Coupling of Wireless LANs like IEEE 802.l1a1b or HIPERLANI2 systems with Public Land Mobile Networks (PLMNs) like GSM or UMTS offers benefits for both operators and users. Coverage and capacity of an operator's network can be extended with equipment expected to be considerably cheaper than for PLMN coverage. Thus, the operator can participate in the expected WLAN boom. Its customer experiences homogenous access to services with high data rate. PLMNs and WLANs should be considered as complementary systems: PLMNs provide universal coverage and high mobility support, while pico-cellular WLANs will be applied in hot spot areas, offering high data rates (up to about 45 Mbitls/cell in HIPERLANI2). Several levels of interworking are currently being defined by the relevant standardisation bodies. Tight and Very Tight Coupling integrate the WLAN into a unified Radio Access Network. But PLMNs and WLANs are based on different design philosophies. Chances, problems, and possible solutions are discussed in this paper.

This paper presents handover procedures for the Broadband Cellular ATM Access (C-ATM) system which shall provide wireless ATM access for mobile users. Beside the description of the architecture and protocol stack of the system this paper describes message sequence charts for two different handover types: radio and network handover. This differentiation allows most of the handovers to be performed seamlessly, i.e., without loss of ATM cells. This feature is especially important for real time applications such as video.

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