the_handbook_of_ad_hoc_wireless_networks_7 ppt

© 2003 by CRC Press LLC RREQs the y receive for the first time, some of the rebroadcasts may be redundant. As shown in Fig. 17.1a, suppose Node A broadcasts a new RREQ to Nodes B and C, which in turn rebroadcast to Node D. Hence, Node D receives two copies of the same RREQ, one of which is redundant . Moreover, if Nodes B and C are close to each other and both transmit at the same time, channel contention could occur. Further, RTS/CTS exchange is not used in broadcast transmission. If the underlying MAC does not provide collision detection capability (i.e., CSMA/CA cannot listen while sending), packet collisions could be damaging. The resulting redundancy, contention and collisions constitute what is called the “broadcast storm” problem [8]. Several schemes are proposed by the authors to alleviate this problem: 1. Probabilistic 2. Counter-based 3. Distance-based 4. Location-based 5. Cluster-based Below is a concise description of these schemes. In the probabilistic scheme, each node rebroadcasts the message it received for the first time with some fixed probability p . F IGURE 17.1 (a) B roadcast storm; (b) Counter-based scheme: Note that the extra coverage area by A (shown in gray) is too little to warrant a transmission; (c) Distance-based scheme: If the distance d between A and B is small, then there is only marginal difference in their coverage areas. (a) Broadcast storm (b) Counter-based scheme: Note that the extra coverage area by A (show in grey) is too little to warrant a transmission (c) Distance-based scheme: If the distance d between A and B is small, then there is only marginal difference in their coverage areas Simpo PDF Merge and Split Unregistered Version - http://www.simpopdf.com © 2003 by CRC Press LLC I n the counter-based scheme, each node rebroadcasts the message only if the same message has not been heard for more than C times, before it itself can transmit. The assumption made by the node is that if the message has been rebroadcast several times by its neighbors, then the extra coverage contribution from its own rebroadcast is probably too low to be worth transmitting. This is illustrated in Fig. 17.1b. In the distance-based scheme, each node rebroadcasts the message only if the physical distance between itself and the node from which it received the message is not less than d (Fig. 17.1c). The node uses the signal strength of the received message to estimate this distance. In the location-based scheme, the message is rebroadcast only if the extra area expected to be covered from this broadcast is greater than A . The node uses the location information from GPS to determine the area of this extra coverage. As for the cluster-based scheme, only cluster-heads and gateway nodes are able to rebroadcast the message. The nodes may use any of the other schemes to determine whether or not to rebroadcast the message. The schemes proposed in the above are effective, in particular for densely populated networks, in which nodes are communicating in close proximity of each other. One problem, however, is that all the threshold values are fixed , which may result in some messages not being broadcast to the destination under certain conditions, i.e., when the network is sparse. Thus, some improvements have been proposed to adapt the threshold values to changing node density [13]. 17.3.4.2 Query Localization Que ry localization [9] is a technique that exploits the knowledge of some previously known route to restrict the query flooding to a specific region of the network. The basic premise behind such technique is that the topology has not changed drastically soon after a link failure and thus many of the nodes on the previous route may be used to reconstruct a new route to the destination. Two schemes for query containment are proposed: The first scheme assumes that the new route cannot be very different from an older route, with at most k nodes different (path locality). The second scheme assumes that the destination is within k hops away from any nodes on the older route (node locality). In both schemes, every query packet carries a counter that is initialized to zero and then incremented each time the query encounters a node that was not on the previous route to the destination. When the query does encounter a node on the previous route, only the second scheme resets the counter to zero. Once the counter exceeds the threshold value k , the query is dropped. This technique should be useful for the source to initiate route rediscovery soon after a link failure. For such cases, the initial value of k is given some small value, i.e., two hops. If a route to the destination cannot be found with this value, then k is increased, and the process repeats until the maximum threshold for k is reached. For a new route discovery in which no previous route to the destination is known, the initial value of k is set to the network diameter, thus flooding the query over the entire network. As with local repair, the query localization shares the possibilities of reconstructing a longer path, as well as increasing the route discovery latency when the initial route rediscovery fails. 