Shirish S. Karande

Network-embedded FEC for optimum throughput of multicast packet video☆

Forward error correction (FEC) schemes have been proposed and used successfully for multicasting realtime video content to groups of users. Under traditional IP multicast, application-level FEC can only be implemented on an end-to-end basis between the sender and the clients. Emerging overlay and peer-to-peer (p2p) networks open the door for new paradigms of network FEC. The deployment of FEC within these emerging networks has received very little attention (if any). In this paper, we analyze and optimize the impact of network-embedded FEC (NEF) in overlay and p2p multimedia multicast networks. Under NEF, we place FEC codecs in selected intermediate nodes of a multicast tree. The NEF codecs detect and recover lost packets within FEC blocks at earlier stages before these blocks arrive at deeper intermediate nodes or at the final leaf nodes. This approach significantly reduces the probability of receiving undecodable FEC blocks. In essence, the proposed NEF codecs work as signal regenerators in a communication system and can reconstruct most of the lost data packets without requiring retransmission. We develop an optimization algorithm for the placement of NEF codecs within random multicast trees. Based on extensive H.264 video simulations, we show that this approach provides significant improvements in video quality, both visually and in terms of PSNR values.

Cross-layer protocol design for real-time multimedia applications over 802.11 b networks

Inherent vulnerability of the wireless medium renders it more susceptible to errors and losses than classical wired media. In this paper, we evaluate the suitability of protocols and strategies across different layers of the stack to provide real-time services over 802.11b wireless LANs. More specifically, within the context of cross-layer design, we compare the performance of UDP with UDP lite - a proposed framework, which improves bandwidth utilization by delivering partially damaged packets to the realtime application. First, we study the high-level end-to-end throughput improvement achieved by making cross-layer modifications to support a UDP lite framework. We compare the quality of perceived media rendered by UDP (dropped packets only) and UDP lite (dropped and corrupted packets). This formulates one of the key findings of this study, that is, although UDP lite improves the overall high-level throughput by relaying corrupted packets to the real-time application, it fails to provide significant enhancement in perceived media quality. This can, in part, be attributed to the bursty nature of errors and losses that we observed at the application layer regardless of the selected transport protocol. Finally, we compare the error-recovery/concealment overhead required by UDP and UDP lite in order to deliver lossless multimedia. We conclude that the overhead required by UDP lite is considerably lower than UDP, since the received corrupted packets that are delivered by UDP lite (but not by UDP) facilitate error-recovery.

Fundamental limits of information dissemination in wireless ad hoc networks-part I: single-packet reception

We present capacity and delay scaling laws for random wireless ad hoc networks under all information dissemination modalities (unicast, multicast, broadcast and anycast) when nodes are endowed with multi-packet reception (MPR) capabilities. Information dissemination modalities are modeled with an (n, m, k)-cast formulation, where n, m, and k denote the number of nodes in the network, the number of destinations for each communication group, and the actual number of communication group members that receives the information (i. e., k ≤ m ≤ n), respectively. We show that Θ(R(n)\√m/k), Θ(1/k), and Θ(R2(n)) bits per second constitute a tight bound for the throughput capacity of random wireless ad hoc networks under the protocol model when m = O(R-2(n)), Ω(k) = R-2(n)= O(m), and k = Ω(R-2(n)), respectively. R(n) denotes the receiver range which depends on the decoding complexity of the nodes. For the minimum receiver range of Θ(√(log n/n)) to guarantee network connectivity, a gain of Θ(log n) for (n, m, k)-casting is attained with MPR compared to the capacity attained when receivers can decode at most one transmission at a time in . Furthermore, we derive the capacity-delay tradeoff of (n, m, k)-casting when MPR is used. We show that the use of MPR can lead to both increased network capacity and reduced delays in wireless ad hoc networks.

Hybrid Erasure-Error Protocols for Wireless Video

Many recently proposed cross-layer protocols for wireless video, have advocated the relay of corrupted packet to higher layers. Such protocols lead to both errors and erasures at the compressed video application layer. We generically refer to such schemes as hybrid erasure-error protocols (HEEPs). In this paper, we analyze the utility of HEEPs for efficient transmission of video over wireless channels. In order to maintain the generic nature of the deductions in this paper, we base our analysis on two (rather abstract) communication schemes for wireless video: hybrid error-erasure cross-layer design (CLD) and hybrid error-erasure cross-layer design with side-information (CLDS). We make a comparative analysis of the channel capacities of these schemes over single and multi-hop wireless channels to identify the conditions under which the HEEPs can provide improved performance over conventional (CON) protocols. In addition, we employ Reed Solomon (RS) and low-density parity check (LDPC)-code-based forward-error correction (FEC) schemes to illustrate that the improvement in capacity can easily enable an FEC scheme employed in conjunction with a HEEP to provide improved throughput. Finally we compare the performance of CON, CLD, and CLDS in terms of video quality using the H.264 video standard. The simulation results show a significant advantage for the HEEPs

Analysis and modeling of errors at the 802.11b link layer

In this paper, we analyze the errors observed at the link layer of an 802.11b network. Our analysis at all supported bitrates (i.e., 2, 5.5. and 11 Mbps) establishes that the error patterns are not memoryless, and therefore, they exhibit a certain level of temporal dependencies. Thus, we evaluate the suitability of a two-state Markov model to capture the channel behavior. Non-stationarity of the error patterns renders such a simplistic model inadequate, and hence, we consider higher order models. This formulates a key contribution of this paper, and that is, a hierarchical Markov model, which captures the non-stationarity of the channel while employing real-time application-specific considerations to determine state-transition probabilities.

