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
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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. We address the self-stabilizing unison problem in tree networks. We propose two self-stabilizing unison protocols without any reset correcting system. The first one, called Protocol SU_Min, being scheduled by a synchronous daemon, is self-stabilizing to synchronous unison in at most D steps, where D is the diameter of the network. The second one, Protocol WU_Min, being scheduled by an asynchronous daemon, is self-stabilizing to asynchronous unison in at most D rounds. Moreover, both are optimal in space. The amount of required space is independent of any local or global information on the tree. Furthermore, they work on dynamic trees networks, in which the topology may change during the execution. A cluster based hierarchical wireless sensor network architecture is proposed to facilitate more than one application sharing the whole or a part of a wireless sensor network, where each application may have its own network, processing requirements and protocols. Such a network is divided into clusters and the clusters are organized hierarchically. For synchronizing the CHWSN there is the requirement for a cluster based hierarchical time synchronization algorithm. None of the already existing time synchronization algorithms satisfy the needs of time synchronization in our CHWSN architecture. Thus the existing time synchronization algorithms TPSN (time synchronization protocol for sensor networks) and FTSP (Flooding Time Synchronization Protocol) are modified to fulfill the needs of time synchronization in CHWSN architecture and developed the cluster based hierarchical, flooding time synchronization algorithm (CBH-FTS). It is a hybrid algorithm, where instead of flooding the synchronization messages to neighbor's node, the root node multicasts the time-sync message to selected cluster-heads using the relevant semantics. Hierarchy of cluster-heads could transmit the synchronization messages down the hierarchy of cluster-heads thus synchronizing only the required part of the network associated with an application. The synchronization could be a result of a decision at root node or could be a result of a request for synchronization from a node to a cluster-head.The CBH-FTS Protocol is semantic driven and covers multiple levels in hierarchy. Previously proposed host-based privacy protection mechanisms use pseudorandom or disposable identifiers on some or all layers of the protocol stack. These approaches either require changes to all hosts participating in the communication or do not provide privacy for the whole protocol stack or the system. Building on previous work, we propose a relatively simple approach: protocol stack virtualization. The key idea is to provide isolation for traffic sent to the network. The granularity of the isolation can be, for example, flow or process based. With process based granularity, every application uses a distinct identifier space on all layers of the protocol stack. This approach does not need any infrastructure support from the network and requires only minor changes to the single host that implements the privacy protection mechanism. To show that no changes to typical applications are required, we implemented the protocol stack virtualization as a user space daemon and tested it with various legacy applications.