The MS supports two radio subsystems (or interfaces) for transporting GPRS signaling and user data (see Figure 22.8).
Figure 22.8 Protocol stack for tight coupling over Gb interface.
The first interface is implemented with the GPRS-specific radio link control (RLC)/MAC and physical layers, whereas the second interface is implemented with 802.11-specific MAC and physical layers. These two interfaces provide two alternative means for transporting LLC packet data units (PDUs).
Typically, when the MS is outside a WLAN area, LLC PDUs are transmitted over the GPRS interface (Um). However, when the mobile enters a WLAN area, LLC PDUs are transmitted over the WLAN interface. This switching is performed with the help of WAF and could be completely transparent to the user and to the upper GPRS layers.
The WAF operates in both MS and GIF. It provides an adaptation function for interworking between LLC and 802.11 MAC (in the mobile) and between 802.3 MAC and BSSGP (in GIF). The GIF also implements the GPRS protocols specified on Gb interface, frame relay (FR), network service (NS), and base station subsystem GPRS protocol (BSSGP). The AP implements the 802.11 and 802.3 protocols and a simple interworking function that provides bridging between them.