An Efficient Network Coding-based Air Index for Queries in Road Networks

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An Efficient Network Coding-based Air Index for Queries in Road Networks

  1. Veeresha1, M. Sugumaran2, D. Sandeep3 Associate Professor1, Professor2, Assistant Professor3 Department of Computer Science and Engineering1,2,3,

    Santhiram Engineering College1,3, Nandyal, India. Pondicherry Engineering College2, Pondicherry,India

    AbstractWireless broadcast is an efficient strategy for disseminating data to the clients in road networks. Network Partition Index (NPI) doesnt address how to handle various data objects are increased to a large number in broadcast system and link errors. To address these problems, an NPI based air index, called Network Coding-based Spatial Air Index (NCSAI) has been proposed by using Hybrid Broadcast (HB) scheduling and adaptive XOR-based network coding for spatial queries in road networks. We conducted various experiments for evaluating query performance. The experimental results show that NCSAI has better query performance compared with state-of-the-art NPI.

    KeywordsWireless broadcast, network coding-based spatial air index, hybrid broadcast, adaptive XOR-based network coding, spatial queries, road networks.

    1. INTRODUCTION

Due to rapid advances in both wireless communication and mobile computing, location-based services are more useful to the clients in real-time and day-to-day applications. Recently, most of the GPS (i.e., Geographical Positing System) enabled devices such as smart phones, laptops and PDAs request nearest information via spatial queries in road networks. The road network has been modeled into time-independent and time-dependent and computed travelling time based on the distance and traffic in a network respectively. However, these models have been suffered from scalability and security [1], [2], [3], [4]. To address these issues, wireless broadcast strategy has been adopted [5]. Wireless broadcast strategies are classified into push-based, pull-based and hybrid scheduling. However, these strategies have been suffered from sequential data access [6], [7]. More research has been done on query processing in Euclidian space rather than road networks [8], [9]. However, in most of the real-time applications query processing in road networks is essential. Air index adopted for shortest path queries in a road network and doesnt address range queries and kNN queries [10]. Recently, NPI based air index adopted for various spatial queries in road networks. However, NPI doesnt addressed well scheduling strategy when the data objects are increased to a large number in broadcast system and link errors [11]. To address these problems, we propose NCSAI using HB scheduling and adaptive XOR- based network coding for spatial queries in road networks and improve query performance. The overall idea of this work is explained as follows. The original road networks may contain various data objects such as restaurants, shopping-malls, hospitals, gas-stations and schools etc., and these are classified into two data sets namely general and specific based on clients requirement, and computed cut-off point [12]. Using grid partition strategy, road network is partitioned into small grid cells, and pre-computed some general information such as diameter of each cell and minimum/maximum network distance between every pair of cells that will be carried by NCSAI. At server site, we incorporate adaptive XOR-based network coding into broadcast scheduling program, and broadcast general and specific data objects of NCSAI with network connectivity information of each cell using HB scheduling. On the client site, once a client receives a query request from a mobile user then it searches query result in its cache, if found then it return the result. If not found, then the client tunes into channel, retrieves required information and decode the data. Finally, process the query using Dijkstras shortest path algorithm. If required data doesnt broadcast while tuning then the client send query request to server via pull-based scheduling. While broadcasting, if any free time-slots exist then server broadcast encoded form of clients requested data objects (i.e., specific data objects) along with general data objects based on longest waiting time in the queue on priority order, and updates optimal cut-off point.

The contribution of this paper can be summarized as follows:

  • Propose NCSAI using HB scheduling and adaptive XOR-based network coding for spatial queries in a road networks.

  • Propose search algorithms at client-site, which includes range query, k-nearest neighbor query and continuous k-nearest neighbor query.

  • Experiments are conducted using real road networks data and compared the performance of NCSAI with state-of-the-art NPI.

