchronous Optical Network, 2007)
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