Fu-Min Chang

Modified vacation policy for M/G/1 retrial queue with balking and feedback

This paper studies a general retrial queue with balking and Bernoulli feedback, where the server operates a modified vacation policy. If the server is busy or on vacation, an arriving customer either enters an orbit with probability b, or balks (does not enter) with probability 1-b. Otherwise the service of the arriving customer commences immediately. At any service completion epoch, the test customer may either enter the orbit for another service with probability p or leave the system with probability 1-p. If the orbit is empty, the server takes at most J vacations until at least one customer is recorded in the orbit when the server returns from a vacation. This retrial system has potential applications in e-mail system and WWW server. By applying the supplementary variable technique, some important performance measures are derived. The effect of various retrial/vacation time distributions and different values of J on the system performance measures is also investigated. The analyses and results presented in this paper may be useful for network system designers and software system engineers.

On a batch retrial model with J vacations

This paper considers a batch arrival retrial queue with general retrial times under a modified vacation policy. Any arriving batch finding the server busy or on vacation enters an orbit. Otherwise, one customer from the arriving batch obtains the service immediately while the rest join the orbit. If the orbit is empty, the server takes at most J vacations repeatedly until at least one customer appears in the orbit upon returning from a vacation. If the orbit is nonempty when the server returns from a vacation, the server waits idly for customers from the orbit or new arrivals. On the other hand, if no customers are recorded in the orbit at the end of the Jth vacation, the server waits for new customers to arrive. This retrial system has potential applications in packet-switched networks. By applying the supplementary variable technique, some important system characteristics are derived. The results presented in the paper may be useful for network system designers and software system engineers.

Analysis of a discrete-time queue with server subject to vacations and breakdowns

This paper studies a Geo/G/1 queue in which the server operates multiple vacations and may break down while working. After all messages have been served in the system, the server leaves for a vacation. Upon returning from the vacation, the server activates immediately to serve if there are some messages waiting in the queue. Otherwise, the server takes another vacation. This queueing system has potential application in the inbound email contact center. By using the generating function technique, we derive some important system characteristics, such as system size distributions of server on vacation/busy/broken-down, the expected lengths of server vacation period and complete period, and messages waiting time in the queue. In particular, a cost model is developed to determine the optimal parameter values of different vacation distributions that minimize the cost, and some numerical examples are presented for illustrative purpose.

An Efficient Handover Mechanism by Adopting Direction Prediction and Adaptive Time-to-Trigger in LTE Networks

In this paper, we propose an efficient handover mechanism for Long Term Evolution (LTE) networks. Both moving direction prediction and Time-to-Trigger (TTT) adjustment are taken into consideration for processing to lower the unnecessary handover. By referencing previous locations, user equipment’s (UE) moving direction is estimated by the cosine function to determine a better destined E-UTRAN NodeB (eNB). With adaptive TTT, we categorize UEs into 3 levels according to the velocity. Initially, TTT value is given for each level, and the value is decremented when the handover failure rate gets higher. In addition, we also combine both approaches of moving direction prediction and adaptive TTT into handover procedure. Simulation results show that the proposed method can effectively improve the handover efficiency. As compared with the standard handover procedure, an average reduction of 31% handovers can be obtained by applying the moving direction prediction. And, as compared with fixed TTT, up to the 36.5% of handover trigger failure rate can be expected by using the adaptive TTT handover triggering approach. The performance can be further improved when both approaches are taken in dealing with the handover procedure.

A slot-based BS scheduling with maximum latency guarantee and capacity first in 802.16e networks

IEEE 802.16e standard defines a wireless broadband access network technology. The standard specifies the QoS support at the MAC level. To meet the QoS requirements, the scheduling algorithm at base station (BS) has to efficiently allocate slots for each connection request. The standard does not specify the detailed algorithm but leaves it open for implementation. In this paper, a Slot-based BS scheduling algorithm with Maximum Latency Guarantee and Capacity First (SMLG-CF) is proposed. With SMLG-CF, the connection is chosen with highest slot capacity first. With the assistance of dynamic sub-frame adjustment, the average system transmission rate can be enhanced. Through the finer slots calculation and proper allocation of transmission time and frequency, the maximum latency guarantee can be achieved for urgent requests. We simulate and compare the proposed mechanism with deficit fair priority queue (DFPQ) scheduling algorithm and Highest Urgency First (HUF) scheduling algorithm in terms of maximum latency violation rate and average transmission rate. The simulation results reveal that the applicability of the proposed mechanism.