The rest of this paper is organized as follows. Section 2 describes brief overview and previous works of both wireless broadcast and network coding, Section 3 presents NCSAI broadcast strategy for spatial queries in road networks, Section 4 discuss various searching algorithms, Section 5 reports performance evolution, and Section 6 concludes the research paper.

2 RELATED WORKS

    1. Wireless Data Broadcast

      Wireless broadcast is an efficient strategy for disseminating various data objects to clients via wireless channel. Various interleaving strategies has been adopted for disseminating air index data to clients, among these (1, m) data interleaving strategy widely used in wireless environments is shown in Fig.1.In this strategy, data is divides into m equal data segments and broadcast each data segment preceded by air index via wireless channel [5]. For evaluating performance, (1, m) data interleaving strategy has been used two performance parameters and these are tuning time and access latency. Tuning time defined as amount of time spends by a client tune into channel and retrieves required data. Access latency defined as amount of time spends by a client to submission of a query and gets query result.

      Fig.1. (1, m) interleaving strategy

    2. Query Processing in Wireless Broadcast Environment

      In wireless broadcast environment, various indexing structures such as R*-tree, D-tree, Grid-index, Hilbert Curve Index (HCI) and Broadcast Grid Index (BGI) are adopted for processing spatial queries in Euclidian space [13], [14], [15], [16], [17]. However, these indexing structures are not suitable for spatial queries in road networks. In road networks, air indexing structures has been adopted for spatial queries and supports only shortest path queries [18], [19].

    3. Network Coding

In wireless environment, while disseminating data to clients, some of the data is lost due to unreliable nature of wireless links. In order to address this problem, Automatic Repeat Request (ARQ) has been adopted. In ARQ, after each transmission, clients send feedback information to the server, be it a positive or negative acknowledgement. However, ARQ is not efficient if there is an increase in the size of the network. The alternative strategy to address this problem is referred as forward error control with channel coding, in which the server broadcasts coded packets (i.e., data packets are coded using network coding schemes) to clients, and then clients perform decoding on coded packets and conclusively get the original data packets. The noteworthy advantages of this strategy are scalability and throughput improvements and robustness of the networks. Network coding has been widely used in wireless multicast environment for improving query performance. Network coding has been primarily introduced for improvin performance in multicast routing [20]. Network coding is regarded a general strategy in packet routing that allows an intermediate router to encode an outgoing packet by mixing multiple incoming packets appropriately. In a multi- hop wireless network, to meet unicast demands, game theoretic framework is introduced using network coding [21]. A cross- layer optimization framework is formulated to derive the maximum throughput for multicast traffic. In a random network, the optimization strategy considers network coding in network layer and conflict-free transmission schedules in MAC layer [22]. In a wireless network, one-hop opportunistic network coding is adopted and argued for making rate control and scheduling network-coding aware [23]. The server disseminates a set of data blocks over a broadcast channel to a set of caching clients called index coding problem [24]. The index coding problem is motivated by various applications in wireless networking and distributed computing [25]. For instance, one can find such applications in satellite communication networks where the clients have limited storage and limited maintenance of the received information. Efficient index codes are useful for the wireless network architectures that utilize the network coding and opportunistic listening techniques [26]. Feedback-based adaptive network coding schemes are designed for minimizing decoding delay in each transmission of the packet-based erasure networks [27]. All traditional scheduling algorithms in data broadcast are based on the assumption that only clients requesting the same data item can be satisfied in one broadcast tick. Coding problem is adopted in on-demand broadcast for reducing accessing time. In this approach, each broadcast data item is encoded from utmost two data items and doesnt fully utilize the advantages of data broadcast and coding [28]. However, it is found in this approach that no restriction is considered on the number of encoded data items [29][33][34]. Approaching differently in a proposed frame work where in a grid cell, availability of data objects in each adjacency cell can be encoded and as such it is a significant factor by which different performance objectives can be achieved.