17.3.4.3 Location-Aided Routing L ocation-Aided Routing (LAR) [10] is a technique that proposes using location information obtained from GPS to confine the route search to a region where the destination is likely to be found. Two variants of this protocol are proposed. Figure 17.2 illustrates the concepts of LAR1. By knowing the physical location L and average speed v of the destination at time t0 , the source defines at time t1 a circular region of radius v (t1 – t0) called “expected zone.” This is the region in which the destination may be found. In addition, the source defines the smallest rectangle that includes the expected zone and itself as the “request Simpo PDF Merge and Split Unregistered Version - http://www.simpopdf.com © 2003 by CRC Press LLC z one,” in which only nodes that reside in this zone can forward the RREQ. The source attaches this information on request zone to the RREQ. In LAR2, the source uses location information to compute the distance between the destination and itself and then attaches this distance value to the RREQ. When a node receives the RREQ, it computes its own distance to the destination and then forwards the RREQ only if it is closer to the destination than the node from which the RREQ is received. Hence, the RREQ will only get progressively closer to the destination after each relay. In both schemes, nodes may attach their location information onto any packets they are sending (i.e., RREQ) in order to allow other nodes to learn about their location. Moreover, if a RREP is not received after some timeout period, the source initiates a new route discovery using flooding. One potential weakness of the protocol is the dependence on GPS for obtaining one’s location, since direct line-of-sight access to GPS satellites may not always be possible due to blockage by objects such as buildings and foliage. Further, prior knowledge of the destination’s location may not always be available at the source. For the former, the problem may be remedied by using some non-GPS techniques as proposed in [4]. For the latter, the protocol may require more mobile nodes to communicate their locations more frequently, or alternatively enlist the aid of a distributed location service [5] if necessary. Savings from the reduced flooding of RREQs when LAR is performed may far outweigh the costs of retrieving location information. 17.3.4.4 Unicast Query Mechanism W e now discuss another location-based optimization, which is the unicast query mechanism [11,12]. This is a mechanism that can be used to improve the overhead performance of LAR. Consider the case when the source and target are not in proximity: a significant portion of the network may be flooded with RREQs, i.e., due to a larger request zone. The unicast query mechanism can help to mitigate this problem by allowing the source to use location information to select an existing route for unicasting its RREQ to a node in the neighborhood of the target. This node, which is known as the “target neighbor,” in turn broadcasts the RREQ to the nearby target, i.e., one or two hops away, as shown in Fig. 17.3. 4 H ence, the RREQ is broadcast near the target and not at the source as in LAR, which helps reduce the FIGURE 17.2 LAR’s request and expected zone concepts. 4 The request and expected zones are not used by this mechanism but are shown to highlight its potential to improve LAR. Simpo PDF Merge and Split Unregistered Version - http://www.simpopdf.com © 2003 by CRC Press LLC number of RREQ and RREP messages generated. Besides, if any intermediate nodes along the query path are allowed to respond to the RREQ when they have a route to the target, then a broadcast of the RREQ may not be required, and the overhead can be further reduced. As with expanding ring search, one potential drawback of this mechanism is the increase in route discovery latency when the initial attempt to discover a route using this mechanism fails, i.e., when the unicast query path has been invalidated as a result of node movements. Extra latency thus can be incurred when the source retries route discovery, either using a different unicast query path (if