A moving direction prediction-assisted handover scheme in LTE networks

The 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) defines a wireless network standard for high packet transmission rate and low packet latency provisions. Handover is one of the important features for helping user equipments (UEs) to roam between LTE networks. However, LTE networks adapt a make-before-break handover procedure, which may cause a brief disconnection, therefore results in the packet transmission delay and packet loss problems. In this paper, we propose a moving direction prediction-assisted handover scheme for LTE networks to lower the number of handovers. We first track the location of user equipments (UEs) to predict their moving direction. By referencing previous locations, the next moving direction of UEs is estimated with the cosine function in order to determine the candidate E-UTRAN NodeBs (eNBs) for handover. Then, a target eNB is selected from the candidate eNBs through an angle-based dynamic weight adjustment scheme. By selecting a proper target eNB for handover, thus the quality of network transmission can be enhanced. Simulation results demonstrate the ability of the proposed scheme in reducing 17% average handover times, compared with the standard handover procedure, thereby reducing 12% average number of packet loss and 5% average packet delay time.

Economic application in a Bernoulli F-policy queueing system with server breakdown

For maintaining the service quality of working place, some kinds of arriving customers control policy are needed. This paper investigates a randomised arriving control policy of finite capacity queuing system with an unreliable server. In the investigated system, when the number of customers reaches the system’s capacity, further arriving customers are not allowed to enter the system. As the queue length decreases to a predefined threshold value, customers may either be allowed entering the system with an exponential startup time or the customers are still unable to enter the system. For such a system, we develop the steady-state solutions and some important system characteristics, such as the probability that the server is busy, effective arrival rate, expected waiting time in the system, etc. We also develop a cost model to determine the optimal control policy at a minimum cost. A sensitivity analysis is also conducted to investigate the effects of changes in the system parameters on the expected cost function. Finally, we present some managerial insights through an application example of the printed circuit boards manufacturing system.

A beacon-based handover scanning mechanism with QoS support in WiMAX FemtoCell architecture

Deploying FemtoCells in WiMAX network, also called WiMAX FemtoCell architecture, gains a lot of attention due to better indoor services can be provided and WiMAX network traffic can be shared by the FemtoCell. However, since the signal of WiMAX base station (BS) is stronger than FemtoCell Base Station (fBS), the handover procedure may not be triggered even though fBS is within the BS coverage. Besides, since the coverage of FemtoCell is small, it is possible that a huge number of fBSs are deployed in a WiMAX BS coverage, which causes a large volume of power consumption to scan all indoor fBSs for a handover. In this paper, we propose a beacon-based handover scanning mechanism with Quality of Service (QoS) support. In the proposed mechanism, through the beacon and modified MOB_NBR-ADV messages, a neighbor cell list with QoS parameters is generated automatically at mobile station (MS). The decision criteria for triggering the handover procedure and selecting an appropriate target indoor cell from many candidate fBSs are developed. Through the adoption of the proposed mechanism, the system throughput can be improved and the traffic load through the WiMAX BS can be reduced effectively. A simulation was conducted using QualNet simulator. The simulation results demonstrate that along with the number of FemtoCells increases, both the system throughput and the utilization of FemtoCells increase significantly, as compared with conventional handover mechanisms.

An efficient uplink scheduling mechanism with enabling Multi-device Transmission and Maximum Latency Fulfillment in IEEE 802.16j networks

IEEE 802.16j spreads out the coverage of the WiMAX network via extending the signal transmission by using the relay technology. To take the advantage of relaying in IEEE 802.16j network, an efficient schedule with quality of service (QoS) provision for multiple link transmissions is necessary, especially when link interference exists. In this paper, we propose an uplink scheduling mechanism with enabling Multi-device Transmission and Maximum Latency Fulfillment (MT-MLF). In the mechanism of MT-MLF, an interference detection mechanism is first carried out, then a resource distribution algorithm and a dynamic frame adjustment method are developed. Through the adoption of the proposed scheduler, both the uplink resource utilization and the requirement of real time applications can be better enhanced. Two simulation experiments were conducted with different levels of interference. The results demonstrate that under a fixed QoS type of connections, when the total number of connections goes up to 360 and 420 and the maximum latency violation rate approaches 20%, the average uplink transmission rate of MT-MLF can achieve 6.67Mbps and 7.92Mbps, which apparently outperform 4Mbps and 3.91Mbps as compared with regular relay scheduling schemes.

Optimization of Machine Repair System with Controlling Arrival and Switching Failure

This study investigates the machine repair problem involving a controlling arrival policy and standbys switching failure. The operation of controlling arrival policy is that when the number of customers reaches the maintenance facility’s capacity, no further arriving failed machines are allowed to enter the system until the system capacity decreases to a certain threshold value. We develop the steady-state solutions and some import system characteristics, such as the expected number of failed machines in the system, machine availability, the average switching failure rate, effective arrival rate, and expected waiting time in the system, etc. A cost model is developed to determine the optimal control policy at a minimum cost. A sensitivity analysis is also conducted to investigate the effect of changes in the system parameters on the expected cost function. Finally, we present some managerial insights through an application example.