63 research outputs found
Partially shared buffers with full or mixed priority
This paper studies a finite-sized discrete-time two-class priority queue. Packets of both classes arrive according to a two-class discrete batch Markovian arrival process (2-DBMAP), taking into account the correlated nature of arrivals in heterogeneous telecommunication networks. The model incorporates time and space priority to provide different types of service to each class. One of both classes receives absolute time priority in order to minimize its delay. Space priority is implemented by the partial buffer sharing acceptance policy and can be provided to the class receiving time priority or to the other class. This choice gives rise to two different queueing models and this paper analyses both these models in a unified manner. Furthermore, the buffer finiteness and the use of space priority raise some issues on the order of arrivals in a slot. This paper does not assume that all arrivals from one class enter the queue before those of the other class. Instead, a string representation for sequences of arriving packets and a probability measure on the set of such strings are introduced. This naturally gives rise to the notion of intra-slot space priority. Performance of these queueing systems is then determined using matrix-analytic techniques. The numerical examples explore the range of service differentiation covered by both models
Regularity and asynchrony when tapping to tactile, auditory and combined pulses
This research is carried out with the aim to develop assistive
technology that helps users following the beat in music, which is of
interest to cohchlear implant users. The envisioned technology would
use tactile feedback on each musical beat. However, this raises
fundamental questions about uni- and cross-modal perception which
are not addressed in similar context in the literature. The aim of this
study was i) to find out how well users are able to follow tactile
pulses. ii) To gain insights in the differences between auditory,
tactile and combined auditory-tactile feedback. A tapping experiment
was organized with 27 subjects. They were requested to tap along
with an auditory pulse, a tactile pulse and a combined auditory-tactile
pulse in three different tempi. An evaluation with respect to
regularity and asynchrony followed. Subjects were found to perform
significantly better in terms of reqularity and asynchrony for the
auditory and auditory/tactile condition with respect to the tactile only
condition. Mean negative asynchrony (MNA) for auditory and
combined (auditory and tactile) conditions were in the range of
previous studies. The MNA’s for the tactile conditions showed a
remarkable dependence on tempo. In the 90BPM condition a clear
anticipation (-20ms) was reported, for the 120BPM condition the
mean was around zero, the 150BPM condition showed a positive
MNA (a reaction vs anticipation). An effect that could be
encorporated into the design of an assistive technology
On the effect of combining cooperative communication with sleep mode
Cooperation is crucial in (next-generation) wireless networks as it can greatly attribute to ensuring connectivity, reliability, performance, ... Relaying looks promising in a wide variety of network types (cellular, ad-hoc on-demand), each using a certain protocol. Energy efficiency constitutes another key aspect of such networks, as battery power is often limited, and is typically achieved by sleep mode operation. As the range of applications is very broad, rather than modelling one of the protocols in detail, we construct a high-level model capturing the two essential characteristics of cooperation and energy efficiency: relaying and sleep mode, and study their interaction. The used analytical approach allows for accurate performance evaluation and enables us to unveil less trivial trade-offs and to formulate rules-of-thumb applicable across all potential scenarios
Uncovering the evolution from finite to infinite high-priority capacity in a priority queue
Infinite capacity queues are often used as approximation for their finite real-world counterparts as they are mathematically tractable. It is generally known that tail probabilities of low-priority system content in a two-class priority queue with infinite capacity for customers of both priority classes can be non-exponential, even if the interarrival time and service time distributions are exponentially decaying. In contrast, when the capacity for the high-priority customers is finite, tail probabilities of low-priority system content are always exponentially decaying. Therefore, using the results for one as an (accurate) approximation for the other is not obvious. From an analytical point of view, the non-exponentiality in the infinite case is caused by the arisal of an implicitly defined function, a root of the kernel, in the probability generating function for the low-priority system content. However, up till now, it has been unclear how this non-exponentiality suddenly emerges when taking the limit from to the finite to the infinite case. Our main contribution is that, under the restriction of a maximum of two arrivals per slot, a recurrence relation in the high-priority capacity is constructed resulting in an explicit expression for the corresponding generating function for the finite case. Amazingly, this expression contains all roots of the kernel in the infinite case. Taking the limit of this expression leads to the well-known behavior for the infinite case as the root inside the complex unit circle dominates the other roots uncovering the evolution from the finite to the infinite case. Furthermore, we investigate under which circumstances the standard tail characterizations are inaccurate
