Sangjin Hong

Performance Optimizations for Deploying VoIP Services in Mesh Networks

In the recent past, there has been a tremendous increase in the popularity of VoIP services as a result of huge growth in broadband access. The same voice-over-Internet protocol (VoIP) service poses new challenges when deployed over a wireless mesh network, while enabling users to make voice calls using WiFi phones. Packet losses and delay due to interference in a multiple-hop mesh network with limited capacity can significantly degrade the end-to-end VoIP call quality. In this work, we discuss the basic requirements for efficient deployment of VoIP services over a mesh network. We present and evaluate practical optimizing techniques that can enhance the network capacity, maintain the VoIP quality and handle user mobility efficiently. Extensive experiments conducted on a real testbed and ns-2 provide insights into the performance issues and demonstrate the level of improvement that can be obtained by the proposed techniques. Specifically, we find that packet aggregation along with header compression can increase the number of supported VoIP calls in a multihop network by 2-3 times. The proposed fast path switching is highly effective in maintaining the VoIP quality. Our fast handoff scheme achieves almost negligible disruption during calls to roaming clients

Collaborative QoS architecture between DiffServ and 802.11e wireless LAN

As the multimedia applications such as voice over IP (VoIP) and audio/visual (AV) streaming across the Internet emerge, many are working on the network architecture to extend such applications to the wireless networking domain. The emerging IEEE 802.11e quality-of-service (QoS)-enabled wireless LAN (WLAN) is considered a strong candidate for the air interface for such multimedia applications thanks to the IP-centric network paradigm along with its inherent high-speed transmission capability. This paper provides an integrated wired/wireless network architecture interfacing QoS between user level traffic over IP using differentiated service (Diffserv) and transport level traffic using IEEE 802.11e WLAN. Our study investigates the correlations in end-to-end traffic management between Diffserv and 802.11e, and presents the hierarchical QoS signaling interface between Diffserv and 802.11e, in terms of traffic classifying, shaping and policing.

VoMESH: voice over wireless MESH networks

In this work, we focus on increasing network utilization to support real-time applications, especially voice over IP, in 802.11b wireless mesh network. For increasing capacity, we investigate on two directions: use of packet aggregation and utilize of header compression. Although aggregation reduces 802.11 MAC overhead, it also increases delay. We present a distributed multihop aggregation algorithm that uses the natural waiting time in the interface queue of packets in a loaded network. Header compression can relax the VoIP protocol overhead, but it also introduces the signaling overhead. In our work, we propose a zero-length header compression algorithm integrated with packet aggregation, which doesnt need to depend on the signaling mechanism to recover the context discrepancy between compressor and decompressor. The above performance optimization techniques are experimented in the NS2 simulator. The experimental results show an increase of 12 times for a six hop string when all optimizations are used

On Packet Aggregation Mechanisms for Improving VoIP Quality in Mesh Networks

Performance in multihop wireless networks is known to degrade with the number of hops for both TCP and UDP traffic. For VoIP, the wireless network presents additional challenges as the perceived quality is dependent on loss, delay and jitter. Furthermore, small size of VoIP packets aggravates network utilization of the 802.11 based wireless mesh network. In this work, we evaluate the packet aggregation algorithms to reduce VoIP protocol overhead, compare the aggregation algorithms and propose a distributed packet aggregation mechanism. The experimental results demonstrates that the proposed distributed packet aggregation algorithm improves the VoIP quality and results in significant increase in the number high quality VoIP calls supported over multihop mesh

Coexistence of VoIP and TCP in wireless multihop networks

When supporting both voice and TCP in a wireless multihop network, there are two conflicting goals: to protect the VoIP traffic and to completely utilize the remaining capacity for TCP. We investigate the interaction between these two popular categories of traffic and find that many solution approaches, such as enhanced TCP variants, priority queues, bandwidth limitation, and traffic shaping, do not always achieve the coexistence goals. Enhanced TCP variants (Reno, Vegas, C-TCP, CUBIC, Westwood) generally fail to protect VoIP in wired-wireless multihop scenarios. Priority schemes, including those built into the 802.11 MAC such as RTS/CTS or 802.11e, do not account for the interference nature of wireless multihop. Finally, bandwidth shaping and window control are valid tools to control TCP, but come with their own trade-offs.

Passive sensor based dynamic object association method in wireless sensor networks

This paper proposes and presents a novel algorithm for dynamic data association in wireless sensor networks. The sensor node incorporates RFID reader and acoustic sensor where the signals are fused for tracking and associating multiple objects. The RFID tag is used for object identification and acoustic sensor is used for estimating object movement. The threshold based association algorithm is proposed and its performance is analyzed. The association performance under single node coverage and multiple node coverage is evaluated as a function of sampling time and the dynamic behavior. The algorithm minimizes the failed association and provides association recovery.

