Hui-Nien Hung

An Adaptive GTS Allocation Scheme for IEEE 802.15.4

IEEE 802.15.4 is a new standard uniquely designed for low-rate wireless personal area networks. It targets ultralow complexity, cost, and power for low-rate wireless connectivity among inexpensive, portable, and moving devices. IEEE 802.15.4 provides a guaranteed time slot (GTS) mechanism to allocate a specific duration within a superframe for time-critical transmissions. This paper proposes an adaptive GTS allocation (AGA) scheme for IEEE 802.15.4, which considers low latency and fairness. The scheme is designed based on the existing IEEE 802.15.4 medium access control protocol, and IEEE 802.15.4 devices can receive this AGA service without any modification. A simulation model and an analytical model are developed to investigate the performance of our AGA scheme. The numerical results show that the proposed scheme significantly outperforms the existing IEEE 802.15.4 implementation.

Modeling UMTS Power Saving with Bursty Packet Data Traffic

The universal mobile telecommunications system (UMTS) utilizes the discontinuous reception (DRX) mechanism to reduce the power consumption of mobile stations (MSs). DRX permits an idle MS to power off the radio receiver for a predefined sleep period and then wake up to receive the next paging message. The sleep/wake-up scheduling of each MS is determined by two DRX parameters: the inactivity timer threshold and the DRX cycle. In the literature, analytic and simulation models have been developed to study the DRX performance mainly for Poisson traffic. In this paper, we propose a novel semi-Markov process to model the UMTS DRX with bursty packet data traffic. The analytic results are validated against simulation experiments. We investigate the effects of the two DRX parameters on output measures including the power saving factor and the mean packet waiting time. Our study provides inactivity timer and DRX cycle value selection guidelines for various packet traffic patterns.

A comprehensive analysis of low-power operation for beacon-enabled IEEE 802.15.4 wireless networks

ZigBee, a unique communication standard designed for low-rate wireless personal area networks, has extremely low complexity, cost, and power consumption for wireless connectivity of inexpensive, portable, and moving devices. ZigBee uses the IEEE 802.15.4 standard as its communication protocol for medium access control (MAC) layer and physical (PHY) layer. The IEEE 802.15.4 MAC layer achieves duty-cycle operations by setting two system parameters, macBeaconOrder (BO) and macSuperFrameOrder (SO), to achieve low power consumption for ZigBee devices. This study comprehensively analyzes IEEE 802.15.4 duty-cycle operation. Specifically, a novel analytical model that accommodates a general traffic distribution is developed. An NS-2 based simulation model, which is validated by the developed analytical model is also proposed. Through the experiments conducted by the analytical and simulation models, some important performance-evaluation insights are gained that can be used as guidelines for future low-power ZigBee network deployment.

A Bayesian procedure for process capability assessment

The usual practice of judging process capability by evaluating point estimates of some process capability indices has a flaw that there is no assessment on the error distributions of these estimates. However, the distributions of these estimates are usually so complicated that it is very difficult to obtain good interval estimates. In this paper we adopt a Bayesian approach to obtain an interval estimation, particularly for the index Cpm. The posterior probability p that the process under investigation is capable is derived; then the credible interval, a Bayesian analogue of the classical confidence interval, can be obtained. We claim that the process is capable if all the points in the credible interval are greater than the pre-specified capability level ω, say 1.33. To make this Bayesian procedure very easy for practitioners to implement on manufacturing floors, we tabulate the minimum values of Ĉpm/ω, for which the posterior probability p reaches the desirable level, say 95%. For the special cases where the process mean equals the target value for Cpm and equals the midpoint of the two specification limits for Cpk, the procedure is even simpler; only chi-square tables are needed. Copyright

Supplier selection for one-sided processes with unequal sample sizes

In this paper, we consider the supplier selection problem which deals with comparing two one-sided processes and selecting the one that has a significantly higher capability value. We present an exact approach to tackle the supplier selection problem. Testing hypotheses with two phases for comparing two processes are considered. Critical values of the tests are calculated to determine the selection decisions. Sample size required for a designated selection power and confidence level is also investigated. The results provide useful information to practitioners. The proposed approach generalizes the existing method by not requiring equal sample sizes for the two processes. An application example on comparing two wavelength division multiplexer (WDM) manufacturing processes is presented to illustrate the practicality of our approach to actual data collected from the factories.

