ised server that is managed by the carrier. Real-time service control systems provide a single control point between the carrier network, content providers and the end-user. Such systems enable carriers to maintain control over services, customer relationships and billing, while offering a wide service. A central challenge for wireless data service carriers - maintaining control of user access to content and applications while offering personalisation and freedom of choice - is reflected in the developing service/billing models. Models used in fixed Internet service and at opposite ends of the spectrum are the "walled garden" and the "open garden" models. Under the walled garden model, users are restricted to a menu of services, within which users may use any service for some fixed price. 20 This model can entice new customers to wireless IP services. Under the open garden model, users may access all applications and content each for a premium. Charging schema may be based on time, content, QoS or other criteria. Between the walled garden and open garden extremes, users may define service boundaries and decide on the value of the services that they wish to buy based on charging parameters. Carriers will sign up customers under the walled garden model and seek to move customers through the middle ground to the open garden. A real-time service 19 When designing services with new technologies, marketing executives must seek desires felt by con- sumers which consumers themselves do not yet know that they have. This is a result of new technologies permitting services which did not previously exist and so which are beyond the imagination of many would-be customers. 20 This is similar to the model of some ISPs where particular web content is available exclusively to subscribers. 6 Service Provisioning and Activation control system allows a carrier to support all models simultaneously because any particular customer can be signed for a unique, customised service combination. The success of SMS prompted service providers 