Sangki Yun

Towards Zero Retransmission Overhead: A Symbol Level Network Coding Approach to Retransmission

We present SYNC, a physical layer transmission scheme that drastically reduces the cost of retransmission by introducing network coding concepts to symbol level operation. It piggybacks a new packet on each retransmitted packet, and exploits the previously received packet (possibly with error) at the receiver to recover both the retransmitted packet and the piggybacked packet. The piggybacking is achieved through higher modulation, but it does not decrease the decodability of the mixed packets owing to the previously received packet at the receiver, which can be analytically shown. SYNC works independently of other PHY level performance boosting schemes such as channel coding and spatial diversity. The proof-of-concept SYNC prototype has been implemented on a software defined radio (SDR) platform. The measurement data shows that under the same channel condition SYNC achieves 110 percent and 42 percent median throughput gain over traditional retransmission and SOFT, respectively. We also show that SYNC can be used proactively where the feedback as to the success of the previous transmission is not available, such as in broadcast.

Multi-Room IPTV Delivery through Pseudo-Broadcast over IEEE 802.11 Links

The IEEE 802.11 wireless LAN (WLAN) is a timetested technology, but it is still evolving towards even higher speeds and richer features. For multi-room IPTV delivery, where multiple IPTVs are fed from a main set-top box (STB), the IEEE 802.11 WLAN is an attractive choice for wirelessly connecting the STB and the IPTVs. In this paper, however, we first show that both the standard 802.11 unicast and broadcast/multicast delivery mechanisms have efficiency and reliability issues when they are applied to the multi-room IPTV delivery. In order to resolve the issues, we propose to use so called the pseudo-broadcast, where a unicast transmission is promiscuously received by multiple receivers. By dynamically changing the unicast destination based on the 802.11k measurement, the scheme also minimizes losses experienced by the promiscuous receivers. The scheme can be implemented without any change in the 802.11 standard, and provides a cost-effective solution to high-reliability and highefficiency multi-room IPTV delivery.

Boosting Video Capacity of IEEE 802.11n through Multiple Receiver Frame Aggregation

The technology based on the IEEE 802.11 standard has been hugely successful, and is evolving towards even higher speeds and richer features. In particular, the 802.11n amendment of the standard aims to achieve the physical layer (PHY) rate of 600 Mbps. Although the 802.11n offers sufficient bandwidth to support high-resolution video applications such as high definition TV (HDTV), we find that the number of video streams that can be supported on the IEEE 802.11n networks depends heavily on how the frame aggregation is implemented. In addition to the frame aggregation scheme stipulated in the amendment, we explore a multiple-receiver frame aggregation scheme for video traffic. The comparative study reveals through extensive simulation that the proposed multiple-receiver aggregation scheme increases the number of supported video streams by a factor of 2 or higher. We also shed light on the qualitative difference in the dynamics of the two approaches. Whereas the aggregation efficiency worsens with traffic increase in the point-to-point aggregation (which is highly undesirable), the proposed multiple- receiver aggregation exhibits resiliency against congestion, by matching the aggregation efficiency to the traffic load.

100+ VoIP Calls on 802.11b: The Power of Combining Voice Frame Aggregation and Uplink-Downlink Bandwidth Control in Wireless LANs

The bandwidth efficiency of voice over IP (VoIP) traffic on the IEEE 802.11 WLAN is notoriously low. VoIP over 802.11 incurs high bandwidth cost for voice frame packetization and MAC/PHY framing, which is aggravated by channel access overhead. For instance, 10 calls with the G.729 codec can barely be supported on 802.11b with acceptable QoS - less than 2% efficiency. As WLANs and VoIP services become increasingly widespread, this inefficiency must be overcome. This paper proposes a solution that boosts the efficiency high enough to support a significantly larger number of calls than existing schemes, with fair call quality. The solution comes in two parts: adaptive frame aggregation and uplink/downlink bandwidth equalization. The former reduces the absolute number of MAC frames according to the link congestion level, and the latter balances the bandwidth usage between the access point (AP) and wireless stations. When used in combination, they yield superior performance, for instance, supporting more than 100 VoIP calls over an IEEE 802.11b link. The authors demonstrate the performance of the proposed approach through extensive simulation, and validate the simulation through analysis.

