Multi-Tier Mobile Ad Hoc Networks: Architecture, Protocols, and Performance

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1 Multi-Tier Mobile Ad Hoc Networks: Architecture, Protocols, and Performance Bo Ryu Tim Andersen Tamer Elbatt Yongguang Zhang Network Analysis and Systems Dept. HRL Laboratories, LLC. Malibu, CA, USA. Abstract While mobile ad-hoc networking (MANET) research has received a considerable attention in recent years, the majority of them have focused on single-tier (e.g., ground) and homogeneous (e.g., same radio for every node) MANET. Few have investigated the potential implications of multi-tier and heterogeneous natures of MANET on the design and performance of MANET protocols in a systematic manner. For example, one of the stark differences between single-tier and multi-tier MANET environments is that the multi-tier MANET naturally creates coverage asymmetry due to the much larger coverage area by airborne nodes compared to ground nodes. Consequently, the number of neighbors an airborne node sees can be potentially several orders of magnitude larger than that of a ground node. Treating this airborne node same as any ground node will adversely affect the performance of medium access control (MAC) and/or routing MANET protocols. In this paper, we present a detailed multi-tier MANET architecture, associated issues, and protocols developed. Our novel protocols and simulation tools designed for multi-tier MANET are described with preliminary performance results. Keywords-mobile ad hoc networks; Multi-tier MANET; virtual dynamic backbone; Airborne Communication Node. I. INTRODUCTION Future battlefield networks will consist of various heterogeneous networking systems and tiers with disparate capabilities and characteristics, ranging from ground ad hoc mobile and sensor networks to airborne-rich sky networks to satellite networks. It is an enormous challenge to create a suite of novel networking technologies that efficiently glue these disparate systems. By doing so, the resulting network must offer unprecedented capacity, flexibility, connectivity, reliability, and scalability for meeting even the most challenging needs of the future warfighters. The primary goal of the ACN networking effort is to develop innovative networking protocols to be employed at airborne nodes that enable assured delivery of large volumes of critical data within a battlefield. Specifically, we attempt to achieve the following set of objectives (see Figure 1): To reduce required network configuration tasks at a pre-planning stage to a minimum so that rapid deployment of new forces or mission changes can be made quickly and dynamically. To provide varying Quality of Service (QoS) needs (priorities, delay, throughput, reliability, etc.) dynamically to a wide array of applications (realtime audio and video, bursty data transfers, S A, C2, etc.) with low control overhead. To maintain the performance of popular Internet protocols with closed-loop controls such as TCP and RMTP (Reliable Multicast Transport Protocol) at the same level as in a static (wired) network (versus the mobile wireless nature of ACN network). To provide seamless airborne connectivity to disparate current and future ground networks (e.g., FCS, SUO, EPLRS, Sensor Nets, etc.) with minimum changes required in their hardware and software. To support 10,000+ ground nodes scattered over a wide battlefield area without significant increase in the protocol overhead. Enable Large-Scale MANET (10,000+) Achieve High Bandwidth Efficiency (under bursty, asymmetric traffic demands) Link Heterogeneous Ground Networks seamlessly Support High Assurance of Service (QoS) Have Low Signaling and Implementation Overhead SUO ACN Cloud Must: FCS EPLRS Ships Sensors Figure 1. ACN Mobile Ad Hoc Networking (MANET): Design Objectives Figure 2 shows an overall ACN networking architecture, protocols and services. The core of the ACN networking architecture is a set of novel protocols (ACN baseline

