as introduced; this was based on the same reed-relay design as the 1ESS but in a smaller package aimed at suburban and rural communities. The smaller rural-market but still analog 3ESS was introduced in the late 1970s, although very few were ever installed; by then, Nortel's small digital DMS-10 was an obviously better choice. The entire 1ESS base was also updated with a new CPU, becoming the 1AESS, which con- tinued to be installed into 1983. Western Electric's first digital switch was the 4ESS, a large toll switch first introduced in 1976. This became the workhorse of the Long Lines division; the RBOCs also acquired a number of them after divestiture as tandem switches. But end office digital switching in the Bell System began with foreign switches. Most prominent was the Northern Telecom DMS family. The DMS-10, a small digital switch said to be loosely derived from the pioneering SL-1 PBX architecture, won rapid acceptance in smaller line size applications (up to several thousand lines). Introduced in Canada in the late 1970s, it entered the United States in 1981; although most U.S. sales were to independent telephone compa- nies such as GTE, it also penetrated the Bell System. The DMS-100, a large digital switch, was introduced in 1979. The predivestiture Bell System bought a few, but once divestiture occurred, it was a hot item. Western Electric first introduced its digital end office ("Class 5") switch, the 5ESS, in 1980, but the initial design had problems that led volume shipments to be delayed until 1982. The newly divested RBOCs began to replace their analog switches with digital ones and divided the bulk of their switching business between the 5ESS and DMS-100. Not that this digitization happened very rapidly. Depreciation schedules were slow, because slow depreciation was seen to help hold down local tele- phone rates. Some old central office switches had been in place for 40 years or more; 1ESS-class analog switches were designed for a 40-year service life, with typical depreciation lives in the 20-y