Ting-Chao Hou

Transmission Range Control in Multihop Packet Radio Networks

This paper presents a model for analyzing the performance of transmission strategies in a multihop packet radio network where each station has adjustable transmission radius. A larger transmission radius will increase the probability of finding a receiver in the desired direction and contribute bigger progress if the transmission is successful, but it also has a higher probability of collision with other transmissions. The converse is true for shorter transmission range. We illustrate our model by comparing three transmission strategies. Our results show that the network can achieve better performance by suitably controlling the transmission range. One of the transmission strategies, namely transmitting to the nearest forward neighbor by using adjustable transmission power, has desirable features in a high terminal density environment.

Analyzing the throughput of IEEE 802.11 DCF scheme with hidden nodes

In this paper, we analyze the throughput of the IEEE 802.11 DCF scheme using the RTS/CTS access mechanism in multi-hop ad hoc networks. The mathematical analysis is much more involved than that in a single-hop network due to the hidden node problem. A more realistic carrier sense range is considered in this paper. The impact of parameters, such as the number of stations, the packet size, and the carrier sense range, is also considered. Simulation results show good agreement with the analytical results.

Performance characterizations of traffic monitoring, and associated control, mechanisms for broadband 'packet' networks

A number of performance issues associated with real-time traffic-monitoring schemes are addressed. These schemes will likely be used in broadband packet-transport-based networks such as broadband ISDN (B-ISDN) with asynchronous transfer mode (ATM). The issues of an appropriate performance framework for traffic-monitoring scheme comparisons, appropriate control actions based on traffic-monitoring, and how traffic parameters (that could enter strongly into service contracts between the network and users) should best be specified are treated in an overview summary. Some of the more commonly suggested traffic-monitoring schemes are described, and the leaky-bucket scheme is analyzed. A relationship between traffic peakedness and traffic-monitoring schemes is introduced.<<ETX>>

Impact of node density on throughput and delay scaling in multi-hop wireless networks

This paper studies the impact of node density on the end-to-end throughput and delay in multi-hop wireless networks. In existing work, each packet is most assumed to be relayed through one cell at each hop and the hop progress is approximated by the square root of a cell area, which does not correspond to the actual hop progress in the real network. In this paper, we calculate the hop progress by taking into account the effect of node density (i.e., the number of nodes within the transmission range of each node), and obtain the required hop count for a multi-hop path. Based on the result, we further discuss the scaling relations between node density and throughput and delay in multi-hop wireless networks. The effects of power control on the scaling relations are also examined. The results show that the impact of node density on the throughput and delay scaling is significant. Specifically, with a larger node density, the required hop count is reduced, resulting in exponential growth of the throughput. However, larger node density incurs more contentions among neighboring nodes. Consequently, it causes linear degradation in throughput. With our model, this trade-off is readily observed.

Efficiency analyses on routing cache replacement algorithms

Recent research on router architectures focuses on speeding up the time-consuming routing-lookup procedure by elaborate algorithms and data structures to match the ever-increasing wire-speed of fiber links. However, we observed strong temporal locality in the traffic traces collected from two TANet backbone routers. Thus, a routing cache, which is used to reuse previous routing-lookup results, can significantly offload the routing-lookup module. In this paper, we first introduce our temporal locality analysis. Then we investigate the efficiency of several routing cache replacement algorithms, which includes FIFO, LRU, random and our proposed LFU implementation alternative. The simulation results show that our proposed exponentially decayed LFU scheme provides better performance than the other algorithms, especially under small-size caches.

