Ad-hoc networks are self-organizing network architectures that are rapidly deployable and that adapt to the propagation conditions and to the traffic and mobility patterns of the networks nodes. The most distinguishing characteristic of ad-hoc networks is the lack of fixed infrastructure. Other characteristics include a distributed peer-to-peer mode of operation, multi-hop routing, and relatively frequent changes in nodal constellation. Precursors of the ad-hoc networking technology were the DARPA Packet Radio Networks and the Survivable Adaptive Networks (SURAN) programs in the 1970s and 1980s. Although, military tactical communication is still considered as the primary application for ad-hoc networks, commercial interest in this type of networks continues to grow. Applications such as rescue missions in times of natural disasters, law enforcement operation, commercial and educational use, and sensor networks are just few possible commercial examples. It is, however, not clear whether the technology developed for military applications is directly transferable to the commercial market.
A panel on "Ad-Hoc Networks" was held at the IEEE MILCOM conference, which took place this year in Monterey, California, November 2-5. The theme of the conference was Integrating Military and Commercial Communications for the Next Century. Thus the topic of ad-hoc networks was especially well suited for the conference. The panelists were:
The panel moderator was Zygmunt J. Haas. After introductions, each of the panelists delivered a 10-minutes position presentation.
In his talk, Zygmunt J. Haas outlined a number of ad-hoc network design choices:
In a hierarchical architecture, nodes are grouped in clusters, with one of the nodes assuming the function of a cluster head. Cluster heads create a second-tier network that operates at much higher transmission power. Routing between nodes in different clusters is always performed through the cluster heads. In contrast, in a flat architecture there are no clusters - "all nodes are created equal." Neighbor nodes can communicate directly, without restriction, to traverse any specific nodes (i.e., cluster heads).
Zygmunt claimed that the flat-routed networks are much more applicable to the military communication environment, since the routing is more optimal (close-by nodes do not have to go through the hierarchy), they provide improved security (Low Probability of Detection / Low Probability of Intercept) as the transmission is limited to adjacent nodes only, and the network tends to better balance the load among multiple paths, thus reducing the traffic bottlenecks that occur at the cluster nodes in the hierarchical approach. He also introduced the term Reconfigurable Wireless Networks (RWN), which are a subgroup of an ad-hoc network architecture, characterized by large range of nodal mobility, large network span, and large number of network nodes. Finally, a brief comparison was made between the proactive (such as OSPF) and reactive (such as flooding) routing protocols. Both of these approaches are not effective in the RWN environment, the first introducing too much overhead traffic and the second suffering from too much delay. A reference was made to a hybrid scheme - the Zone Routing Protocol (ZRP).
J.J. Garcia-Luna-Aceves addressed the lack of seamless extension of the Internet to support "unplug-and-play" operation, unrestricted by the existing infrastructure. In particular, the currently used protocols, such as OSPF and BGP, do not provide a satisfactory solution to ad-hoc networking. There is no support for Quality-of-Service (QoS) in routing and multicasting, and the protocols assume fairly static configurations. Furthermore, existing protocols do not take into account battery life.
J.J. predicted in his talk that the future solutions will involve IP. However, some modifications are required, especially to reduce the IP overhead and to support some QoS measures. The characteristics of future ad-hoc networks will be influenced by cheap processing and storage and by relatively expensive transmission. The protocol stack will be integrated; i.e., the physical layer will affect the higher layer protocols. Multiple channels will be used based on open etiquettes for cooperation among the network nodes. MAC protocols will incorporate some form of time scheduling to provide delay guarantees to higher layers. Future routing protocols will use link-state information to accommodate QoS, but will not be based on flooding. Future multicasting protocols for ad-hoc nets will be based on more flexible routing structures than the multicast routing trees used in the Internet. Finally, reliable multicast solutions for ad-hoc networks will be different than those being developed for the Internet.
David B. Johnson discussed the use of ad-hoc networks for applications where no infrastructure is available, such as remote areas, unplanned meetings, or emergency relief operation. Furthermore, sometimes users may not want to use the existing infrastructure even when it is available, such as when it takes too long to register or the service or when the use of the service is too expensive. David pointed out that the existing traditional routing protocols are insufficient for ad-hoc routing, since the amount of update traffic may waste a large portion of the wireless bandwidth, especially for protocols that use periodic updates and when the network topology changes too fast. Current limits to ad-hoc networking include the problems of resource consumption forwarding packets for others, etiquette, and lack of services. The possible commercial applications of ad-hoc networking include mission-oriented networking, community-based networking, and arbitrary groups of strangers.
