Locating nodes in Ad Hoc Networks: a Survey Giovanni Turi IIT-CNR Pisa

06/03/2003MobileMAN Meeting2 Agenda Introduction Survey Conclusions and questions

06/03/2003MobileMAN Meeting3 Routing Node Location Service Provides an estimation (more or less updated an accurate) of the specified node’s current location Node location services have often been proposed as building blocks for position- based routing on large scale networks Location Service Forwarding Strategy

06/03/2003MobileMAN Meeting4 Node Location Service Traditional routing, based on message flooding, does not scale on large networks, then: First, get the current location L of node A (location service) Second, forward the message towards L If the location service is performant, then there are some advantages: No need to keep routing tables Estimation of the network topology (depending on how the location service is made) Answers to questions like: Where is node A? Who is around location L?

06/03/2003MobileMAN Meeting5 Node Location Service A location services for ad hoc networks is a distributed database, with basically two important operations: Location update, to store a new location in the DB Location lookup, to retrieve the location of a given node A classification could be based on how the database is distributed: All-for-All, Some-for-All, All-for-Some, Some-for- Some

06/03/2003MobileMAN Meeting6 Node Location Service Some aspects to evaluate: Time complexity for updates and lookups VS. network size Communicational complexity for updates and lookups VS. network size Robustness: how many node failures it takes to let a given node unreachable (one, some, all) Localized information: provided or not

06/03/2003MobileMAN Meeting7 Distance routing effect algorithm for mobility (DREAM) All-for-all approach Each node is able to retrieve its own position (using GPS for example) Each node performs location updates by flooding packets. Accuracy controlled by: Time resolution, variable update frequency. The faster the node moves, the more frequent the position updates Spatial resolution, flooding area controlled by packet lifetime (number of hops). Distance effect: closer nodes get more updates than far away nodes, as they appear to move faster ACB BC  <<

06/03/2003MobileMAN Meeting8 Distance routing effect algorithm for mobility (DREAM) Location updates: Time O(sqrt(n)), network diameter Communication O(n), span all nodes Location lookups are constant O(c), as the information is locally stored Very robust system (all-for-all approach) and localized information is provided On large scale networks presents the same problems of routing with flooded messages

06/03/2003MobileMAN Meeting9 Uniform Quorum System The idea is identify and maintain k subsets of nodes, called quorums, such that the intersection of any two quorums is not empty The location database is maintained by quorum nodes, which are connected by a virtual backbone built over the flat network topology at run-time Normal nodes perform location updates and lookups querying a close backbone node backbone nodes propagate updates and lookups through the entire quorum. Success of lookups is guaranteed by the intersection property of quorums

06/03/2003MobileMAN Meeting10 Uniform Quorum System 3 quorums in the example (A, B, C), made by red nodes, forming the virtual backbone (bold lines) Node D updates its position to node 4, which propagates the information to its quorum (2, 3, 4, 5, 6) A B C S D As quorum B intersects with A (node 2) and with C (node 6), D’s position is available to node S, which queries backbone node 1 Node 1 doesn’t have the information directly, but propagates the request inside its quorum (1, 2, 8)

06/03/2003MobileMAN Meeting11 Uniform Quorum System UQS is a some-for-some approach, but quorums can be chosen to have bigger intersections In the worst case, time and communicational complexity for both updates and lookups scales with the network diameter O(sqrt(n)) Localized information is not provided and the system is medium robust: the failure of a quorum brakes the system No description of how the quorums are created and maintained is provided

06/03/2003MobileMAN Meeting12 Grid Location Service The network has a flat addressing scheme, and addresses are uniformly distributed in a range [1,..,N] The network area is partitioned using a quad tree: 4 * n-order squares make one (n+1)-order square. This partitioning is widely known, and each node is able to understand in which k-order square is currently located 1-order 2-order 3-order

06/03/2003MobileMAN Meeting13 Grid Location Service Nodes broadcasts their current position to inside the 1-order square: each node in a 1-order square knows its neighborhood Each nodes selects other nodes where to update its position (location servers) using a notion of near node ID: the least greater ID that the node’s own ID The selection interests the three (n+1)-order squares surrounding the node’s current n-order square, for each n=1, 2, … until the all area is covered The density of location servers decreases logarithmically with the distance from the node The same selection procedure is used for lookups: it is provable by induction, that given a node A the selection list for update and lookup from two different places intersect. The number of steps needed to find the first common node is logarithmic

