“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).” 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.
“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…” and is “expected to provide the transport infrastructure for worldwide telecommunications for at least the next two or three decades.” The SONET network is 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 in case one of them stops working. Information comes and goes out of the Add-Drop Multiplexers.
The Synchronous Transport Signal has 810 bytes, and the information is distributed in a vertical manner. “The two-dimensional figure is just for convenience. Actual transmission takes place serially, i.e., the leftmost 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 leftmost byte in the second row is transmitted, and it goes on.”
Digital signals have to be synchronized to be sent out. Just like 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.”
Plesiochronous means that when a shift occurs in two signals, the variation in them is very less and is controlled to not go over a particular limit. “For example, if two networks must interwork, their clocks may be derived from two different PRCs. Although these clocks are extremely accurate, there is a difference between one clock and the other. This is known as a plesiochronous difference.” Asynchronous signals are where the shifts do not happen at the same rate. These signals are less accurate because their difference, when compared to each other, is way higher.
“SONET defines technology for carrying many signals of different capacities through a synchronous, flexible, optical hierarchy. This is accomplished by means of a byte-interleaved multiplexing scheme. Byte-interleaving simplifies multiplexing and offers end-to-end network management.” SONET multiplexing is complicated and takes many steps. First, the base signal is started, and it transports signals in a synchronous manner. 51.84 Mbps is the beginning speed at level 1. The first step in the SONET multiplexing process involves the generation of the lowest level or base signal. In SONET, this base signal is referred to as synchronous transport signal–level 1, or simply STS–1, which operates at 51.84 Mbps.
The difference between SONET and SDH is not a big one. In the SDH, the STS-1 is not the first level. The first level is the Synchronous Transport Mode 1 and is equal to the third SONET level. In SDH, overhead bytes are a little different. “A common misconception is that STM-Ns is formed by multiplexing STM-1s. STM-1s, STM-4s, and STM-16s that terminate on a network node are broken down to recover the VCs which they contain. The outbound STM-Ns are then reconstructed with new overheads.”
“SONET was developed in the United States through ANSI T1X1.5 committee. ANSI work commenced in 1985, with the CCITT (now ITU) initiating a standardization effort in 1986. The US wanted a data rate close to 50Mbps. But the Europeans wanted the data rate to be around 150 Mbps. A compromise was reached, and the US data rates were made a subset of ITU specification, known formally as Synchronous Digital Hierarchy (SDH).” Basically, the SONET and SDH are quite similar, aside from a few differences. The SONET is used in America and Canada, and the rest of the world uses SDH. After the compromise, the SDH is used as the proper protocol.