Kan Cai

Efficient Mesh Router Placement in Wireless Mesh Networks

The placement of mesh routers (MRs) in building a wireless mesh network (WMN) is the first step to ensure the desired network performance. Given a network domain, the fundamental issue in placing MRs is to find the minimal configuration of MRs so as to satisfy the network coverage, connectivity, and Internet traffic demand. In this paper, the problem is addressed under a constraint network model in which the traffic demand is non-uniformly distributed and the candidate positions for MRs are pre-decided. After formulating the MR placement problem, we first provide the theoretical analysis to validate the traffic demand and determine the optimal position of Internet gateway (IGW). To reduce complexity of determining the locations of MRs while satisfying the traffic constraint, we propose an effective heuristic algorithm to obtain an close-to-optimal solution. Finally, our simulation results verify our analytical model and show the effectiveness of our proposed algorithm.

Non-intrusive, dynamic interference detection for 802.11 networks

In densely packed 802.11 environments, access-point domains significantly overlap and wireless hosts interfere with each other in complex ways. Knowing which devices interfere is an essential first step to minimizing this interference, improving efficiency and delivering quality connectivity throughout the network. This knowledge, however, is extremely difficult to obtain without either taking a running network offline for measurements or having client hosts monitor and report airspace anomalies, something typically outside the control of network administrators. In this paper we describe a technique we have developed to reveal wireless-network interference relationships by examining the network traffic at wired routers that connects wireless domains to the Internet. This approach, which we call VOID (Vvirless Online Interference Detection), searches for correlated throughput changes that occur when traffic from one node causes a throughput drop at other nodes in its radio range. In one analysis round we identify each nodes interference neighbours using a single set of performance data collected from a wired-network router. We have evaluated VOID in Emulab testbeds consisting of tens of nodes as well as a six-node testbed in a live wireless network. The initial results have shown the promise of VOID to accurately correlate interfering devices together and effectively discriminate interfering devices from non-interfering ones.

Wireless unfairness: alleviate MAC congestion first!

It is well known that competition between 802.11 wireless flows can lead to severe unfairness problems. In this paper we show that certain flows can be denied from their fair share of network access for long periods of time due toinequitable channel conditions. We argue that this unfairness problem is likely to get worse as the use of wireless in terms of number of devices and bandwidth continues to grow. To address unfairness, we rely on a common network management technique called traffic shaping. Instead of designing new MAC protocols or introducing complex wireless fair queuing or adaptation algorithms, we argue that 802.11, TCP and a well-understood wired fair queuing scheme can provide fairness even in unfavourable network topologies as long as we can prevent MAC-layer congestion from happening. This shaping approach has the advantage of being easily deployed in existing network management systems, requiring no change to the end-user devices or access points. We have proven the effectiveness of this cross-layer approach using a wireless testbed, and are currently in the process of incorporating this scheme into a large-scale campus wireless network.

Trade: Cooperation without Trust in 802.11 Networks

Blasting garbage packets in a wireless network is an anti-social and undesirable behaviour. Nevertheless, we propose a cooperation scheme called Trade in which wireless devices use the threat of blasting to force other nodes to fairly share network airspace. We have used game theory to prove that trade is a Nash-equilibrium strategy that guarantees participating nodes negotiate faithfully. This paper describes the Trade framework, its benefits and its implementation challenges.

Understanding Performance for Two 802.11 Competing Flows

It is well known that 802.11 suffers from both inefficiency and unfairness in the face of competition and interference. This paper provides a detailed analysis of the impact of topology and traffic type on network performance when two flows compete with each other for airspace. We consider both TCP and UDP flows and a comprehensive set of node topologies. We vary these topologies to consider all combinations of the following four node-to-node interactions: (1) nodes unable to read or sense each other, (2) nodes able to sense each other but not able to read each others packets and nodes able to communicate with (3) weak and with (4) strong signal. We evaluate all possible cases through simulation and show that the cases can be reduced to 9 UDP and 10 TCP 802.1 Ig models with similar efficiency/fairness characteristics. We also validate our simulation results with extensive experiments conducted in a laboratory testbed. These more detailed models improve on previous work such as hidden-Zexposed-terminal categorization and are thus better suited as a basis for adaptive techniques to improve performance in 802.11 multi-hop WLAN or Mesh Networks.

Routing Transient Traffic in Mobile Ad Hoc Networks

Recent research shows that the traffic in public wireless networks is mostly transient and bursty. There is good reason to believe that ad-hoc traffic will follow the same pattern as its popularity grows. Unfortunately transient traffic generates route discoveries much more frequently than the long-term, constant-bit-rate traffic, causing network congestion problems for existing routing protocols. This paper describes the design of a new routing algorithm, called ECBR, that uses hybrid backbone routing in a manner that is well suited to workloads that include transient traffic. We explain three key features of our algorithm and demonstrate their roles in greatly improving the performance compared to existing reactive and backbone routing techniques.

The impact of transient traffic on mobile, ad-hoc routing

The current ad-hoc routing algorithms perform poorly when dealing with spontaneous, transient communication. This traffic pattern generates route discoveries much more frequently than long-term communication and thus causes the reactive component of flat routing algorithms to flood the network. Similarly, the increased number of route discoveries also makes the backbone a bottleneck for the hierarchical algorithms.We designed a backbone routing protocol, called DCDS, that handles both short-term and long-term traffic well. This paper compares it extensively with three other algorithms, two variants of DSR and a model hierarchical algorithm. Our results show that DCDS performs substantially better than these other algorithms for the transient workloads we studied.