We propose a hybrid coupling scheme to support interworking between UNITS and WLAN networks. Under the Tight-coupled system, it is expected that WLAN users can also use UNITS services with guaranteed QoS and seamless mobility. However, the interworking is problematic. The capacity of UNITS core network nodes cannot accommodate the bulky data traffic from WLAN, since the core network nodes are designed to handle the small-sized data of circuit voice calls or short packets. The proposed coupling scheme differentiates the data paths according to the type of the traffic and can accommodate traffic from WLAN efficiently, with guaranteed QoS and seamless mobility. We compare the handover procedures of the proposed coupling strategy with those of the loose and tight coupled schemes. In addition, we analyze the delay based on signaling costs during vertical handover. It is shown that the handover latency decreases when the UNITS and WLAN are coupled in the proposed way. (C) 2006 Elsevier GmbH. All rights reserved.