Sathya Narayanan

CoopMAC: A Cooperative MAC for Wireless LANs

Due to the broadcast nature of wireless signals, a wireless transmission intended for a particular destination station can be overheard by other neighboring stations. A focus of recent research activities in cooperative communications is to achieve spatial diversity gains by requiring these neighboring stations to retransmit the overheard information to the final destination. In this paper we demonstrate that such cooperation among stations in a wireless LAN (WLAN) can achieve both higher throughput and lower interference. We present the design for a medium access control protocol called CoopMAC, in which high data rate stations assist low data rate stations in their transmission by forwarding their traffic. In our proposed protocol, using the overheard transmissions, each low data rate node maintains a table, called a CoopTable, of potential helper nodes that can assist in its transmissions. During transmission, each low data rate node selects either direct transmission or transmission through a helper node in order to minimize the total transmission time. Using analysis, simulation and testbed experimentation, we quantify the increase in the total network throughput, and the reduction in delay, if such cooperative transmissions are utilized. The CoopMAC protocol is simple and backward compatible with the legacy 802.11 system. In this paper, we also demonstrate a reduction in the signal-to-interference ratio in a dense deployment of 802.11 access points, which in some cases is a more important consequence of cooperation

MRTP: a multiflow real-time transport protocol for ad hoc networks

Real-time multimedia transport has stringent quality of service requirements, which are generally not supported by current network architectures. In emerging mobile ad hoc networks, frequent topology changes and link failures cause severe packet losses, which degrade the quality of received media. However, in such mesh networks, there usually exist multiple paths between any source and destination nodes. Such path diversity has been demonstrated to be effective in combating congestion and link failures for improved media quality. In this paper, we present a new protocol to facilitate multipath transport of real-time multimedia data. The proposed protocol, the multiflow real-time transport protocol (MRTP), provides a convenient vehicle for real-time applications to partition and transmit data using multiple flows. We demonstrate through analysis that data partitioning, which is an essential function of MRTP, can effectively reduce the short-range dependence of multimedia data, thus improving its queueing performance in underlying networks. Furthermore, we show that a few flows are sufficient for MRTP to exploit most of the benefits of multipath transport. Finally, we present a comprehensive simulation study on the performance of MRTP under a mobile ad hoc network. We show that with one additional path, MRTP outperformed single-flow RTP by a significant margin.

Implementing a Cooperative MAC Protocol for Wireless LANs

In wireless LANs that provide multi-rate support (IEEE 802.11a, 802.11b), stations that experience poor channel quality tend to use low transmission rates to reduce the bit-error-rate (BER) of each transmission. This phenomenon usually leads to a throughput fairness problem between the stations with good channel quality and those without. This fairness problem has been shown to result in throughput degradation for the whole network [8]. The MAC protocol proposed in [5] addresses this issue using an efficient cooperative scheme. Under this scheme, low rate stations are assisted by a high rate station, referred to as helper stations, in its transmissions. With such assistance, the low rate station will be able to transmit data at a higher rate in a two-hop manner using the helper station. We implemented this new protocol in a Linux testbed. This paper describes the assumptions, the implementation process and the challenges we were presented with. We evaluated the protocol using our testbed through experiments. The implementation of the protocol shows that it performs efficiently in supporting TCP applications.

MRTP: a multiflow realtime transport protocol for ad hoc networks

Transporting multimedia data over ad hoc networks is a challenging problem. However, the mesh topology of ad hoc networks implies the existence of multiple paths between two nodes. In our previous work, we have shown that path diversity provides an effective means of combating transmission errors and topology changes that are typical in ad hoc networks. Moreover, data partitioning techniques, such as striping and thinning, have been demonstrated to improve the queueing performance of realtime data. Recognizing the advantages of these techniques, as well as the increasing need of video services in ad hoc networks, we propose a new transport protocol to support multipath transport of realtime data. The new protocol, called multiflow realtime transport protocol (MRTP), provides a convenient vehicle for realtime applications to partition and transmit data using multiple flows. Analysis results from a bottleneck mobile node and simulation results from multiple path video transport over a 16-node ad hoc network illustrate the benefits of MRTP.

On the advantages of multi-hop extensions to the IEEE 802.11 infrastructure mode

IEEE 802.11 specifies two modes of operation, an infrastructure mode where nodes communicate to/through an access point, and an ad-hoc mode, where nodes communicate with each other directly. Neither mode supports multiple hop transmissions between these nodes. In this paper we present two advantages in extending 802.11 MAC to support multiple hops in the infrastructure mode. One advantage is higher available bandwidth in a multi-rate 802.11 network. IEEE 802.11 allows hosts to select different transmission rates based on the quality of the signal received by the host. Based on performance results both from analytical modeling and simulations, we demonstrate that the total available bandwidth can be improved by using multiple hops instead of reducing the transmission rates of nodes. We present the results in terms of both the increase in total throughput of the network and the available throughput for the forwarding node. The second advantage presented is that by using multi-hop transmissions, the power of transmission at the edges of 802.11 cells can be reduced resulting in lower interference with nodes at the edges of other 802.11 cells. This leads to a more uniform coverage, with increased throughput experienced by nodes at the cell edges.