06/03/2003MobileMAN Meeting14 Grid Location Service Node 10 selects 15, 73, 18, 25, 29 and 14 to store its current position Node 78, willing to initiate a communication with 10, selects 36, 43, 64 and 29. The last one is a location server for 10 and knows its current position

06/03/2003MobileMAN Meeting15 Grid Location Service GLS is an all-for-some approach Time and communicational complexity for updates and lookups is equal to the network’s diameter O(sqrt(n)) The space needed at each node to host the DB goes logarithmically O(log(n)) (but space is not really an issue for a location DB…) The system is medium robust and provides localized information

06/03/2003MobileMAN Meeting16 Virtual Home Region A VHR is a set of nodes, close to each other, located in a circular area with center C and radius R. These nodes are responsible for keeping the updated position of a specified node A called the owner of the region The link between node A and its VHR is provided by a hash function H, returning C when applied to A The hash function is widely known, the addressing scheme is flat and addresses are supposed to be uniformly distributed inside a range [1,..,N], where H operates consistently

06/03/2003MobileMAN Meeting17 Virtual Home Region When node A needs to update its current position, it forwards an update packet towards its VHR. Once the packet is in the VHR’s center proximity, A’s location is replicated in a quorum of nodes in relation with the radius R of the region Another node B, willing to start communicating with A, at first retrieves A’s current position by forwarding a lookup packet towards the center C of A’s VHR (returned by the same hash function H). Eventually, some node inside A’s VHR will respond to B, which in turn will directly communicate with A C R A B C R A B

06/03/2003MobileMAN Meeting18 Virtual Home Region VHR is an all-for-some approach Time and communicational complexity for updates and lookups is equal to the network’s diameter O(sqrt(n)) Localized information is not provided and the system is medium robust

06/03/2003MobileMAN Meeting19 Network Environment Wireless Service (NEWS) It’s a location service based on the concept of mobile agents Each node maintains a local location table, and periodically broadcasts an “Hello” message to its neighborhood The “Hello message” contains the node’s current position with a timestamp and it is stored in the neighbors local location tables Another table, called the global location table, is also maintained and passed around in the network As a node receives the global location table, it merges local and global location tables, taking the most recent and new entries of both The result is locally stored and forwarded to a node that wasn’t previously visited (control through a visited bit): first closest unvisited neighbor, then closest unvisited node

06/03/2003MobileMAN Meeting20 Network Environment Wireless Service (NEWS) It is an all-for-all approach, but the failure of the node currently holding the global table, compromises the system Time and communication for location updates is equal to O(n) (global table travels the all network), while lookups are constant (local table) The system provides localized information

06/03/2003MobileMAN Meeting21 Conclusions There are of course other location services: Stojmenovic proposed a system where updates travel on a north-south column including the node, while lookups travel on a est-west column including the requester Grossglauser and Vetterli proposed a system where updates and lookups are not exchanged: a node only maintains the history of all the other nodes it has encountered in the past. Nodes mobility augments encounters and works like a vehicle of position estimates. This is not really a location service (works tightly-coupled with routing and like very much random mobility), but could support a real service

06/03/2003MobileMAN Meeting22 Conclusions For the purpose of a “MobileMAN” network, maybe an all-for-all approach could work fine: Flooding on small networks is not so costly Communication is mainly localized as the network has this “nice” 2-3 hops horizon

06/03/2003MobileMAN Meeting23 References Mauve, Widmer and Hartenstein. A survey on position-based routing in mobile ad hoc networks. IEEE Network, vol. 15 (2001) Basagni, Chlamtac, Syrotiuk and Woodward. A distance routing effect algorithm for mobility (dream). MOBICOM 1998 Haas and Liang. Ad hoc mobility management with uniform quorum systems. IEEE/ACM Trans. On Networking (1999) Stojmenovic. A routing strategy and quorum based location update scheme for ad hoc wireless networks. Tech. report (1999) Li, Jannotti, De Couto, Karger and Morris. A scalable location service for geographic ad hoc routing. MOBICOM 2000 Giordano and Hamdi. Mobility management: the virtual home region. Tech. Report (1999) Jiang and Camp. An agent based location service for ad hoc networks. Submitted (2003) Grossglauser and Vetterli. Locating node with EASE: mobility diffusion of last encounters in ad hoc networks. INFOCOM 2003