nt each other to meet both the user's erable resource, financial and human. This coincides with these and the PTO's requirements for service and ease of deployment. The organisations seeking to create strategic advantage in their core ISDN interface is an enhancement to the business set interface, business activity using sophisticated telecommunications. In the which in turn offers an improvement in functionality over the POTS UK this has resulted in a proliferation of private networks and large interface. departments to manage the telecommunications service. An alterna- The attraction of the POTS interface stems from its historical tive service from the public network which fulfils the same require- standardisation and simple implementation at both the subscriber ments allows these organisations to outsource their and central office ends of the loop. The limitations of the interface telecommunications and concentrate on their core, revenue genera- result from its single analogue channel. Consequently, multiple line ting, activities. An additional benefit for multi-sited corporations is appearances require a key system at the customer premises. Limita- that their various sites can be linked together over a public telephone tions of the interface also make access to most Centrex features network based on sophisticated central office exchanges such as the difficult; the user has to use 'feature' codes, for example, *74" is Northern Telecom DMS range, to form a 'virtual private network' used to invoke call forward. With the more advanced sets users can (VPN). This gives a simpler internal dialling plan as well as network take advantage of assignable feature keys, which eliminate the need wide use of features such as call forwarding, ring again and call to remember the feature access codes. Some terminals also offer the waiting. user hands-free capability, message waiting features, line status Centrex is also attractive to business that do not have enough lines monitoring and visual ringing. However, these features a