A delay-based transport layer mechanism for fair TCP throughput over 802.11 multihop wireless mesh networks

Cities worldwide have planned and implemented large-scale wireless mesh network (WMN) deployments. These mesh deployments are expected to provide broadband mobile access to the Internet at a low cost to the user. This paper considers the fairness problem affecting nodes in multihop WMNs. Many existing approaches to coping with the fairness problem are unsuitable because they necessitate modifications to the IEEE 802.11 MAC protocol or node queueing operations. Such modifications require a change in hardware/firmware at every node in the WMN. Thus, these approaches are less favorable from the points of view of interoperability and cost. Without modifying the lower layer protocols, this study identifies TCP parameters that impact throughput fairness and proposes an adjustment to these parameters to reduce frame collisions and to improve throughput fairness. Using simple mathematical formulations and ns2 simulations, this study shows that the frame transmissions from each node can be effectively controlled by properly controlling the delayed ACK timer and by using a suitable advertised window. The proposed method, in addition to fairness, requires fewer buffer resources than other methods. Moreover, it is not sensitive to the carrier sense range. It is also simple and easy to deploy. Copyright

A Suburban Femtocell Model for Evaluating Signal Quality Improvement in WiMAX Networks with Femtocell Base Stations

The femtocell base station is a low power and low cost small base station in the customer premise. It connects to the core network via cable, DSL or similar backhaul technology. In this paper, we focus on how received signal quality can be improved by deploying femtocell base stations in a WiMAX system. We propose a suburban femtocell model to analyze the potential improvement on the SINR when using different channel assignment schemes and different deployment densities of femtocell base stations within a macrocell. The simulation results show that we can enhance SINR by deploying large number of femtocell base stations within macrocells, and the Least-Interfered-First (LIF) scheme we proposed can be used to significantly reduce the co-channel interference induced by neighboring femtocell base stations.

Performance analysis of TCP over wireless link with dedicated buffers and link level error control

To evaluate the behavior of TCP on a path with a wireless link, we build a wireline-wireless network model, and assume the wireless link is the bottleneck which causes packets to be buffered at the wireline-wireless interface buffer. A reliable link layer protocol is assumed on the wireless link which hides the fluctuations of the wireless medium from TCP. We first build the link layer model and the wireless channel Markov chain model, and use the matrix geometric method to evaluate the packet loss probability. We then analyze the long-term steady-state send rate and throughput of a TCP connection.

Location-Dependent Power Setting for next generation femtocell base stations

In this paper, we study how the downlink received signal quality and throughput will be affected by deploying the Open Subscriber Group (OSG) Femtocell [1] using Location-Dependent Power Setting (LDPS) scheme with spectrum allocation (1,3,3) in the next generation mobile network. The LDPS scheme aims to use a suitable power for femtocell base station to maintains its indoor user coverage. The design of mandating indoor users to connect to femtocells ensures the transmission quality of indoor users and reduces the load of the macrocell base stations. Furthermore, by introducing transmit power upper bound (Pmax) and lower bound (Pmin), LDPS can better save power or trade power with throughput. Our study uses a large-scale macrocell/femtocell deployment model and considers many aspects of the next generation mobile communication systems, including OFDMA, adaptive modulation and coding, diverse propagation loss models, etc. This study sheds new insight on the self-optimization/self-configuration aspect of the next-generation femtocell deployment.

On the cluster based dynamic channel assignment for multihop ad hoc networks

In order to ensure an efficient network control for mobile ad hoc networks, nodes are usually aggregated into clusters to provide a convenient framework. However, an important issue on how to properly assign channels over a multi-cluster structure has not been studied in depth. In this paper, we first propose a new clustering algorithm to facilitate subsequent channel assignment. Each cluster has one clusterhead. Between clusterheads, there are gateways that relay control information and help eliminate the hidden terminal problem. Then two dynamic channel assignment (DCA) strategies are designed to make the best use of available channels by taking advantage of the spatial reuse concept. One strategy tries approaching a compact-channel pattern so that the channel reuse efficiency can be enhanced. On the other hand, the other strategy tries reducing the control overhead on the channel assignment so that the users can have a lower power consumption. We evaluate our cluster structure via simulation in terms of cluster stability and overheads. Furthermore, the two DCA strategies are compared to provide a guideline on how to trade off between the channel reuse efficiency and the control overhead.