[show abstract][hide abstract] ABSTRACT: With the increased popularity of mobile multimedia services, efficient and robust video multicast strategies are of critical importance. In a conventional multicast system, the source station transmits at the base rate of the underlying network so that all the nodes can receive the data correctly. The performance of such a multicast system is limited by the node with the worst channel conditions, which usually corresponds to the nodes at the edge of the multicast coverage range. To overcome this problem, we propose a two-hop cooperative transmission scheme where in the first hop the source station transmits the packets and the nodes who receive the packets forward the packets simultaneously in the second hop using randomized distributed space time codes (R-DSTC). We further integrate this randomized cooperative transmission with layered video coding to provide users different video quality based on their channel conditions. The performance of the system is evaluated and compared with a conventional multicast system. Our results show that the proposed cooperative system significantly improves the performance compared to conventional multicast.
[show abstract][hide abstract] ABSTRACT: Carrier sensing has been used as an effective way to reduce collisions and exploit spatial reuse in wireless networks. Previous research has attempted to tune the carrier sensing range to maximize the network throughput. However, the impact of carrier sensing threshold on the probability of successful transmission has been ignored. In this paper, we derive an analytical model to calculate the successful transmission probability. We then calculate the throughput of routing protocols using different link metrics. To the best of our knowledge, this perhaps is the first attempt to derive the throughput of routing protocols like expected transmission count (ETX)  and expected transmission time (ETT)  in wireless networks. We also investigate the impact of some other important factors, such as node density, average contention window size and packet length. Our results show that optimal routing protocols that are using ETT as the path metric can achieve around 30% more throughput than those using ETX and End-to-end delay. Compared to the minimum hop count protocols such as DSR and AODV, the optimal routing protocol can improve the throughput by up to 100%.
Wireless Communications and Networking Conference, 2009. WCNC 2009. IEEE; 05/2009
[show abstract][hide abstract] ABSTRACT: Cooperative networking, by leveraging the broadcast nature of the wireless channel, significantly improves system performance and constitutes a promising technology for next-generation wireless networks. Although there is a large body of literature on cooperative communications, most of the work is limited to theoretical or simulation studies. To impact the next generation of wireless technologies and standards, it is essential to demonstrate that cooperative techniques indeed work in practice. This article describes two programmable cooperative communication testbeds built at Polytechnic Institute of NYU to achieve this goal. The testbeds are based on open-source platforms and enable implementation of cooperative networking protocols in both the physical and the medium access control layer. Extensive experiments carried out using the testbeds suggest not only that cooperative communication techniques can be integrated into current wireless technologies, but also that significant benefits of cooperation can be observed in terms of network throughput, delay, and video quality in real applications.
[show abstract][hide abstract] ABSTRACT: Wireless video multicast enables delivery of popular events to many mobile users in a bandwidth efficient manner. However, providing good and stable video quality to a large number of users with varying channel conditions remains elusive. A promising solution to this problem is the use of packet level (FEC) mechanisms. However, the adjustment of the FEC rate is not a trivial issue due to the dynamic wireless environment. This decision becomes more complicated if we consider the multi-rate capability of the existing wireless LAN technology that adjusts the transmission rates based on the channel conditions and the coverage range. In this paper, we explore the dynamics of Forward Error Correction (FEC) schemes in multi-rate wireless local area networks. We study the fundamental behavior of a 802.11g network which already has embedded error correction in physical layer, under unicast and broadcast modes in a real outdoor environment. We then explore the effectiveness of packet level FEC over wireless networks with multi-rate capability. In order to evaluate the system quantitatively, we implemented a prototype using open source drivers, and ran experiments. Based on the experimental results, we provide guidelines on how to efficiently use FEC for wireless multicast services in order to improve the overall system performance. We argue that even there is a physical layer error correction, using a higher transmission rate together with stronger FEC is more efficient than using a lower transmission rate with weaker FEC for multicast.