2 protocols suite) enabling a true multi-tier ad hoc networking among heterogeneous ACN platforms. The suite consists of (i) multi-tier-friendly, QoS-aware ad hoc routing protocol called Multi Virtual backbone Protocol (MVP) based on the notion of the virtual dynamic backbone algorithm [1], (ii) dynamic multicast routing protocol to operate over MVP, and (iii) cross-tier medium access control protocol (MAC) that enables higher-tier ACNs to optimally allocate channel bandwidth to lower-tier ACNs or ground nodes; see [4] and Section II-A for more details on the cross-tier MAC protocol. In addition, we have developed advanced networking and user functions to further increase the performance the ACN network and ensure its seamless operation under high mobility of ACNs. They include: (i) active traffic control mechanisms to combat the weakness of TCP and other feedback-based transport protocols in wireless ad hoc network like ACN; (ii) dynamic frequency reassignment for ensuring spectrum orthogonality that may be violated by the uncontrolled or unexpected mobility of ACNs; and (iii) enhancing MVP to operate seamlessly with directional antenna. In this paper, we present an overview of MVP, Cross-Tier MAC, and Multi-mode TCP protocols. Despite the fact that the ACN network is 3-D in nature, simulation tools such as OPNET and ns2 provide little support to develop, test, and visualize the protocols under realistic ACN network dynamics. The lack of 3-D support while designing and evaluating protocols for the ACN network has been a major obstacle to the full understanding of the interaction between protocol behaviors and topology dynamics. We have developed new 3-D ACN Network Simulation Testbed and Visualization tools based on popular ns2 simulator. These new capabilities enable researchers to develop innovative networking protocols and fine-tune them by conducting rigorous tests in a realistic 3-D ACN environment. Figure 3 shows a snapshot of this tool with two ground nodes, two low-tier ACNs, and a single high-tier ACN. The picture inside shows a realization of 3-tier networks and the coverage of 4 low-tier and 1 high-tier ACNs. Multi-Tier Ad Hoc Networking Load-Balancing, QoS-aware Multi-Tier Ad Hoc Routing - Multi Virtual Dynamic Backbone Protocol (MVP) Dynamic Multicast Routing Cross-tier MAC Protocol - Optimal Resource Allocation in FDD/TDMA or TDD/TDMA - Bandwidth-on-demand with Diff. Services Support (e.g., Priority) - Legacy Radio Support with Mobility Management and Handoff 2 nd -Tier ACN 1 st -Tier ACN Terrestrial Advanced Features Active Traffic Control - Multi-modal TCP - Explicit BW Notification Dynamic Frequency Reassignment QoS Routing and Signaling 3-D ACN Network Simulation and Testbed Emulation Figure 2. Overall ACN Networking Architecture, Protocols, Features, and Tools. H igh -altitu d e A C N Directional Beam Packet Transmission on Directional Beam Packet T ransm issions on O m ni-d irectional Beam Low-altitude ACN Ground Nodes Figure 3. A snapshot of 3-D ACN network simulation and visualization.

3 II. MULTI-TIER MANET PROTOCOLS A. Cross-Tier Medium Access Control Protocol In this section we introduce a multiple access protocol that facilitates communication between an ACN tier and a higher ACN tier according to the following requirements: efficient bandwidth utilization, priority support, handoff support, and QoS support for multimedia applications. In addition, the candidate protocol is expected to support short bursts of data that have less stringent quality of service requirements. Accordingly, we propose a TDMA-based multiple access protocol that has the following two types of information slots: Contention-free Slots: guarantee QoS for multimedia applications and support handoff of ground nodes. Contention-based Slots: carry short message bursts (e.g. Route & Topology Updates) with less stringent quality of service constraints. The uplink and downlink frame structures of the proposed protocol are given in Figure 4. Each uplink frame is divided into two sub-frames, namely Reservation Sub-Frame and Information Sub-Frame. As shown in Figure 4, the reservation sub-frame is divided into n slots where each slot is dedicated to one of the ground nodes. Furthermore, each reservation slot is divided, in turn, into mini-slots. These mini-slots are dedicated to carry the reservation requests of various traffic streams to the overlaying ACN node. The reservation requests contain the network state information necessary for the slot