S to GSM and vice versa. The handover function is vital for the offered QoS. 14.3.4 WLAN ISP (WISP): ROAMING Islands of public WLAN coverage have already established roaming between their respec- tive networks, based on unilateral agreements. There are also companies establishing themselves as intermediary roaming players. That can be a choice for many WISP operators who are very local or who want to establish global roaming quickly. Supposing that visited and home WISPs do not have any roaming relations; the roaming broker can make direct agreements with each involved WISP. 14.3.5 MOBILE OPERATOR: WLAN ROAMING For a mobile operator, the well-established GSM (and GPRS) roaming concept can be extended to also cover WLAN roaming traffic. WLAN roaming traffic costs can be integrated with other GSM roaming traffic charges and billed on regular invoices by the home operator. Understanding Changing Telecommunications - Building a Successful Telecom Business 14.3.6 CHARGING The servers provide flexible charging mechanisms. The existing charging capabilities allow the operator to charge a subscriber for network access and network usage. The system collects data, creates call detail records (CDR) and delivers them to external media, such as billing systems or a mediation system that can process the collected data for different purposes. Charging is treated in more depth in Chapter 5. 14.3.7 SECURITY AND QoS CSCF provides predefined PDP context parameters via SIP signaling to user equipment. Just as with dial-up roaming, proxy RADIUS is a common method for remote authen- tication for WISP roaming - at least in the short to medium term (See also Chapters 6 and 7). 14.3.8 ROUTING AND NUMBERING/ADDRESSING See Chapter 8. 14.3.9 IN SERVICES In general IN services were successful during the 1990s. However, many services have lost popularity with some exceptions, for example: cashless calling with prepaid, payment by card or billing afterwards. Prepaid is a 'killer application' access screening number portability freephone, which