Joon Yoo

EBA: an enhancement of the IEEE 802.11 DCF via distributed reservation

The IEEE 802.11 standard for wireless local area networks (WLANs) employs a medium access control (MAC), called distributed coordination function (DCF), which is based on carrier sense multiple access with collision avoidance (CSMA/CA). The collision avoidance mechanism utilizes the random backoff prior to each frame transmission attempt. The random nature of the backoff reduces the collision probability, but cannot completely eliminate collisions. It is known that the throughput performance of the 802.11 WLAN is significantly compromised as the number of stations increases. In this paper, we propose a novel distributed reservation-based MAC protocol, called early backoff announcement (EBA), which is backward compatible with the legacy DCF. Under EBA, a station announces its future backoff information in terms of the number of backoff slots via the MAC header of its frame being transmitted. All the stations receiving the information avoid collisions by excluding the same backoff duration when selecting their future backoff value. Through extensive simulations, EBA is found to achieve a significant increase in the throughput performance as well as a higher degree of fairness compared to the 802.11 DCF.

An opportunistic relay protocol for vehicular road-side access with fading channels

In the drive-thru Internet access systems, vehicles connect to road-side access points (APs) to use IP-based services, such as web-browsing, e-mail, and file download, in addition to the customized vehicular applications. However, the mobility of vehicles and the limited coverage of APs result in the short connectivity duration and low throughput, thus leading to low availability of Internet to vehicle services. Vehicle-to-vehicle (V2V) relay support is an attractive backup solution that can address these limitations by extending the coverage. To fully realize the benefit of V2V relay support, however, the vehicle that gives the best performance must be selected as relay, yet the dynamic wireless channel conditions and the high speed of vehicles render relay selection a challenging problem. In this paper, we evaluate several relay strategies in an analytic framework to compute the resulting overall network capacity with fading channels. We then propose and devise an efficient opportunistic relay protocol that exploits multiuser diversity and effectively copes with the dynamic channel. Through both capacity analysis and Qualnet simulations, we show that the opportunistic relay scheme significantly outperforms others.

A novel performance analysis model for an IEEE 802.11 wireless LAN

This letter presents a novel analytic model that accurately evaluates the performance of a single-hop IEEE 802.11 wireless LAN (WLAN). By using a closed queuing network, we model an IEEE 802.11 WLAN system that consists of a fixed number of stations and derive the saturated throughput of the IEEE 802.11 distributed coordination function (DCF). The ns-2 simulation results show that our new analysis model is very accurate in evaluating the performance of the IEEE 802.11 DCF.

IA-TCP: A rate based incast-avoidance algorithm for TCP in data center networks

In recent years, the data center networks commonly accommodate applications such as MapReduce and web search that inherently shows the incast communication pattern; multiple workers simultaneously transmit TCP data to a single aggregator. In this environment, the TCP performance is significantly degraded in terms of goodput and query completion time, as a result of the severe packet loss at Top of Rack (ToR) switches. The TCP senders aggressively transmit packets causing throughput collapse even though the network pipe size, i.e., bandwidth-delay product, is extremely small. In this paper, we introduce a novel end-to-end congestion control algorithm called IA-TCP that avoids the TCP incast congestion problem effectively. IA-TCP employs the rate-based algorithm at the aggregator node, which controls both the window size of workers and ACK delay. Through extensive NS-2 simulations, we validate that our algorithm is scalable in terms of the number of workers achieving enhanced goodput and zero timeouts.

A Distributed Fair Scheduling Scheme With a New Analysis Model in IEEE 802.11 Wireless LANs

In this paper, we study the performance of 802.11 wireless local area networks (WLANs) and propose a distributed packet scheduling scheme that improves the short-term fairness among stations in a WLAN. First, we present a completely different analytical model to analyze the performance of 802.11 distributed coordination function (DCF), which models the channel contention dynamics of an IEEE 802.11 system, rather than the individual user perspective adopted by most previous analytical works based on Markov chains. Our model is much simpler than previous Markov chain models that have been used popularly to model the DCFs binary exponential backoff (BEB) procedure. In our model, the backoff stage and the value of the backoff counter are represented as a queueing system and the residual service time at the queue, respectively. Then, we model an entire 802.11 WLAN as a closed queuing network and derive the performance of the 802.11 DCF based on the queueing theory. Next, by exploiting the analytic results, we develop a simple distributed runtime estimation scheme that effectively calculates the number of contending stations. Finally, we propose a distributed packet scheduling scheme that improves the short-term fairness to the IEEE 802.11 network. The scheme utilizes the token bucket mechanism and controls the packet arrival rate sent down to the medium access control (MAC) layer from the logical link control (LLC) layer by adjusting the token-generation rate according to the measured channel contention level. Performance evaluation results show that our scheduling scheme provides a higher degree of short-term fairness as well as throughput improvement.

