Linda Zeger

MAC Centered Cooperation — Synergistic Design of Network Coding, Multi-Packet Reception, and Improved Fairness to Increase Network Throughput

We design a cross-layer approach to aid in developing a cooperative solution using multi-packet reception (MPR), network coding (NC), and medium access (MAC). We construct a model for the behavior of the IEEE 802.11 MAC protocol and apply it to key small canonical topology components and their larger counterparts. The results obtained from this model match the available experimental results with fidelity. Using this model, we show that fairness allocation by the 802.11 MAC can significantly impede performance; hence, we devise a new MAC that not only substantially improves throughput, but provides fairness to flows of information rather than to nodes. We show that cooperation between NC, MPR, and our new MAC achieves super-additive gains of up to 6.3 times that of routing with the standard 802.11 MAC. Furthermore, we extend the model to analyze our MACs asymptotic and throughput behaviors as the number of nodes increases or the MPR capability is limited to only a single node. Finally, we show that although network performance is reduced under substantial asymmetry or limited implementation of MPR to a central node, there are some important practical cases, even under these conditions, where MPR, NC, and their combination provide significant gains.

Multi-Path TCP with Network Coding for Mobile Devices in Heterogeneous Networks

Existing mobile devices have the capability to use multiple network technologies simultaneously to help increase performance; but they rarely, if at all, effectively use these technologies in parallel. We first present empirical data to help understand the mobile environment when three heterogeneous networks are available to the mobile device (i.e., a WiFi network, WiMax network, and an Iridium satellite network). We then propose a reliable, multi-path protocol called Multi-Path TCP with Network Coding (MPTCP/NC) that utilizes each of these networks in parallel. An analytical model is developed and a mean-field approximation is derived that gives an estimate of the protocols achievable throughput. Finally, a comparison between MPTCP and MPTCP/NC is presented using both the empirical data and mean-field approximation. Our results show that network coding can provide users in mobile environments a higher quality of service by enabling the use of multiple network technologies and the capability to overcome packet losses due to lossy, wireless network connections.

Speeding Multicast by Acknowledgment Reduction Technique (SMART)

We present a novel feedback protocol for wireless broadcast networks that utilize linear network coding. We consider transmission of packets from one source to many receivers over a single-hop broadcast erasure channel. Our method utilizes a predictive model to request feedback only when the probability that all receivers have completed decoding is significant. In addition, our proposed NACK-based feedback mechanism enables all receivers to request, within a single time slot, the number of retransmissions needed for successful decoding. We present simulation results as well as analytical results that show the favorable scalability of our technique as the number of receivers, file size, and packet erasure probability increase. We also show the robustness of this scheme to uncertainty in the predictive model, including uncertainty in the number of receiving nodes and the packet erasure probability, as well as to losses of the feedback itself. Our scheme, SMART, is shown to perform nearly as well as an omniscient transmitter that requires no feedback. Furthermore, SMART, is shown to outperform current state of the art methods at any given erasure probability, file size, and numbers of receivers.

Effects of MAC approaches on non-monotonic saturation with COPE - a simple case study

We construct a simple network model to provide insight into network design strategies. We show that the model can be used to address various approaches to network coding, MAC, and multi-packet reception so that their effects on network throughput can be evaluated. We consider several topology components which exhibit the same non-monotonic saturation behavior found within the Katti et. al. COPE experiments. We further show that fairness allocation by the MAC can seriously impact performance and cause this non-monotonic saturation. Using our model, we develop a MAC that provides monotonic saturation, higher saturation throughput gains and fairness among flows rather than nodes. The proposed model provides an estimate of the achievable gains for the cross-layer design of network coding, multi-packet reception, and MAC showing that super-additive throughput gains on the order of six times that of routing are possible.