[show abstract][hide abstract] ABSTRACT: Video multicast over wireless local area networks (WLANs) faces many challenges due to varying channel conditions and limited bandwidth. A promising solution to this problem is the use of packet level forward error correction (FEC) mechanisms. However, the adjustment of the FEC rate is not a trivial issue due to the dynamic wireless environment. This decision becomes more complicated if we consider the multi-rate capability of the existing wireless LAN technology that adjusts the transmission rates based on the channel conditions and the coverage range. In order to explore the above issues we conducted an experimental study of the packet loss behavior of the IEEE 802.11b protocol. In our experiments we considered different transmission rates under the broadcast mode in indoor and outdoor environments. We further explored the effectiveness of packet level FEC for video multicast over wireless networks with multi-rate capability. In order to evaluate the system quantitatively, we implemented a prototype using open source drivers and socket programming. Based on the experimental results, we provide guidelines on how to efficiently use FEC for wireless video multicast in order to improve the overall system performance. We show that the Packet Error Rate (PER) increases exponentially with distance and using a higher transmission rate together with stronger FEC is more efficient than using a lower transmission rate with weaker FEC for video multicast.
[show abstract][hide abstract] ABSTRACT: Cooperative communication fully leverages the broadcast nature of the wireless channel and spatial diversity, thereby achieving tremendous improvements in system capacity and delay. A cross-layer implementation approach has been pursued in this demonstration, in order to confirm the viability and efficacy of cooperation at the MAC layer, in conjunction with the routing layer, in multi-hop ad-hoc networks. In the cooperative MAC protocol, a station would use a neighboring helper station for MAC layer forwarding, if the two-hop relaying yields to a better performance than a direct single-hop transmission. In this cross layer scheme, the DSDV routing protocol defines a multihop path from the source to the destination, while the cooperative MAC scheme, eventually selects two hop forwarding for each routing layer hop, in order to boost the performance of the routing protocol. The Cooperative MAC scheme has been implemented in the MadWiFi driver, while the DSDV routing protocol has been implemented in the Click modular router. In the demo, a video clip is streamed from a server to a remote client, where the received video is played out in real time. The basic route is discovered by the DSDV routing protocol that runs on every station. The underlying MAC implementation would dynamically alternate between IEEE 802.11 g and the cooperative MAC protocol, for each route hop. In the multi-hop, ad-hoc network, the cooperative cross-layer scheme delivers a smooth user experience while the video playout over the legacy IEEE 802.11 g has noticeable freezes and frequent distortions. The demo verifies the extensibility of the cooperative MAC protocol into multi-hop ad-hoc networks, where in conjunction with the routing protocol, can achieve superior performance, compared to the legacy IEEE 802.11 g.
Testbeds and Research Infrastructures for the Development of Networks & Communities and Workshops, 2009. TridentCom 2009. 5th International Conference on; 01/2009
[show abstract][hide abstract] ABSTRACT: Cooperative communication is a promising approach to improve the reliability of a received signal at the physical layer. Cooperating nodes create a virtual MIMO system that provides spatial diversity even though the nodes have a single antenna. Therefore, tremendous improvements in system capacity and delay can be achieved. An implementation approach has been pursued in this demonstration to confirm the viability and efficacy of cooperation at the physical layer. The implemented cooperative physical layer scheme is called CoopPHY and is based on cooperative coding. In this technique, the source node transmits only a portion of its encoded data bits based on channel conditions among cooperating nodes. Helper nodes transmit the rest of the encoded bits. The destination node, finally, combines and decodes signals received via different channels and therefore increases the decoding reliability. In this demo, a Matlab video clip is streamed frame by frame from a source to destination node using sequentially the direct communication scheme, relaying via one helper, relaying via another helper and finally cooperative scheme. CoopPHY protocol delivers a smooth user experience, while the video playout over the direct scheme shows frequent distortions. In addition to this cooperative coomunication shows better quality than just relaying via any of the helpers.
[show abstract][hide abstract] ABSTRACT: There has been a tremendous increase in demand for real-time video applications over military networks. Multicast provides an efficient solution for simultaneous content delivery to a group of users. It is especially valuable for military applications, as it saves network resources by sharing the data streams across receivers. Even with ever increasing channel bandwidth and computation power, efficiently multicasting video over the tactical edge is still challenging due to factors such as higher packet loss ratio, bandwidth variations and the heterogeneity of the users. In this paper, we explore the use of omni-directional relays to improve the performance of wireless video multicast in tactical environments. We focus on assessing the trade-off between total relay energy, coverage area and video quality. The results provide achievable operational regions, which can serve as a reference and a starting point for system design.