available), or by broadcast. Another potential source of latency is introduced when the intermediate nodes are allowed to respond to RREQs. Though network-wide broadcast is expensive, it enables the source to discover multiple routes to the destination. But more importantly, it allows many nodes, i.e., intermediate nodes that forward RREPs to the source, to discover a route to the destination, as well as to other nodes along the route (i.e., by virtue of source routing). This greatly increases a node’s ability to respond to others’ RREQs. Con - versely, this ability diminishes when the searching space of many route discoveries is constrained, thus increasing the RREQ’s traversal time. 5 Further, since the RREQs are not broadcast end-to-end, i.e., from source to destination, the routes constructed by the mechanism may not always be the shortest. But this shortcoming can be remedied with route maintenance features such as “automatic route shortening” from DSR, which makes possible the self-optimization of path length over time. 17.4 Thoughts and Suggestions for Future Research One of the key objectives of most optimization techniques is to minimize the amount of control traffic generated in a route discovery. But in the process, they often impact other aspects of performance in ways that are not always desired. Expanding ring search, for example, compromises packet latency for bandwidth efficiency, and so do other techniques that confine the search space for routes or limit the query to only a subset of nodes. Early quenching of route requests by intermediate nodes may result in fewer control packets and a shorter query time. However, the routes obtained can be obsolete or non - optimal, which results in both increased packet loss and latency. Inherently, there exists a tradeoff between FIGURE 17.3 The unicast query mechanism. 5 We expect this phenomenon to occur as well (though to different extents) in LAR and other similar optimizations. Simpo PDF Merge and Split Unregistered Version - http://www.simpopdf.com © 2003 by CRC Press LLC overhead of route discovery and other performance areas. This is not unexpected, but a question arises as to whether such tradeoffs in performance can be averted. For example, if the message to be sent is urgent, or the current network utilization is low, then flooding may be used to discover a better route in a shorter time. If not, better resource conserving techniques such as the unicast query mechanism may be employed for route discovery. Conceivably, some adaptive methods of optimizing route discovery would be useful to allow a flexible tradeoff between efficiency and performance. This may be interesting for future investigation. In the previous section, we also discussed techniques that utilize geographic location information for directional route discovery. Underlying these techniques is the notion that a route to the destination can be found by searching in the general direction of the destination. Terrain features such as buildings, hills, and foliage are currently not considered in these techniques. The presence of such objects can obstruct or substantially weaken the transmission of radio signals, making communication across them difficult if not impossible even though the communicating nodes may be close physically. Lack of terrain awareness may render the use of these techniques less effective in real-life scenarios. As an example, we examine a case where route discovery using LAR may be problematic when obstructions are not considered. In Fig. 17.4, we represent the obstructions by rectangular objects in gray and assume them to be impenetrable by radio waves. Suppose that Node S initiates a route discovery to Node D by broadcasting a query message. Node S floods this query to its request zone only to find that Node D is unreachable because no queries rebroadcast by other nodes in the request zone have reached Node D due to obstructions. In fact, a route does exist through Node K. However, this route is not discovered since Node K is lying beyond Node S’s request zone. If the obstructions are known a priori, then Node S may (for example) increase the search space around edges of the obstruction at Node D, so that Node K can be encompassed within Node S’s request zone. There are, of course, other solutions possible. But in general, knowing the terrain over which communication is to take place is expected to yield greater success in route discovery. It is also possible that some obstructions are semipenetrable (i.e., forested areas), where radio signals are weakened but not completely obstructed. If a direct route is desired over one that makes a detour around the