Improving the performance of TCPIP Over Wireless Links

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Improving the performance of TCP/IP Over Wireless
Links

• Presented By
• 
  Aditya Mahajan
• Nabhendra Bisnik

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TCP/IP over Wired Links

• TCP designed to provide reliability over Internet
• Low Pe due to channel error
• Most packet loss due to congestion
• TCP Congestion Control Algorithms assume that if
  the ACK does not come, the packet was lost due to
  congestion.

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Congestion Control in TCP

• Slow Start
• Congestion Avoidance
• Fast Retransmit
• Fast Recovery

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Trouble with Wireless Links

• High BER
• Pe due to channel error is large
• False Activation of Congestion Control Mechanism
• Low Throughput
• Handoff
• Packet Loss due to Handoff

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ssthresh
cwnd
6
ssthresh
cwnd
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Packet Loss Due to Network Congestion cwnd
reduced Load on Network Reduces
ssthresh
cwnd
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ssthresh
cwnd
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ssthresh
cwnd
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Channel Error in Packet Misinterpreted as
network congestion cwnd size reduces
ssthresh
cwnd
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Mechanisms to Improve Performance

• End to End Schemes
• Distinguish between congestion losses and
  non-congestion losses
• Split Connection Schemes
• Hides the unreliable wireless link from the wired
  host
• Link Layer Schemes
• Hides losses from TCP sender by local
  retransmission and FEC

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End-to-End Schemes

• Explicit Loss Notification (ELN)
• Selective ACK (SACK)
• Drawbacks
• Complete TCP stack has to be changed for all
  connected hosts.

Can only be used in Isolated Networks
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Split Connection Schemes

• Splits the end-to-end TCP connection at base
  station
• M-TCP
• Mobility Support Router (MSR)
• Drawbacks
• End to end semantics are violated
• Large delay and Latency during handoffs
• Each packet goes through TCP processing twice

TCP implementation needs to be changed only at
the BS and MH
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Link Layer Schemes

• Hide link related losses from TCP sender by using
  local retransmission.
• AIRMAIL (FEC)
• Snoop (Duplicate ACK suppression)
• Drawback
• Cache of Unacknowledged packets maintained at BS

Gives better performance than the other schemes
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W-TCP

• Uses Header Compression
• Adaptive MAC frame size
• QoS can be implemented
• Direction Based Selective Multicast

Same drawbacks as Split-Connection
Enhancement in performance due to Link Layer
retransmission
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Wireless Link
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Conclusion

• TCP/IP Performance can be improved by
  modification in TCP
• All schemes have some advantages and drawbacks
• W-TCP gives best performance by combining
  advantages of all schemes.

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• THANK YOU