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Figure 6-1 The supply/demand curve.
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SONET and SDH, as it turns out, are squarely positioned to address both of these issues. Consider the evolution of the typical optical network. When service providers first began to build long-haul optical transport systems in the late 1970s and early 1980s, the networks were largely point-to-point, as shown in Figure 6-2, because SONET, SDH, and the concept of a ring architecture had not yet arrived. Traffic that needed to be transported between Dallas and Denver, for example, would travel across a dedicated point-topoint optical link between the two end points. It wasn t the least bit flexible, but it offered bandwidth at high volumes and satisfied the nascent demand for something beyond copper transport. The problem with this dedicated model was twofold. First, it was highly vulnerable. A carefully situated backhoe digging in the wrong place could easily disrupt massive volumes of sensitive corporate traffic as well as voice, video, and other customer information streams. Recovery meant either a lengthy dispatch and repair job or some form of temporary reroute via digital or manual cross-connect.
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Figure 6-2 A point-to-point circuit.
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The second problem was that the model was limited in capacity. Naturally, the optical span offered tremendous bandwidth, but everything is finite, including an optical fiber. If the span were to be exhausted, the only solution was to provision additional spans, illustrated in Figure 6-3. This solution had two key downsides. First, it was inordinately expensive. To add fiber capacity, the trench had to be opened, the fiber, amplifiers, and repeater equipment had to be installed, and the hole had to be filled again. That process involved enormous cost and time because of the humanintensive nature of the installation process. A solution came in two forms, one well before the other. The first was optical time-division multiplexing (TDM), handled well by SONET and SDH, which enabled service providers to transport multiple data streams across a facility by assigning time slots to them. The second was Dense Wavelength Division Multiplexing (DWDM), which permitted service providers to transport multiple higher bit rate streams by assigning wavelengths to them, facilitating extremely high bit rate transport. SONET/SDH and DWDM, then, played a major role in early cost reduction efforts.
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Network Evolution Begins
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The next stage in the evolutionary process saw linear networks give way to rings as the criticality of high-speed network transport became obvious and the capabilities of the SONET/SDH overhead are realized. You will recall that the K bytes give SONET and SDH the capability to monitor the health of the network connections between multiplexers, the multiplexers themselves, and in the event of a failure to provide automatic protection switching as a way of surviving a cable cut. As the SONET/SDH overhead became better understood, service providers began to explore alternative network architectures that could take best advantage of the capabilities of that overhead. Two architectures in particular emerged: the two- and four-fiber ring.
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Players and Futures in the SONET/SDH Game
Players and Futures in the SONET/SDH Game
Figure 6-3 Multiple circuits.
Ring Architectures
As we described earlier, ring architectures have several advantages. The first is the redundancy and survivability that the architecture provides. In a two-fiber implementation, shown in Figure 6-4, the rings are typically designated as a primary or active ring and a protect or backup ring. Live traffic is carried on the active path under normal operations, whereas the backup ring simply monitors the status of the overall system and transports keep-alive messages. In the event of a failure of the primary ring, automatic protection switching kicks in, causing the devices on either end of the ring breach to switch traffic (usually within 50 ms) to the backup span. Under normal circumstances, the user will be unaware of the switchover. This particular two-fiber architecture is known as a Unidirectional Path-Switched Ring (UPSR). In the event of a dual-span failure (due to the efforts of our overly diligent backhoe driver described in 2), the ring wraps at the multiplexer nodes on either side of the breach, resulting in the creation of a single span ring, but preserving the integrity of the transmission path. This is shown in Figure 6-5.
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