David described the CMU Dynamic Source Routing protocol, which is based on on-demand route discovery and route maintenance. The features of the protocol include absence of periodic exchanges of messages and no reaction to movements by nodes whose communications are not affected by the movement. The protocol is being implemented on FreeBSD 2.2.2 in an environment of IBM infrared wireless LAN cards. The current maximum size of the network is about 10 machines, limited by the amount of hardware and physical space available for the prototype. A commercial application that is being considered is to support ad-hoc connectivity at a large mining or construction site. Caterpillar is cooperating with CMU on a project in this area.
Joseph Macker's presentation started with description of the work done at the Naval Research Laboratory in mobile routing. The work include the pioneering work on Linked Cluster Routing (Baker et al), the more recent work on the Temporally-Ordered Routing Algorithm (TORA) protocol, and involvement in the IETF IP-based Ad-hoc Mobile Network Working Group (MANet). He then outlined the desirable properties of a well-suited routing protocol: executes distributedly, provides loop-free routes, supplies multiple routes, establishes routes quickly, and minimizes algorithmic reaction/communication overhead. The TORA protocol was introduced and simulation results displayed in terms of the reduced bandwidth utilization for the protocol's control traffic and the mean packet delay.
Joseph then discussed mobile routing for the Internet protocol suite, indicating that wireless networking poses technical challenges at the link and the physical layer, in addition to the effect on performance of the upper layers. Related IETF work was described. This work extends the current standardized protocols, as those were not intended for use in a mobile environment. IEFT Mobile Ad-hoc Networking (MANet) was chartered. MANet addresses improved, peer-to-peer mobile routing technology in a heterogeneous wireless fabric. Some issues include performance evaluation criteria and infrastructure integration. In particular, performance criteria could include data throughput and delay and efficiency and robustness, under different network context, such as size, connectivity, rate of change, and capacity. As concluding remarks, Joseph suggested that there is a need for increased support for standardized routing solutions for mobile networking, that the future holds some interesting possibilities, such as use of heterogeneous wireless "fabrics" and inexpensive wireless routing nodes, and that some initial concepts are ready for technology transition.
The next position talk was given by Charlie Perkins, who discussed routing considerations for ad-hoc networking. He advocated the use of Distance Vector algorithms for routing, since they use less memory, constrain the updates to local operations, are simple to program, and can be made loop free. Routing protocols have to be based on demand-driven route establishment procedures. They have to be scalable for large node populations in memory and search time, in the overhead for maintenance, and in the overhead on data traffic.
Charlie described the Ad-Hoc On-Demand Distance Vector (AODV) routing protocol, which uses destination sequence numbers for route updates. Destination sequence numbers ensure loop-freeness and eliminate the well known "counting to infinity" problem of Bellman-Ford algorithms. In AODV, a route is cached by intermediate nodes for some time since its last use. A limited-range broadcast message is used for neighbor discovery. Upon discovery of a link failure, a notification is issued to the users of the link. The protocol involves little overhead for data traffic and enables future aggregation of computations. Open questions for future consideration include the location of network services (e.g., DNS, certificates, agent directory for service location) and whether functions should be independent of location and hostname.
Robert Ruth described the DARPA GloMo (Global Mobile Information Systems) project. The goal of the project is to make the mobile environment a first-class citizen in the Defense Information Infrastructure by providing user friendly connectivity and access to services for wireless mobile users. The defense wireless environment is characterized by lack of pre-deployed infrastructure, by significant changes in the connectivity and link quality due to weather, terrain, foliage, or EMI, and by mobile operations. The focus of the GloMo program is in the areas of: mobile applications support, end-to-end networking, wireless networking, and wireless node design. Hard issues in mobile ad-hoc networking include: set up and settling time, dynamic reconfiguration, overhead vs. throughput, reliable multicast, QoS, variation in network density, highly asymmetric communications, and heterogeneous networking. Network security and survivability is especially a hard and important issue that deserve more attention.
Robert briefly described a number of GloMo projects, concentrating on the new ideas in each of them. GloMo initiatives include: self organizing/self healing networks; both flat and hierarchical multihop routing algorithms; ATM over wireless; Georouting; Satellite communications networks; heterogeneous networking with IP overlays; end-to-end network enhancements; and security & survivability for ad-hoc networks. GloMo technologies are applicable to Wide-Area Information Systems, Information Systems for Dismounted Forces, and Information System for Rapid Deployment of Forces.
Robert stressed two points in his presentation. The first was that much can be gained by designing the radio/wireless node and networking algorithms to work together. The second was that one protocol may not be optimum for all situations. Instead, we may need to move to a multi-mode networking stack that addresses: stealth, low power, low data rate, voice and low data rate, multi-media, and antijam.