  1. NETWORK CODING-BASED SPATIAL AIR INDEXE FOR QUERIES IN ROAD NETWORKS

    NCSAI is very useful in wireless broadcast environment for processing spatial queries in road networks as it derives the advantages of network coding. Based on the study vide [11], the partition of the road networks into small grid cells, pre-

    computing of the diameter of cell and border point, and formation of NCSAI is shown below in Fig.2. Furthermore, this section describes 1) data segment; 2) adaptive XOR-based network coding and 3) NCSAI broadcasting method on wireless channel.

      1. Data Segment

        Broadcast program contains two major components. They are air index and data. An air index represents NCSAI and data represents data objects respectively. In this work, let us assume that structure of road network is captured by adjacency lists, and each client has unique ID (i.e. identification). From the Fig.2, the grid cell C2 contains three clients i.e., v1, v3 and v4, and two data objects O3 and O4 respectively. In this cell, the adjacency lists of all the vertices are broadcast based on the order of the clients IDs. For example, consider the structure of v1 adjacency list, it contains two neighbors v3 and v4, and each neighbor is represented by four-tuples i.e., (v3, 1, 1, O4), where the first field v3 indicates adjacent node i.e.,v3 connected to v1, the second field 1 indicates a weight of an edge (v1, v3), the third field 1 indicates the number of data objects lying on this edge, and the fourth field is a pointer which points to the first data object on this edge. If there is no object on this edge, the pointer is set to null. The overall structure of grid cell C2 is shown in Fig.3.

        Fig.2. Road Network Partition into 2 × 2 Grid cells

        Fig.3. Data structure of Grid cell C2

      2. Adaptive XOR-based Network Coding

        In grid cell C2, we should consider the structure of v1 adjacency list. It contains two neighbors v3 and v4. v1 to v3 contains data object O4 and v1 to v4 contains data object O3. For example, v1 to v3 contains one or more than one data objects; then these data objects are encoded using adaptive XOR-based network coding and broadcast. At client site, decoding takes place. NPI doesnt satisfy multiple client requests in a single tick as network coding concept has not been used. NCSAI, being a more flexible concept satisfies multiple client requests in a single tick using adaptive XOR-based network coding. The salient futures of the concept are stated here. 1) Adaptive XOR-based network coding is a simple one with fewer overheads; 2) Server should know what type of data objects are being requested by clients as well as the clients cached data;

        3) In a grid cell, based on the availability of data objects in a adjacency cell, data objects are encoded using adaptive XOR- based network coding and broadcast by the server and 4) Server and clients site use simple XOR operation for encoding and decoding the data.

      3. How to broadcast NCSAI with data segment on Wireless channel

    The server broadcasts NCSAI with data segment via wireless channel using (1, m) data interleaving strategy. Then the interested client tunes into channel, retrieve the data and decode it.

  2. PROCESSING OF SPATIAL QUERIES AT CLIENT SITE USING NCSAI

    This section describes various searching algorithms at client site which include range query, k-nearest neighbor query, and continuous k-nearest neighbor query. The objective of these algorithms is to process the query efficiently and return result to the client with the help of NCSAI. For processing queries in road networks, Dijkstras shortest path algorithm has been followed because it is simple and efficient for small sub-graph retrieved and decoded by clients. In this task, it is assumed that the clients locations are located at network nodes and easily extended to support the case where the clients locations are put up along the network edges [4], [18], [19].

      1. Range Query

        In road networks, range query retrieves all data objects within a network distance d from a query point q, and it is denoted as (q, d, S) = {o |o S ||o, q|| d}, where q is a query point, o is a data object, d is a network distance and S is a dataset in road networks.

