For TCP/AQM systems, the issue of buffer sizing has recently received much attention. The classical rule-of-thumb suggests O(N) buffer size to ensure full link utilization when N TCP flows share a bottleneck link of capacity O(N), while recent empirical study shows the buffer of size O(√N) is enough to yield high utilization (say, 95%) for large N. However, these results are all limited to the drop-tail scheme and there has been no systematic modeling framework for any buffer sizing between O(√N) and O(N). In this paper, we study the limiting behavior of a TCP/AQM system for an intermediate buffer sizing of O(Nγ) (0.5 ≤γ< 1). We develop a stochastic model in a discrete-time setting to characterize the system dynamics and then show that we can have 100% link utilization and zero packet loss probability for a large number of flows when the buffer size is chosen anywhere between O(√N) and O(N). Our model is general enough to cover any queue-based AQM scheme with ECN marking (including the drop-tail) and various generalized AIMD (Additive-Increase-Multiplicative-Decrease) algorithms for each TCP flow. We also provide arguments showing that the discrete-time based modeling can effectively capture all the essential system dynamics under our choice of scaling (0.5 ≤γ< 1) for buffer size as well as AQM parameters.

Stability is one of the important issues for a TCP/AQM (Active Queue Management) system. In this paper, we study the local and global stability of TCP-newReno/RED under many flows regime. The existing results of the local stability are mostly for TCP-Reno, not for new Reno. These results are obtained based on a small scale model with a few number of flows and thus cannot be blindly applied to a large system with many flows. Moreover, traditional approaches for the global stability based on Lyapunov functions is not suitable for a system with a large amount of flows due to its complexity. Motivated by this, we present a normalized discrete-time model to capture the essential dynamics of TCP-newReno/RED with many flows and obtain its local stability criterion. The normalized model allows us to proceed numerical iterations to analyze the global stability in an efficient manner. Our results show that by properly choosing some ‘free’ parameters, we can always ensure that a locally stable TCP-newReno/RED system is in fact globally stable. Our results become more accurate as the number of flows increases. Finally, we extend our normalized model to the case of heterogeneous RTTs.

Performance of TCP-Vegas is not satisfactory in multihop ad hoc networks over IEEE 802.11 MAC protocol. We analyze the problem with a unified network model and a TCP source window model. We observe that Vegas cannot maintain the optimal window with maximum average throughput when the network capacity is smaller than the reset slow start threshold of Vegas. The aggregate throughput of all traffics decreases as the load of network increases. The main reasons lie in Vegas’s large minimum congestion window, large reset slow start threshold and aggressive window increase policy. All of them induce overload of the network, which cause packet losses at MAC layer and over-reaction at routing layer. These in return result in the breakup of end-to-end connections and reduce the throughput. To fix these problems, we propose a modified TCP protocol based on TCP-Vegas for multihop ad hoc networks, called Vegas-W. We extend congestion window to fraction with a rate control timer under the TCP sending process. Probing mechanisms of legacy TCP-Vegas in both slow start and congestion avoidance phases are changed to increase congestion window after receiving more than one ACK. Furthermore, we update slow start threshold by tracking stable window. We evaluate the performance of Vegas-W through ns-2. Extensive simulation results show that Vegas-W can improve the throughput up to 87% over legacy TCP-Vegas over a variety of topologies including chain, grid, star, dumbbell and hammer, etc.

In this paper, we present a new energy-efficient bandwidth allocation scheme for wireless networks. First of all, we investigate the intrinsic relationship between the energy consumption and transmission rates of mobile terminals, in which transmission rate is determined through channel allocations. Then, we propose two schemes for connection admission control: Victim Selection Algorithm (VSA) and Beneficiary Selection Algorithm (BSA) with the intent to reduce energy consumption of each terminal. Moreover, we introduce an adjustment algorithm to statistically meet the demands for quality of service (QoS) during the resource allocation. The performance of the proposed schemes is evaluated with respect to energy consumption rate of each successfully transmitted bit, throughput and call blocking probabilities. An extensive analysis and simulation study is conducted for Poisson and self-similar, multi-class traffic.