Space Communication | Optical Networking | Wireless Ad Hoc Networks
While the field of communications and networks is rapidly advancing due to the increased popularity of the Internet, air and space communication systems are at a much more immature state of development. Certain attributes of the open air and satellite channels make techniques previously developed for terrestrial networks inefficient or entirely unsuitable. For example, satellite systems often have longer propagation delays and higher bit error rates than their terrestrial counterparts; and the open air interface for satellite channels lends itself to the concept of dynamic sharing of resources. In addition, space based systems involve additional issues such as spacecraft technologies, launch costs and complex system characteristics that are intertwined and complicate the design of communication systems. The following specific areas of research are particularly important within the context of aerospace information system technology and architecture.My research activities in this area are sponsored by the Defense Advance Research Project Agency (DARPA), and the NASA Glenn Research Center.
Increasingly, space systems are being deployed for the purpose of providing a communications infrastructure for mobile users as well as users with small antennas. Hence, it is necessary to develop a communication systems architecture that can efficiently deliver data to mobile users and high rate data to users with small terminals. In addition, an effective space communication and network architecture is critical to the design of future air and space systems, supporting such functions as high level commands to autonomous vehicles and delivery of vehicle guidance information. This gives rise to a range of problems including satellite constellation design, resource allocation (such as links and processing), the design of media access control schemes and system management and control. Research activities in this area will address these large systems issues and will also include the creation of new Physical Layer architectures using advanced technologies (such as agile beam forming, signal processing etc.), the invention of new protocols to deal with special channel properties, dynamic routing in the presence of changing topologies and fluctuating loads, and interconnection with terrestrial and wireless networks
The space to ground communication link is a particularly fruitful area of possible innovation for space communications. This is largely due to the fact that on-board transmitter power and frequency allocation are precious and the space-ground channel quality is highly variable due to atmospheric conditions. The design of a communication layer that makes efficient use of power and bandwidth is thus of critical importance. Also, because of the variability of data traffic, overall system capacities can be significantly increased through the use of an efficient uplink channel access scheme that takes advantage of the bursty nature of data traffic.
With the rapid maturity of key technology building blocks, it is now very possible to construct a number of high performance space-network architectures that can support a variety of services. These services can be provided by a single or a small number of networks that are shared among a multitude of spaceborne, airborne, mobile and fixed terrestrial users with small terminals. The research program will consider architectures and protocols for heterogeneous networks that include both space and terrestrial segments. Typically, satellite open-air links are different from wired terrestrial links in a number of ways: they have higher bit-error rates, larger propagation delays, fading due to weather conditions; as physical layer as well coding and interleaving. As a result, existing protocols often fail to operate efficiently over air or space links. In order to efficiently connect space and ground networks the wide disparity in transmission capacity and channel quality that exists between these two segments must be addressed. This gives rise to a range of issues including: space/ground network architectures, the design of efficient end-to-end protocols, quality of service assurance and the design of efficient interfaces between the ground and space portions of the network.
The communication systems that are essential to the internal control of an aircraft or spacecraft are similar to traditional local area networks. However, for mission critical information, real-time delivery of data with very high reliability is a must. Traditional LANs such as Ethernet or token rings provide little guarantees in terms of delivery and reliability. The design of high-capacity intra-aircraft/spacecraft communication systems that are based on and compatible with the packet network architecture; yet provide the reliability and timeliness required by mission critical data is an important area for future development. Another thrust of the research agenda will address the design using advanced technologies, of aircraft and spacecraft LANs that are light, power-efficient and easily upgradeable. Such systems will have a first order impact on aircraft and spacecraft design and airborne and spaceborne system architectures of the future.