        Fig.4. Processing of Range Query

        Vol. 9 Issue 04, April-2020

        Algorithm 1: Range Query

        Procedure Range_query(q, d, S);

        Input: q query point, d distance, S data set

        Output: valid data objects within the range d

        % receive_query( ) a client waits for receiving a query from the mobile user, perform encoding and decoding at server and client sites respectively using XOR-based network coding, t time, Cq valid grid cells from a query point q, Rq valid range query result from a query point q, q,i minimum or maximum network distance from a query point q, retriveIndexHeader( ) the client retrieves index header, SleepUntilCellBroadcast( ) client sleeps until required data broadcast, retriveCell client retrieves the cells containing query result, adjacencyLists adjacency lists are forms as sub graph, Dijkstra(subGraph, q, d) process the query using Dijkstras shortest path algorithm and return query result; compute a cut-off point cp using [12] %

        % perform encoding and decoding at server and client sites respectively using adaptive XOR-network coding %

        begin

        1: divide the data set S into general and specific; 2: compute a cut-off point cp;

        3: query = receive_query( );

        4: if query results found at client cache then

        5: return result;

        6: else

        7: Listen_channel( );

        8: return result;

        9: end if

        10: end

        1: Procedure Listen_channel( ); 2: begin

        3: wait t seconds for required data;

        4: if a clints required data doesn't broadcast then

        5: client sends query to server; 6: for each time slot do

        7: if empty slot exist then

        8: select specific data objects based on waiting time and priority; 9: end if

        10: end for

        11: update cut_off point cp;

        12: else if required data is received from the server then

        13: NCSAIHeader = retriveIndexHeader( ); 14: find out the grid cells Cq containing query q;

        15: read the qth row Rq corresponding to Cq in the matrix; 16: for each cell Ci do

        17: if q,i<= d then add Ci to the candidate cell

        18: sort the candidate cells by their arrival times; 19: for each candidate cell do

        20: SleepUntilCellBroadcast( );

        21: Listen_channel( );

        22: adjacencyLists = retriveCell( );

        23: subGraph.add(adjacencyLists);

        24: return Dijkstra(subGraph, q, d);

        25: end for

        26: end if

        27: end for

        28: end if

        29: end

        In the light of the above mentioned algorithm, it may be assumed that the client being located at node v5 requests range query to find four restaurants from query point q with network distance d = 3. Then the range query result obtained is O1, O2, O3 and O4.

      2. k-Nearest Neighbor Query

        K-Nearest Neighbor Query defined as a mobile client retrieves k-nearest data objects from a query point q. Like a range query, the range is not fixed in a k-nearest neighbor query and it will be depend on the value of k and location of query point q. In this work, we estimate network distance dmax based on NCSAI in order to find candidate cells. Once dmax is estimated then k-nearest neighbor query is converted into range query and retrieve candidate data objects within a network distance dmax from a query point q. The dmaxis estimated based on the upper bounds of network distance between any object in Cu and any objects in Cv and denoted as UB (Cu, Cv).

        Lemma: Given two cells Cu and Cv, the network distance between any object u in Cu and any object v in Cv is bounded by (diameter(Cu)+diameter(Cv)+u,v) and it is denoted as UB(Cu,Cv) diameter(Cu)+diameter(Cv)+ u,v.

        Based on above lemma, dmax is estimated as follows. For example a query point q is located in cell Cq, we access Ci based on the non-descending order of q,i that is Ci is visited earlier than Cj if q,i< q,j there is tie, UB(Cq,Ci) and UB(Cq,Cj) are used as tie- breaker and one with smaller UB value will be visited first. In this process two parameters are used, one is count i.e., the total number of data objects in all the cells visited so far, and the other is UB(dmax) for each visited cell Ci. We calculate UB(Cq,Ci) and set UB(dmax) to the maximum UB(Cq,Ci) found so far and mean while we check |Ci|, and update count. This process is continuing until count reaches to value k.