Military Communications Conference, 2008. MILCOM 2008. IEEE; 12/2008
[show abstract][hide abstract] ABSTRACT: It has been shown in the literature that many MAC protocols for wireless networks have a considerable control overhead, which limits their achievable throughput and delay performance. In this paper, we study the problem of improving the efficiency of MAC protocols. We first analyze the popular p- Persistent CSMA scheme and show that it does not achieve 100% throughput.Motivated by insights from polling system theory, we then present three polling service-based MAC schemes, termed PSMACs, for improved performance. The main idea is to serve multiple data frames after a successful contention resolution, thus amortizing the high control overhead and making the protocols more efficient. We present analysis and simulation studies of the proposed schemes. Our results show that PSMAC can effectively improve the throughput and delay performance of p-Persistent CSMA, as well as providing energy savings. We also observe that PSMAC is more efficient for handling the more general and challenging bursty traffic and outperforms p-Persistent CSMA with respect to fairness.
IEEE Transactions on Wireless Communications 12/2008; · 2.42 Impact Factor
[show abstract][hide abstract] ABSTRACT: We consider the design of an open P2P live-video streaming system. When designing a live video system that is both open and P2P, the system must include mechanisms that incentivize peers to contribute upload capacity. We advocate an incentive principle for live P2P streaming: a peerpsilas video quality is commensurate with its upload rate. We propose substream trading, a new P2P streaming design which not only enables differentiated video quality commensurate with a peerpsilas upload contribution but can also accommodate different video coding schemes, including single-layer coding, layered coding, and multiple description coding. Extensive trace-driven simulations show that substream trading has high efficiency, provides differentiated service, low start-up latency, synergies among peers with different Internet access rates, and protection against free-riders.
Network Protocols, 2008. ICNP 2008. IEEE International Conference on; 11/2008
[show abstract][hide abstract] ABSTRACT: In the next-generation WiMAX system, cooperative communication is being considered as an advanced technique to increase the throughput and improve the signal quality. In a cooperative scenario, multiple stations can jointly emulate the antenna elements of a multi-input multi-output (MIMO) system in a distributed fashion. Unlike conventional space-time coding (STC) mechanisms used by a IEEE 802.16e antenna array, distributed space-time coding (DSTC) is employed across the cooperating stations to achieve a higher spatial diversity gain. In this paper, we present the framework for DSTC in the emerging relay-assisted WiMAX network, and develop a cooperative MAC layer protocol, called CoopMAX, for DSTC deployment in a WiMAX system. Through extensive simulations, we evaluate the performance of CoopMAX and show that DSTC can yield capacity gains of up to about 50% for the uplink of an IEEE 802.16 network.
Broadband Communications, Networks and Systems, 2008. BROADNETS 2008. 5th International Conference on; 10/2008
[show abstract][hide abstract] ABSTRACT: Cooperation in wireless networks has shown significant performance gains in comparison to legacy wireless networks. Cooperative wireless protocols achieve such efficiency by enabling cooperation among nodes to exploit spatial diversity. CoopMAC is a medium access control (MAC) protocol that enables cooperation by using an intermediate node as a helper to a direct communication under poor channel conditions. The helper is typically located in a position where it experiences a good channel with both the source and destination. Therefore, it increases the efficiency of the communication by forwarding a packet from the source to the destination using high transmission rates. In an earlier attempt, we demonstrated the benefits of cooperation at the MAC layer by implementing the CoopMAC protocol using an open source wireless driver platform. However, due to some limitations posed by the hardware, the full potential of the protocol could not be explored. In this paper, we proceed with a complete implementation of the cooperative MAC protocol using an OFDM based software defined radio (SDR) platform. We investigate the benefits of the SDR approach, describe the details of the implementation, as well as the experiments we run in order to evaluate the protocol. Experimental results show that CoopMAC can easily be implemented and can lead to a significant improvement in the performance of wireless networks.
