The Number One Electronic Switching System was the first large scale Stored Program Control telephone exchange or Electronic Switching System in the Bell System, entering service in the late 1960s. The switching fabric was composed of reed matrixes controlled by wire spring relays which in turn were controlled by a central processing unit (CPU). The 1AESS version of the late 1970s was a plug compatible upgrade, using smaller remreed switches, fewer relays, a faster CPU, and disk storage.