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use of IEEE 802.11 MAC protocol, when a node successfully obtains the channel and performs its transmission, any other node (either belonging to the same ow or other ows) that is within the node s transmission range (and the node s sensing range) should not perform any transmission and must defer their transmissions for a later time. Although this conservative transmission policy can reduce the chance of packet collisions, it introduces the exposed node problem [3]. An exposed node is a node that is within the transmission range of the sender but out of range of the destination. According to the above-mentioned transmission policy, the exposed node is restricted from transmitting even though its transmission will not hinder the communication between the sender and the destination. Therefore, the available bandwidth is underutilized. Moreover, contention allows an aggressive sender to capture the channel which reduces the chance of transmissions of the other senders in the vicinity. Inter ow and intra ow contention typically lead to increased packet transmission delays. Heavy contention is usually caused from using an inappropriately large congestion window or when a bad transmission scheduling policy is used. Previous studies have shown that the maximum congestion window size should be kept small and should be maintained at a level proportional to some fraction of the length (in terms of the hop count on the path) of the connection [4, 12] in order to alleviate the effect of contention. Unfortunately, as indicated in reference 4, conventional TCP does not operate its congestion window at an appropriate level and thus, its performance is affected by the contention severely. 9.3.4 Delay Spike Causes TCP to Invoke Unnecessary Retransmissions The RTT estimation of TCP is adequate in a stable network in which the RTT uctuations are small. However, a sudden increase in the packet transmission time will cause TCP to invoke spurious retransmissions [13]. A sudden delay spike may be caused by several factors: (1) a sudden change in the link quality that leads to a burst of transmission errors and many link-level retransmission attempts, (2) route changes or intermittent disconnections due to mobility that lead to a higher delay experienced by the transmitted packets, and (3) an increase in the contention along the route that leads to a longer waiting time before the packets in the queue are transmitted. Under these circumstances, the sender may not receive an acknowledgment within the timeout period and thus the sender will regard all the transmitted but not acknowledged packets as lost. As a consequence, the sender will unnecessarily reduce the sending rate and retransmit those packets that are deemed lost but are merely delayed. 9.3.5 Inef ciency Due to the Loss of Retransmitted Packet Due to the instability of routes and the error-prone wireless channel, packet drops can occur more frequently in MANETs (as compared to wireline networks). In other words, a TCP sender may need to perform more retransmissions in MANETs. However, most TCP implementations do not have a mechanism to ef ciently detect or recover from the loss of retransmitted packets without using the inef cient timeout
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VARIOUS TCP SCHEMES
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mechanism. If a retransmitted packet is lost, the TCP sender can only wait until an expiration of the retransmission timer to detect the loss. Since the retransmission timeout value is usually large (due to the doubling of RTO for every retransmission attempt), the extended waiting period of the TCP sender will lead to poor performance. In wireline networks, due to the low BER, one might expect that the chance of the retransmitted packet being lost again is extremely low and therefore the aforementioned problem is considered insigni cant. However, this problem can severely affect the performance of the TCP in MANETs due to the high BER of the wireless channel.
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9.4 VARIOUS TCP SCHEMES This section presents some of the major TCP enhancements that have been proposed to alleviate the TCP performance issues that were discussed in the previous section. In contrast with many of the previous efforts that classify and compare TCP enhancements by their type [5, 6] (i.e., end-to-end proposals, link-layer proposals, and split-connection proposals), they are classi ed as per the strategy used in order to improve the TCP performance. Strategy using which they offer improvements4 : 1. 2. 3. 4. 5. Estimating the available bandwidth. Determining route failures and wireless errors. Reducing contention. Detecting spurious retransmissions. Exploiting buffering capabilities.
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For each of the above strategies, the representative schemes are presented and their operating mechanisms are described, along with discussions on their strengths and weaknesses in improving TCP performance in MANETs. 9.4.1 Estimating the Available Bandwidth As discussed in Section 9.3, the traditional loss-based congestion control mechanism of TCP cannot accurately adjust the sending rate when it is used in MANETs. Packet loss is not always a sign of congestion; it could be due to mobility. To this end, several TCP schemes have been proposed recently to address the problem by estimating better the bandwidth of the connection. The representative schemes that take advantage of bandwidth estimation to enhance TCP performance include TCP-Vegas [14], TCPWestwood [15], and TCP-Jersey [16].
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4 Some schemes can be matched to several types of strategies but are only classi ed according to their main
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