        Algorithm 2: kNN Query

        Procedure kNN _query(q, k, S);

        Input: q – query point, k an integer, S – data set

        Output: valid k nearest data objects from query point q

        % receive_query( ) a client waits for receiving a query from the mobile user, perform data encoding and decoding at server and client sites respectively using XOR-based network coding, t time, Cq valid grid cells from a query point q, Rq valid range query result from a query point q, q,i minimum or maximum network distance from a query point q, Q queue, Canq candidates cells from a query point q, retriveIndexHeader( ) client retrieves index header, rangeQuery(q, UB(dmax), S) process the range query and return sub graph with candidate cells, Dijkstra(subGraph, Canq, q, k) process the query using Dijkstras shortest path algorithm and return query result, compute a cut-off point cp using [12] %

        % perform encoding and decoding at server and client sites respectively using adaptive XOR-network coding %

        begin

        1: divide the data set S into general and specific; 2: compute a cut-off point cp;

        3: query = receive_query( );

        4: if query results found at client cache then

        5: return result;

        6: else

        7: Listen_channel( );

        8: return result;

        9: end if

        10: end

        1: Procedure Listen_channel( ); 2: begin

        3: wait t seconds for required data;

        4: if a client required data doesn't broadcast then

        5: client sends query to server; 6: for each time slot do

        7: if empty slot exist then

        8: select specific data objects based on waiting time and priority; 9: end if

        10: end for

        11: update cut_off point cp;

        12: else if required data is received from the server then

        13: Q = 0, count = 0, UB(dmax) = 0;

        14: NCSAIHeader = retriveIndexHeader( ); 15: find out grid cells Cq containing query q;

        16: read the qth row Rq corresponding to Cq in the matrix;

        17: sort the cells Ci based on non-descending order of ,q,i and maintained in Q; 18: while Q is not empty do

        19: Ci = Q.pop();

        20: count =count + |Ci| ;

        21: UB(dmax) = MAX(UB(dmax),UB(Cq,Ci));

        22: end while

        23: if count > = k then break;

        24: (subGraph, Canq) = rangeQuery(q, UB(dmax), S);

        25: return Dijkstra(subGraph, Canq, q, k);

        26: end if

        27: end if

        28: end

        Based on the above cited algorithm, it may be assumed that the client is located at node v5 and requests k-nearest neighbor query to find four nearest neighbor restaurants from a query point q. Then k-nearest neighbor query result is O1, O2, O3 and O4.

      3. Continuous k-Nearest Neighbor Query

    Continuous k-Nearest Neighbor (Ck-NN) [30], [31] query widely used in many real-time applications. For example, mobile user continuously sends a query request to other mobile users while moving on the road networks called Ck-NN query. In Ck-NN query, once a client receives query from a mobile user, then it initially finds its located grid cell and the query path (v1, v2,vL). Thats why; based on the query path the client can find data objects in each segment in road network. Each segment in the network is called as valid interval, and all valid intervals contain start/end points and these points are called as split points. Let us assume that if a client located in the same safe region requests the kNN query, the obtained result is the same as the partial road network is the same. So unless the client tunes into another channel and retrieves a new candidates cell leaving the current safe region with maximum distance, the condition ( ||q, q1|| (UB(dmax) – dk)/2)) goes unsatisfied.

    Algorithm 3: Space Validation for Ck-NN Query

    Input: a query path P(v1,v2,…,vL)

    Output: valid space segments (VSS1, VSS2,,VSSL)

    % receive_query( ) a client waits for receiving a query from the mobile user, perform data encoding and decoding at server and client sites respectively using XOR-based network coding, t time, Cq valid grid cells from a query point q, Rq valid range query result from a query point q, q,i minimum or maximum network distance from a query point q, start starting point, L ending point, vstart starting vertex, n and i integer, G1 valid query path, G11 query path from starting point to ending point, P path, |vstart,vi|> (UB(dmax)-dk)/2 if the condition is true then client need to tune into channel and retrieve new candidate cells, VSSn(vstart,…,vi-1) valid space segments from vstart to vi-1, compute a cut-off point cp using [12] %

    % perform encoding and decoding at server and client sites respectively using adaptive XOR-network coding %

    begin

    1: divide the data set S into general and specific; 2: compute a cut-off point cp;

    3: query = receive_query( );

    4: if query results found at client cache then

    5: return result;

    6: else

    7: Listen_channel( );