obstruction, then nodes may instead increase their transmit power to get the queries across, i.e., Node C may increase its transmit power to send to Node D. However, increasing transmit power would cause greater interference with surrounding nodes. Hence, the use of directional antennas can be envisaged . FIGURE 17.4 Route discovery with LAR in the presence of obstructions. Simpo PDF Merge and Split Unregistered Version - http://www.simpopdf.com © 2003 by CRC Press LLC Surface terrain information can be obtained from computerized terrain databases such as geographic information systems (GIS). Intuitively, nodes may also learn about their physical environment by examining other nodes’ geographic location and their logical connectivity, i.e., though Node C and Node D are close in physical space, Node C requires two hops to reach Node D via Nodes J and K. If the type of radio interface used is the same between the nodes (having equal transmit power and bi- connected links), then one may infer that an obstruction exists in the region bounded by Nodes C, J, K, and D. This is a simplistic inference that has not considered other important factors such as node distribution and density, and clearly much work remains to be done. By itself, this may be another interesting topic that is worth exploring. 17.5 Summary and Concluding Remarks In this chapter, we discussed several techniques to limit the extent and effects of query flooding during a route discovery. However, as we noted earlier, there is always a tension between the conflicting goals of efficiency and performance. Hence, the gain in bandwidth efficiency is not without its costs. We also explored some ideas for future research, including adaptive methods of optimizing route discovery and LAR with terrain awareness. References 1. Perkins, C.E., Royer, E.M., and Das, S.R., Ad Hoc On-Demand Distance Vector (AODV) Routing, Internet-Draft, draft-ietf-manet-aodv-10.txt, Jan. 2002, work in progress. 2. Johnson, D.B., Maltz, D.A., Hu, Y C., and Jetcheva, J. G., The Dynamic Source Routing Protocol for Mobile Ad Hoc Networks (DSR), Internet-Draft, draft-ietf-manet-dsr-06-txt, Nov. 2001, work in progress. 3. Internet Engineering Task Force (IETF) Mobile Ad Hoc Networking (MANET) Working Group, http://www.ietf.org/html.charters/manet-charter.html. 4. Capkun, S., Hamdi, M., and Hubaux, J P., GPS-free Positioning in Mobile Ad-Hoc Networks, Proc. 34th Hawaii Int. Conf. System Sciences, Maui, 2001, p. 3481. 5. Li, J., Jannotti, J., De Couto, D.S.J., Karger, D.R., and Morris, R., A Scalable Location Service for Geographic Ad Hoc Routing, Proc. 6th Int. Conf. Mobile Computing and Networking, Boston, MA, 2000, p. 120. 6. Lee, S J. and Gerla, M., Split Multipath Routing with Maximally Disjoint Paths in Ad Hoc Net- works, Proc. IEEE Int. Conf. Communications, Helsinki, 2001, p. 3201. 7. Sucec, J. and Marsic, I., An Application of Parameter Estimation of Route Discovery by On-Demand Routing Protocols, Proc. 21st Int. Conf. Distributed Computing Systems, Phoenix, 2001, p. 207. 8. Ni, S Y., Tseng, Y C., Chen, Y S., and Sheu, J P., The broadcast storm problem in mobile ad hoc networks, Proc. 5th ACM/IEEE Int. Conf. Mobile Computing and Networking, Seattle, 1999, p. 151. 9. Castaneda, R. and Das, S.R., Query localization techniques for on-demand routing protocols in ad hoc networks, Proc. 5th ACM/IEEE Int. Conf. Mobile Computing and Networking, Seattle, 1999, p. 186. 10. Ko, Y. and Vaidya, N., Location-aided routing (LAR) in mobile ad hoc networks, Proc. 4th ACM/ IEEE Mobile Computing and Networking, Dallas, 1998, p. 66. 11. Seet, B C., Lee, B S., and Lau, C T., Route discovery optimisation for dynamic source routing in mobile ad hoc networks, IEE Electronic Lett., 36, 1963, 2000. 12. Seet, B C., Lee, B S., and Lau, C T., Study of a unicast query mechanism for dynamic source routing in mobile ad hoc networks, Lecture Notes on Computer Science, 2094, Springer-Verlag, Berlin, 2001, p. 168. Simpo PDF Merge and Split Unregistered Version - http://www.simpopdf.com © 2003 by CRC Press LLC 13. Tseng, Y C., Ni, S Y., and Shih, E Y., Adaptive Approaches to Relieving Broadcast Storms in a Wireless Multihop Mobile Ad Hoc Network, Proc. 21st Int. Conf. Distributed Computing Systems, Phoenix, 2001, p. 481. Simpo PDF Merge and Split Unregistered Version - http://www.simpopdf.com © 2003 by CRC Press LLC 18 Location-Aware Routing and Applications of Mobile Ad Hoc Networks 18.1 Introduction 18.2 Location-Assisted Routing Protocols L AR (Location-Assisted Routing) • GPSR (Greedy Perimeter Stateless Routing) • GRA (Geographical Routing Algorithm) • GEDIR (Geographic Distance Routing) 18.3 Zone-Based Routing