On the hidden terminal problem in multi-rate ad hoc wireless networks

Multi-hop ad hoc wireless networks generally use the IEEE 802.11 Distributed Coordination Function (DCF) MAC protocol, which utilizes the request-to-send/clear-to-send (RTS/CTS) mechanism to prevent the hidden terminal problem. It has been pointed out that the RTS/CTS mechanism cannot completely solve the hidden terminal problem in ad hoc networks because the interference range could exceed the basic rate transmission range. In this paper we provide a worst-case analysis of collision probability induced by the hidden terminal problem in ad hoc networks with multi-rate functionality. We show that the interference caused by the nodes in the area that is not covered by the RTS/CTS is bounded by C′R−4, where C′ is a constant and R is the distance between the two transmitting nodes. The analytic result showed that the interference could shorten the data transmission range up to 30 percent. We then propose a simple multi-rate MAC protocol that could prevent the hidden terminal problem when transmit power control (TPC) is employed.

Joint uplink/downlink opportunistic scheduling for Wi-Fi WLANs

Recent advances in the speed of multi-rate wireless local area networks (WLANs) and the proliferation of WLAN devices have made rate adaptive, opportunistic scheduling critical for throughput optimization. As WLAN traffic evolves to be more symmetric due to the emerging new applications such as VoWLAN, collaborative download, and peer-to-peer file sharing, opportunistic scheduling at the downlink becomes insufficient for optimized utilization of the single shared wireless channel. Furthermore, without proper scheduling on the uplink, the downlink throughput gain diminishes proportionally to the increasing number of clients transmitting on the uplink. However, opportunistic scheduling on the uplink of a WLAN is challenging because wireless channel condition is dynamic and asymmetric. Each transmitting client has to probe the access point to maintain the updated channel conditions at the access point. Moreover, the scheduling decisions must be coordinated at all clients for consistency. This paper presents JUDS, a joint uplink/downlink opportunistic scheduling for WLANs. Through synergistic integration of both the uplink and the downlink scheduling, JUDS maximizes channel diversity at significantly reduced scheduling overhead. It also enforces fair channel sharing between the downlink and uplink traffic. Through analysis and extensive QualNet simulations, we show that JUDS improves the overall throughput by up to 127% and achieves both fairness between uplink and downlink traffic.

Packet scheduling for Multipath TCP

Multipath TCP (MPTCP) has been an emerging transport protocol as it can greatly improve application throughput by utilizing multiple network interfaces at the same time, e.g., both of WiFi and 3G/LTE. While MPTCP is generally beneficial for long-lived flows, it shows worse performance than SPTCP that exploits the best path when the flow size is small, e.g., only hundreds of KB. In this case, it would be better to use only the fastest path since the delay is much more important than network bandwidth in such small data delivery. The problem is that the existing default MPTCP packet scheduler may choose a slow path if the congestion window of the fast path is not available, resulting in a long flow completion time. To avoid this problem, we propose a new MPTCP packet scheduler that freezes the slow path temporarily when the delay difference between the slow and fast paths is significant, so that the small amount of data can be transmitted quickly via the fast path. We implement the proposed scheduler into the MPTCP Linux kernel and evaluate on our testbed and compare to the default packet scheduler. Through the experiments, we confirm that the proposed scheme significantly reduces the flow completion time for short flows.

A VANET routing based on the real-time road vehicle density in the city environment

The intelligent transportation system (ITS) can enhance the drivers safety by providing safety-related information such as traffic conditions and accident information to drivers. The vehicular ad hoc network (VANET) is an essential technology for the deployment of ITS. And, for the reliable delivery of safety-related information to vehicles in the VANET, a reliable VANET routing protocol is required. VANET routing is challenging since it is fundamentally different from conventional ad hoc networks; the vehicles move fast, and the network topology changes rapidly causing intermittent link connectivity. In this paper, we propose a routing protocol that works based on the real-time road vehicle density in order to provide fast and reliable communications so that it adapts to the dynamic vehicular city environment. In the proposed routing mechanism, each vehicle computes the vehicle density of the road to which it belongs by using beacon messages and the road information table. Based on the real-time road vehicle density information, each vehicle establishes a reliable route for packet delivery. In order to evaluate the performance of the proposed mechanism, we compare our proposed mechanism with GPSR through NS-2 based simulations and show that our mechanism outperforms GPSR in terms of delivery success rate and routing overhead.

An on-demand energy-efficient routing algorithm for wireless ad hoc networks

Ad hoc networks are non-infrastructure networks which consist of mobile nodes. Since the mobile nodes have limited battery power, it is very important to use energy efficiently in ad hoc networks. In order to maximize the lifetime of ad hoc networks, traffic should be sent via a route that can be avoid nodes with low energy while minimizing the total transmission power. In addition, considering that the nodes of ad hoc networks are mobile, on-demand routing protocols are preferred for ad hoc networks. However, most existing power-aware routing algorithms do not meet these requirements. Although some power-aware routing algorithms try to compromise between two objectives, they have difficulty in implementation into on-demand version. In this paper, we propose a novel on-demand power aware routing algorithm called DEAR. DEAR prolongs its network lifetime by compromising between minimum energy consumption and fair energy consumption without additional control packets. DEAR also improves its data packet delivery ratio.