Roberto Beraldi

A reversible hierarchical scheme for microcellular systems with overlaying macrocells

Future cellular systems are expected to use multilayered, multisized cells to cover non-homogeneous populated areas. An example in literature is given by a 2 level hierarchical architecture in which an overlaying macrocell provides a group of overflow channels utilized when a microcell, which covers a densely populated area, is not able to accommodate a new call, or a handover from another microcell. The macrocell has the higher hierarchical position, meaning that it can receive handover requests from microcells, lower in the hierarchy, as well as from other macrocells. On the contrary, a call served by the macrocell cannot handover to a microcell. This paper proposes a reversible hierarchical scheme characterized by the presence of handover attempts from macrocells to microcells. The scheme is conceived so that the microcells are given the majority of the traffic load as they are able to operate with very high capacity, while the macrocells, having lower channel utilization, can better carry out their support task. An analytical study is carried out showing that the system performance can be improved, at the expense of relatively little increase of network control overhead, when compared with the classical, i.e. nonreversible hierarchical scheme.

Performance of a Reversible Hierarchical Cellular System

In order to provide mobile worldwidecommunication services, cellular systems can be based ona multilayered, multisized cells architecturecharacterized by the use of different technologies.Satellites can cover lightly populated areas, as well asthose areas where other means cannot be used (like thesea); fixed base stations, with different transmissionpowers and thus different cell sizes, can cover relatively highly populated areas. As a result,a given area could simultaneously be served by more thanone Base Station, and the structure of such a systemmust be carefully designed. A Hierarchical Architecture(HA) scheme is a possible solution. HA is based on amultilevel cell configuration: microcells (or evenindoor picocells) cover more densely populated areas andare given the majority of the traffic load as they are able to operate with very highcapacity, while overlaying macrocells (or satellitarcells) provide a group of overflow channels.Occasionally, when microcells are not able to satisfychannel requests, the overflow channels are consumed.HA schemes are divided in two categories: ReversibleHierarchical (RH) and Non Reversible (NRH). Thedifference is given by the handover directions between cells allowed. In a RH handover, attemptsbetween macrocells and microcells occur in bothdirections. This paper compares RH and NRH schemes bymeans of an analytical model based on birth-deathprocesses. The main performance indexes and the controloverhead are shown.

Selective BECN schemes for congestion control of ABR traffic in ATM LAN

There is a growing interest in high speed data transfer by adopting the asynchronous transfer mode (ATM) technique on a local basis. To this end ATM based LANs must provide available bit rate (ABR) services in which a source does not specifies its traffic characteristics, but only its peak rate, and transmits without any bandwidth reservation. Congestion control is a critical issue for such a service. It can be achieved by regulating the actual source transmission rate using backward explicit congestion notification (BECN) information, i.e. control cells transmitted back to the source. Although this feedback mechanism can be used due to the low distance its effectiveness must be carefully studied in order to avoiding wasted bandwidth. Two selective BECN schemes are proposed and extensive simulation results are presented. It is shown that a selective scheme assures a remarkable reduction in the number of BECN cells necessary to obtain the same performance when compared to trivial schemes.

Temporal uncertainty time warp: An implementation based on Java and ActorFoundry

This article describes temporal uncertainty time warp (TUTW), a distributed control engine designed for exploitation of temporal uncertainty (TU) in general optimistic simulations. Novel in TUTW is an event model in which time intervals are attached to events instead of classical punctual or “precise” time stamps. All of this complies with system specifications where the occurrence time of events cannot be known with complete certainty. TUTW is able to take advantage of TU by resolving events in such a way as to reduce the number of rollbacks. The simulator performance can thus be improved without necessarily compromising the accuracy of the results. An agent-based implementation of TUTW was achieved that enables simulations to be carried out over the Internet. The implementation depends on Java and the ActorFoundry middleware. The implementation is totally portable and simplifies configuration and control of a distributed simulation virtual computer that includes heterogeneous computing platforms. The article reports performance and accuracy results of TUTW applied to a large QNET simulation model.