Protocols Z one-Based Routing Protocol • GRID • Comparison 18.4 Location-Aware Applications of MANET Ge ocast • Location Services • Location-Assisted Broadcasting in MANET • Location-Assisted Tour Guide 18.5 Conclusions Acknowledgments References 18.1 Introduction W ireless communications have made great progress recently. Computing technologies have also advanced quickly as we see a variety of portable, small, light devices appearing on the market. These together have made computing and communication anytime, anywhere possible. One of the promising wireless network architectures that can realize communication anytime, anywhere is the mobile ad hoc network (MANET). A MANET consists of a set of mobile hosts without the support of base stations. It is attractive since it can be quickly deployed and operated by batteries only. We have observed that wireless networks typically operate in a three-dimensional real space because wireless communications must rely on signals traveling in the space. On the contrary, in traditional wireline networks, cables may interconnect hosts into (ideally) any kind of topology. Thus, we may say that wireline networks are not limited to humans’ three-dimensional world. This interesting observation has led to many researchers working on location-aware MANETs. By location awareness, we mean that a host is capable of knowing its current physical location in the three-dimensional world. In traditional networks, hosts only have logical names (such as IP addresses) and do not know exactly what their current physical locations are. GPS (G lobal Positioning System) is the most widely used tool to calculate a device’s physical location. GPS is a worldwide, satellite-based radio navigation system. The GPS system consists of 24 satellites in Y u-Chee Tseng National Chiao-T ung University Chih-Sun Hsu National Central University Simpo PDF Merge and Split Unregistered Version - http://www.simpopdf.com © 2003 by CRC Press LLC six o rbital planes. The satellites transmit navigation messages periodically. Each navigation message contains the satellite’s orbit element, clock, and status. After receiving the navigation messages, a GPS receiver can determine its position and roaming velocity. To determine the receiver’s longitude and latitude, we need at least three satellites. If we also want to determine the altitude, another satellite is needed. More satellites can increase the positioning accuracy. The positioning accuracy of GPS ranges in about a few tens of meters. GPS receivers can be used almost anywhere near the surface of the Earth. By connecting to a GPS receiver, a mobile host will be able to know its current physical location. This can greatly help the performance of a MANET, and it is for this reason that many researchers have proposed to adopt GPS in MANETs. For example, mobile hosts in a MANET can avoid using naïve flooding to find routes; neighbors’ or destinations’ locations may be used as a guideline to find routing paths efficiently. Several works have addressed location-aware routing protocols for MANETs [Jain et al., 2001; Karp and Kung, 2000; Ko and Vaidya, 1998; Lin and Stojmenovic, 1999; Mauve and Widmer, 2001; Stojmenovic and Lin, 2001]. Proposals that partition the physical area into nonoverlapping zones to facilitate routing have also been proposed [Joa-Ng and Lu, 1999; Liao et al., 2001]. One interesting feature of such zone-based protocols is that a host can easily decide which zone it belongs to, and only one representative host needs to be active to collect routing-related information. The route search cost can be reduced significantly too since nonrepresentative hosts will not flood the route request packets. The applications of location information are not limited to routing protocols. Navigation systems, which already incorporate GPS, can further combine MANET for group communications. Geocast, the goal of which is to deliver a message to a target area, is another potential service [Ko and Vaidya, 1999; Liao et al., 2000]. A computer-assisted tour guide system may take advantage of location information as well as the wireless communication capability of ad hoc networks. The rest of the chapter is organized as follows. Section 18.2 discusses several location-assisted routing protocols. Section 18.3 reviews two zone-based routing protocols. Section 18.4 presents some location- aware applications. Conclusions are presented in Section 18.5. 