Jong-Suk Ahn

Dynamic calendar queue

Discrete event simulations need a priority queue sorting events according to their timestamp to process events in their time order. As the number of events increases, the choice of data structure for this event list can affect the simulation performance significantly. A calendar queue is a data structure popularly used in most discrete event simulators due to its O(1) time complexity regardless of the number of stored events. Calendar queues, however, perform poorly over skewed event distributions due to the static resize algorithm and the inappropriate selection of events for measuring the degree of the event distribution. To improve the calendar queues performance over uneven event distributions, this paper proposes two new mechanisms. We call our calendar queue adopting these two mechanisms DCQ (dynamic calendar queue). Our experiment results showed that DCQ can achieve an order of magnitude speedup for uneven distributions while performing as well over even distributions as the conventional calendar queue.

TCP-like flow control algorithm for real-time applications

This paper proposes an efficient flow control algorithm for RTP/RTCP which has been employed as a transport layer protocol for real-time applications over the Internet. As the Internet becomes a common transport vehicle for various applications, some of them require a new flow control different from TCP since they do not need the expensive error recovery inseparable from the current TCP. The new flow control algorithm for real-time applications should efficiently utilize network bandwidth and also minimize the delay jitter while not hampering the performance of the existing TCP applications. This paper introduces a flow control algorithm which dynamically adjusts the sending rate similarly to TCP based on the network status evaluated by two network parameters; packet loss and round trip time (RTT) reported by RTP/RTCP. For the reduction of delay jitters, the algorithm tries to maintain RTTs periodically reported by RTCP within a predetermined range by either additively increasing or multiplicatively decreasing the sending rate as much aggressively as TCP. For evaluation, we compare the behavior of our algorithm to that of RAP, one of representative flow control algorithms for real-time applications. The simulation experiments show that our algorithm achieves the same degree of performance for TCP as RAP but without overloading the network with ack packets like RAP.

An Adaptive FEC Algorithm for Mobile Wireless Networks

large portion of packets due to propagation errors. To improve reliability over noisy wireless channels, wireless networks can employ forward error correction (FEC) techniques. Static FEC algorithms, however, can degrade the performance by poorly matching their overhead to the degree of the underlying channel error, especially when the channel path loss rate fluctuates widely. This paper investigates the benefits of an adaptable FEC mechanism for wireless networks with severe packet loss. We show that our adaptive FEC technique improves the performance by dynamically tuning FEC strength to the current amount of wireless channel loss. We quantify these benefits through a hybrid simulation integrating packet-level simulation with bit-level details and validate the simulation model through experimentation.

Performance and energy consumption analysis of 802.11 with FEC codes over wireless sensor networks

This paper expands an analytical performance model of 802.11 to accurately estimate throughput and energy demand of 802.11-based wireless sensor network (WSN) when sensor nodes employ Reed-Solomon (RS) codes, one of block forward error correction (FEC) techniques. This model evaluates these two metrics as a function of the channel bit error rate (BER) and the RS symbol size. Since the basic recovery unit of RS codes is a symbol not a bit, the symbol size affects the WSN performance even if each packet carries the same amount of FEC check bits. The larger size is more effective to recover long-lasting error bursts although it increases the computational complexity of encoding and decoding RS codes. For applying the extended model to WSNs, this paper collects traffic traces from a WSN consisting of two TIP50CM sensor nodes and measures its energy consumption for processing RS codes. Based on traces, it approximates WSN channels with Gilbert models. The computational analyses confirm that the adoption of RS codes in 802.11 significantly improves its throughput and energy efficiency of WSNs with a high BER. They also predict that the choice of an appropriate RS symbol size causes a lot of difference in throughput and power waste over short-term durations while the symbol size rarely affects the long-term average of these metrics.

Performance Analysis of a Non-Overlapping Binary Exponential Backoff Algorithm over IEEE 802.15.4

This paper evaluates the effects of the Extended Non-Overlapping Binary Exponential Backoff (ENO-BEB) algorithm over IEEE 802.15.4 by building its performance models based on a 2-dimensional Markov chain. This algorithm differs from the previously proposed Non-Overlapping Binary Exponential Backoff (NO-BEB) algorithm for IEEE 802.11, where the next backoff range is fixed as the second half of the conventional exponentially enlarged range. The ENO-BEB algorithm maps the next backoff range to the last 1/2 j -th subrange of the conventional range where j is an integer standing for the number of consecutive channel capture failures. To measure its impacts of the degree of separation between two backoff ranges at two adjacent backoff stages, we generalize the conventional IEEE 802.15.4 Markov chain model by including the behavior of the ENO-BEB algorithm. The analytical performance model predicts that the ENO-BEB technique achieves better throughput for larger j, for example, up to 113 % and 21 % than the conventional BEB and NO-BEB algorithm, respectively when j and the total number of nodes are 3 and 60. Simulations confirm these numerical results with a 7 % difference.

