TCP Performance in Wireless multi-hop Networks

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Introduction

  
  • Early research showed TCP suffers poor Performance in wireless networks because of packet losses and corruption caused by wireless inducted errors
  • Further studies searched for mechanism to improve TCP performance in cellular wireless systems
  • Other researches investigated other network problems that negativly affect TCP performance, such as bandwidth asymmetry and large round trip times, which are prevalent in satelite networks
  • During the presentaition we adress another network charackteristic that impacts TCP performance, which is common in mobile ad hoc networks: link failures due to mobility
  • First present performance analysis of standart TCP over mobile ad hoc networks
  • Then present analysis of the use of explicit notification techniques to counter the affects of link failures
  • Simulation Environment and Methodology

      
  • For simulations the ns network simulator from Lawrence Berkles National Laboratory was used, with extensions from the MONARCH project at Carnegie Mellon
  • Extensions include a set of mobile ad hoc network routing protocols, an implementation of BSDs ARP protocol, an 802.11 MAC Layer, a radio propagation model and mechanisms to model node mobility using pre-computed mobility patterns that are fed to the simulation at run time
  • No modifications were made to the simulator (accept minor bug fixes that were necessary)
  • All results based on a network configuration consisting of TCP-Reno over IP on an 802.11 wireless network, with routing provided by the Dynamic Source Routing (DSR) protocol and BSDs ARP protocol (used to resolve IP adresses to MAC adresses)
  • Objective was to observe TCPs performance in the presence of mobility inducted failures in a plausible network environment, for which any of the proposed mobile wireless ad hoc routing protocols would have sufficed
  • Network model consists of 30 nodes in 1500x300 meter flat, rectangular area
  • Nodes move according to random waypoint mobility model
  • In random waypoint model each node x picks a random destination and speed in the area and travels to destination in straight line
  • Once x arrives, it pauses, picks another destination and goes on
  • No pause, so every node is always in moving
  • All nodes communicate with identical half duplex wireless radios, which are modeld after 802.11 based Wave Lan wireless radios, with a bandwith of 2Mbps and nominal transmission radius of 250m
  • TCP packet size was 1460bytes, maximum window was eight packets
  • All simulation results based on average throughput of 50 scenarios or patterns
  • Each pattern, generated randomly, designates the initial placement and heading of each of the nodes over simulated time
  • Used same pattern for different mean speeds
  • For a given pattern at different speeds, same sequence of movements (and link failures) occur
  • Speed of each node is uniformly distributed in an interval of 0,9v - 1,1v for some mean speed v
  • Performance Metric

      
  • Throughput as performance metric used
  • TCP throughput ussually less than optimal due to TCP senders inability to accurate determine the cause of a packet loss
  • TCP sender assumes that all packets losses are caused by congestion
  • When link on TCP route breaks, TCP sender reacts as if congestion was the cause, reducing its congestion window and, in instance of a timeout, backing-off its retransmission timeout (RTO)
  • Therefore, route changes due to host mobility can detrimental impact on TCP performance
  • To gauge impact of route changes on TCP perfomance, we derived an upper bound on TCP throughput, the expected throughput
  • TCP throughput measure obtained by simulation is then compared with expected throughput
  • Expected throughput was obtained as the following:
    - First simulated a static (fixed) network of n nodes that formed a linear chain containing n-1 wireless hops
    - Nodes used 802.11 protocol for medium access
    - Then a one-way TCP data transfer was performed between the two nodes at the ends of the linear chain, and the TCP throughput was measured between these nodes
  • HopsThroughput (Kbps)
  • Figure 1 shows measured TCP throughput as a function of number of hops, averaged over ten runs
  • Throughput decreases rapidly when number of hops is increased from 1, then stabilizes once the number of hops becomes large
  • 11463.0
    2729.0
    3484.4
    4339.9
    5246.4
    6205.2
    7198.1
    8191.8
    9185.3
    10182.4

    Measurement of TCP-Reno Throughput

    Mobility Induced Behaviours

    TCP Performance Using Explicit Feedback

    Split-TCP

    Conclusion

    References