18.2 Location-Assisted Routing Protocols I n this section, we review some routing protocols for MANETs that take advantage of location information of the hosts [Jain et al., 2001; Karp and Kung, 2000; Ko and Vaidya, 1998; Lin and Stojmenovic, 1999; Mauve and Widmer, 2001; Stojmenovic and Lin, 2001]. Different levels of knowledge are assumed to be known in advance. Generally, these works assume that a source host knows the destination’s location or all its one-hop neighbors’ locations. Some assume that each mobile host knows the locations of all its two-hop neighbors [Stojmenovic and Lin, 2001]. The location-aided routing (LAR) protocol also exploits roaming speeds of destination hosts [Ko and Vaidya, 1998]. Most of the routing protocols mentioned here do not need to go through the route discovery procedure before sending packets. Mobile hosts can forward packets directly to next hops according to local location information. Greedy approaches are widely adopted by using distance [Jain et al., 2001; Karp and Kung, 2000; Lin and Stojmenovic, 1999] or direction [Lin and Stojmenovic, 1999] as the metric to pick the next host to forward packets. However, greedy solutions may fall into the dilemma of running into a local maximum host (such as a dead end). When trying to avoid local maximum hosts, loops may occur. Solutions are proposed in [Stojmenovic and Lin, 2001]. 18.2.1 LAR (Location-Aided Routing) T he location-aided routing (LAR) protocol [Ko and Vaidya, 1998] assumes that the source host (denoted as S ) knows the recent location and roaming speed of the destination host (denoted as D ). Suppose that S obtains D ’s location, denoted as ( Xd, Yd ), and speed, denoted as v, at time t 0 and that the current time is t 1 . We can define the expected zone in which host D may be located at time t 1 (r efer to the circle in Fig. 18.1). The radius of the expected zone is R = v(t 1 – t 0 ). Simpo PDF Merge and Split Unregistered Version - http://www.simpopdf.com © 2003 by CRC Press LLC F rom the expected zone, we can define the request zone to be the shaded rectangle as shown in Fig. 18.1 (surrounded by corners S, A, B, and C ). The LAR protocol basically uses restricted flooding to discover routes. That is, only hosts in the request zone will help forward route-searching packets. Thus, the searching cost can be decreased. When S initiates the route-searching packet, it should include the coordinates of the request zone in the packet. A receiving host simply needs to compare its own location to the request zone to decide whether or not to rebroadcast the route-searching packet. After D receives the route-searching packet, it sends a route reply packet to S. When S receives the reply, the route is established. If the route cannot be discovered in a suitable timeout period, S can initiate a new route discovery with an expanded request zone. The expanded request zone should be larger than the previous request zone. In the extreme case, it can be set as the entire network. Since the expanded request zone is larger, the probability of discovering a route is increased with a gradually increasing cost. 18.2.2 GPSR (Greedy Perimeter Stateless Routing) T he greedy perimeter stateless routing (GPSR) protocol [Karp and Kung, 2000] assumes that each mobile host knows all its neighbors’ locations (with direct links). The location of the destination host is also assumed to be known in advance. Different from the LAR protocol, the GPSR protocol does not need to discover a route prior to sending a packet. A host can forward a received packet directly based on local information. Two forwarding methods are used in GPSR: greedy forwarding and perimeter forwarding. Figur e 18.2 shows an example of greedy forwarding. When host S needs to send a packet to host D, it picks from its neighbors one host that is closest to the destination host and then forwards the packet to it. In this example, host A is the closest one. After receiving the packet, host A follows the same greedy forwarding procedure to find the next hop. This is repeatedly used until host D or a local maximum host is reached. F IGURE 18.1 R equest and expected zones in the LAR protocol. F IGURE 18.2 A n example of greedy forwarding in the GPSR protocol. Simpo PDF Merge and Split Unregistered Version - http://www.simpopdf.com . instead of the MAC address of host C, as the next hop leading to host D. This provides an interesting “handoff” capability in the sense that if C roams away, the next leader (if any) in the. computer-assisted tour guide system may take advantage of location information as well as the wireless communication capability of ad hoc networks. The rest of the chapter is organized as follows. Section. nodes are able to rebroadcast the message. The nodes may use any of the other schemes to determine whether or not to rebroadcast the message. The schemes proposed in the above are effective,