Comparison of the Chondrogenic Potential of Mesenchymal Stem Cells Derived from Bone Marrow and Umbilical Cord Blood Intended for Cartilage Tissue Engineering
International audienceOsteoarthritis (OA) remains incurable in humans or horses and mesenchymal stromal/stem cells (MSCs) represent an attractive solution for producing a neocartilage substitute. However, the best MSC source still needs to be identified. This study compared the chondrogenic potential of equine MSCs derived from bone marrow (BM) and umbilical cord blood (UCB), at their undifferentiated status to check if one cell source is better proned, and after chondrogenic-induced differentiation. Chondrogenesis was induced by culture in collagen scaffold with BMP-2 + TGF-ß1 in hypoxia or normoxia. MSCs chondrogenic potential was evaluated using the mRNA and corresponding protein levels for osteogenic, hypertrophic and chondrogenic markers. MSCs characterization demonstrated that BM- and UCB-MSCs differ in proliferation and tripotencies. At undifferentiated status, they also showed differences in their expression of osteogenic, chondrogenic and hypertrophic markers. Upon chondrogenesis induction, both MSCs sources exhibited increased chondrogenic expression and produce an extracellular matrix (ECM) of better quality in hypoxia, although collagen I remained expressed. UCB-MSCs produced higher amounts of collagen II, particularly its IIB isoform, than BM-MSCs, but also collagen I and Htra1, regardless of the oxygen condition. Finally, immunohistochemistry revealed that the BM-MSCs synthesized an ECM of higher quality, regarding the more homogenous distribution of type IIB collagen, compared to UCB-MSCs. Considering collagen I as the major undesirable component in the neo-synthesis of in vitro cartilage, we recommend using BM-MSCs for horse cartilage engineering
Influence of real-time queue capacity on system contents in Diffserv's expedited forwarding per-hop-behavior
This paper studies a single-server non-preemptive priority queue with two traffic classes in order to model Expedited Forwarding Per-Hop Behavior in the Differentiated Services (DiffServ) architecture. Generally, queueing models assume infinite queue capacity but in a DiffServ router the capacity for high priority traffic is typically small to prevent this traffic from monopolizing the output link and hence causing starvation of low-priority traffic. The presented model takes the exact (finite) high-priority queue capacity into account. Analytical formulas for the system content of each class are determined as well as the high-priority packet loss ratio. For each class, service of a packet takes a (different) general independent distribution. The issues this causes are resolved by using spectral decomposition. Numerical examples indicate the considerable impact of the finite capacity on system performance
Tail behaviour of a finite-/infinite-capacity priority queue
Priority queues have been studied extensively throughout the last few decades. One of the main performance measures of interest is the loss probability, caused by the evident finiteness of queues. From analytical point of view, finite-capacity queues are more complex to study than queues with infinite capacity. Therefore, the most common approach for studying the loss probability is to analyze the tail behaviour of the corresponding infinite-capacity system. From this behaviour, the loss probability of the finite-capacity system can be approximated. However, this approximation can be rather inaccurate.
Consider a system with two priority classes where the low-priority queue has infinite capacity but the high-priority queue can only accommodate N packets. In the system where both queues have infinite capacity, the tail
behaviour of the queue content of the low-priority class can be geometric or non-geometric, dependent on whether the dominant singularity of the respective generating function is a pole or a branch point. However, in the system with a finite high-priority queue the tail is always geometric,
even when N is large. Investigating this discrepancy is the topic of our contribution.
In the finite system, the low-priority queue content can be expressed by a matrix equation. These matrices contain (partial) generating functions. Finding the poles corresponds to solving a non-linear eigenvalue problem. However, finding all poles is infeasible for larger N. Therefore, a numerical method is used to obtain the poles in a certain region (around the dominant pole)
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