    8: return result;

    9: end if

    10: end

    1: Procedure Listen_channel();

    2: begin

    3: wait for t seconds for required data;

    4: if the required data is not broadcast then

    5: client sends query to server; 6: for eachtime slot do

    7: if empty slot exist then

    8: select specific data objects based on waiting time and priority; 9: end if

    10: end for

    11: update cut_off point cp;

    12: else if required data is received from the server then

    13: start =1, G11 = 0, n =1;

    14: while start <= L do

    15: find out the grid cells Cq containing vstart and G1 = 0;

    16: compute UB(dmax) of vstart using Algorithm 2;

    17: for each grid cell Ci with q,i<=UB(dmax) do

    18: if Ci belongs to G11 then

    19: copy content of Ci to G1 ; 20: else

    21: retrieve Ci and add to G1 and G11; 22: end if

    23: end for

    24: search the kNN of vstart based on G1

    25: find the first node vi in P that |vstart,vi|> (UB(dmax)-dk)/2;

    26: output VSSn(vstart,…,vi-1);

    27: n++;

    28: start = i;

    29: end while

    30: end if

    31: end

    According to the above cited algorithmic study, it may be assumed that the client is located at node v5 and requests continuous k-nearest neighbor query to find continuous four nearest neighbor restaurants from a query point q. Then the ascertained continuous k-nearest neighbor query result is O1, O2, O3 and O4.

  3. PERFORMANCE EVALUATIONS

    Experiments have been conducted for evaluating query performance using ns-2 simulator with window 7 platform, 2.33G Intel Core 2 CPU and 3.2 GB RAM.

      1. Experimental Process

        In this simulation, real road networks data set namely Oldenburg (OL) and California (CAL) have been considered. The OL contains 6,105 nodes and 7,035 edges, and CAL contains 21,048 nodes and 21,693 edges [32] respectively. The evaluation is run on simulator which contains server, broadcast channel and clients. For simulating results, 100 clients and 500 random queries are used. In this work, we cared that 1) the size of data object is fixed at 128 bytes; 2) a set of data objects are randomly generated and uniformly distributed; 3) query issuing points are always at the network nodes; 4) network bandwidth is dynamic;

        5) tuning time and access latency are measured in terms of number of bytes of data transferred in a wireless channel.

      2. Cycle Length

        Cycle length plays an important role in this simulation because it has directly impacts on the access latency. The original road network is partitioning into 2i× 2i uniform grid cells, where i vary from 0 to 4, (i.e., the number of grids ranges from 1, to 4, to 16, to 64, and to 256). The number of grid cells in a network is set to 4i and mapped into Hilbert curve order. The server disseminates data using (1, m) strategy by setting m to its optimal value [5], we compared the performance of NCSAI with state- of-the-art NPI based on grid sizes 42, 43, and 44 respectively, and these are denoted as NCSAI/NPI-16, NCSAI/NPI-64 and NCSAI/NPI-256.

        Table 1. Parameter Settings

        Table 2. Broadcast Cycle Length

      3. Range Query

        In a range query, a mobile client is located at query point q and issue a query with distance d then it retrieves all data objects within a d distance. In this simulation, we ignore an object density DN but fixed at 0.1 because more number of data objects will not affects on query performance, and the radius of range query is varied from 0.01DN, to 0.05DN, to 0.1DN and to 0.2DN. From Fig.5, we observe that with smaller radius, NCSAI-16 and NCSAI-64 have better tuning time compare to state-of-the-art NPI- 16 and NPI-64 respectively. As for the access latency, NCSAI-16 has shorter access latency than NPI-16 consistently; similarly NCSAI-64 also has smaller access latency than NPI-64 in some cases because NCSAI has used both XOR based network coding

        and cache at server and client sites respectively. This set of experiments are gives a comparison between the performance of NCSAI-16 and NCSAI-64. NCSAI-16 has smaller index size, so the cycle is shorter, hence access latency is smaller. Similarly, NCSAI-64 has narrow grid cells, which provides tighter upper bound, hence smaller search space for range query, resulting in shorter tuning time. By observing range query experimental results NCSAI-64 expected to produce a better system performance compared to NCSAI-16.