The next speaker was Paul Sass, who presented the Army perspective on ad-hoc wireless networks, based on four layers, from the bottom up: the so-called "Plasma Net," the Terrestrial Transport, the High Altitude/Endurance UAV, and the Satellite Transport. He described a "canonical brigade," which includes 1000 tactical platforms (tanks, APCs, helicopters, etc), with one or more tactical radios per platform, each with hemispherical coverage, and with platform-to-platform direct line of sight range of less than 16 km. The deployment area of the brigade is 3 km wide by 5 km deep in offensive posture and 15 km wide by 10 km deep in defensive posture. While platoon platforms are relatively close, brigade platforms may be widely separated. Communication today is currently supported by SINCGARS, the Army's VHF combat net radio designed primarily for digitized voice. He described near-term plans to experiment with the NTDR, an IP-based packet radio, to support increasing needs for IP datagram service in the Tactical Internet.
Paul pointed out that although commercial protocols must be the starting point for military R&D, the commercial network attributes differ from the military networks. The military environment requires high survivability, lack of fixed infrastructure, fast response time to failures, reliable communications, and support for real-time traffic. Multicast and "eavesdropping" are also important attributes for military networks. Thus, the commercial cellular wireless model does not represent the tactical world. At the brigade level and below, tactical networks are multi-hop. There are a lot of small networks (i.e., 20 nodes per network) and the number of networks is much larger than the number of nodes per network. Since both hosts and routers are mobile, IP addressing must be dynamically managed. Effective dynamic routing schemes are needed at the subnet layer. Open questions include how connectivity information could be shared with IP routers? How can mobility be managed without IP address changes? Can on-demand routing replace the Distributed Spanning Tree (e.g., SINCGARS) or Link State (e.g., NTDR) routing? How data and voice should be handled? Can a single routing scheme handle connectionless and connection-oriented traffic?
Jay Strater's presentation started with outlining design challenges for future tactical radio systems. The challenges include support for mixed traffic, increase in network capacity, support of unrestricted mobility, and simplification of configuration and management procedures. Jay then addressed the topic of the network architecture considerations: routing architecture, cluster management, address/mobility management, interface/protocol stack, and security. In particular, the routing architecture could be either shared or separate band (which is related to the previously-mentioned flat-routed vs. hierarchically-routed networks). Cluster management includes issues related to cluster formation and its maintenance and connectivity within the cluster. Address/mobility management can be performed with distributed or with centralized management algorithms and can be implemented through address servers or forwarding agents.
On the physical layer, Jay discussed the issues of frequency bands, the choice of spread spectrum technique (direct sequence and frequency hopping), and modulation and coding schemes. On the link layer, reference was made to the MACA and the PRMA schemes. Finally, on the network layer, the routing schemes and router resource reservation topics were addressed. In particular, a distinction was made between the topology-driven and the on-demand routing protocols.
The last, but not least, talk was given by John Zavgren, which described the Near-Term Digital Radio (NTDR) mobility management protocols and procedures. NTDR is based on layered architecture: Media Layer, Internet Layer, and Internet Layer. The Intranet layer is based on two-level hierarchical architecture (with clusters and cluster heads), while the Internet layer is based on peer NTDR communication. The routing in the Intranet is based on forwarding tables for the backbone network (the network that interconnects cluster heads) and on augmenting the forwarding tables with cluster members information. The clustering is implemented through the following concepts: neighbor discovery, backbone formation, and affiliation of nodes with cluster heads. The cluster beacon format was presented and the affiliation protocol was described. The routing protocol is based on the Shortest Path First (SPF) link-state algorithm, with the link-state information aged over 3 minutes.
John presented the BBN modeling strategy, which included scenario of 50-node testbed simulated by OPNET Modeler. (OPNET is a trademark of MIL3, Inc.) In particular, the shared modeling scenario, based on ITT and BBN models, was described. Although OSPF has a large overhead due to the HELLO protocol and due to its flooding operation, modifications were made to reduce this overhead. For example, it was claimed that for 500-node NTDR network, the OSPF HELLO protocol and the link-state flooding would consume in excess of 1.4 Mbps each. The Radio OSPF was defined that includes such features as elimination of OSPF HELLO protocol and elimination of designated-router flooding.
The above presentations were followed by about an hour and a half of questions and comments from the audience to the panel members. Examples of topics discussed were:
In summary, this was one of the most successful panels at the conference with large audience participation. Comments of the attendees were extremely positive and indicate significant interest in the ad-hoc networking technology, both for the military and commercial markets.
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