Ashish Raniwala

Centralized channel assignment and routing algorithms for multi-channel wireless mesh networks

The IEEE 802.11 Wireless LAN standards allow multiple non-overlapping frequency channels to be used simultaneously to increase the aggregate bandwidth available to end-users. Such bandwidth aggregation capability is routinely used in infrastructure mode operation, where the traffic to and from wireless nodes is distributed among multiple interfaces of an access point or among multiple access points to balance the traffic load. However, bandwidth aggregation is rarely used in the context of multi-hop 802.11-based LANs that operate in the ad hoc mode. Most past research efforts that attempt to exploit multiple radio channels require modifications to the MAC protocol and therefore do not work with commodity 802.11 interface hardware. In this paper, we propose and evaluate one of the first multi-channel multi-hop wireless ad-hoc network architectures that can be built using standard 802.11 hardware by equipping each node with multiple network interface cards (NICs) operating on different channels. We focus our attention on wireless mesh networks that serve as the backbone for relaying end-user traffic from wireless access points to the wired network. The idea of exploiting multiple channels is particularly appealing in wireless mesh networks because of their high capacity requirements to support backbone traffic. To reap the full performance potential of this architecture, we develop a set of centralized channel assignment, bandwidth allocation, and routing algorithms for multi-channel wireless mesh networks. A detailed performance evaluation shows that with intelligent channel and bandwidth assignment, equipping every wireless mesh network node with just 2 NICs operating on different channels can increase the total network goodput by a factor of up to 8 compared with the conventional single-channel ad hoc network architecture.

MiNT: a miniaturized network testbed for mobile wireless research

Most mobile wireless networking research today relies on simulations. However, fidelity of simulation results has always been a concern, especially when the protocols being studied are affected by the propagation and interference characteristics of the radio channels. Inherent difficulty in faithfully modeling the wireless channel characteristics has encouraged several researchers to build wireless network testbeds. A full-fledged wireless testbed is spread over a large physical space because of the wide coverage area of radio signals. This makes a large-scale testbed difficult and expensive to set up, configure, and manage. This paper describes a miniaturized 802.11b-based, multi-hop wireless network testbed called MiNT. MiNT occupies a significantly small space, and dramatically reduces the efforts required in setting up a multi-hop wireless network used for wireless application/protocol testing and evaluation. MiNT is also a hybrid simulation platform that can execute ns-2 simulation scripts with the link, MAC and physical layer in the simulator replaced by real hardware. We demonstrate the fidelity of MiNT by comparing experimental results on it with similar experiments conducted on a non-miniaturized testbed. We also compare the results of experiments conducted using hybrid simulation on MiNT with those obtained using pure simulation. Finally, using a case study we show the usefulness of MiNT in wireless application testing and evaluation.

MiNT-m: an autonomous mobile wireless experimentation platform

Limited fidelity of software-based wireless network simulations has prompted many researchers to build testbeds for developing and evaluating their wireless protocols and mobile applications. Since most testbeds are tailored to the needs of specific research projects, they cannot be easily reused for other research projects that may have different requirements on physical topology, radio channel characteristics or mobility pattern. In this paper, we describe the design, implementation and evaluation of MiNT-m, an experimentation platform devised specifically to support arbitrary experiments for mobile multi-hop wireless network protocols. In addition to inheriting the miniaturization feature from its predecessor MiNT [9], MiNT-m enables flexible testbed reconfiguration on an experiment-by-experiment basis by putting each testbed node on a centrally controlled untethered mobile robot. To support mobility and reconfiguration of testbed nodes, MiNT-m includes a scalable mobile robot navigation control subsystem, which in turn consists of a vision-based robot positioning module and a collision avoidance-based trajectory planning module. Further, MiNT-m provides a comprehensive network/experiment management subsystem that affords a user full interactive control over the testbed as well as real-time visualization of the testbed activities. Finally, because MiNT-m is designed to be a shared research infrastructure that supports 24x7 operation, it incorporates a novel automatic battery recharging capability that enables testbed robots to operate without human intervention for weeks.

