Munhwan Choi

Performance analysis of IEEE 802.16m sleep mode for heterogeneous traffic

We numerically analyze the performance of the emerging 802.16 ms sleep mode operation in order to gain a new insight regarding its power consumption and traffic transmission delay when a Mobile Station (MS) in the sleep mode is served with both non-realtime and realtime traffic simultaneously. We validate the analysis via the comparison with simulation results.

Multiple WNIC-based handoff in IEEE 802.11 WLANs

We propose a novel scanning scheme for IEEE 802.11 by equipping Access Points (APs) with multiple Wireless Network Interface Cards (Multi-WNICs), one of which is set to operate in an exclusively reserved channel for the scanning purpose. In this environment, a STAtion (STA) can easily search neighboring APs by scanning the reserved channel. Our simulation results demonstrate that the proposed scheme ultimately reduces the overall scanning time to improve the handoff latency.

Video multicast over WLANs: Power saving and reliability perspectives

Video streaming service via multicast is one promising technology for multimedia services over wireless local area networks. Given that many portable devices are battery-powered while video quality relies on the reliable delivery of video traffic, video multicast frames need to be reliably delivered to power-saving stations. In this article, we study video multicast from the power saving and reliability perspectives. The IEEE 802.11 standard allows multicast receivers to enable power management while receiving multicast frames. However, from measurements with several commercial WLAN devices, we observe that many devices are not standards- compliant, thus severely degrading video multicast performance when multicast receivers are in the power saving mode. We identify various standards-non-compliant malfunctions of both access points and stations, and then categorize these malfunctions. Meanwhile, the legacy 802.11 standard is known to support only unreliable multicast service due to the lack of retransmissions. We present various reliable multicast schemes including application layer forward error correction and reliable multicast protocols defined in the emerging IEEE 802.11v and 802.11aa, and comparatively evaluate them via simulations.

Empirical analysis of video multicast over WiFi

Video multicast service is becoming one of the most important applications over WiFi, due to the increasing popularity of WiFi for multimedia communication. On the one hand, energy efficient operation is required to WiFi technology due to the limited battery power of most WiFi-equipped devices. In this paper, we empirically study the video multicast operation, especially, along with power management operation in two aspects: (1) whether commercial WiFi devices correctly operate as defined in IEEE 802.11 standard and (2) what problem the standard-compliant operation can induce. From our experimental results, we first figure out that some of commercial WiFi devices do not follow the standard with respect to the power saving operation, and this noncompliance worsens interoperability. We also find that the standard-compliant operation may cause significant delay of voice over IP (VoIP) traffic, when video multicast coexists with VoIP. Through this experimental study, we provide the guidelines for energy-efficient video multicast service.

Fast scanning schemes for IEEE 802.11 WLANs in virtual AP environments

Abstract Recently, provisioning a fast handoff in IEEE 802.11 Wireless Local Area Network (WLAN) has been an attractive research issue since the incurring latencies are unsatisfactory to support today’s fast emerging realtime services with stringent Quality of Service (QoS) requirements. The 802.11 handoff consists of (1) scanning, (2) authentication, and (3) reassociation. Particularly, the scanning has been studied intensively since it accounts for a major portion of the handoff latency due to potential overheads in the scanning operation. That is, (1) a scanning STAtion (STA) should stay in a scanned channel waiting for Access Points (APs)’ responses without any assurances of APs’ existence after request frame transmissions; moreover, (2) per-channel waiting time is not explicitly defined in the 802.11 standard. In order to reduce the overheads, we propose novel scanning schemes composed of two phases: (1) channel selection phase; and (2) AP search phase. In the channel selection phase, a scanning STA identifies and selects the channels suitable for the 802.11 scanning by assessing all employed channels with Request-to-Send/Clear-to-Send (RTS/CTS) handshaking in a virtual AP environment. Then, in the AP search phase, it performs a unicast-based scanning in order to search for an AP suitable for its handoff in a selected channel. For a proper handoff decision depending on STA’s requirement, we build two algorithms referred to as first-fit and near best-fit algorithms, respectively. We demonstrate the superiority of our proposed schemes to existing approaches via simulations with realistic time-varying channel models in various simulation environments. The simulation results demonstrate that the proposed scanning schemes provide relatively shorter handoff latencies as (1) a scanning STA has shorter channel switching time; (2) an 802.11 WLAN employs more channels; and (3) the APs are deployed more densely.

