was first implemented using disk technology but was replaced by the 4E attached processor system (4EAPS). The 4EAPS is a 3B computer running 4EAPS application software on the DMERT operating system. The 4EAPS interfaces to the 4ESS processor via the attached processor interface (API) units. The "1A file store" became partitions on the 3B computer disks. At first the 4EAPS just provided "file store" but soon it also provided access to the common-network interface ring (CNI ring) to provide common-channel signaling (CCS). The 4EAPS originally used the 3B20D computer. These were all converted to the 3B21D around 1995. Peripheral units The peripheral units include units that interface to the central control over the peripheral unit bus. This includes the common channel interface signaling (CCIS) terminal, signal processors, time-slot interchanges (TSI) and time multiplexed switches (TMS). It also includes equipment not directly on the PUB including terminating equipment used to connect the switch to the transport network and the TSIs and TMSs, which actually perform the "time-space-time" switching function. Timing is provided by a high speed, high accuracy network clock. History 4ESS development began circa 1970, mainly in Naperville, Illinois under the direction of Henry Earle Vaughan. AT&T Long Distance was the primary customer for the switch. Driving development from the customer's perspective was AT&T VP Billy Oliver. Previous tandem switching systems, primarily the No. 4 Crossbar switch, used analog voice signaling. The decision to switch in a digital voice format was controversial at the time, both from a technical and economic viewpoint. Nevertheless, visionaries such as Vaughn and Oliver recognized that the network would eventually become digital, requiring digital switching technologies. The last 4ESS was installed in suburban Atlanta, GA in 1999 as a toll tandem for AT&T. At the peak of the product's life time in 1999, AT&T employed 145 4ESS switches in its long-haul network, and several were owned by various Regional Bell Operating Companies (RBOCs). AT&T replaced or supplemented the 4ESS toll tandem switches with 5ESS switches, which featured an advanced design, and are used as edge switches in the network. Most RBOCs who used 4ESS tandems have replaced them with Class 5 systems of other manufacturers, e.g., Nortel. As of 2014, AT&T operates and maintains approximately one hundred 4ESS switches in the public switched telephone network. Next-generation 4ESS The Nokia N4E-N1B (New 4ESS) is the ATCA-based next-generation toll switch for AT&T. The N4E-N1B includes the 4E