This paper reports on the design and implementation of an IP-centric vertical handoff decision algorithm and execution scheme between a wireless wide area network (WWAN) and a wireless local area network (WLAN) as part of the "IP-centric Field Workforce Automation". The project aims to ensure free movement of field workers between networks while always connected in an IP mode, and to allow a user to use one device across many types of networks. We use a fuzzy logic inference system to process a multi-criteria vertical handoff decision metrics, and present a mobility management scheme for IP-based networks using the transport layer protocol SCTP and the application layer protocol SIP. The multi-homing capability and DAR extension of SCTP are applied to provide vertical handoff between heterogeneous networks. SIP is used to provide location management when a fixed node initiates an association with a mobile node.