reservation algorithm to allocate various information slots as described in more detail in section VI. It is worth mentioning that due to the propagation delays to and from the ACN in addition to the processing delays, reservations are made for the next frame, i.e. reservations at the beginning of frame K, request slots in frame K+1. Uplink (ground -> ACN) Contention-Free Information Slots Reservation slots: One slot per node Contains request for slots for the next TDMA frame Contains handoff request Downlink (ACN -> Ground) Synchronization information Packet Reservation Multiple Access: Route Request/Updates Node Registration Data from ACN this boundary is movable. Downlink signaling information: Slot allocation table for the next uplink frame Downlink slot assignment table Boundary between Contention-free and PRMA for the next uplink frame Figure 4: Uplink and downlink TDMA frame structure for the proposed cross-tier MAC protocol On the other hand, the information sub-frame is divided into information slots. Each information slot is assumed to carry one packet. As pointed out earlier, these information packets are expected to carry three possible traffic types, namely multimedia and handoff traffic streams with strict QoS constraints, in addition to bursty traffic with less strict requirements. Therefore, N information slots are contentionfree and dedicated to serving multimedia and handoff traffic streams, while the remaining slots are contention-based in order to serve short message bursts. In this work we focus on the contention-free slots, more specifically we develop the algorithm that optimally allocates the N contention-free slots to various traffic streams depending on the network state and the priorities of various services. Our algorithm is fundamentally different from previous approaches in the following aspects: Supporting multiple classes of priority. Introducing a novel cost function that has direct relation to packet latencies and packet loss ratio. Solving the optimization problem on a frame-by-frame basis. An optimal scheduling algorithm with the above requirements has been developed and its performance has been fully analyzed via simulation in [4]. B. Multi-Virtual Backbone Protocol (MVP) This section describes multi virtual backbone protocol (MVP), a new mobile ad hoc routing protocol designed for heterogeneous multi-tier mobile ad hoc networks such as battlefield networks. The primary goal of this protocol is to enable assured delivery of large volumes of critical data within a battlefield by ground nodes and ACNs at various altitudes. MVP is based on the concept of Virtual Dynamic Backbone (VDB) [1][5][6]. A VDB is an approximation to the Minimum Connected Dominating Set (MCDS) in a graph theory defined as a subset of the graph such that it is connected, reaches non- MCDS nodes within the graph with a single hop, and has the minimum size. By definition, VDB optimally carries broadcast traffic because it minimizes the number of forwarding actions by limiting it to only those in the MCDS. Details of this algorithm, along with its efficiency under various mobility scenarios, are presented in [1]. MVP operates by creating and maintaining multiple VDBs with minimum overlap among them. This concept utilizes the key strength of the original VDB algorithm in creating and maintaining a stable and smallsize virtual backbone with a simple distributed algorithm. Note that the metric employed to compute a VDB is based on the combination of degree, frequency of neighbor changes, and a few other minor factors [1]. By replacing this metric with a QoS metric (instead of degree), the same algorithm can be seamlessly utilized to form different backbones. This approach enables the ACN (or even ground) tier to create and maintain multiple virtual backbones such as Delay-VDB for delaysensitive traffic, Loss-VDB for loss-sensitive traffic, Energy- VDB for energy-conserving nodes, etc. We argue that this is an ideal structure for multi-tier heterogeneous ad hoc networks since MVP naturally provides multiple robust virtual backbones with distinctive QoS features, each via a separate tier, providing mobile nodes multiple choices of routes at any time depending on its QoS needs at that moment.

