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Channel Assignment in Wireless Mobile Ad Hoc Networks
Published in Jonathan Loo, Jaime Lloret Mauri, Jesús Hamilton Ortiz, Mobile Ad Hoc Networks, 2016
Javad Akbari Torkestani, Mohammad Reza Meybodi
The polling is an ODCA scheme in which a centralized controller queries the hosts, in a cyclic predetermined order, whether they have data to transmit or not. Due to the recent advances in communication systems, some other variations of the polling scheme have also been considered. These variations deal with noncyclic allocation policies, which include random, Markovian, or, more generally, nondeterministic allocation policies. In a polling scheme, controller polls (one by one) the hosts to give them an opportunity to access the medium. The hosts that have no packet to be transmitted (or do not need the channel access) decline, and the other hosts begin the packet transmission upon receiving the query. In polling scheme, the centralized controller is responsible for coordinating the transmissions, and so polling is a collision-free scheme. In this scheme, the entire bandwidth is available for the host that is permitted to transmit data. Although in realistic scenarios, traffic load of the different hosts is not the same, the major drawback of the basic polling scheme is to give the same importance (or equal access to the channel) to all hosts. A prioritized polling system may provide better results. Furthermore, the polling scheme suffers from the substantial overhead caused by the large number of messages generated by the controller to query the communicating hosts. As mentioned earlier, polling is based on a centralized control system. Therefore, in ad hoc networks, due to the lack of fixed infrastructures and centralized administrations, polling cannot be a practical channel assignment policy. Clustering the ad hoc networks in which the network is subdivided into several nonoverlapping groups is a promising approach to solve the above-mentioned problem. In clustered multihop ad hoc networks, the cluster head assumes the role of a centralized controller.
Analysis and approximation for the performance of a workstation with various types of setups
Published in International Journal of Production Research, 2018
A typical polling system consists of a number of queues attended by a single server who visits the queues in some order to render service to customers waiting in the queues, typically incurring a changeover time when moving from one queue to another. Federgruen and Katalan (1996) characterised the effect of the mean and distribution of changeover setup times. They proved that all of the moments of waiting times and queue lengths are reduced if the higher order moment of the changeover setup times is reduced, while maintaining their means. Takagi (1988, 2000) summarised the applications of polling models under the assumptions of cyclic order and Poisson arrivals, and discussed queueing models with exhaustive, gated, limited or decrementing service dispatching rules. The cyclic order can be generalised and replaced by a periodic service order table or a polling table (Eisenberg 1972; Baker and Rubin 1987). However, these models generally assume that the setup time distribution only depends on the future (or current) product type but do not consider the case where the distributions of changeover setups depend on both the current and future product types in manufacturing systems.