In a wireless network, mobile nodes (MNs) repeatedly perform tasks such as layer 2 (L2) handoff, layer 3 (L3) handoff and authentication. These tasks are critical, particularly for real-time applications such as VoIP. We propose a novel approach, namely Cooperative Roaming (CR), in which MNs can collaborate with each other and share useful information about the network in which they move. We show how we can achieve seamless L2 and L3 handoffs regardless of the authentication mechanism used and without any changes to either the infrastructure or the protocol. In particular, we provide a working implementation of CR and show how, with CR, MNs can achieve a total L2+L3 handoff time of less than 16 ms in an open network and of about 21 ms in an IEEE 802.11i network. We consider behaviors typical of IEEE 802.11 networks, although many of the concepts and problems addressed here apply to any kind of mobile network.
In @cite_9 suggest an algorithm called syncscan which does not require changes to either the protocol or the infrastructure. It does require, however, that all the APs in the network are synchronized and only accelerates unauthenticated L2 handoffs.
@cite_9
1742958260 1911929785 2022645219 1520594164
Wireless access networks scale by replicating base stations geographically and then allowing mobile clients to seamlessly "hand off" from one station to the next as they traverse the network. However, providing the illusion of continuous connectivity requires selecting the right moment to handoff and the right base station to transfer to. Unfortunately, 802.11-based networks only attempt a handoff when a client's service degrades to a point where connectivity is threatened. Worse, the overhead of scanning for nearby base stations is routinely over 250 ms - during which incoming packets are dropped - far longer than what can be tolerated by highly interactive applications such as voice telephony. In this paper we describe SyncScan, a low-cost technique for continuously tracking nearby base stations by synchronizing short listening periods at the client with periodic transmissions from each base station. We have implemented this SyncScan algorithm using commodity 802.11 hardware and we demonstrate that it allows better handoff decisions and over an order of magnitude improvement in handoff delay. Finally, our approach only requires trivial implementation changes, is incrementally deployable and is completely backward compatible with existing 802.11 standards. Consider a complete communication network on n nodes. In synchronous 2-counting, the nodes receive a common clock pulse and they have to agree on which pulses are "odd" and which are "even". Furthermore, the solution needs to be self-stabilising (reaching correct operation from any initial state) and tolerate f Byzantine failures (nodes that send arbitrary misinformation). Prior algorithms either require a source of random bits or a large number of states per node. In this work, we give fast state-optimal deterministic algorithms for the first non-trivial case f = 1 . To obtain these algorithms, we develop and evaluate two different techniques for algorithm synthesis. Both are based on casting the synthesis problem as a propositional satisfiability (SAT) problem; a direct encoding is efficient for synthesising time-optimal algorithms, while an approach based on counter-example guided abstraction refinement discovers non-optimal algorithms quickly. We develop computational techniques to find algorithms for synchronous 2-counting.Automated synthesis yields state-optimal self-stabilising fault-tolerant algorithms.We give a thorough experimental comparison of our two SAT-based synthesis techniques.A direct SAT encoding is more efficient for finding time-optimal algorithms.An iterative CEGAR-based approach finds non-optimal algorithms quickly. In this paper a distributed clock synchronization algorithm is proposed. The algorithm requires asymmetric gossip communications between the nodes of the network, and is based on an PI-like consensus protocol where the proportional part compensates the different clock speeds while the integral part eliminates the different clock offsets. Convergence of the algorithm is proved and analyzed with respect to the controller parameter, when the underlying graph is the complete graph. Simulations results show the effectiveness of the proposed strategy also for more general communication topologies. Consider a distributed network of n nodes that is connected to a global source of "beats". All nodes receive the "beats" simultaneously, and operate in lock-step. A scheme that produces a "pulse" every Cycle beats is shown. That is, the nodes agree on "special beats", which are spaced Cycle beats apart. Given such a scheme, a clock synchronization algorithm is built. The "pulsing" scheme is self-stabilized despite any transient faults and the continuous presence of up to f < n 3 Byzantine nodes. Therefore, the clock synchronization built on top of the "pulse" is highly fault tolerant. In addition, a highly fault tolerant general stabilizer algorithm is constructed on top of the "pulse" mechanism. Previous clock synchronization solutions, operating in the exact same model as this one, either support f < n 4 and converge in linear time, or support f < n 3 and have exponential convergence time that also depends on the value of max-clock (the clock wrap around value). The proposed scheme combines the best of both worlds: it converges in linear time that is independent of max-clock and is tolerant to up to f < n 3 Byzantine nodes. Moreover, considering problems in a self-stabilizing, Byzantine tolerant environment that require nodes to know the global state (clock synchronization, token circulation, agreement, etc.), the work presented here is the first protocol to operate in a network that is not fully connected.