Header Detection to Improve Multimedia Quality Over Wireless Networks

Wireless multimedia studies have revealed that forward error correction (FEC) on corrupted packets yields better bandwidth utilization and lower delay than retransmissions. To facilitate FEC-based recovery, corrupted packets should not be dropped so that maximum number of packets is relayed to a wireless receivers FEC decoder. Previous studies proposed to mitigate wireless packet drops by a partial checksum that ignored payload errors. Such schemes require modifications to both transmitters and receivers, and incur packet-losses due to header errors. In this paper, we introduce a receiver-based scheme which uses the history of active multimedia sessions to detect transmitted values of corrupted packet headers, thereby improving wireless multimedia throughput. Header detection is posed as the decision-theoretic problem of multihypothesis detection of known parameters in noise. Performance of the proposed scheme is evaluated using trace-driven video simulations on an 802.11b local area network. We show that header detection with application layer FEC provides significant throughput and video quality improvements over the conventional UDP/IP/802.11 protocol stack

Optimal scaling of multicommodity flows in wireless ad hoc networks: Beyond the Gupta-Kumar barrier

We establish a tight max-flow min-cut theorem for multicommodity routing in random geometric graphs. We show that, as the number of nodes in the network n tends to infinity, the maximum concurrent flow (MCF) and the minimum cut-capacity scale as Theta(n2r3(n)/k) for a random choice of k ges Theta(n) source-destination pairs, where r(n) is the communication range in the network. We exploit the fact, that the MCF in a random geometric graph equals the interference-free capacity of an ad-hoc network under the protocol model, to derive scaling laws for interference-constrained network capacity. We generalize all existing results reported to date by showing that the per-commodity capacity of the network scales as Theta(1/r(n)k) for the single-packet reception model suggested by Gupta and Kumar, and as Theta(nr(n)/k) for the multiple-packet reception model suggested by others. More importantly, we show that, if the nodes in the network are capable of multiple-packet transmission and reception, then it is feasible to achieve the optimal scaling of Theta(n2r3(n)/k), despite the presence of interference. This result provides an improvement of Theta(nr2(n)) over the highest achieved capacity reported to date. In stark contrast to the conventional wisdom that has evolved from the Gupta-Kumar results, our results show that the capacity of ad-hoc networks can actually increase with n while the communication range tends to zero!

Natural growth codes: Partial recovery under random network coding

Growth codes (GC) improve the disruption tolerance of zero-configuration sensor networks by providing graceful data recovery. Here, we highlight the existence of periphery monitoring topologies which are conducive for graceful recovery. In such networks, the performance of random network coding (RNC) is observed to be superior to that of GCs. RNC increases the data persistence, while maintaining a lower delay.

On the capacity improvement of multicast traffic with network coding

In this paper, we study the contribution of network coding (NC) in improving the multicast capacity of random wireless ad hoc networks when nodes are endowed with multi-packet transmission (MPT) and multi-packet reception (MPR) capabilities. We show that a per session throughput capacity of Theta (nT3(n)), where n is the total number of nodes and T(n) is the communication range, can be achieved as a tight bound when each session contains a constant number of sinks. Surprisingly, an identical order capacity can be achieved when nodes have only MPR and MPT capabilities. This result proves that NC does not contribute to the order capacity of multicast traffic in wireless ad hoc networks when MPR and MPT are used in the network. The result is in sharp contrast to the general belief (conjecture) that NC improves the order capacity of multicast. Furthermore, if the communication range is selected to guarantee the connectivity in the network, i.e., T(n) ges Theta (radic(logn/n)), then the combination of MPR and MPT achieves a throughput capacity of Theta (log3/2 n/radicn) which provides an order capacity gain of Theta (log2 n) compared to the point-to-point multicast capacity with the same number of destinations.

Design and analysis of Generalized LT-codes using colored ripples

Research has shown that fluid limits of Markov processes can be used to obtain closed form expressions for the evolution of the ripple-size. In this work we extend the above analysis to generalized LT (GLT) codes, which can be used to represent LT encoding (with priorities) over multiple data segments. In our analysis, we segregate the ripple into multiple colored ripples, where each color corresponds to a segment. We derive closed form expressions for the size of each ripple. We utilize these expressions to design GLT distributions, optimized for a desired intermediate and unequal recovery.