Local and Metropolitan Area Networks, 2008. LANMAN 2008. 16th IEEE Workshop on; 10/2008
[show abstract][hide abstract] ABSTRACT: Crosspoint buffered switches are emerging as the focus of research in high-speed routers. They have simpler scheduling algorithms, and achieve better performance than a bufferless crossbar switch. Crosspoint buffered switches have a buffer at each crosspoint. A cell is first delivered to a crosspoint buffer, and then transferred to the output port. With a speedup of two, a crosspoint buffered switch has previously been proved to provide 100% throughput. In this paper, we propose a 100% throughput scheduling algorithm without speedup, called SQUID. With this design, each input/output keeps track of the previously served virtual output queues (VOQs)/crosspoint buffers. We prove that SQUID, with a time complexity of O(log N), can achieve 100% throughput without any speedup. Our simulation results also show a delay performance comparable to outputqueued switches. We also present a novel queuing model that models crosspoint buffered switches under uniform traffic.
High Performance Switching and Routing, 2008. HSPR 2008. International Conference on; 06/2008
[show abstract][hide abstract] ABSTRACT: Cooperative communications is an innovative technique that is expected to change the behavior of wireless networks in the near feature. In the MAC layer, this technique defines new protocols by enabling additional collaboration from stations that otherwise will not directly participate in the transmission. A typical example of such a protocol is CoopMAC, a cooperative MAC protocol that involves an intermediate station or helper in the communication between a transmitter and a receiver. Under this scheme, the transmitter sends its packets to the receiver by forwarding them through the helper. In this way the protocol takes advantage of spatial diversity and faster two- hop transmission, significantly improving the performance of the network. In such an environment, where the sender relies on an intermediate helper to forward its packets to the original destination, numerous security issues may arise. The present security schemes need to be adapted in order to support end- to-end security in the source-helper-destination communication model. In this paper we discuss the potential security issues that cooperation may raise and propose two new security schemes to address those concerns. To evaluate the feasibility of the proposed algorithms, we implement them using open source drivers platform, which is explained in the paper in detail. Moreover, the paper also discusses the design challenges encountered and share the experience and insights gained during implementation. Our implementations of the suggested techniques allow the WEP, WPA and WPA2 (802.11i) security protocols to successfully operate in the new cooperative environment.
[show abstract][hide abstract] ABSTRACT: In multicast/broadcast services over infrastructure- based/cellular wireless networks (e.g. 3G cellular networks, WiMax, DVB), data is transmitted to multiple recipients from an access point/base station. Multicast greatly improves the network efficiency to distribute data to multiple recipients as compared to multiple unicast sessions of the same data to each receiver individually, by taking advantage of the shared nature of the wireless medium. However it is difficult to guarantee the reception reliability of multiple multicast/broadcast recipients because the wireless medium is error prone and each receiver experiences different channel conditions. An additional difficulty is that multicast/broadcast services in many networks such as 3G multimedia multicast services do not provide a reverse communications channel for the receivers to request the retransmission of lost data packets. This research proposes a novel method to provide QoS support by using an assistant network to recover the loss of multicast data in the principal network. Wireless devices are connected to the principal network to receive the multicast data. A wireless device may lose some of the multicast data sent over the principal network. The wireless devices form an assistant network to recover the lost multicast data cooperatively from their peers. The performance of this recovery mechanism has been investigated using extensive simulation experiments.
Proceedings of the Fifth Annual IEEE Communications Society Conference on Sensor, Mesh and Ad Hoc Communications and Networks, SECON 2008, June 16-20, 2008, Crowne Plaza, San Francisco International Airport, California, USA; 01/2008
[show abstract][hide abstract] ABSTRACT: In a P2P VoD system, the rate at which peers receive video fluctuates due to peer churn. Although scalable video coding has the potential to adapt to long-term rate variations, existing scalable video schemes have not been tailored for P2P systems for which substreams emanate from churning peers. In this paper we propose a new multi-stream coding and transmission scheme, Redundancy-Free Multiple Description (RFMD) Coding and Transmission, that has been designed for P2P VoD systems. Unlike layered video, with RFMD all substreams have equal importance. Thus, video quality gracefully degrades as substreams are lost, independently of which particular substreams are lost. Furthermore, only the source bits are collectively transmitted by the supplying peers. Thus, all transmitted bits contribute to improve video quality. Finally, RFMD can be used to create any number of descriptions. We conduct an extensive simulation study, comparing single layer coding with highrate erasure codes, layered coding, multiple description coding (MD-FEC) and RFMD. The simulations show that RFMD performs best in a variety of representative scenarios.