Modeling Credit Reservation Procedure for UMTS Online Charging System

The IP multimedia core network subsystem (IMS) provides real-time multimedia services for Universal Mobile Telecommunications System (UMTS). Through recharge threshold-based credit reservation (RTCR) mechanism, prepaid IMS services can be supported by the online charging system (OCS) in UMTS. In RTCR, when the remaining amount of prepaid credit is below a threshold, the OCS reminds the user to recharge the prepaid account. It is essential to choose an appropriate recharge threshold to reduce the probability that the in-progress service sessions are forced-terminated. An analytic model is developed to investigate the performance of RTCR for the OCS. Based on our study, the network operator can select the appropriate parameter values for various traffic conditions.

Modeling VoIP Call Holding Times for Telecommunications

Voice over IP is one of the most popular applications in broadband access networks. It is anticipated that the characteristics of call holding times (CHTs) for VoIP calls will be quite different from traditional phone calls. This article analyzes the CHTs for mobile VoIP calls based on measured data collected from commercial operation. Previous approaches directly used the Kolmogorov-Smirnov (K-S) test to derive the CHT distributions, which may cause inaccuracy. In this article we propose a new approach to derive the CHT distributions for mobile VoIP calls and other call types. Specifically, our approach uses hazard rate to select an appropriate distribution, and then utilizes the K-S test to validate our selection. We show that the mobile VoIP CHT distribution can be accurately approximated by a mix of two log-normal distributions. Based on the derived distributions, we compare the mobile VoIP CHTs with those for non-VoIP calls and fixed-network VoIP calls. Our study indicates that the characteristics for mobile VoIP calls are quite different from those of the non-VoIP mobile phone calls and are more close to those of fixed-network phone calls.

A note on Bayesian estimation of process capability indices

Process capability indices are useful for assessing the capability of manufacturing processes. Most traditional methods are obtained from the frequentist point of view. We view the problem from the Bayes and empirical Bayes approaches by using non-informative and conjugate priors, respectively.

Random number generation for excess life of mobile user residence time

In a mobile telecommunications network, the period when a mobile station (MS) resides in a cell (the radio coverage of a base station) is called the cell residence time of that MS. The period between when a call arrives at the MS and when the MS moves out the cell is called the excess life of the cell residence time for that MS. In performance evaluation of a mobile telecommunications network, it is important to derive the excess life distribution from the cell residence times. This distribution determines if a connected call will be handed over to a new cell, and therefore significantly affects the call dropping probability of the network. In mobile-telecommunications-network simulation, generating the excess-life random numbers is not a trivial task, which has not been addressed in the literature. This paper shows how to generate the random numbers from the excess life distribution, and develop the excess-life random number generation procedures for cell residence times with gamma, Pareto, lognormal, and Weibull distributions. This paper indicates that the generated random numbers closely match the true excess-life distributions

Resolving mobile database overflow with most idle replacement

In a personal communications service (PCS) network, mobility databases called visitor location registers (VLRs) are utilized to temporarily store the subscription data and the location information for the roaming users. Because of user mobility, it is possible that the VLR is full when a mobile user arrives. Under such a circumstance, the incoming user has no VLR record and thus cannot receive PCS services. This issue is called VLR overflow. To resolve the VLR overflow problem, a VLR record can be selected for replacement when the VLR is full and then the reclaimed storage is used to hold the record of the requesting user. This paper considers the most idle replacement policy to provide services to mobile users without VLR records. In this policy, the record with the longest idle time is selected for replacement. We propose an analytic model to investigate the performance of this replacement policy. The analytic results are validated against simulation experiments. The results indicate that our approach effectively resolves the VLR overflow problem.