this chapter we will explore telephony and the telephone network. Described as "the largest machine ever built," the telephone network is an impressive thing. We begin with some history, then explore the net- work itself, and finally explore telephony service-how it works, how it makes its way across the network, and how it is managed. We will exam- ine the process of voice digitization followed by the multiplexing and high-speed transport of voice through such technologies as PCM, SONET, and SDH. Some people feel that voice is something of an anachronism and has no place in a book about the new, slick world of telecommunications. I dis- agree: Without an understanding of how the voice network operates, it is impossible to understand or appreciate how data networks operate. Fur- thermore, as voice moves inexorably toward an all-IP model, the voice stream becomes nothing more than one more component of the overall corporate data stream. Additionally, since the IT organization manages data in most corporations, voice is rapidly becoming just another corpo- rate data application. This chapter is designed to bridge the gap between the two, and we begin our tale in New York City. Miracle on Second Avenue February 26, 1975 was a business-as-usual day at New York Telephone's lower Manhattan switching center. Located at 2nd Avenue and 13th Street, the massive 11-story building was the telephony nerve center for 12 Manhattan exchanges that provided service to 300 New York City blocks, including 104,000 subscribers and 170,000 telephones. Within the service area were six hospitals, 11 firehouses, three post offices, one police station, nine schools, and three universities. The building was massive, but like most central offices was completely invisible to the pub- lic. It was just a big, windowless structure that belonged to the telephone company. No one really knew what went on in there; nobody cared. When night fell, most of the building's employees went home, leaving a small crew to handle maintenance tasks and minor se