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In recent years, many different kinds of wireless access networks have been deployed for the Internet and have become inseparable parts of the Internet. But TCP, the most widely used transport protocol of the Internet, was designed for stationery hosts. In particular, TCP faces severe challenges when user moves around in these networks and handoff occurs frequently.In this paper, TCP HandOff (TCP-HO), a practical end-to-end mechanism, is proposed for improving TCP performance in heterogeneous mobile environments. TCP-HO assumes that a mobile host is able to detect the completion of handoff immediately and has a coarse estimation of new wireless link's bandwidth. When a mobile host detects handoff completion, it will immediately notify the server through two duplicate ACKs, whose TCP option also carries the bandwidth of new wireless link. After receiving this notification, the server begins to transmit immediately and keeps updating ssthresh according to the bandwidth from mobile host and its new RTT samples. This updating will be stopped after four RTT samples or after congestion is detected.TCP-HO has been implemented in FreeBSD 5.4. Experimental results show that TCP-HO does improve TCP performance without adversely affecting cross traffic in a heterogeneous mobile environment.

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Abstract-A wireless sensor network is normally a multi-hop wireless ad hoc network whose purpose is to transmit events sensed by sensors to the sink. Since sensor nodes are normally powered by non-rechargeable and non-replaceable battery, an energy efficient event transport service is crucial for extending the life time of a wireless sensor network.Due to the possibility of crossing multiple lossy links between the source and the sink, many events may be lost before they arrive at the sink. In addition, the overhead of low layer protocols is very large due to the short event data. These two factors can result in significant event loss and waste of energy. Many mechanisms have been proposed on other networks to improve reliability and reduce overhead. Due to scarce memory and poor computing ability of sensor nodes in a sensor network, these mechanisms may not be feasible to implement and they may not work efficiently with respect to energy.In this paper, PMC (Pipelined Multiple Copies) is proposed for transmitting events in a wireless sensor network, with the goals of achieving low event loss rate and high energy efficiency. In PMC, multiple copies of an event are pipelined and transmitted in several consecutive frames according to Silent CSMA, a variant of CSMA/CA. Multiple copies of an event reduce end-to-end event loss rate and Silent CSMA reduces packet energy consumption significantly. Mathematical analysis shows that PMC achieves the best trade-off between packet energy consumption and event loss rate compare to the Silent CSMA and CSMA/CA/ARQ schemes. It also shows that PMC is really an energy efficient event transport service for resource-limited wireless sensor networks.