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).