CCITT Hierarchy Nomenclature Quaternary center (highest ranking) Class 1 Office Tertiary center Class 2 Office Secondary Class 3 Office Primary Class 4 Office Local Class 5 Office Over the last decade, more and moer of the telephone switching environments have been under the control of stored programs residing withing the realm of ESS. These switching systems are controlled by intricate software modules exactly as any other sophisticated computers. The reprogramming of these switching systems is relatively straightforward in light of the requirements of the intelligent functions expected from the network. For the circuit switching part of the facility, the existing #1ESS (introduced in 1965 to local switching and to toll switching in 1970) and #1AESS (introduced in 1976 to local switching) have hte basic switching capacity to switch and be programmed accordingly. However, the common channel signaling aspects need to be upgraded to accommodate the intelligent network functions, especially those of the ISDN. The flexible and highly programmable digital switches are key components in the distributed and intelligent networks. In Norht America, the advent of the more sophisticated ESS facility (#5ESS) was a major step in the realization of intelligent networks. Its modular hardware and software building blocks permit the capacity to customize these switching systems to perform the flexible and programmable functions with inherent mechanized intelligence. The more recently introduced #5ESS switch will meet the CCITT requirements for the (2B + D), (23B + D), and the (30B + D) rate interfaces. In Europe, the steady evolution of the switching systems (Ericsson, AXE10) for intelligent network functions has been well along its way since 1984. The modularity for customization and for CCITT signaling system No. 7, which facilitates the customer services integrated in the network functions, has been incorporated into the newer switching systems being built in Europe. These switches also provide for the (2B + D), (23B + D), and the (30B + D).