relay-driven rows and columns of connecting rods. These were magnificent machines offering machine-operated telephone service in a time before computers, when few other things were automated. In 1947, Bell Telephone Laboratories helped open the age of digital electronics with its invention of the transistor. Less than 20 years later, the digital computer brought a new era to telephone switching with introduction of the electronic switching system (ESS). Electronic switching systems use computer software to control the electronic sig- nal pathways. In 1965, Western Electric, the technology and manufacturing arm of the Bell Telephone System, came out with the 1ESS, which could handle 60,000 lines. In 1976, the 1AESS more than doubled the 1ESS capacity with its more advanced 1A processor. By 1985, the 1AESS switch was able to handle 250,000 subscriber lines. The 1AESS is an analog switch. While the 1A processor is a digital computer, the telephone wires going in and out of the 1AESS are analog wires, and the connections inside the 1AESS are mechanical switches. These reed switches are controlled by tiny electromagnets that open and close circuits. 8 The early Advanced Mobile Phone Service (AMPS) cellular systems used the 1AESS as their mobile telephone switching offices (MTSOs), so AMPS was a fully analog sys- tem, from the mobile telephone along the analog FM radio link, through analog base station electronics, along analog facilities to the MTSO, through an analog telephone switch, and out to the PSTN. Today's AMPS calls go through digital facilities and digi- tal equipment, but the air interface remains analog FM. A digital switch not only has a digital computer but also has the ability to interface directly to digital facilities, which we will discuss in Chap. 13. The Northern Telecom DMS-10 switch was introduced into small local exchange offices in 1981, and the DMS- 100 was introduced for larger offices. AT&T's Western Electric Division also introduced a digital local exchange office switch, the 5ESS. Because 