Design considerations for a multihop wireless network testbed

Limited fidelity of simulators has prompted researchers to build wireless network testbeds for realistic testing. Unlike simulators, which have broad applicability, most of these testbeds are tailored to specific projects and cannot be used by a wider research community. Recognizing the growing importance of testbeds, this article is one of the first attempts to identify a comprehensive set of requirements for a general-purpose multihop wireless network testbed and the challenges therein. The issues range from initial testbed deployment and routine management to individual experimental configuration and data collection. We survey state-of-the-art wireless testbeds and highlight their salient features. The article is intended to provide an initial reference for researchers, application developers, and administrators dealing with various aspects of wireless network testbeds.

Design of a Channel Characteristics-Aware Routing Protocol

Radio channel quality of real-world wireless networks tends to exhibit both short-term and long-term temporal variations that are in general difficult to model. To maximize the utilization efficiency of radio resources, it is critical that these temporal fluctuations in radio signal quality be incorporated into wireless routing decisions. In this paper, we explore the design considerations in leveraging accurate real-time radio channel quality information when making routing decisions. Specifically, we propose a channel characteristics-aware routing protocol (CARP) that (1) uses per-packet transmission time to estimate the effective residual capacity of a wireless link, (2) employs a bandwidth probability distribution model to better approximate a wireless paths capacity profile, and (3) applies multi-path routing to exploit diversity among alternative paths and deliver more robust throughputs despite temporal fluctuations in wireless link quality. We evaluated the performance gains of incorporating each of these mechanisms on a miniaturized multi-hop wireless network testbed- MiNT-m.

Globally fair radio resource allocation for wireless mesh networks

Network flows running on a wireless mesh network (WMN) may suffer from partial failures in the form of serious throughput degradation, sometimes to the extent of starvation, because of weaknesses in the underlying MAC protocol, dissimilar physical transmission rates or different degrees of local congestion. Most existing WMN transport protocols fail to take these factors into account. This paper describes the design, implementation and evaluation of a coordinated congestion control (C3L) algorithm that guarantees fair resource allocation under adverse scenarios and thus provides end-to-end max-min fairness among competing flows. The C3L algorithm features an advanced topology discovery mechanism that detects the inhibition of wireless communication links, and a general collision domain capacity re-estimation mechanism that effectively addresses such inhibition. A comprehensive ns-2-based simulation study as well as empirical measurements taken from an IEEE 802.11a-based multi-hop wireless testbed demonstrate that the C3L algorithm greatly improves inter-flow fairness, eliminates the starvation problem, and at the same time maintains high radio resource utilization efficiency.

Phoenix: a low-power fault-tolerant real-time network-attached storage device

Phoenix is a real-time network-attached storage device (NASD) that guarantees real-time data delivery to network clients even across single disk failure. The service interfaces that Phoenix provides are best-effort/real-time reads/writes based on unique object identifiers and block offsets. Data retrieval from Phoenix can be serviced in server push or client pull modes. Phoenixs real-time disk subsystem performance results from a standard cycle-based scan-order disk scheduling mechanism. However, the disk I/O cycle of Phoenix is either completely active or completely idle. This on-off disk scheduling model effectively reduces the power consumption of the disk subsystem, without increasing the buffer size requirement. Phoenix also exploits unused disk storage space and maintains additional redundancy beyond the generic RAID5-style parity. This extra redundancy, typically in the form of block replication, reduces the time to reconstruct the data on the failed disk. This paper describes the design, implementation, and evaluation of Phoenix, one of the first, if not the first, NASDs that support fault-tolerant, real-time, and low-power network storage service.

An empirical comparison of throughput-maximizing wireless mesh routing protocols

Communication quality of wireless network links is heavily dependent on various external factors such as physical geometry of environmental objects and interference among radio signal sources. As a result, the radio channel quality of real-world wireless networks tends to exhibit both short-term and long-term temporal variations that are in general difficult to model analytically. There has been a large body of research on maximizing the overall throughput of wireless mesh networks through dynamic load/capacity measurement and adaptive routing. However, so far there is no comprehensive evaluation of different protocol mechanisms on a real wireless network testbed. In this paper we first identify the major design dimensions of throughput-maximizing wireless mesh network routing protocols: wireless link capacity estimation, routing path selection, and adaptation to temporal link quality fluctuation, and empirically quantify the performance comparison of various alternatives in each dimension using both software simulations and a miniaturized multi-hop wireless network testbed-MiNT-m.