Mehmet Akif Yazici

The workload-dependent MAP/PH/1 queue with infinite/finite workload capacity

We propose a numerical algorithm for finding the steady-state queue occupancy distribution for a workload-dependent MAP/PH/1 queue in which the arrival process and the service rate depend continuously on the instantaneous workload in the system. Both infinite and finite queue capacity scenarios are considered, including partial rejection and complete rejection policies for the latter. Using discretization, this system is approximately described by a multi-regime Markov fluid queue for which numerical algorithms are available. The computational complexity of the proposed method is linear in the number of regimes used for discretization. We provide numerical examples to validate the proposed approach.

Analysis of continuous feedback Markov fluid queues and its applications to modeling Optical Burst Switching

Optical Burst Switching (OBS) has been proposed as a candidate technology for the next-generation Internet. In OBS, packets are assembled into a burst, and a burst control packet is sent in advance to inform and reserve resources at the optical nodes in the path of the burst. In this study, we analyze the horizon-based reservation scheme in OBS using Markov fluid queues. First, we provide a solution to continuous feedback Markov fluid queues, then we model the horizon-based reservation scheme as a continuous feedback Markov fluid queue and numerically study it. We provide numerical examples to validate our model and its solution technique as well as to obtain some insight on the horizon-based reservation mechanism.

The finite/infinite horizon ruin problem with multi-threshold premiums: a Markov fluid queue approach

We present a new numerical method to obtain the finite- and infinite-horizon ruin probabilities for a general continuous-time risk problem. We assume the claim arrivals are modeled by the versatile Markovian arrival process, the claim sizes are PH-distributed, and the premium rate is allowed to depend on the instantaneous risk reserve in a piecewise-constant manner driven by a number of thresholds, i.e., multi-threshold premiums. We introduce a novel sample path technique by which the ruin problems are shown to reduce to the steady-state solution of a certain multi-regime Markov fluid queue. We propose to use the already existing numerically efficient and stable numerical algorithms for such Markov fluid queues. Numerical results are presented to validate the effectiveness of the proposed method regarding the computation of the finite- and infinite-horizon ruin probabilities for risk models including those with relatively large number of thresholds.

Performance modeling of QoS differentiation in optical packet switching via FDL access limitation

We present an exact analytical model for single-wavelength quality of service (QoS) differentiation in a two-class optical packet switch. In this system, QoS differentiation is achieved by limiting the set of fiber delay lines (FDLs) to the low-priority class, whereas the high-priority class is allowed to access the entire FDL bank. The analytical model is based on multi-regime Markov fluid queues and is extensible to multi-class systems with more than two classes. Markovian arrival process packet arrivals and phase-type distributed packet sizes are considered for the purpose of generality. The proposed analytical model is validated through simulations. The numerical results provide insight into determining appropriate subsets of FDLs allowed for the access of the low-priority class. The results also show that it is possible to direct almost all the packet losses to the low-priority class under moderate loads.

Fluid flow analysis and buffer management of video playout

The playout buffer occupancy of a mobile user watching a video on a cellular wireless network is modeled analytically as Markov fluid queue. The probability of video freeze that is observed upon buffer drain is computed depending on adaptive playout rate, rate adaptation threshold and pre-play buffering. An optimization problem concerning these three parameters is formulated.

A hybrid asymmetric traffic classifier for deep packet inspection systems with route asymmetry

A flow is said to be asymmetrically routed if its packets follow separate paths for forward and reverse directions. Routing asymmetry leads to problems in flow identification, policy enforcement, quota management, traffic shaping etc. in DPI systems. There are two existing approaches to battle routing asymmetry: clustering and state sharing. The latter fails with stateless traffic, while clustering leads to large traffic overhead. We propose the Hybrid Asymmetric Traffic Classifier (HATC) method that merges the best aspects of the two existing methods. HATC is able to handle all types of asymmetric traffic with reduced overhead compared to clustering. Numerical evaluation of HATC using two real traffic traces is also presented.

A distributed solution for air-time fairness in multi-rate WLANs

WLANs employing IEEE802.11 standards suffer from a performance anomaly that drags down the fast nodes in the WLAN due to the slow nodes. Air time fairness has been proposed to battle this anomaly. We propose a novel distributed method to achieve air time fairness in this study.

Running Multiple Instances of the Distributed Coordination Function for Air-Time Fairness in Multi-Rate WLANs

Conventional multi-rate IEEE 802.11 Wireless LANs (WLANs) are associated with the so-called performance anomaly to describe the phenomenon of high bit rate nodes being dragged down by slower nodes. This anomaly is known to be an impediment to obtaining high cumulative throughputs despite the employment of effective link adaptation mechanisms. To cope with the performance anomaly, air-time fairness has been proposed as an alternative to throughput fairness, the latter being a main characteristic of the IEEE 802.11 Distributed Coordination Function (DCF). In this paper, we propose a novel distributed air-time fair MAC (Medium Access Control) without having to change the operation of the conventional DCF. In the proposed MAC, each node in the system runs multiple instances of the conventional DCF back-off algorithm where the number of DCF instances for the nodes can be chosen in a distributed manner. Both analytical and simulation-based results are provided to validate the effectiveness of the proposed air-time fair MAC.

Markov zinciri tabanlı Güvenilirlik modellemesi

Çok-akişli eşler-arasi video, gelecek nesil isteğe bağli video sistemleri iÇin kullanilmaya aday teknolojilerdendir. Çok-akişli video sistemlerinin sorunlarindan birisi, eş dalgalanmasi adi verilen, eşlerin ağla olan bağlantilarinin kopup düzelmesi durumudur. Eş dalgalanmasinin bütşn eşler-arasi sistemlerde olumsuz etkileri olduğu bilinmektedir. Bu etkinin hafifletilmesi iÇin Çeşitli hata kontrollü alt-akiş kodlama teknikleri önerilmiğtir. Bu makalede, eş dalgalanmasi ve hata kontrolünün rolünü incelemek amaciyla sürekli zamanli Markov zinciri tabanli bir rassal model ünerilmektedir. Bu model kullanilarak, hata kontrolünün rolü, gelecek nesil Çok-akişli eşler-arasi isteğe bağli video sistemlerinin tasariminda kullanilmak üzere nicel olarak ortaya konmaktadir.