Sonet/SDH Demystified

The year was 1984, and Bill McGowan had a problem. Freshly bloodied and scarred from the AT&T divestiture battlefield, where as the head of MCI, he served as the attacking general who, in many people s minds, single-handedly drove the breakup of the Bell System. McGowan realized that the toppling of the titan and subsequent shattering of AT&T into eight distinct pieces (seven regional providers plus one long-distance provider), shown in Figure 1-1, only resolved one of the challenges that would lead to the creation of a truly competitive marketplace.
Although the best-known impact of divestiture was the breakup of AT&T (one result of which was the liberalization of the telecommunications marketplace in the U.S.), a second decision that was tightly intertwined with the Bell System s breakup was largely invisible to the public, yet was at least as important to AT&T competitors, MCI and Sprint, as the breakup itself. This decision, known as Equal Access, had one seminal goal: to make it possible for end customers to take advantage of one of the products of divestiture, the ability to select one s long-distance provider from a pool of available service providers in this case AT&T, MCI, or Sprint. This, of course, was the realization of a truly competitive marketplace in the long-distance market segment.
To understand this evolution, it is helpful to have a high-level understanding of the overall architecture of the network. In the pre-divestiture world, AT&T was the provider for local service, long-distance service,...