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Vertical Optimization Of Data Transmission For Mobile Wireless Terminals MICHAEL METHFESSEL, KAI F. DOMBROWSKI, PETER LANGENDORFER, HORST FRANKENFELDT, IRINA BABANSKAJA, IRINA MATTHAEI, AND ROLF KRAEMER, IHP IEEE Wireless Communications • December 2002 邱家偉 Outline Introduction Protocol Optimization On A Mobile End Device Optimization Strategies For The Mobile Device Interference Between TCP And The Wireless Link Simulation Setup: TCP Running Over The IEEE 802.11 MAC Optimizing The Uplink Retransmission On The TCP And MAC Layers Packet Fragmentation Summary And Conclusions Introduction The integration of wireless and mobile systems. A major bottleneck is enough processing power and battery life. This article discusses a more conservative approach in which only the mobile device is available to be modified. Optimization of the wireless communication in the uplink and in the downlink. Protocol Optimization On A Mobile End Device The end-to-end TCP connection between a mobile device and a remote server runs through a wireless link in a local WLAN and the Internet. Protocol Optimization On A Mobile End Device TCP runs as an end-to-end protocol on the mobile device and server, while a wireless protocol such as IEEE 802.11 controls transmission over the local wireless link. Data transmission through the Internet can involve longer and unpredictable delays, and can collapse when congestion occurs. Optimization Strategies For The Mobile Device A main part of the incentive for mobile devices is the ability to roam through different networks. The mobile device is completely available for modification as long as it obeys the MAC and TCP/IP standards itself. If control is only over the mobile device, it cannot be assumed that the base station will do the same in the downlink. The base station will choose its own transmission characteristics by an algorithm. Interference Between TCP And The Wireless Link The throughput can be decreased drastically even if only a few packets are lost in the channel. Consequently, losses in the wireless transmission are incorrectly interpreted as a sign of congestion, leading to a reduction of the data flow by the TCP congestion control mechanism. The fast retransmit mechanism is not able to cope, and data transfer recommences only after a retransmission timeout (RTO). Interference Between TCP And The Wireless Link Each value shown is the average of 10 simulations for the transfer of 2 Mbytes. The TCP packet length was 500 bytes. Simulation Setup: TCP Running Over The IEEE 802.11 MAC Use the BONeS tool A C implementation of TCP in the New Reno variant was used. It has been checked by extensive interoperability tests against Linux, FreeBSD, and Microsoft versions of TCP under a lossy channel. The MAC layer was implemented in the BONeS language, based on a model of the PHY layer that takes account the correct timings and delays. Simulation Setup: TCP Running Over The IEEE 802.11 MAC The network delay was kept at 1000 ms, and the network data rate was set to 0.08 Mb/s, yielding a pipe capacity of 20 kbytes. The data rate of the wireless transmission was set to 2Mb/s. This parameter does not play a very large role since the overall rate is restricted by the much lower network data rate. The main difference is that data is sent in separated bursts instead of a continuous stream. Optimizing The Uplink Considerable earlier work has been done on adaptive link layer strategies as a means to improve transmission over a timevarying faulty wireless channel. By sensing the quality of the channel and adjusting the transmission characteristics accordingly, power can be saved and data transfer improved. Specific mechanisms include packet fragmentation, switching to a slower but more robust data rate, modifying the error correction mechanism, and channel probing to detect the end of a channel downtime efficiently. Optimizing The Uplink One can increase the error correction only at the price of a lower data rate, and fragment the packets only at the price of increased overhead. A decision must be made based on at least two thresholds: how bad the channel must be before switching to a different method and the maximum permitted transmission delay. Retransmission On The TCP And MAC Layers The MAC layer discards a packet, the consequence is that TCP must retransmit it, albeit at a later time. It is the delays associated with these events (retransmission& fast) that cause the reduced data rate when LRL is smaller. Timeouts at the TCP level are generally much larger than the delays in the wireless channel, so there is enough time available for numerous attempts. Retransmission On The TCP And MAC Layers TCP throughput as a function of the BER in the wireless channel for various values of the MAC long retry limit. Retransmission On The TCP And MAC Layers Total number of bytes transmitted into the wireless channel as a measure of the consumed energy Packet Fragmentation In the case of good channel conditions, the only effect of reducing the packet length is to increase the overhead needed to transmit a certain chunk of data. The packet length can be changed by either reducing the size of a TCP packet or introducing fragmentation at the MAC level. That basically the same conclusions apply when the focus is on energy consumption instead of throughput. Summary And Conclusions Specific targets are to reduce power consumption, increase throughput and availability, and reduce delay and jitter. As a major benefit, it is then possible to optimize the system performance without any modifications to the base stations or to the TCP protocol on the remote hosts.