An analysis of speeding multicast by acknowledgment reduction technique (SMART) with homogeneous and heterogeneous links - a method of types approach

We present a novel feedback protocol for wireless broadcast networks that use linear network coding. We consider transmission of packets from a single source to many receivers over a single-hop broadcast erasure channel with heterogeneous links. We propose a predictive model to minimize feedback as well as extraneous data transmissions by the source. The predictive model will schedule feedbacks only when there is a significant probability that all receivers have completed the download. We demonstrate analytically as well as empirically that reliable multicast can provide a good completion time characteristic for all users, if the initial feedback is delayed until the expected download completion time. We show that with SMART, counter to conventional wisdom the average users download completion time improves slightly as the number of users increases. Furthermore, we show that completion time of the worst user in a multicast session is not very sensitive to the number of users; however it is very sensitive to imbalanced effective rate and heterogeneity among users. Moreover, we show that SMART performs nearly as well as an omniscient transmitter that requires no feedback.

Lists that are smaller than their parts: A coding approach to tunable secrecy

We present a new information-theoretic definition and associated results, based on list decoding in a source coding setting. We begin by presenting list-source codes, which naturally map a key length (entropy) to list size. We then show that such codes can be analyzed in the context of a novel information-theoretic metric, ϵ-symbol secrecy, that encompasses both the one-time pad and traditional rate-based asymptotic metrics, but, like most cryptographic constructs, can be applied in non-aymptotic settings. We derive fundamental bounds for ϵ-symbol secrecy and demonstrate how these bounds can be achieved with MDS codes when the source is uniformly distributed. We discuss applications and implementation issues of our codes.

Packet erasure coding with random access to reduce losses of delay sensitive multislot messages

Many commercial and military systems use some form of random access. ALOHA type protocols are particularly useful for multicast traffic and have low complexity; however, they suffer from low capacity and large loss probabilities. The inclusion of packet level erasure coding in single channel ALOHA protocols is a new area. This paper demonstrates a novel combination of a medium access layer tailored to and used with packet level erasure coding; this new protocol not only reduces message loss, but also decreases delays of multislot messages. It is shown how packet level erasure coding can be used with a tailored random access protocol to render multislot messages more robust against collisions. This resilience to collisions is shown to hold both when limited feedback and no feedback are available. Despite the increased utilization of the collision channel with the additional traffic produced by the erasure code, it is demonstrated to result in a significant decrease in the probability of loss of multislot and single slot messages. The tradeoffs between message loss probability and delays are shown for a range of medium access transmission designs used in conjunction with the erasure coding.

Survivability and recovery of degraded communication networks

When multiple nodes in a network are subject to failure or loss, the question arises as to whether communication across the resulting degraded network is feasible. Percolation theory and random graph theory have been previously used to answer this question. Here we extend random geometric graph theory to the case of networks with some randomness in bond or edge formation, and we derive a lower bound for bond formation probability. In addition, practical methods to address the little studied question as to how to recover from failures that destroy network connectivity are proposed here.

Efficient methods for broadcasting multi-slot messages with random access with capture

A new technique for best effort delivery of broadcast bursty multi-slot messages is investigated, using random access with capture. It is shown that a combination of packet level coding and random spreading of a messages packets can improve message reception probability, satisfaction of delay constraints, and throughput in many cases. Furthermore, we show how the specific transmission strategy employed should depend on the message length, total offered traffic, and the specific delay constraints. In addition, we show how edge effects, the finite size of the network, and receiver capture significantly impact performance not only at the edges of the network, but also near the center of the network. Finally, we demonstrate how to leverage these edge effects so that the introduction of minimal feedback from only one or two nodes can substantially improve the probability of message reception for all nodes in the network.

Capacity in Wireless Systems with Random Access and Delay Constraints

It can be desirable in some wireless systems, including mobile ad hoc networks (MANETs), to use a random access multiple access scheme. In this case, the uncoordinated arrival of packets at a node that receives transmissions from two or more other nodes results in contention. This contention is particularly high at a node that aggregates traffic transmitted from a number of nodes. When delay sensitive information is carried, the physical layer packet loss probability must be low; this requirement, in conjunction with the multiple access contention, severely limits the receive capacity of a node that aggregates traffic from multiple transmitting nodes. Design tradeoffs to increase the capacity are explored, which are applicable to a broad class of packet arrival processes and are applied to a wide range of the number of transmitting nodes.