Hop-by-Hop Frame Aggregation for VoIP on Multi-Hop Wireless Networks

The already severe inefficiency problem in running VoIP on the IEEE 802.11-based wireless LAN is exacerbated in multi-hop networks due to spatial interference. This paper presents a novel scheme that significantly mitigates the effect of the interference by combining the inter-call aggregation and the pseudo-broadcast technique as used in network coding. As such, the proposed scheme works effectively not just for calls traveling the same routed path but also for calls crossing each other inside the multi-hop networks. We demonstrate through extensive simulation that the IEEE 802.11-based multi-hop network can be boosted up to 700% in terms of the number of supportable VoIP calls, with acceptable QoS, through the proposed scheme.

Maximizing Differentiated Throughput in IEEE 802.11e Wireless LANs

The throughput performance of the distributed coordination function (DCF) of the IEEE 802.11 MAC protocol quickly degrades as the number of contending stations increases. To solve this problem, it has been shown recently that adaptive contention window modulation based on channel idle time tracking can be used, generating near optimal throughput. In this paper, we extend the approach for the IEEE 802.11e network, where different QoS classes are defined. We show how to find the class-specific optimal contention window sizes that yield the maximum aggregate throughput while maintaining the target throughput difference between classes

WLC34-1: A Simple Congestion-Resilient Link Adaptation Algorithm for IEEE 802.11 WLANs

Algorithmic approach to link adaptation for IEEE 802.11 networks such as Automatic Rate Fallback (ARF) is known to suffer from the inability to differentiate between collision and channel-induced error. In this paper, we propose a novel algorithm called COLA that overcomes the shortcoming and achieves near-optimal throughput over wide range of channel and load conditions. The result is significant since the throughput is achieved without any hardware support. Moreover, COLA does not require any optional or extra-protocol mechanisms support, either, such as RTS/CTS exchange, Clear Channel Assessment (CCS), and promiscuous channel monitoring. Finally, the COLA algorithm has a short critical path of just 10 instructions, and it is free of heuristic parameters, which will facilitate practical use.

Solving the Coupon Collector's Problem for the Safety Beaconing in the IEEE 802.11p WAVE

For the WAVE applications built on safety beacons, it is imperative that the neighboring vehicle information be collected as quickly and efficiently as possible. But the unreliability of broadcast transmission and the randomness of channel access in the IEEE 802.11p MAC hamper the collection process. Specifically, the process suffers from essentially a form of the classic Coupon Collectors Problem, where it takes longer and longer to obtain the remaining information. In this paper, we solve the problem by introducing the application-level acknowledgement of the safety beacons. We demonstrate that this optimization drastically reduces the collection completion time, eventually contributing to the safety and efficiency in WAVE- based systems.

Sidewalk: A RFID Tag Anti-Collision Algorithm Exploiting Sequential Arrangements of Tags

Although predicting the RFID tag distribution before a read cycle begins would be generally difficult and even futile, a likely and interesting scenario is where the tags have a sequential arrangement. In large-scale applications such as supply chain management, for instance, hundreds or thousands of the same type of product made by the same manufacturer can be stacked together in the stock. In this paper, we propose a simple RFID tag anti-collision algorithm that exploits such sequential structure of the given tag space to achieve a higher read efficiency. The proposed algorithm is also designed to still achieve the level of performance comparable to that of existing algorithms if tags exhibit little sequential structure. Through exhaustive simulations over various tag distributions, we demonstrate that the proposed algorithm achieves at least 10% improvement even in the worst case (i.e., when tags are completely randomly distributed), but yields significantly higher improvement when the number of tags and/or the correlation increases.

Boosting VoIP capacity of wireless mesh networks through lazy frame aggregation

A novel method of voice frame aggregation for wireless mesh networks is presented. In the method, the degree of aggregation is automatically regulated by the congestion level on the wireless link. On the IEEE 802.11-based mesh network, it is shown to yield approximately twice the call capacity, while incurring no additional delay for frame aggregation.