Over the past decade the growth in the use and capabilities of communication networks has transformed the way we live and work. As we progress further into the information age, the reliance on networking will increase. With the expected explosive growth in data traffic, networks will be strained in terms of both transport and processing requirements. Wavelength Division Multiplexing (WDM) is emerging as a dominant technology for use in backbone and access networks. With WDM, the capacity of a fiber is significantly increased by allowing simultaneous transmission on multiple wavelengths (channels), each operating at the maximum electronic rate. Systems with between 40 and 80 wavelengths are presently being deployed for point-to-point transmission. With tens of wavelengths per fiber and transmission rates of up to 10 Gbps per wavelength, capacities that approach a Tera-bit per second can be achieved. My research activities in this area are sponsored by the Defense Advance Research Project Agency (DARPA), the National Science Foundation (NSF), and Intel Corporation.
Modern communication networks are constructed using a layered approach, with one or more electronic layers (e.g., IP, ATM, SONET) built on top of an optical fiber network. This multitude of layers is used in order to simplify network design and operations and to enable efficient sharing of network resources. However, this layering also gives rise to certain inefficiencies and interoperability issues. In this project we focus on the impact of layering on network survivability, i.e., protection and restoration in the face of link failures. Networks often rely on the electronic layers to provide most protection and restoration services. However, in a layered network, the protection mechanisms provided at the electronic layer may not be robust in the face of failures in the underlying optical layer. For example, SONET networks typically provide protection against single link failures using a ring network architecture, and protection in general “mesh” networks (e.g., ATM, WDM) is typically provided using disjoint paths. However, even electronic topologies that are designed to be tolerant of single link failures, once they are embedded on the physical (e.g., fiber) topology, may no longer be survivable to single physical (fiber) link failures. This is because the failure of a single fiber link can lead to the failure of multiple links in the electronic topology, which may subsequently leave the electronic topology disconnected. Thus, network survivability mechanisms often cannot provide their claimed level of protection and restoration, when embedded on a physical topology. The goal of this project is to develop a fundamental theory for understanding cross-layer survivability, and mechanisms for providing survivability in layered networks.
The goal of this DARPA sponsored project is to develop WDM based access networks that provides economical access to the internet at rates of one to ten Gigabits per second. The access network refers to the portion of the communication infrastructure responsible for reaching the customer premises. Because of the proximity to the end-user, an access network is quite different from a backbone network and hence offers additional technology and economic challenges. The relatively low-rate individual traffic flows need to be multiplexed into the higher-rate backbone trunks, thus requiring multiplexing and grooming equipment. In addition, the data traffic is quite bursty because it has not yet been sufficiently aggregated and could therefore benefit from statistical multiplexing. Complicating matters further, equipment must be cheaper in the access network than in the backbone networks because equipment is shared over a smaller set of customers. Research in access architectures involves the design of low cost passive networks for connecting end-users to an access node (or central offic), and a configurable optical Feeder networks (e.g., WDM rings) for connecting the different access nodes to each other and to a backbone network. Our research is focused on the development of mechanisms that would allow next generation networks to take advantage of the huge bandwidth and configurability of WDM. Some of the issues involved include: the choice of electronic multiplexing and switching between the IP and WDM layers; modifications of IP to interface with WDM in order to make use of new optical layer services; network reconfiguration algorithms that improve network capacity by altering the network logical topology in response to changes in traffic conditions; traffic grooming algorithms to minimize electronic costs at access nodes and to make efficient use of wavelengths in the Feeder; and scaleable protocols for efficient channel access such as optical-layer MAC protocols for Bandwidth-on-Demand services in passive optical networks. We are also investigating the impact of this new, high capacity, reconfigurable WDM layer of higher layer protocols. For example, we will study the interaction of TCP flow control with the underlying WDM-based network; the impact of rapid topology reconfiguration on IP route calculations and the potential for routing table instabilities; and extending multi-layer switching into the optical domain. This effort is a part of the ONRAMP consortium that includes AT&T, Cabletron Systems, JDS Uniphase and MIT.
