et- works. This integration means that IP-PBX features can be available to mobile workers on the road. In addition, productivity can be greatly enhanced by having only one device, one phone number, and one voicemail account to manage, while enabling access to enterprise data from many locations. With such feature integra- tion, the mobile worker has the same level of functionality, accessibility, and pro- ductivity on the road as in the office. Cellular Carrier Cooperation with VoWLAN Aside from technology issues, the real issue that will determine the success of WLAN/cellular integration is whether the cellular carriers will cooperate. The handset Motorola has developed is GSM compatible, but it is unclear how many carriers will actually offer VoWLAN services. The "free" model associated with WLANs is not the traditional philosophy pursued by carriers. In addition, Wi-Fi roaming threatens to take minutes off the cellular carriers' networks. Another potential stumbling block is that many of the emerging handsets, such as the one from Motorola, require Generic Access Network (GAN). GAN technology provides access to GSM and GPRS mobile services over unlicensed spectrum technologies, including Bluetooth and 802.11. By deploying GAN technology, service providers can enable subscribers to roam and hand over between cellular networks and pub- lic and private unlicensed wireless networks using dual-mode mobile handsets. Because WLAN implementation is transparent to carriers, they may not be able to do much about it. In a typical operation, when a call is transferred from a cellular network to a WLAN, all the carrier knows is that the subscriber hung up. It has no way of knowing that the call was actually continued on the WLAN. Similarly, when a call is transferred to a cellular network, the carrier simply sees a new call origination. It has no idea that a WLAN handoff has occurred. Although calls can always be transferred from a WLAN to a cellular network, the reverse is not true. For cellular calls to be transferred in