X-MAC protocol with Collision Avoidance algorithm

This paper introduces X-MAC/CA protocol combining X-MAC protocol with CA (Collision Avoidance) algorithm with an aim to maximally randomize transmissions in overcrowded networks. X-MAC protocol, one of typical asynchronous MAC protocols for WSNs (Wireless Sensor Networks) allowing each node to independently and periodically sleep, is characterized as sending a sequence of short preambles until an early acknowledgement comes back. This feature is designed to save more energy by permitting the receiver to notify the sender that it is awake. X-MAC/CA protocol is designed to reduce collisions, especially when a large number of nodes in densely populated WSNs simultaneously deliver data once they sense some events to report. For its accurate measurement, this paper also extends a conventional mathematical model for X-MAC protocol to include the effect of CA algorithm. The extended model predicts that X-MAC/CA protocol can improve the throughput of X-MAC by up-to 30% in 40-node networks.

Dynamic lazy calendar queue: an event list for network simulation

Proposes a new calendar queue which can improve the conventional calendar queues performance over uneven event distributions. A calendar queue is a multi-list priority queue which is frequently employed in discrete event simulations as the global event list, since its performance shows O(1) time complexity. For O(1) performance, calendar queues maintain only a small number of events at each list of their multi-list by constantly adjusting their multi-list size depending on the number of enqueued events and redistributing events over the newly resized multi-list. Calendar queues, however, perform poorly over skewed event distributions. Our proposed calendar queue can reduce the conventional calendar queues sensitivity to event distributions by adding two new mechanisms. The first mechanism constantly measures the event distribution and, according to the measured metrics, reconfigures the calendar queues multi-list to maintain O(1) performance even for uneven distributions. The second mechanism adopts an additional data structure to save the time wasted in frequent resizing of calendar queues. Our conducted experiments show that our calendar queue can achieve more than a 10-fold speedup for uneven event distributions while maintaining the same performance for other distributions.

Performance Evaluation of an Adaptive Congestion Avoidance Algorithm for IEEE 802.15.4

Like most wireless network protocols, IEEE 802.15.4 adopts a CA (Congestion Avoidance) algorithm to avoid early collisions of new frames by randomizing its first transmission delay instead of delivering them immediately. The traditional CA scheme of IEEE 802.15.4 selects its random access time from the predetermined range without considering the degree of underlying network congestion. This static behavior often incurs a long delay to settle in the suitable range for the current network load or frequent clashes especially when heavy traffic lasts long. This paper proposes an ACA(Adaptive Congestion Avoidance) algorithm which dynamically adjusts the initial back off range according to the measurement of the average collision rate. It also introduces an analytical model to predict the performance spectrum in which ACA algorithm¡¯s throughput falls. With only small deviations from corresponding simulations, our model shows that ACA scheme can improve the throughput of IEEE 802.15.4 by up-to 41% while shortening the frame delays.

Analytical performance evaluation of IEEE 802.15.4 with multiple transmission queues for providing QoS under non-saturated conditions

This paper investigates the effects of adopting multiple transmission queues in IEEE 802.15.4 to differentiate frame sending priorities for providing QoS (Quality of Service) services. By employing different contention window operational parameters for various traffic classes like IEEE 802.11e, the multi-queue system can send frames of higher priority class more rapidly than those of the lower ones. For the performance evaluation of multi-queues behavior under both saturated and unsaturated traffic conditions, this paper proposes an analytical model which integrates models for conventional 802.15.4 and legacy 802.11e. To enhance the accuracy, furthermore, the model accommodates the deferment behaviors and transmission retries that have not been accounted in traditional 802.15.4 models. Our analytical model predicts that the multi-queue scheme separates the throughput of two different classes by up to 46% without wasting the bandwidth when the two classes contend over the wireless channel. Simulation results validate our models accuracy within 7% discrepancy at maximum.

An Analytical Model for GTS Service Delay of IEEE 802.15.4 with Two Priority Queues

ABSTRACT IEEE 802.15.4 reserves transmission time to support real-time transport by sending GTS request packets to the PAN coordinator in advance. This paper introduces GTS-FAT technique to reduce the reservation time by giving a higher sending priority to GTS request packets than data packets. Differently from the conventional scheme where these two kinds of packets share a single transmission queue, GTS-FAT scheme allocates two queues with two different contention window sizes like IEEE 802.11e. This paper also proposes an analytical GTS delay model by combining the two legacy models for 802.15.4 and 802.11e to accurately predict the GTS-FAT delay over a given network topology. Our analysis shows that GTS-FAT reduces GTS service delay by up to 50% at the expense of the data delay by only up to 6.1% when GTS request packets four times outnumber data packets.Keywords:GTS-FAT, 802.15.4, real-time applications, priority, GTS request, delay 1. 서 론 1) 저속의 무선 근거리 개인 네트워크(LR-WPANs: Low Rate Wireless Personal Area Networks)를 지원하기 위해 국제전기전자학회(IEEE: Institute of Electrical and Electronics Engineers)에서는 802.15.4 표준[1]을 정의하였다. 802.15.4 표준은 802.11[2] 랜과는 달리 전원을 교체하기 어려운 저전력