Dragos Niculescu

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

Communication paradigms for sensor networks

When compared with now classical MANETs, sensor networks have different characteristics, and present different design and engineering challenges. One of the main aspects of sensor networks is that the solutions tend to be very application-specific. For this reason, a layered view like the one used in OSI imposes a large penalty, and implementations more geared toward the particular are desirable. This survey presents the three main paradigms for communication in ad hoc networks and discusses their applicability for routing, querying, and discovery. We conclude that the node-centric approach, although the oldest and best understood, is not the most appropriate for large-size low-energy application-specific sensor networks.

Interference map for 802.11 networks

The interference map of an 802.11 network is a collection of data structures that can help heuristics for routing, channel assignment and call admission in dense wireless networks. The map can be obtained from detailed measurements, which are time consuming and require network down time. We explore methods and models to produce the interference map with a reduced number of measurements, by identifying interference properties that help to extrapolate complex measurements from simple measurements. Actual interference in an 802.11a testbed is shown to follow certain regularities - it is linear with respect to packet rate of the source, packet rate of the interferer, and shows independence among interferers. When multiple cards are available, they behave differently, and even different channels of the same card have different performance. We find that while current methods of gathering the interference map may be appropriate for characterizing interference in one card networks, they are unscalable for multiple card networks when considering: 802.11 characteristics (card and channel asymmetries, time variation), required downtime, and complexity of the measurement procedure.

Performance of VoIP in a 802.11 Wireless Mesh Network

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 both loss and delay. We investigate several methods to improve voice quality and present experimental results from an 802.11b testbed optimized for voice delivery. Use of multiple interfaces, path diversity and aggregation are shown to provide a combined improvement of 13 times in number of calls supported in our 15 node 802.11 mesh system. VoIP, 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.

Channel assignment for wireless meshes with tree topology

The capacity of wireless mesh networks can be enhanced with judicious channel assignment. This paper deals with one particular type of mesh network topology - the tree topology. The unique characteristics of this topology is that all the traffic to/from the mesh nodes goes through the root. This enables design of an efficient channel allocation algorithm that utilizes the intrinsic characteristics of the tree topology and the traffic pattern over this topology. We use the unique connection characteristics of the tree topology to create an auxiliary contention graph over which we execute our coloring algorithm. This mitigates the burden of ensuring connectivity that channel allocation algorithms for mesh have to consider and the algorithm can solely focus on the task of capacity maximization. Our algorithm has a low complexity of O(N2) for a mesh network with N nodes.

On time-domain coexistence of unlicensed and licensed spectrum users

This paper aims to exploit spectrum white spaces in time-domain via the Dynamic Spectrum Access (DSA) technology. DSA relies on opportunistic access of licensed spectrum by unlicensed devices. Any “unlicensed-with-licensed” coexistence must ensure “safe” (i.e., un-interfered) communications for the incumbents, while achieving high spectrum-use efficiency for the secondary users. We propose a novel and comprehensive metric called the Coexistence Goodness Factor (CGF) to accurately model the inherent tradeoff between incumbent safety and unlicensed access efficiency for time-domain DSA-based coexistence. To optimize the coexistence performance, we propose a generic, online, dual-mode DSA coexistence protocol. The unlicensed devices attempt to estimate incumbent behavior patterns, and enter the Aggressive Mode (AM) once such a pattern is found, while they stay in the Safe Mode (SM) otherwise. For low-overhead and reliable estimation of incumbent spectrum-usage patterns, we propose algorithms based on Approximate Entropy (ApEn). Further, we design Spectrum-Conscious WiFi (SpeCWiFi), which provisions the proposed DSA coexistence scheme to the base 802.11 MAC. We conduct an extensive experimental evaluation of SpeCWiFi using a MadWifi-based prototype implementation in conjunction with 802.11 wireless cards. The evaluation shows that SpeCWiFi achieves excellent time-domain DSA coexistence in the presence of different types of licensed spectrum, including fast-varying channels that feature short-duration white spaces.

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.

Allocation of channels in wireless tree topologies

Judiciously assigning channels to a wireless mesh network can substantially enhance capacity of the network. One particular flavor of mesh network is that with a tree topology, which has the property that all traffic passes through one central point. Usually the allocation problem is linked to problems of routing, load, and measurements of interference. In this paper we take advantage of the restrictions on the routing, and on potential load imposed by the topology, to present an allocation algorithm that is tailored for wireless trees. Using the connection characteristics of the topology, we define a conflict graph over which a coloring heuristic can provide better performance by not having to focus on connectivity/routing/interference. We show that the algorithm has a low complexity (quadratic in the number of nodes), and in simulation shows significant gains in performance when compared to simpler solutions.

Voice Adaptive Gateway Pacer for 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 does not easily coexist because of TCP aggressiveness and data burstiness, and the self-interference nature of multihop traffic. We found that enhanced TCP variants (Reno, Vegas, C-TCP, CUBIC, Westwood) 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, internet delays, different patterns of interference, different multihop topologies, different traffic patterns.