Distributed simulation of timed Petri nets. A modular approach using actors and Time Warp

Focuses on using a modular and actor-based approach to the development of time-dependent distributed systems that separates functional aspects from timing. The Time Warp mechanism regulates the interaction policy among logical processes, shifting the overhead from communication to computation.

A new dynamic reservation multiple access protocol for supporting multimedia traffic in third generation cellular system

A dynamic reservation multiple access (D-RMA) protocol for future third generation cellular mobile radio systems, which is explicitly designed for supporting multimedia traffic, is proposed and its behavior is investigated under variable traffic conditions. Its structure is based on traditional PRMA++ protocol. In addition to PRMA++, a dynamic adaptation of percentage of bandwidth to use for reservation within a frame to traffic condition, is permitted, this having the main goal to guarantee the required grade of service (GoS) to multimedia services. The goal is also achieved by providing for a suitable bandwidth allocation strategy, which assigns to different broadband services a different amount of frequency resource, and by coupling it with a suitable management scheme for the base station reservation buffer. Obtained results show that performance of D-RMA protocols are superior when compared with a traditional non-dynamic protocol, both in terms of offered GoS to carried services and number of activable connections in a microcell at the same time.<<ETX>>

Exploiting temporal uncertainty in time warp simulations

This paper describes a time warp mechanism designed to exploit temporal uncertainty (TU) in distributed simulation. Novel in the proposed approach are: a formal event model where events are assigned time intervals instead of usual punctual timestamps; an aggressive cancellation technique which shifts overheads from communications to computation; and an implementation in Java which deploys a framework for distributed simulations over the Internet. The paper introduces the time warp mechanism and reports some experimental results using a large PCS model. The experiments confirm that TU is able to speedup simulation without compromising the accuracy of the results.

Temporal uncertainty time warp: an agent-based implementation

This paper introduces TUTW - Temporal Uncertainty Time Warp - a control engine designed for an exploitation of temporal uncertainty (TU) in general optimistic simulations, and concentrates on an agent-based implementation of TUTW which enables distributed simulations over the Internet. A novel concept in TUTW is an event model where time intervals are attached to events instead of classical punctual or precise timestamps. All of this reflects typical system specifications where the occurrence time of events cannot be known with complete certainty. TUTW takes advantage of TU by resolving events so that the number of rollbacks is reduced. The simulator performance can thus be improved without necessarily compromising the accuracy of the results. The current agent-based implementation is based on Java and the ActorFoundry middleware. The implementation is totally portable and makes it possible to configure a distributed simulation virtual machine which includes heterogeneous computing platforms. The paper includes an example to illustrate the application of TUTW.

Performance of a Time Warp based simulator of large scale PCS networks

Abstract This paper describes an object-oriented Time Warp (TW) mechanism which supports general parallel simulation on a distributed, possibly heterogeneous, computing environment. As a significant application of the developed TW, a simulation model adequate for large personal communication services (PCS) networks is proposed and its performance results given. Special attention is paid to such TW critical issues as load balancing and checkpointing interval tuning which strongly affect the achievement of good speedups. The experimental results confirm that good performance can be obtained on an heterogeneous distributed system provided an accurate parameter tuning is accomplished.

Analysis of queueing strategies for handoff and originating calls in macro/micro/pico cellular systems

This paper analyzes two handoff stream management algorithms suitable for mobile cellular networks. In order to obtain a high total admitted traffic while guaranteeing the protection of handoff stream, handoff calls (HC) and originating calls (OC) are both queued. The strategies, named buffer rigid partitioning (BRP) and push out of OC (POC), are studied for different percentages of HC, selected according to the radius of the cells. Numeric computations aiming at estimating the optimum buffer size have been carried out. The results are obtained by an exact analytical model for the BPR scheme and by means of simulations for the POC scheme. The performance achieved with BRP and POC schemes is superior with respect to classic schemes based only on queueing of HC.