To Forward or not to Forward --- that is the Question

We introduced the use of two-hop forwarding to increase the throughput of an 802.11 network in our earlier work (Narayanan et al., Proceedings of IEEE WCNC’05, March 2005). Other researchers have also considered the benefits of forwarding in the 802.11 infrastructure mode to increase the total network throughput. But the high-data rate node that forwards data for other nodes will have to spend its energy transmitting this data. Previous work on forwarding implicitly assumed that in an enterprise network, the collective good is sufficient to justify this increased energy expense. However, it is important to address the advantages and the cost of participating in such schemes from the individual forwarding node’s perspective. Since a node cannot know whether there are other high-data rate nodes in the network capable and willing to forward data, it needs to assume that it is the only node with the capability to do so. In this paper, we focus our analysis on the cost benefit for such a forwarding node. We quantify the throughput improvement, medium access delay reduction and energy consumption for the forwarding node in a saturated network. The analysis and simulation results demonstrate that in a saturated network, participation in forwarding provides the high-data rate node with significant benefits in throughput and media-access-delay, while increasing the number of bits-per-joule even if it is the only node participating in data forwarding as suggested in this paper. The increase in the bits-per-joule is due to the reduction in the total amount of time needed by the high data rate node to transmit a given number of its own application bits. This results in savings in energy expenditure for the forwarding node. Based on these benefits, we conclude that it is unequivocally in the interest of a high data rate node to participate in two-hop forwarding schemes in 802.11 networks.

UP2P: a peer-to-peer overlay architecture for ubiquitous communications and networking

Approaches to building an intelligent consumer-friendly network have evolved over time from centralized switch-based to router- and server-based Internet architectures. We propose to drive this evolution further with a new highly scalable architecture that provides features to users derived from the computational and networking capabilities of very large populations of sophisticated terminals. This architecture relies on emerging peer-to-peer overlay technology. We describe a peer-to-peer overlay design that addresses requirements crucial for consumer applications, including overlay federation, peer heterogeneity, peer mobility, and service discovery. In addition, we introduce the concept of an overlay operator and describe the requirements for managed overlays. We have designed and implemented both a middleware and a peer-topeer platform that illustrates these concepts.

Peer-to-peer streaming for networked consumer electronics [Peer-to-Peer Multimedia Streaming]

Applications such as multimedia communications and entertainment make media streaming a key feature for peer-to-peer (P2P) technology embedded in networked consumer electronics. In this article, we discuss some key issues that are relevant to enabling peer-to-peer streaming in networked consumer electronics and address possible technical solutions to the issues of interoperability, NAT/firewall traversal, and codec inflexibility. We also address how to improve overall system performance by introducing a notion of node coordinates into the discovery of services on a P2P network and confirm the effectiveness of our approach using simulation. We conclude with a discussion of our prototype CE-oriented P2P streaming system.

Supporting MPEG video transport on DOCSIS-compliant cable networks

A novel quality of service (QoS) scheduling mechanism suitable for transporting variable bit rate video in the upstream direction over a DOCSIS (data over cable system interface specification)-compliant cable network is presented. It is shown, via simulation using real life video traces, that the proposed scheduling service provides significant improvements as compared to the existing DOCSIS QoS scheduling services, with regard to bandwidth utilization and latency distribution. The proposed scheduling service is also applicable to transport integrated services over the Internet and can be utilized by other emerging multimedia applications, where data are bursty in nature and variable in bit rate.

When two-hop meets VoFi

In a paper titled Can I add a VoIP call?(1), the authors calculate the maximum number of VoIP calls that a WiFi (VoFi) network can support. In this paper we extend their analysis to calculate the maximum number of VoFi calls when two-hop forwarding is used in order to avoid rate adaptation at nodes with reduced received signal strength. We calculate this number for different combinations of data rates for slow node transmissions and potential two-hop transmissions. These calculations demonstrate that the use of two-hop forwarding increases the maximum number of VoIP calls in a multi-rate 802.11b network. We validate the analysis by means of simulation of G711 codec sources in a 802.11b network. Even though the earlier discussions focus on specific combinations of high data rate two-hop and low data rate one-hop transmissions in an 802.11b network, we conclude the paper by calculating the maximum number of VoIP calls that can be supported with uniform node distri- bution in 802.11b and 802.11g networks. These calculations show a significant increase in the number of VoIP calls when two-hop forwarding is used in an 802.11g network. This sig- nificant increase is due to the higher data rates and the low PHY overhead of the 802.11g MAC.