Improving reliable transport and handoff performance in cellular wireless networks Hari Balakrishnan Srinivasan Seshan and Randy H Katz Presented by Liang Chen Outline Problems in cellular wireless networks Local retransmission Low latency handoff Measurements Conclusion Discussions Problems in cellular wireless networks Ordinary TCP is turned for wired network so that it assumes packet losses are caused by congestion TCP has no knowledge of network condition when it waits for ACKs When packet losses occur in the wireless link the reactions of ordinary TCP causing a significant degradation in throughput and interactive delays Network type Main reason for packet loss Solution wired congestion Reduce transmission rate wireless bit error and handoff Retransmit as soon as possible How to effective recover from bit errors in wireless networks Goals Improve the end to end performance on networks with wireless links Don t change existing TCP implementation at hosts in the fixed network Don t recompile or re link existing applications Ideas Cache packets at the base station Base station retransmits unacknowledged packets locally across the wireless links Wired Networks Base Station Fixed Host FH Mobile Host MH Local Re transmission across the wireless link FH BS with TCP packet cache Wired Network TCP SEQ 1000 TCP SEQ 2000 TCP SEQ 3000 MH TCP SEQ 1000 TCP SEQ 2000 TCP SEQ 3000 TCP ACK 2000 TCP ACK 2000 TCP SEQ 4000 TCP ACK 2000 TCP SEQ 2000 TCP SEQ 3000 TCP SEQ 4000 TCP ACK 5000 TCP ACK 5000 TCP SEQ 10000 TCP SEQ 10000 TCP SEQ 12000 TCP SEQ 11000 TCP SEQ 12000 TCP NAK 11000 TCP SEQ 11000 TCP SEQ 11000 data Control message Flow charts of snoop module at the base station Low latency handoff Problem Traffic is disrupted when MH switches between base stations Ideas Reduce message transactions and packet forwarding delay during handoff HA multicasts MH s packets to the neighbors or the primary BS Each neighbors cache the most recent packets When MH switches to the new BS the new BS forwards its buffered packets to MH Handoff routing Measurement of TCP Throughput with Bit Error 1 Measurement of TCP Throughput with Bit Error 2 A poisson distributed bit error rate of 3 9 10 6 Measurements of Handoff Performance 1 Measurements of Handoff Performance 2 Handoff occurs every 10 seconds Measurements of Handoff Performance 3 Traffic distribution time 3 20ms Performance with bit error and handoff Conclusions The cache at BS and the local retransmission across the wireless link increase the throughput when packets lost at the wireless links Retransmit packets as soon as possible Maintain the TCP window size at end hosts The multicast groups among neighboring BSs reduce the handoff time of MH Discussions The comparison of link layer retransmission The paper did not evaluate the affects of their proposal with congested wired network How can we reduce the MH s TCP window size when MH sends packets to FH across a congested wired network UDP traffic No ACK information for local retransmission Can MH communicates with two BS simultaneously Configuration of multicast groups The handoff delay can be further reduced with multiradio MH
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