1. DataTAC 5000 Network Base station ARLAN 640 ARLAN 690 InfoTAC Mobile workstation Server InfoTAC Ethernet Wireless LAN SAHWW Architecture

2. Mobile WorkstationServer Application Layer handover initiation signal handover request / disconnect request handover initiation signal handover request ; number of last packet received System Agent -client Handover Agent -client Data Link Layer Data Link Layer WLAN Interface WAN Interface Application Layer System Agent -server Data Link Layer Data Link Layer WAN Interface WLAN Interface beacon client beacon server recovery request ; number of last packet received buffer Handover Agent -server beacon no beacon / beacon detected signal signal acknowledgment Architecture of WLAN-WAN handover

3. Handover from WLAN to WAN  Beacons are continuously transmitted from beacon-server to beacon-client through WLAN. The interval between two beacons it t ms.  When beacon-client cannot hear n consecutive beacons, the wireless LAN connection is assumed to be broken.  “No beacon” signal is sent to handover-agent-client (HA- client) which would send handover initiation signal to application layer.  “Wireless LAN link failure” message would be displayed on the user screen and the handover to WAN decision is left to the user.  Two types of messages could be passed from the application layer to HA-client:  Handover request  Disconnect request

4. Handover from WAN to WLAN   Beacon-client and beacon-server would try to communicate through the WLAN. If n consecutive beacons are received by the beacon- client, WLAN link is assumed to be ready. Then the connection would be switched back to WLAN - reverse handover to WLAN.   ‘Beacon detected’ signal would be sent to the HA-client which would send the signal to the SA-client.   SA-client would stop receiving data and send the handover signal together with the number of last packet received to SA-server. the message would pass through the WAN connection.   HA-client would switch the network connection from WAN to WLAN and listen to the WLAN link.   On the server side, after the SA-server has received the message, it would send the handover request to the HA-server. It would instruct the HA-server to switch to WLAN.   An acknowledgment would be sent back to the SA-client.   SA-server would start to re-send the packets from the indicated number.   The process is resumed through WLAN connection.