Improving the Performance of TCP in Wireless Communications
Date Issued
2004
Date
2004
Author(s)
Huang, Jeng-Ji
DOI
en-US
Abstract
Growing popularity of both wireless communications and Internet access
have led to that TCP (transmission control protocol) in wireless links
dooms to play an important role in the design of future wireless
communication systems.
Compared to its wireline counterpart, wireless communications has its
own problems and limitations: lacking bandwidth, multipath fading,
propagation loss, cochannel interference, and etc.
Due to that bandwidth is limited, a wireless link is usually the
bottleneck along the transmission path of a TCP connection.
On the other hand, due to multipath fading, propagation loss, and
cochannel interference, the bit error rate of data transmission in a
wireless link is often several orders higher than an optical wireline.
In this study several schemes are proposed to alleviate the performance
degradation of TCP due to channel errors.
The wireless systems considered include satellite, cellular, and
wireless LAN (local area network).
To solve the performance degradation problem of wireless TCP, three
major approaches have been widely discussed in the literature:
1. Splitting: This approach splits a TCP connection into two separate
connections at the base station -- one between the fixed host and the
base station, and the other between the base station and the mobile host.
2. Link-layer retransmission: In this approach, a retransmission
protocol together with FEC (forward error correction) at the data link
layer is employed for the wireless segment.
3. Differentiation: This approach manages to make the TCP sender
capable of telling the cause of a packet loss, in order to prevent TCP
from triggering unnecessary congestion control and avoidance mechanisms
when packet loss is due to wireless channel errors.
Among these approaches, we considered ``link-layer retransmission' and
``differentiation' in this study.
We propose schemes that are able to effectively improve the performance
of wireless TCP.
To obtain results, the ns-2 (network simulator) is used as a tool.
Five topics are studied in this thesis.
An asymmetric Internet accessing system consisting of a high speed
satellite link and a slow terrestrial link is considered in the first
problem.
The satellite link is used for forwarding TCP data and the terrestrial
link for returning acknowledgements.
In this environment, a ground retransmission mechanism that takes the
advantage of small latency of the terrestrial connection is proposed
to enhance TCP performance.
The proposed mechanism effectively prevents TCP from triggering
unnecessary fast retransmits, thus achieving a much better throughput
performance than regular TCP.
The second topic deals with the problem of spurious timeouts in the
fully-reliable retransmission mechanism in mobile computing environments.
An ACK buffering method is proposed to effectively suppress the
occurrence of TCP spurious timeouts.
In this method a small number of ACKs are buffered at the base station
prior to the emergence of a bad state period in the wireless channel,
and these ACKs are henceforth released by the base station one at a
time to reset the TCP sender's retransmission timer.
This effectively prevents TCP from spurious timeouts and alleviates the
performance degradation of TCP.
In the third and the fourth topics, two different differentiation
schemes are proposed for TCP SACK.
In the first differentiation scheme, a support is required at routers
to incorporate a new dropping policy.
Although it significantly improves the performance of TCP SACK over
both low and long delay noisy links, the implementation cost is high.
In contrast, the second differentiation scheme requires only
modification at the TCP sender.
However, the performance gain is lower, compared to the first
differentiation scheme.
The last topic discusses the problem relating to the buffer management
at a base station in a heterogeneous network when Snoop protocol is
used to improve TCP performance over wireless links.
We notice that FDA (forced duplicate acknowledgement), which is an
active flow control scheme proposed to handle the buffer management
when multiple TCP flows share a common wireless link, would fail to
control the flow of TCP SACK, causing the wireless bandwidth to be
unfairly distributed when TCP SACK coexists with Tahoe/Reno.
An improved FDA flow control based on the key features of TCP SACK is
then proposed to solve this problem.
Subjects
無線通訊
傳輸控制規約
TCP
wireless communication
Type
thesis
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