 The DMS also has the capability to support existing simple data departments before queuing the call. Credit card validation systems networks based on drop and insert multiplexors utilising spare also use the same technology, which can, for example, be provided capacity on private leased lines for voice networking. Changing the on the NT Meridian Mail system. voice network over from a PBX to Centrex will not call for an immediate re-appraisal of the overlay data network. 35/6 Centrex A key benefit resulting from taking the Centrex route to ISDN is Mercury's initial service was limited to customers directly con- future-proofing of the corporate network. New ISDN features are nected to their central London optical fibre network. In four years, continually being defined and are likely to force expensive upgrades features have been enhanced to make sure that the service continues on PBXs. In contrast, upgrades by the PTO to central office switches to meet or exceed leading PBX offerings and tariff reductions have are immediately available to all users. made the service even more competitive against PBXs. Early users included members of the London financial community, who saw 35.2.4 Network management customer benefits as: The existence of a virtual private network means that the telephone Speed of installation and service; less than 4 months from company is able to offer customers advanced network management signing of lease to full operation in a new building. features. The user specifies the features required and retains direct Financial flexibility; the equivalent of a £2M installation with- control over their use. In addition, the operator can provide central out capital outlay. monitoring, measurement and control, allowing more sophisticated Operational flexibility; the ability to add or alter functions, and management than possible on a network of PBXs. The management to cope with expansion and possible office moves. systems will capture comprehensive details of every call, which can Maintenance; 24 hour monito