[show abstract][hide abstract] ABSTRACT: In this paper, we consider applying multiple description coding in mesh-pull P2P live streaming networks to provide incentives for redistribution. In our system, a video is encoded into multiple descriptions with each description having equal importance. We consider a heterogeneous system with peers having different uplink bandwidths. We design a distributed protocol in which a peer contributing more uplink bandwidth receives more descriptions and consequently better video quality. Previous approaches consider single-layer video, where each peer receives the same video quality no matter how much bandwidth it contributes to the system. The simulation results show that our approach can provide differentiated video quality commensurate with a peer's contribution to other peers.
Multimedia and Expo, 2007 IEEE International Conference on; 08/2007
[show abstract][hide abstract] ABSTRACT: Advances in cooperative communications and directional antenna design so far have taken place in parallel, if not in isolation from each other. To explore the role they may play in the next generation of wireless networks, it is important to design and evaluate protocols that can provide co-opdirectionality, the capability of tapping into the combined potential of both cooperation and transmission directionality. Inspired by the protocols presented in  and , we propose a novel co-opdirectional MAC that fully leverages both cooperation diversity and transmission directionality1. Special attention has been paid to the protocol design so that the new MAC would not only inherit the advantages of two previous protocols, but also avoid their weaknesses in the wireless ad hoc environment. The protocol thus delivers a superior performance, as our extensive simulations confirm. To the best knowledge of the authors, this paper represents the first effort to deal with the challenges of integrating cooperation and directional capabilities at the MAC layer.
Modeling and Optimization in Mobile, Ad Hoc and Wireless Networks and Workshops, 2007. WiOpt 2007. 5th International Symposium on; 05/2007
[show abstract][hide abstract] ABSTRACT: It has been shown in the literature that many MAC protocols for wireless networks, such as the IEEE 802.11 MAC, have a considerable control overhead, which limits their achievable throughput and delay performance. In this paper, we study the problem of improving the efficiency of MAC protocols. We first analyze the popular p-persistent CSMA scheme which does not achieve 100% throughput. Motivated by insights from polling system theory, we then present three polling service-based MAC schemes, termed PSMAC, for improved performance. The main idea is to serve multiple data frames after a successful contention resolution, thus amortizing the high control overhead and making the protocols more efficient. We present analysis and simulation studies of the proposed schemes. Our results show that the proposed algorithms can effectively improve the throughput and delay performance of p-persistent CSMA, as well as providing energy savings. The proposed schemes are more efficient for handling bursty traffic typically found in wireless networks. Finally, we observe that the proposed PSMAC schemes significantly outperform p-persistent CSMA with respect to fairness.
Modeling and Optimization in Mobile, Ad Hoc and Wireless Networks and Workshops, 2007. WiOpt 2007. 5th International Symposium on; 05/2007
[show abstract][hide abstract] ABSTRACT: Buffered crossbar switches have received great attention recently because they have become technologically feasible, have simpler scheduling algorithms, and achieve better performance than a bufferiess crossbar switch. Buffered crossbar switches have a buffer placed at each crosspoint. A cell is first delivered to a crosspoint buffer and then transferred to the output port. With a speedup of two, a buffered crossbar switch has previously been proved to provide 100% throughput. We propose what we believe is the first feasible scheduling scheme that can achieve 100% throughput without speedup and a finite crosspoint buffer. The proposed scheme is called SQUISH: a Stable Queue Input-output Scheduler with Hamiltonian walk. With SQUISH, each input/output first makes decisions based on the information from the virtual output queues and crosspoint buffers. Then it is compared with a Hamiltonian walk schedule to avoid possible "bad" states. We then prove that SQUISH can achieve 100% throughput with a speedup of one. Our simulation results also show good delay performance for SQUISH.
High Performance Switching and Routing, 2007. HPSR '07. Workshop on; 01/2007