Voice Transmission Enhancing Model on Wireless Mesh Networks

This paper initially shows ROHC and packet aggregation significantly improve the number of successful voice calls. However, the improvement does not include processors processing overhead, which is identified by measuring ROHC processing time from Intel Pentium 4 and RouterBOARD, and applying the results into NS-2 simulations. Simulation results indicate the number of successful voice calls is seriously affected by the processing overhead. For the solution of the processing limitation, we propose a hardware model of the two algorithms and numerically analyze the model. Simulation results show the hardware model makes the number of successful voice calls approach the ideal (no-delay) case, and the numerical model exactly characterizes the hardware behaviors.

Gateway Strategies for VoIP Traffic over Wireless Multihop Networks

When supporting both voice and TCP in a wireless multihop network, there are two conflicting goals: to protect the VoIP traffic, and to completely utilize the remaining capacity for TCP. We investigate the interaction between these two popular categories of traffic and find that conventional solution approaches, such as enhanced TCP variants, priority queues, bandwidth limitation, and traffic shaping do not always achieve the goals. TCP and VoIP traffic do not easily coexist because of TCP aggressiveness and data burstiness, and the (self-) interference nature of multihop traffic. We found that enhanced TCP variants fail to coexist with VoIP in the wireless multihop scenarios. Surprisingly, even priority schemes, including those built into the MAC such as RTS/CTS or 802.11e generally cannot protect voice, as they do not account for the interference outside communication range. We present VAGP (Voice Adaptive Gateway Pacer) - an adaptive bandwidth control algorithm at the access gateway that dynamically paces wired-to-wireless TCP data flows based on VoIP traffic status. VAGP continuously monitors the quality of VoIP flows at the gateway and controls the bandwidth used by TCP flows before entering the wireless multihop. To also maintain utilization and TCP performance, VAGP employs TCP specific mechanisms that suppress certain retransmissions across the wireless multihop. Compared to previous proposals for improving TCP over wireless multihop, we show that VAGP retains the end-to-end semantics of TCP, does not require modifications of endpoints, and works in a variety of conditions: different TCP variants, multiple flows, and internet delays, different patterns of interference, different multihop topologies, and different traffic patterns.

Network/hardware cross-layer evaluation for ROHC and packet aggregation on wireless mesh networks

This paper proposes a profile-based network and hardware co-simulation method to investigate the overall performance and real-timing characteristics of a wireless mesh network (WMN) affected by hardware capabilities, speed and complexity. For the sophisticated algorithms to be assisted by a hardware realization, we adopt the RObust Header Compression (ROHC) and packet aggregation that provide high and reliable data transmission over unstable wireless links, which is proven in the preliminary works. To verify the hardware support needs to get the benefit of the two algorithms, we measure the ROHC processing time from Intel Pentium 4 and RouterBOARD, and identify the deterioration of sensor throughput and successful voice calls under various NS-2 simulation scenarios. The co-simulation method integrates the network level simulator, NS-2 and hardware level simulator, SystemC. In this approach, we first insert the modules of ROHC and packet aggregation algorithms into the network simulator hierarchy, and measure the packet arrival times. Then, the corresponding hardware architecture is designed by SystemC for profiling the hardware delay appeared in encoding and decoding packets. The hardware is suitably designed to reduce the complexity and make a sufficient speedup in the packet processing. Finally, the traced hardware delays are applied into the network level simulator to extract real-timing WMN behaviors changed by the hardware operations in each mesh router.

On Achieving High Performance Wireless Mesh Networks With Data Fusion

Wireless Mesh Network (WMN) supports various types of mesh clients such as sensor networks, cellular; 802.11/e Wireless LAN, etc. In this case, it needs to be suitably organized and adopt well-known or newly proposed algorithm to guarantee high and reliable mesh performance. This paper applies RObust Header Compression (ROHC) originate from cellular data network plus packet aggregation to WMNs when mesh clients generate small-size fusion data, i.e., sensor (delivered by UDP) and voice over IP (VOIP) data. We preliminarily analyze ROHC and packet aggregation improve UDP throughput and the number of supported voice calls. However; the improvement does not include a processors processing overhead for ROHC and packet aggregation, which is examined by measuring ROHC execution time from Intel Pentium 4 and RouterBOARD, and applying the results into NS-2 simulations. The results indicate UDP and voice performance are seriously affected by the processing overhead. As a solution for the performance degradation, we introduce a hardware design for the two algorithms. By using SystemC Hardware Description Language (HDL), we first design hardware specification composed of 1Ghz master clock and 333Mhz BUS system. Then, we integrate the hardware model into previous NS-2 network model, and propose profile-based network/hardware co-simulation method which gives insight how to investigate real-timing characteristics of the hardware model in the view of network behaviors.