The goal of this NSF sponsored project is to significantly increase the capacity of the internet by providing optical bypass for most of the traffic. While WDM systems are likely to meet future transport demands, electronically processing all of the traffic at network nodes will present a significant bottleneck. Fortunately, it is not necessary to electronically process all traffic entering and leaving each node. For example, much of the traffic passing through a node is neither sourced at that node nor destined to that node. To reduce the amount of traffic that must be electronically processed at intermediate nodes, future WDM systems will employ WDM Add/Drop multiplexers (WADMs) and cross-connects, that allow each wavelength to either be dropped and electronically processed at the node or to optically bypass the node's electronics. Our research in this area is focused on developing mechanisms for providing optical bypass to the electronic layer thereby reducing the size and cost of electronic switches and routers in the network. A number of techniques will be explored, each of which is appropriate for different traffic scenarios. For the case of low rate stream traffic, grooming algorithms will be developed to selectively multiplex multiple low rate traffic streams onto wavelengths such that the number of wavelengths that must be processed at each node is minimized. For bursty packet traffic, topology reconfiguration algorithms will be developed to reduce the load on the electronic switches and routers via dynamic load balancing. Lastly, for large data transfers, Optical Flow Switching protocols that bypass all of the electronics in the network using all-optical end-to-end connections will be developed. The combination of the above mechanisms will reduce the size, cost and complexity of electronic switches and routers and will lead to a dramatic increase in the traffic capacity that can be supported by the Next Generation Internet (NGI).
This project, investigate issues in the control and management of mobile ad hoc networks. Such networks are of critical importance for future combat systems, sensor networks and autonomous systems involving mobile ground and air vehicles. These systems heavily depend on cooperative control between mobile vehicles and consequently on the availability of a communication capability between the vehicles. In a dynamic, mobile environment one cannot assume that such communication capabilities are always present. In this effort we will develop architectures and protocols fo providing reliable communications in this environment. Existing communications protocols and architectures typically rely on a stationary network infrastructure. In a battlefield environment network nodes are typically mobile and have a wide range of capabilities. Those can include high altitude aircraft or satellites, low flying UAVs, ground vehicles, and ground or air-based sensors. Some of these vehicles, such as satellites may have robust communication capabilities while others such as low flying UAVs or sensors may have limited and time varying communication connectivity (due to rapid mobility) and limited transmission power. This project will address issues in providing communication capabilities in this hostile environment.
In order to effectively design an ad hoc network we must first develop fundamental understanding of these networks. We will try to characterize the performance of an ad hoc network in terms of critical design parameters such as the number of nodes in the network and the transmission power available to the nodes. For example, we will attempt to characterize the capacity region of a mobile ad hoc network. We will also attempt to characterize the robustness of the network by obtaining bounds on the connectivity of the network (i.e., bounds on the probability that two nodes are within reach of one another). These performance measures will in turn allow us to effectively design the system. For example, in a sensor network, we will be able to examine the tradeoff between the number of nodes (or sensors) and the connectivity of the network. Similarly, we will be able to tradeoff the battery capacity of the nodes with the transmission capacity of the network.
These networks are likely to include a heterogeneous mix of vehicles. While some may be highly mobile and have constrained communications capability, others may be relatively stationary or have significantly more power for communications. This mix of capabilities can be used to design a hierarchical network where some of the stationary and more capable nodes provide a "backbone" over which reliable end-to-end networking can take place. This project will explore ad hoc network architectures and more importantly mechanisms for dynamically establishing a hierarchical network in a mobile ad hoc environment.
Most previous work on routing in ad hoc networks has focused almost entirely on the problem of route discovery. Little, if any, attention has been paid to the problem of reliable communications in a mobile network. In a network this is often accomplished by providing "backup" routes. However, recovery using backup routes in ad hoc networks is very different from a static fiber networks due to the high degree of mobility that results in rapid topology changes. This project will develop efficient recovery mechanisms in a mobile ad hoc environment.
