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Synchronization and Timing in SONET Networks
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When the telephone company first began to deploy networks, they realized that they had a significant problem to overcome. The networks they deployed had a clear hierarchy to them, as shown in Figure 2-32. The box at the top labeled 1 represents the highest level of the switching hierarchy in a region (for example, North America). In this model, it provides a
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synchronization and timing signal to the devices immediately below it, labeled 2 . These switches, in turn, provide timing to the third tier, and so on down the line. The highest level of the network hierarchy required the most accurate timing signal of all because they were responsible for providing the reference-timing signal for all the subtending layers in the network. As a result, at the top of the network heap lived an extremely accurate cesium clock that beat out a standard-timing signal called the Bell System Reference Frequency (BSRF) that all tier-one network devices used as their timing source. These tier-one devices would in turn provide a timing signal to their downstream partners, which would do the same for theirs, and so on. The only problem with this model is that all devices in the network are required to derive timing from a sole source, which involves enormous complexity when one considers the magnitude of this problem. Consider how many central office switches are available, not to mention digital cross-connect systems, carrier devices, and other components that require synchronization. So why didn t the offices simply generate their own timing signals The answer is quite simple. The first digital-transmission systems came about in the early 1960s and at that time, the state of the art in power supplies and timing circuitry was not good enough to guarantee the stability required to time a device as critically dependent on stable clocking as a high-end switch or multiplexer. The clocks would drift, which would in turn lead to timing slips, and the ultimate result was errors in the traffic being transported across the network. The only solution was to rely on a single, highly reliable (and enormously expensive) clock that could provide the necessary stability.
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Figure 2-32 Timing hierarchy in early networks.
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As time went on, power supplies became more stable, clock sources became more accurate, and alternatives to the BSRF became available. The most commonly used clock signal today derives from the Global Positioning System (GPS) satellites, and most central offices in North America have the ability to derive a GPS timing signal using a rooftop receiver (Figure 2-33) and feed the signal to a device known as a Building Integrated Timing Supply, or BITS clock, shown in Figure 2-34. This clock is accurate enough to provide timing to all devices in the office, and serves as the Primary Reference Clock (PRC). An office may actually have more than one feed from an accurate timing source to ensure redundancy, but it will only use one of the sources at a time as a timing signal for the devices in that office. In data networks, lower-level devices routinely transmit traffic between each other, meaning that the overall system must ensure that two or more communicating devices in a transmission system be synchronized relative to one another to within certain tolerances. If we consider the example of a network of multiplexers that are communicating with each other, we see that the ability to synchronize the transmitted signal from a multiplexer is relatively simple. After all, the mux is generating the voltage pulses that indicate the mix of ones and zeroes to the receiver. How, though, can a multiplexer ensure that the received signal, that is, the signal arriving from another multiplexer, is synchronized as well After all, the signal is coming
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