VPAL: Video Packet Adaptation Layer for reliable video multicast over IEEE 802.11n WLAN

In this paper, we propose a scheme, called Video Packet Adaptation Layer (VPAL), for reliable video multicast over the IEEE 802.11n WLAN. VPAL is composed of (1) Raptor coding for reliable video transmission, (2) header compression and (3) packet aggregation, both for efficient video transmission. Most of the VPAL functionalities reside above the emerging IEEE 802.11n Medium Access Control (MAC) layer while the packet aggregation requires some changes in the MAC functionalities. The reliability of the video multicast under a strict delay requirement, is provided by achieving the target error probability of video packets, which is done by controlling both the Raptor code rate and the physical (PHY) layer transmission rate. This strategy can provide a satisfactory quality of multicast video service irrespective of the channel condition with a minimum bandwidth use. New features of the 802.11n MAC are utilized for the channel status feedback from the users. Redundant header fields in the video packets are compressed, and then these packets are aggregated to further reduce the protocol overheads. We also consider a reduced version of VPAL which does not require any change in the MAC functionalities and simply works with the IEEE 802.11n MAC. The performance of the proposed systems is comparatively evaluated in terms of the perceived video quality, i.e., peak signal-to-noise ratio (PSNR), as well as the amount of required resources via both numerical analysis and simulations.

Opportunistic Spatial Reuse in IEEE 802.15.3c Wireless Personal Area Networks

The IEEE 802.15.3c wireless personal area network (WPAN) standard is designed to support highly directional wireless communications at the 60-GHz frequency band. Highly directional communications are helpful to reduce interference; hence, great potentials are provided for aggregate network throughput improvement via spatial reuse (SR). However, in 802.15.3c WPANs, the SR is limited in use since the 802.15.3c standard does not specify an explicit procedure for the SR. We design a new and simple scheme exploiting the SR, thus improving aggregate network throughput. In the proposed scheme, the 802.15.3c devices measure and report the measured channel status. Then, a controlling device, which is called the piconet coordinator (PNC) in the 802.15.3c WPAN, schedules resources available for each device. The simulation results demonstrate that the proposed scheme contributes to significant aggregate network throughput improvement. The proposed scheme is so simple that it can be easily employed for practical 802.15.3c devices. In addition, it is guaranteed to work well without conflicting with existing 802.15.3c operations.

Supporting VoIP Services in IEEE 802.11e WLANs

Voice over Internet Protocol (VoIP) over Wireless Local Area Network (WLAN) is becoming popular thanks to its cost efficiency. However, it has been a challenge to provide good quality of VoIP services in WLANs, which is due mainly to (i) the nature of contention-based channel access of WLAN Medium Access Control (MAC); (ii) the presence of coexisting non-real-time data traffic; and (iii) the time-varying WLAN capacity caused by transmission rate diversity and variation of stations over time. In this paper, we propose a simple, effective and viable solution to improve the quality of VoIP services in 802.11e contention-based WLANs, which basically utilizes the advanced features of 802.11e MAC for QoS support. The key ingredients of our solution include (i) a priority queue to serve the VoIP traffic with higher priority than the non-real-time data traffic; and (ii) a conservative history-based admission control scheme for VoIP services, which accommodates the transmission rate diversity and variation of ongoing VoIP sessions over time. Simulation results demonstrate that our solution admits as many VoIP calls as possible without compromising the quality of their services.

Coherence Time Estimation for Performance Improvement of IEEE 802.11n Link Adaptation

IEEE 802.11n standard provides a framework for new link adaptation. A station can request that another station provide a Modulation and Coding Scheme (MCS) feedback, to fully exploit channel variations on a link. However, if the time elapsed between MCS feedback request and the data frame transmission using the MCS feedback becomes bigger, the previously received feedback information may be obsolete. In that case, the effectiveness of the feedback-based link adaptation is compromised. If a station can estimate how fast the channel quality to the target station changes, it can improve accuracy of the link adaptation. The contribution of this paper is twofold. First, through a thorough NS-2 simulation, we show how the coherence time affects the performance of the MCS feedback based link adaptation of 802.11n networks. Second, this paper proposes an effective algorithm for coherence time estimation. Using Allan variance information statistic, a station estimates the coherence time of the receiving link. A proposed link adaptation scheme considering the coherence time can provide better performance.