      4. k-Nearest Neighbor Query

    Fig.5. Performance of range queries with radius

    We evaluate the performance of state-of-the-art and proposed algorithms under various k values, as presented in Fig.11. In this experiment, objects density is fixed at 0.1 and k values are varied from 1, to 5, to 10, to 15. We consider only small k values in our experiments because mobile devices have small screen, and difficult to display a large number of NNs. We observe the performance of tuning time presented in Fig.6.(a) and Fig.6.(c), NCSAI-256 has better tuning time compared to state-of-the-art NPI-256, because it used pre-computation information carried by NCSAI and cache data at clients are enables the clients to prune the search space effectively. But, for the kNN queries with k > 1, tuning time of NCSAI increases as k becomes larger. Because the upper bound of the distance between the query point and kth NN is enlarged by k, so the clients would process range query with larger radius. However, even under a relatively large k, the tuning time of NCSAI is still smaller compared to state- of-the-art NPI. We also observe the access latency presented in Fig.6. (b) and Fig.6.(d), NCSAI-16 has outperformed because of small index size. Similarly NCSAI-64 also has better performance compared to state-of-the-art NPI-64. However, NCSAI- 256 has energy efficient, but it has large access latency because of larger index size.

    We also evaluate performance of different index with various object densities as shown in Fig.7. For simplifying the comparison, we assume that the number of grid cells is fixed at 64 for both state-of-the-art NPI and NCSAI. The performance of NCSAI-64 is more stable to the variation of the object density compared to state-of-the-art NPI-64 because NCSAI enables the clients to check the number of objects in each grid cells and compute the upper bound of the distance of the kth NN. When the object density becomes dense, then the upper bound decreases because it prunes away more unnecessary grid cells.

    Fig.6. Performance of kNN queries with k value

    5 Ck-NN Query

    Fig.7. Performance of 10NN queries with various object densities

    In Ck-NN Query, Query Path Length Ratio (QPLR) is defined as the ratio of the query path length to the diameter of the road network. The query path is generated by the shortest path between two randomly selected nodes in the network. In this experiment, object density is fixed at 0.1 and QPLR ranges from 0.01 to 0.2. The performance for the OL and CAL data sets for answering CNN queries with k = 10 as shown in Fig.8. The tuning time of NCSAI-64 varies as QPLR changes because it enables space pruning. However, tuning time of NCSAI-64 has better than state-of-the-art NPI-64. But NCSAI-64 has suffered from longer access latency because the Ck-NN algorithm under NCSAI index, allows the clients to tune into the channel to update the partial road network and kNN candidates only when they arrive at the first point of a new SVS. We also evaluate the performance of state-of-the-art NPI-64 and NCSAI-64 for supporting CNN queries with different k values. In this experiment, we fixed QPLR at 0.1, k is varying from 1 to 15, and object density is fixed at 0.1 for both OL and CAL shown in Fig.9.

    NCSAI-64 has more energy efficient compared to state-of-the-art NPI-64 for different k because the pre-computation by NCSAI helps to clients prune the needless regions and avoid the clients keeping tuning into the channel rapidly as the clients move to a new position. However, access latency of NCSAI-64 has better than state-of-the-art NPI-64 with different k.

    Fig.8. Performance of CNN queries with QPLR

    Fig.9. Performance of CNN queries with k values

  4. CONCLUSION

In wireless mobile environments, sptial query processing is a challenging issue due to the following limitations such as low bandwidth, limited energy and capacity, and mobility. In this work, we observed that NCSAI has improved query performance compared with state-of-the-art NPI because NCSAI effectively utilized the advantages of network coding and cached data at clients. In future, we can extent this work by incorporating an advanced network coding strategies and spatial queries.

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