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A comparison of mechanisms for improving TCP performance over wireless links

Satisfactory Essays
The paper focuses on the Logical link control(LLC) part of the link layer without taking into account improvements that can be made at the Medium access control(MAC) part to reduce non-congestion related errors. A major reason why Ethernet provides better performance is due to a robust collision detection mechanism called carrier sense multiple access(CSMA) with collision detection(CD). This layer prevents hosts from transmitting simultaneously on the channel resulting in damaged frames. So it doesn't allow damaged packets to propagate up the stack and cause unnecessary retransmissions at the LLC layer which can interfere with TCP retransmissions. We know that collision detection is harder in wireless networks due to the “hidden-terminal” problem. But we can still use “collision avoidance” mechanisms to ensure that no two hosts transmit simultaneously. Collision avoidance mechanisms can be conservative – that is they have back off timers that gradually increase the back off intervals when an ack is not received. Instead of backing off, we can employ a more aggressive approach, so that frames are transmitted as soon as an idle channel is sensed. If there is an error, then the sender will not hear an ack within the timeout interval and can retransmit. Issues with End-to-end schemes The major issue in this scheme is that the paper assumes perfect knowledge about wireless losses to generate explicit loss notification(ELN information). This information is then propagated to the TCP sender which takes corrective action based on the type of error. In a real world implementation, it is impossible to accurately identify packets lost due to errors on a lossy network. This is mostly because the only other entity between the source and the ... ... middle of paper ... ...acket. This will require the TCP layer to be enhanced to identify and handle different types of errors. 802.11 uses CSMA/CA to sense if the channel is idle before making a transmission. The exponential back off strategy can adversely impact performance. So in situations where there are more than a certain number('N') of hosts accessing the same wireless link, we can have an arbiter that decides which host has access to the link. In situations where there aren't enough hosts, they can fall back to an aggressive version of CSMA that doesn't have large back off timers. A hybrid approach like this will help reduce non-congestion errors on a wireless link and boost TCP performance. A robust error handling mechanism will make sure that the TCP congestion window size doesn't fluctuate unnecessarily and also avoid duplicate retransmissions and competing retransmissions.
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