System Description with Tight Coupling
Figure 22.6 shows the system architecture with tight coupling.
Figure 22.6 WLAN-GPRS integration with tight coupling system configuration.
A WLAN is deployed with one or more off-the-shelf access points, which are connected by means of a distribution system. The distribution system is LAN-compliant with IEEE 802.3. The WLAN is deployed in an infrastructure configuration, that is, APs behave like base stations, and mobiles exchange data only with APs.
The service area of an AP is called a basic service set. Each WLAN is typically composed of many basic service sets, which all together form an extended service set (ESS) (see Chapter 21).
The WLAN network is deployed as an alternative RAN and connects to the GPRS core network through the standard Gb interface. From the core network point of view, the WLAN is considered as other GPRS routing areas (RAs) in the system. The GPRS core network does not identify the difference between an RA with WLAN radio technology and one with GPRS radio technology.
The key functional element in the system is the GPRS interworking function (GIF), which is connected to a distribution system and to an SGSN via the standard Gb interface. The main function of the GIF is to provide a standardized interface to the GPRS core network and to virtually hide the WLAN particularities. The GIF is the function that makes the SGSN consider the WLAN a typical routing area.
The existing GPRS protocols in mobile are fully reused. The LLC, subnetwork dependent convergence protocol (SNDCP), GPRS mobility management (GMM), and session management are used in both a standard GPRS cell and a WLAN area. Therefore, the WLAN merely provides a new radio transport for these protocols.
When a mobile station (MS) is outside the WLAN area, its WLAN interface is in passive scan mode, that is, it scans a specific frequency band and searches for a beacon signal. When a beacon is received the service set identifier (SSID) is checked and compared against a pre-configured SSID. The SSID serves as a WLAN identifier and can help mobiles attach to the correct WLAN. For example, an operator could use a unique SSID and request that its subscribers confi gure their mobiles to consider only this SSID valid.
When an MS detects a valid SSID, it performs the typical authentication and association procedures. It then enables its WLAN interface, and further signaling is carried over this interface.
Mobile stations are dual mode, that is, they support both GPRS and WLAN access in a seamless fashion. System mobility is achieved by means of the routing area update (RAU) procedure, which is the core mobility management procedure in GPRS.
Typically, when a mobile enters a WLAN area, an RAU procedure takes place, and subsequent GPRS signaling and user data transmission are carried over the WLAN interface. Similarly, when the mobile exits a WLAN area, another RAU procedure takes place, and the GPRS interface is enabled and used to carry further data and signaling traffic. From the core network point of view, handoff between WLAN and GPRS is considered handoff between two individual cells.
Mobile stations in the WLAN send uplink GPRS traffic to the MAC address of GIF; similarly, downlink GPRS traffic is sent from the GIF to the MAC addresses of mobile stations (see Figure 22.6). The MS has two radio subsystems, one for GPRS access and another for WLAN access (refer to Figure 22.7).
22.7 Tight coupling over Gb interface: a reference diagram.
The WLAN adaptation function (WAF) identifies when the WLAN radio subsystem is enabled (i.e., when the MS associates with a valid AP) and informs the LLC layer, which subsequently redirects signaling and data traffic to the WLAN. Note that all standard GPRS protocols operating on top of the LLC (SNDCP, GMM, SM) function as usual and do not identify which radio subsystem is used. WAF is a key component in a mobile station.