2.1. Locationing Techniques for Mobile Stations in Wireless Networks
Most of the localization technologies in the wireless networks are landmark based. In landmark-based localizations, a number of fixed access points (APs) periodically broadcast their accurate positions. A mobile host could determine its current location through geometric computations using the estimated distances between a mobile station (MS) and the fixed APs. The distances are estimated through various measurements obtained from radio frequency signals that are transmitted between the MS and the fixed APs. The measurements can be categorized into the received signal strength-based metrics and the time delay-based metrics.
The received signal strength (RSS) [4] exploits the relation between power loss of the signals and the distance [5]. RSS is defined as the voltage measured by a receiver's received signal strength indicator (RSSI) circuit [6]. RSS measurements are relatively inexpensive and simple to implement in hardware, though they are easily affected by two sources of error: multipath signals and shadowing [6].
Time delay-based metrics form the finer resolution distance measurements by estimating the propagation time of the line-of-sight (LOS) signals. The time delay-based metrics include time of arrival (TOA), time difference of arrival (TDOA), and round-trip time of arrival (RTOA). An TOA measurement is defined as the time of transmission plus a propagation-induced time delay [7]. TOA measurements require a common time reference between wireless stations, whereas TDOA and RTOA measurements can be obtained in an asynchronous environment. Time delay-based signal metrics are susceptible to two sources of error: additive noise and multipath signals.
The angle of arrival (AOA) measurements [8] are defined as the direction to neighboring sensors (rather than the distance to neighboring sensors). The AOA measurements require each wireless station to be equipped with two (or more) directional antennas that point in different directions. The AOA measurements can be incurred in two approaches. The most common method is to use an array of wireless stations which adopt the array signal processing techniques. AOA is estimated from the differences in arrival times for a transmitted signal at each wireless station in the array. The second approach is to estimate the RSS ratio between two (or more) directional antennas that are located on a wireless station. An AOA measurement is estimated from the ratio of the individual RSS values at two directional antennas.
2.2. Traditional Location-Ignorant Routing Protocols
Traditional ad hoc routing protocols maintain routing tables in order to determine the paths for forwarding packets [9]. Each host proactively maintains its routing table to reflect its current view of the network topology. Each wireless host propagates the content of its routing table to others either periodically or upon content changes. A wireless host updates its own routing table based on the content of a routing table that is propagated by another host, such as the Destination-Sequenced Distance-Vector (DSDV) routing protocol [10]. Route maintenance is performed by each host even when there is no pending data transmissions.
2.3. Location-Aided Routing Protocols
In order to restrict the overhead in maintaining routing tables, some ad hoc routing protocols have been proposed to eliminate or restrict the use of routing tables. Landmark-based routing protocols [11] only make limited use of routing tables. A landmark hierarchy is established in a landmark-based routing protocol. A network is divided into scopes. Each scope maintains one landmark, typically a router, which maintains routing information within the scope. The landmarks in different scopes interconnect themselves. Any packet sent between different scopes goes through the corresponding pair of landmark hosts. Landmarks exchange routing information among themselves. Other landmark-based routing protocols include the LANMAR routing protocol [12] and the Location-Aided Routing (LAR) protocol [13].
Geographically forwarding packets is generally used in location-based routing protocols, such as the GPSR protocol [1]. Geographic forwarding is also used in content delivery networks. The scalable Content-Addressable Network (CAN) [14] also makes use of the geographic forwarding. The key of a content is used as the storage location of the content. Queries to a content are geographically forwarded to the corresponding storage location whose address is the key of the content.
2.4. Indexing Structures
Decentralized index structures have been used in many application scenarios. In the Logarithmic Dictionary Tree (LDT) [15], each entity maintains its own view of an entire distributed tree. An operation is accomplished through the collaboration of a set of entities by forwarding it among these entities. The Chord protocol [16] is a content retrieval service by the content keys. An indexing structure is used in Chord and is organized into a balanced binary search tree. Both content lookups and updates to content only involves a small number of hosts. Tapestry [17] is a peer-to-peer overlay routing infrastructure for sending requests to nearby servers. A decentralized indexing structure provides good scalability because no atomic operation is required across multiple entities.
Other decentralized indexing structures include PGrid [18], Chord [16], and peer-tree [2]. PGrid is a distributed and balanced binary search tree that is used for retrieving data objects. Each host only maintains a small portion of the whole set of data objects, and a retrieval about a data object is served through a collaboration among a small set of hosts with regard to the total number of data objects. Chord is a balanced binary search tree. A content lookup or a content update only involve a small number of hosts. peer-tree is constructed as a distributed complete binary search tree. The peer-tree structure aims to limit the number of times that a service packet is forwarded by defining the cooperative relations among entities.
2.5. Comparison of the PTLS Location Lookup Service with Other Location Lookup Services
The PTLS service has two prominent properties. First, PTLS restrains the bandwidth consumption. This is achieved by making one forwarding hop at the protocol level to correspond to one hop of forwarding in the underlying physical network. Second, PTLS forwards location updates and queries along multiple paths. Multi-path forwarding serves to achieve both the high rates of successfully answering location queries and the robustness to changes of network topology.
The Grid Location Service (GLS) [19] is a location lookup service based on geographically forwarding location updates and queries. Each mobile host distributes its up-to-date location information to a number of location servers across a geographic area. A location query about a target host is served through being geographically forwarded to a location server which holds an answer. Heavy overhead on forwarding packets has been a problem in GLS. One hop of packet forwarding at the protocol level may correspond to multiple hops in the underlying physical network. Without taking into account the connectivity in an underlying network, a large amount of network bandwidth can be consumed in forwarding packets. In the topology-aware Content-Addressable Networks [20], a routing mesh is made to match the underlying network-level connectivity by clustering geographically proximate hosts.
Multi-path forwarding has also been adopted in the wireless ad hoc routing protocols. In the DSR protocol [21], a source host can maintain multiple routes to a destination under highly dynamic network connectivity. The Geocast routing protocol [22] forwards a packet through multiple routes in order to enhance the chances of reaching a destination under dynamic network connectivity. The service-restorable bandwidth-guaranteed routing [23] maintains two routing paths. One is the main path, and the other one is the backup path. Efficient path selection algorithms are desired constrain the computational overhead in arranging the multipath routes. The peer-tree structure adopted in the PTLS service facilitates the computationally efficient selection of paths.