3 g Wireless

Seamless Handoff Between Heterogeneous Wireless NetworksHaipeng Jin and Kameswari Chebrolu, UCSDAditya Pande and Xiaohua Chen, Stanford UniversityJavad Razavilar, Magis Networks Inc., San DiegoBarani Subbiah, 3Com Corp., Santa Clara

IntroductionFuture Wireless systems will be characterized by their heterogeneity.Multiple access technologies provide access to internet content.Indoor Environment 802.11, Bluetooth, HomeRF, IrDA etc.Outdoor Environment GPRS, CDMA2000, Satellite etc.Systems differ in terms of coverage, bandwidth, delay, cost etc.

Universal Wireless Communication SystemAnytime, Anywhere CoverageThree Layers of Coverage coverage decreases from top to bottom.Topmost Layer Satellite SystemsGlobal CoverageMiddle Layer Cellular NetworksMetropolitan AreasBottom Layer Local Area NetworksHot Spot CoverageA common IP based network will connect them to each other.

RequirementsMultimode terminalsSeamless inter-system (Vertical) handoffService negotiation across different systemsSeamless intra-system (Horizontal) handoff

Our FocusVertical HandoffIdentify issues related to handoff between WLAN and WWANRequirements for handoff: small delay, low packet lossImplement a testbedNetwork Layer : Mobile IP with extensionsLink Layer: Handoff Algorithm

Mobile IPIETF standard , handles mobility across IP subnets at the network layer.Mobile Terminal (MT) communicates with other nodes using its home address.MT acquires care-of address and registers with Home Agent (HA).HA intercepts packets destined for the MT and tunnels them.Route optimization - avoids triangle routingCH learns the MTs current care-of address and tunnels packets directly.

Mobile IPCHHAFAMTPackets destined for mobilePackets in reverse directionEncapsulated packets

ArchitectureSeamless handoff between WLAN (IEEE 802.11) and WWAN (CDMA PCS). Mobile IP based ExtensionsBefore handoff, MT notifies old Foreign Agent (FA) to stop transmitting and buffer packets.Connection established to new FANew FA contacts old FA which passes the buffered packets.MT communicates with Correspondent Host (CH) through the new FA.

Handoff Algorithm

DetailsBased on measurement of WLAN signal strength, Signal-to-Noise Ratio (SNR)Connected to WLANSNR above CThresh, MT stays connected to WLANSNR falls below CThresh, MT scans for WWAN services and makes initial connectionsSNR falls below DThresh, MT will switch from WLAN to WWAN

ContConnected to WWANSearches for WLAN signal periodicallySNR above CThresh, MT switches from WWAN to WLANTwo thresholds defined to avoid Ping-Pong effectAn exponential moving average of the SNR is used.Only SNR of the WLAN is monitored as MT prefers WLAN because of the high data rate

Testbed

TestbedTestbed built at 3Com CorpA laptop equipped with 802.11 and CDMA PCS network interfaces served as MT.Protocol Mobile IP with extensionsHome Agent was not implemented.Handoff related processes were implemented using C under linux platform.A text based application was used in the demonstration.

Results Handoff delay-time elapsed between the last packet received on the old connection to the first packet received on the new connection.Handoff delay was about 600ms when switching from WWAN to WLANHandoff delay was about 1040ms when switching from WLAN to WWAN.

SNR measurement on WLAN

Switching from WLAN to WWAN

Delay on WLAN

Delay while handoff

ConclusionsA handoff algorithm to provide seamless roaming between WLAN and WWAN was proposed.Field test results showNo unnecessary handoffs Small packet loss and delay