4 Figure 5 illustrates the concept of creating multiple backbones in response to traffic changes. Suppose that each node disseminates not only their degrees and stability, but also the observed queueing delays. Initially, the nodes 3,4,5, and 6 form a VDB based on the original algorithm utilizing only degree and stability. Since there will be little traffic across the network, the original VDB also serves as Delay-VDB. Now, suppose the traffic between nodes 4 and 5 have doubled due to sudden increase in traffic from lower-tier nodes under 4 and 5. This will delay the traffic from node 1 to 9, thus increasing queueing delay at nodes 3, 4, and 5. Based on measuring their own queueing delays, these nodes declare to be no longer Delay-VDB. Upon detecting this, the node 7 changes to RED (backbone node of Delay-VDB) following the same VDB selection algorithm since its delay value is locally minimum. Since the nodes 6 and 7 are now backbone nodes of Delay- VDB, the backbone connection process makes node 8 red as well. The node 3 later becomes part of the Delay-VDB upon the request of node 1, just like the backbone expansion process of the original algorithm. This example shows how the existing algorithm can be seamlessly used to form a QoS-specific virtual backbone by pushing QoS-sensitive traffic outside the original VDB whenever it needs to. The bottom of Figure 5 illustrates this concept applied to 3-tier ACN ad hoc network Route 1 Route Virtual Backbone 1 Virtual Backbone Figure 5. An illustration of MVP with its application for ACN Network. 6 9 Aerial View H o r iz o n ta l V ie w Tier-2 ACN The key benefit of MVP is that it naturally supports the multi-tier structure of the ACN network. As illustrates in Figure 6, the MVP forms the first virtual backbone (represented as black color) with the tier-2 ACN only since it can see all the nodes under it. The second virtual backbone is formed solely by lower-tier ACNs (represented as red color). The nodes that are green represent the ACNs that are connected to both backbones; see [7] for preliminary performance results of MVP. We have also implemented MVP in linux for testing and validation purpose. The HRL-developed mobile topology emulation tool, MobiEmu [3], was extensively used during the various processes of implementing and testing MVP. This implementation was used and demonstrated as part of DARPA s ACN program. Figure 6: 3-D visualization of 2-tier ACN network with MVP. C. Active Traffic Control: Multi-Modal TCP The ACN network consists of terrestrial wireless segments, UAV segments, and satellite segments (both GEO and LEO). The dynamic change in topology and connectivity among these segments results in constant fluctuation in the characteristics of end-to-end communication. Many of the performance metrics measured at the end points, including round-trip delay and bandwidth, will be highly volatile in a continual basis. Many of the network traffic control mechanisms that make today s network work smoothly, such as TCP congestion control and QoS/diffserv protocols, all base their operations on the values of these metrics. Unfortunately, these mechanisms are not designed for coping with high variation in these parameters in the network, and subsequently they will suffer significant performance

5 degradation. For example, switching between UAV network and GEO network will increase the end-to-end delay several times, causing gross timeout in TCP connections. The closeloop based congestion control mechanisms will be too slow to adapt to such network topology change, resulting in poor resource utilization. To address this problem, we must develop a new notion of network-topology-change and develop new traffic control mechanisms to recognize and to respond rapidly to network topology changes. One import effect of network topology change is the quantization of end-to-end network performance (e.g., delay, bandwidth, loss). In this research, we are developing new algorithms, mechanisms, and system components that model the network-topology-change, and adapt to it. To be specific, we are developing the following active traffic control mechanisms for ACN: Multi-Modal TCP. For example, congestion control mechanisms in this highly volatile network environment should build multiple running averages as opposite to one as in the current TCP. Instead of gross timeout, TCP can have quantized timers. This way, TCP can quickly adapt to rapid route changes due to changes in network topology. Quantized traffic control protocols. Route/path adaptation in many traffic control protocols should also be quantized. We will apply our quantization methodology to traffic control protocols beyond TCP. Proactive network support. For more effective active traffic control, proactive network elements can provide supports for detecting network condition change and assist in protocol quantization. For example, a router notification mechanism can be introduced to