**3.2 L2 handoff procedure**

In this section, we describe the proposed L2 handoff procedure. We consider two handoff scenarios. The first case is that the mobile station is moving from cdma2000 cellular network into WiBro. The second one is the reverse case. We do not consider the cases that handoff is occurring within its own wireless network, i.e., cdma2000 or WiBro. We also do not consider the initial call setup procedures for its own wireless network, since it is followed as described in standardization of each wireless network.

Fig. 4 describes the proposed L2 handoff procedure. When the mobile station moves into a new network area, it processes L2 connection procedure. At this time, handoff information is transmitted to the network through the Origination message of cdma2000 or L2 REG-REQ message of WiBro as shown in (1) of Fig. 4(a) and (1) of Fig. 4(b), respectively. More specifically, the Origination message includes PANID (Previous Access Network ID). If the cdma2000 system receives the Origination message which contains PANID=ANID of WiBro, the cdma2000 system regards it as a vertical handoff from WiBro network. Similary, the REG-REQ message of WiBro can contain PANID=ANID of cdma2000 which means a vertical handoff from cdma2000 network. With such handoff information, the source PDSN or ACR detects the occurrence of handoff and extracts the target ACR or PDSN address for the handoff. Based on this address information, PDSN and ACR requests each other and generates tunnel for the handoff traffic. After the tunnel is setup, the corresponding PDSN or ACR is buffering packets destined to the mobile station while the requested L2 connection is being made.
