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This chapter is from the book

Clarification of Terms

Before we proceed into a more detailed discussion of the subject matter, it is appropriate to pause and define some terms that will be used in subsequent chapters. In the early 1980s, AT&T introduced the digital access and cross-connect system (DACS) as a major enhancement to its digital transport system products. It is also called the digital cross-connect system, or DCS. The subject of DCS is introduced here in order to explain several concepts that are central to subsequent chapters.

The original DACSs were considered complex implementations of microelectronics technology. They had a RAM (random access memory) of 256 words! At that time, they were considered very esoteric machines.

Figure 1–6 shows some of the major operations of a DCS, which uses a combination of time-division and space-division switching techniques. The original DACS terminated up to 127 DS1 signals (3048 DS0 channels) and provided up to 1524 cross-connections.

Figure 6-6Figure 1–6 Terms and concepts.

The DCS permits the assignment and redistribution of DS0 channels for drop-and-insert (also known as ADM in today's technology) services. As Figure 1–6(a) shows, the operation provides for the distribution of traffic to nodes reachable by the DCS (the drop operation). It also allows a node to send traffic to the DCS for delivery to another node in the network (the insert operation). Figure 1–6(b) shows how the DCS performs back hauling (sending traffic downstream and returning it back), which does not require back-to-back channel banks (or the conversion of the digital signals to analog and back to digital again). A DCS also performs groom-and-fill operations (Figure 1–6(c)), a technique in which the machine accepts input from two or more low-speed lines and multiplexes these signals into a higher speed line.

Figure 1–7 shows some examples of equipment and configurations that exist at the customer premises equipment and the telephone central office. Be aware that a wide variety of options are available and these examples are only a sampling of possible services and arrangements.

Figure 6-7Figure 1–7 Examples of configurations.

Figure 1–7(a) shows a voice interface into a channel bank, which converts the analog signal into a digital signal with a CODEC (a coder/decoder). The CO then multiplexes multiple digital signals together for transmission into the network.

Figures 1–7(b) through 1–7(d) show how channel service units (CSUs), digital service units (DSUs), and office channel units (OCUs) may be employed. Before the divestiture of AT&T in 1984, the CSU and and DSU were in separate boxes. They are responsible for coding the user traffic into self-clocking signals and performing a variety of testing and loop-back functions.

In the 1970s and early 1980s, the customer was provided an interface into a digital channel with a Western Electric 500A, a CSU and DSU, or a combination of the two (CSU/DSU). The DSU converts the user equipment signals into signals that are more efficient for use in a digital network. The DSU also performs clocking and signal regeneration on the channel. The CSU performs functions such as line conditioning (equalization), which keeps the signal's performance consistent across the channel bandwidth; signal reshaping, which reconstitutes the binary pulse stream; and loop-back testing, which entails the transmission of test signals between the CSU and the network carrier's office channel unit (OCU).

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