feedback network-topology-change events to multi-modal TCP sender so that the sender can adapt quickly. As the first step, we have implemented Multi-Modal TCP in NS2. We designed Multi-Modal TCP to overcome the problem that traditional TCP could not respond effectively to volatile change in the network. Multi-Modal TCP does this by keeping multiple modal the subnet of TCP parameters that are used in TCP congestion control and switch among them after changes in the end-to-end path characteristics. We have implemented Multi-Modal TCP in ns2 as an extension to TCP New Reno. To verify the implementation demonstrate the benefits of Multi-Modal TCP, we ran it over the following simplified variable link in ns2: 5ms to 100ms. The following figures illustrate how Multi- Modal TCP responds to such change, compared with the normal TCP (which we denote single-modal TCP). For Multi-Modal TCP, we illustrated the two cases: 1) when the network condition is new, i.e., a new modal must be established from scratch, and 2) the network condition has been visited before and the previous modal can be retrieved and used immediately. The following 3 figures compare the TCP congestion window size (cwnd), a measure of how TCP performs. The X-axis is time (in second). Figure 7. Single Modal TCP -- it took > 10 seconds to recover from sudden bandwidth drop Figure 8. Multi-Modal TCP, condition had not been visited before -- it took < 3 seconds to recover source Variable link sink During the simulation, we varied the link characteristics suddenly between high bandwidth and low bandwidth and between long latency and low latency. Example: At simulation time at 5 second, the bandwidth drops from 1.6Mbps to 64Kbps and the latency increase from Figure 9. Multi-Modal TCP, condition had been visited before -- immediate recovery. From these graphs we can see that Multi-Modal TCP did improve on the single-modal TCP for sudden bandwidth

6 drop. This is because it avoided unnecessary retransmissions and timeouts. Even though this is a simple example, our measurement showed that the TCP good put had been improved by 7% during the 30-second interval. We have established a similar pattern for network condition change in other directions, which we have not included here for space consideration; see [8][9] for more extensive simulation results. III. CONCLUSION We have presented a comprehensive architecture of an ACN network which is multi-tier in nature. Multi-tier networks are considerably more challenging than a traditional flat-plane MANET because they contain connectivity asymmetry (i.e., higher-tier nodes have much higher connectivity than lower-tier nodes) and node heterogeneity (in terms of mobility, range, battery power, etc.). To address these challenges, new protocols in the area of ad hoc routing and MAC have been developed. In addition, a change has been suggested for TCP to work properly in ACN network which is volatile and TCPunfriendly. While not included in this paper due to space limitation, we have also investigated dynamic frequency planning algorithms aimed at resolving potential spectrum allocation conflicts due to mobility of ACNs. Providing reliable networking service to heterogeneous, large-scale ground wireless networks is a significant challenge, as various components such as routing, MAC, and host protocols (e.g., TCP) need to work seamlessly without incurring considerable overhead. We believe the work presented here provides an important insight into designing such a complex network. Further work is required in order to fully characterize the performance of the protocols when working together under various unicast and multicast traffic load conditions and realistic channel environment. This is under way, and the results will be reported elsewhere. REFERENCES [1] U. Kozat, G. Kondylis, B. Ryu, and M. Marina, Virtual Dynamic Backbone for Mobile Ad Hoc Network, IEEE ICC 01, June [2] B. Ryu et al., Research Tools for 3-D Mobile Ad-hoc Networking with Directional Antenna, The First Annual Symposium on Autonomous Intelligent Networks and Systems (AINS), May [3] Y. Zhang and W. Li, An Integrated Environment for Testing Mobile Ad-Hoc Networks, ACM MobiHoc, Lausanne, Switzerland, June [4] T. Elbatt and B. Ryu, Priority-based Dynamic Packet Reservation for TDMA Wireless Networks, IEEE MILCOM [5] P. Sinha, R. Sivakumar, and V. Bharghavan, Enhancing Ad hoc Routing with Dynamic Virtual Infrastructure, IEEE INCOCOM 01, April [6] B. Liang and Z. Haas, Virtual Backbone Generation and Maintenance in Ad Hoc Network Mobility Management, IEEE INFOCOM 00, April [7] B. Ryu, T. Elbatt, and Tim Anderson, Multi-Tier Ad Hoc Routing, To appear in VTC 03, Oct [8] D. Dutta and Y. Zhang, An Early Bandwidth Notification (EBN) Architecture for Dynamic Bandwidth Environments, IEEE ICC, Apr [9] D. Dutta and Y. Zhang, An Active Proxy Based Architecture for TCP in Heterogeneous Variable Bandwidth Networks, IEEE GLOBECOM, Nov 2001.