THESIS
1997
xvi, 127 leaves : ill. ; 30 cm
Abstract
Technological advances of VLSI and wireless communication in the 90's have been reshaping the existing form of network computing in the Internet into what is known as "Wireless Mobile Computing?. Host mobility and wireless access are two newly introduced design considerations which pose challenging problems at all layers of the networking protocol stack in the Internet. This thesis investigates their impacts on the designs of the link and transport layer protocols, particularly focusing on the Transmission Control Protocol (TCP)....[
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Technological advances of VLSI and wireless communication in the 90's have been reshaping the existing form of network computing in the Internet into what is known as "Wireless Mobile Computing?. Host mobility and wireless access are two newly introduced design considerations which pose challenging problems at all layers of the networking protocol stack in the Internet. This thesis investigates their impacts on the designs of the link and transport layer protocols, particularly focusing on the Transmission Control Protocol (TCP).
At the transport layer, flow and congestion control are the most important problems. Although MobileIP at the network layer provides continuous network connectivity to mobile hosts, it cannot shield the effects due to host movement and wireless medium characteristics (noisy, long and varying latencies) from the TCP protocol at the transport layer. Mismatches between wired and wireless access thus cause severe performance degradation to TCP whose congestion control mechanisms are optimized for wired networks with stationary hosts.
This thesis makes observations on the effects of noisy channels and temporary disconnections on the control mechanisms in a particular version of TCP, 4.3 BSD TCP-Reno. Three phenomena, namely false congestion control trigger, multiple corruptions in a single window, and corruptions of retransmissions, are found to be the main causes of TCP through-put degradation in noisy medium. Based on these observations, an analytical methodology is developed to evaluate the throughput performance of TCP over noisy channels and to study the influences of the parameter such as fast retransmit threshold.
Making use of a proposed negative acknowledgment implemented in the TCP option, this thesis proposes a scheme called NACK which has simple modifications to the congestion control mechanisms of TCP to improve its end-to-end performance with respect to the noisy channels. Two other designs based on an existing proposal, Last-Hop-ACKnowledgment (LHACK), are also proposed to provide better improvement compared with LHACK.
To avoid throughput degradation due to bulk losses and the subsequent long pauses in communication during handoff, this thesis proposes buffering at the radio interfaces (BUFFER+FREEZE) and the use of probing (PROBE) to achieve better end-to-end TCP throughput.
Although modifications at the transport layer are sufficient to improve the end-to-end performance of individual TCP session, packet retransmissions triggered by corruptions are wasteful to the scarce resource of wireless channels. Exploiting the phenomenon that fading characteristics of different channels are independent and location-dependent, this thesis proposes and investigates a number of scheduling algorithms at the MAC (Medium Access Control) layer of the radio interfaces which take the wireless channel status into account to defer packet transmissions to a destination in fade and use the corresponding resource to transmit packets whose destinations have good channels. It is found to have improvements in the aggregate throughput and channel utilization. Fairness issue is also studied in this thesis.
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