6610-l14- - TCP over Wireless Networks Click to edit Master...

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Click to edit Master subtitle style 11 TCP over Wireless Networks
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Puzzle o In C, what is the output of the following code: main() { int a[5] = {0,1,2,3,4};
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33 Recap o TCP/IP/MAC o Wireless MAC o Wireless Scheduling o Mobility Support o Wireless Transport …
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44 Outline o TCP over wireless networks n TCP assumptions n Wireless network characteristics n Impact on TCP performance o Approaches to improve TCP performance n Link layer approaches n TCP-aware link layer approaches n Split connection approaches n End-to-end approaches n TCP over satellite and ad-hoc networks
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55 Wireless Characteristics o Low bandwidths n 10-20Kbps WWANs, 1-10Mbps WLANs, 100-500Kbps satellite o Random wireless losses n Upto 10% packet loss rates o Hand-offs n Depending on cell coverage and user mobility (30m/s in an 802.11 network will result in a hand-off every 10-15 seconds)
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66 Wireless Characteristics (Contd.) o Black-outs n Fading, temporary disconnections etc. n Can last from a few seconds to less than a minute o Large and Varying Delay n WWANs have typical RTTs of 400ms and deviations of up-to a few seconds o Path asymmetry n Reverse path characteristics different from forward path characteristics (e.g. satellite, WWANs)
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77 Characteristics (Contd.) o Local-area wireless n Frequent hand-offs when mobile o Ad-hoc wireless n Routers mobile! Frequent disconnections n Network partitions? o Satellite wireless n Large bandwidth delay products (why?)
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88 TCP on Wireless – Random Losses o TCP uses losses as indication of congestion o Reduces congestion window by half (LIMD) when it experiences congestion o Even when no congestion, if packets are dropped due to random losses, TCP will cut down its rate (is this right?)
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99 Other Losses o TCP will interpret hand-offs related losses also as congestion based losses o Hence, it will reduce the congestion window every time hand-offs related losses occur o Black-outs will further result in TCP experiencing multiple timeouts of increasing granularity
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1010 Large and Varying Delay o TCP uses RTTavg + 4 * RTTmdev as the retransmission timeout o If there is large variance in delay, mean deviation is high resulting in inflated timeout values o Hence, if there are burst losses resulting in a timeout, the sender would take longer time to detect losses and recover
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1111 Path Asymmetry o TCP relies on ACK arrivals for congestion window progression o If path asymmetry exists, a TCP connection’s performance will be influenced by the reverse path characteristics also o Indirect effects of path asymmetry (ACK bunching)
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1212 Low Bandwidths o TCP uses window based congestion control o If there is free space in the congestion window, TCP will transmit o Hence, TCP’s output can be bursty o This coupled with the low bandwidths can result in queue build-ups in the network adversely affecting RTT calculations and causing packet drops
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1313 Large Bandwidth Delay Products o TCP’s header has 16 bits allocated for receiver window advertisement o Maximum of 500Kb can be advertised
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This note was uploaded on 12/14/2009 for the course ECE 6610 taught by Professor Staff during the Fall '08 term at Georgia Institute of Technology.

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6610-l14- - TCP over Wireless Networks Click to edit Master...

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