Keywords IEEE ; wireless mesh network; Tower height; 2P MAC protocol; Tree enumeration algorithm; CSMA/CA; TDMA; Heuristic algorithm.

Transcription

1 Volume 6, Issue 9, September 2016 ISSN: X International Journal of Advanced Research in Computer Science and Software Engineering Research Paper Available online at: Modified Topology for Long Distance Wireless Mesh Network Dikchha Dwivedi, A.K. Dwivedi, Akhilesh Kosta Kanpur Institute of Technology, Kanpur, Uttar Pradesh, India Abstract Cost optimization is a crucial criterion in the field of technology deployment especially in developing domains. IEEE is a set of media access control (MAC) and physical layer (PHY) specifications for implementing wireless local area network (WLAN) computer communication in the 900 MHz and 2.4, 3.6, 5, and 60 GHz frequency bands. It helps in proving a cost effective option to connect remote villages. In this paper we have tried to study the problem of topology planning considering long-distance based countryside networks. First we tried to formulate this problem in terms of its restrains and optimization metric. Being combinatorial in nature it is not possible to scale it up for larger sets of inputs. So we break the problem into various constituent sub-parts followed by several searching and trimming approaches for generating an optimal solution. Finally, we presented the results generated by the algorithm and able to reduce the cost of transmission by factors of tens. By improving the process of assigning the tower height the transmission cost can be reduced to 15 times. While planning the network we gave special attention to the relationship between the Medium Access Layer (MAC) technology and the throughput requirements of the nodes. Keywords IEEE ; wireless mesh network; Tower height; 2P MAC protocol; Tree enumeration algorithm; CSMA/CA; TDMA; Heuristic algorithm. I. INTRODUCTION Due to advancement of wireless technology and the proliferation of based hand-held wireless terminals, recent years have witnessed an ever-increasing popularity of wireless networks, ranging from Wireless Local Area Networks (WLANs) to Mobile Adhoc Networks (MANETs) But, this growth and popularity is cramped to developed countries, and metropolitan cities of developing countries. This is really ill-fated for developing countries like India, where more than 74% of the population is rural [12]. In the US and Europe, most people started connecting to the internet using dial-up modems when landlines were the norm. But this approach won t work in India, where only about 26 million landlines exist for the population of 1.3 billion. Cellular wireless is also not sustainable because of its business model, which requires more high-paying consumer. In this regard, the Wi-Fi has shown remarkable progress and acceptance as a last hope access solution, because of its low price. Although was chiefly designed for indoor operation, but has recognized the opportunity of using in longdistance networking based long-distance networks has been recommended as a cost-effective choice to provide Internet connectivity to countryside areas in developing countries, to empower ICT (Information and Communication Technology) services [13]. The major cost in based long-distance networks is the cost of antenna towers [2]. While the radio itself costs about U.S.$50 or less, a 30 m antenna tower can cost as much as U.S.$1,000. Topology planning for longdistance networks with stress on cost minimization is the motivation of this paper. Before planning any deployment, the first thing which should be kept in mind is the topology planning, apart from it the other crucial aspects are Medium Access Control protocol, inter-node interference and the resultant application performance seen. II. METHODOLOGY We first break down the problem into parts: (a) once a tree has been formed, optimum height is assigned to tower locations (b) allocating suitable antennas, and (c) allocating transmit power so that all the links formed can function all together. We are using a set of Linear Programming (LP) equations for the first problem, after introducing proper domain based relaxations. For the second problem which is the antenna assignment problem, we first put forward an optimum combinatorial algorithm, and then substitute it with a heuristic algorithm of polynomial time complexity. The third problem of power assignment is cracked by solving an LP formulation HIGH-AVAILABILITY The height assignment problem: This involves allocating optimum heights to the towers to be placed at villages. This step assumes that the parent child relationship between the various nodes is known. Antenna assignment problem: This involves allocating appropriate antennas to the radios to placed at villages. This step should have all the parent child relationship information available. The aim of the step is to ensure coverage within half beam width region of each antenna of the opposing point of the link, while reducing interference. 2016, IJARCSSE All Rights Reserved Page 291

2 The Power Assignment problem: This involves allocating appropriate power to the radios to placed at villages. Power assignment step should take care of the following: (a) excess power would result in high interference among the various links reducing the performance, whereas (b) too less power might result in failure in formation of links. III. MEDIUM ACCESS CONTROL (MAC) PROTOCOL The Medium Access Control (MAC) protocol is used to provide the data link layer of the Ethernet LAN system. The MAC protocol encapsulates a SDU (payload data) by adding a 14-byte header (Protocol Control Information (PCI)) before the data and appending an integrity checksum, the checksum is a 4-byte (32-bit) Cyclic Redundancy Check (CRC) after the data. The entire frame is preceded by a small idle period (the minimum inter-frame gap, 9.6 microsecond (µs)) and a 8 byte preamble (including the start of frame delimiter) [9]. For long distance wireless networks, we have taken Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) consider along with 2p, Time Division Multiple Access (TDMA) based protocols. The Carrier Sense Multiple Access (CSMA) with Collision Detection (CD) protocol is used to control access to the shared Ethernet medium. A switched network (e.g. Fast Ethernet) may use a full duplex mode giving access to the full link speed when used between directly connected NICs, Switch to NIC cables, or Switch to Switch cables. Time division multiple access (TDMA) is a channel access method for shared medium networks. It allows several users to share the same frequency channel by dividing the signal into different time slots. Dependences Dependence of throughput on MAC The throughput that can be attained on a single link depends on the MAC protocol to be used. This can be understood if we consider the case of 2p, TDMA and CSMA/CA MACs [7]. In 2p the links can operate simultaneously, while in the other two MACs the channel is basically time shared. Seeing the identical physical encoding for all three Medium Access Control simplies that the capacity of 2p would be greater than that of the other two MACs. For the planned long-distance networks produced by our algorithm, all the traffic is to be routed through one root node. The link between a child and parent can support only a fixed amount of traffic which depends on the physical layer encoding (PHY) used by the MAC, this constraint is cascaded to all other nodes down the tree as well. The value of maximum traffic is determined by the PHY used, for b PHY this value is 11 Mbps, while for a/g PHY this value is 54 Mbps. Dependence of throughput on Antenna/link type The throughput rest on whether the link is P-to-mP or P-to-P link set, in the previous case the maximum allowed throughput by the MAC is always shared between the constituent links while in the latter the link can use the entire throughput allowed by the MAC [8]. The reason for this is, in a P-to-mP link set one of the ends has only one radio, which can only service the set of many antennas at the further end in turns 2016, IJARCSSE All Rights Reserved Page 292

3 Dependence of transmission power on Antenna/link length The strength of radio signal degrades as its transmission distance increases. Also the antennas have a explicit gain (capacity to enhance the signal power) in a specified direction. The value of the transmit power is calculated while taking care of the preceding points and confirming that high enough signal is received at the other end [4]. Dependence of MAC on transmit power In 2p protocol a set topology desires to be divided, and the two dividers of nodes, switch between simultaneous transmission and simultaneous reception phases, such that if one end of an edge is in reception phase the other is in transmission [1] [11]. While using 2p protocol, for a link A => B the various links which interfere is brief below. If the link is X => A it doesn't interfere. Since according to 2P MAC, the direction of transmission would be from A=>X. We represent this in above figure. If the link is A=> X and the antenna is not the same as that of A=> B it interferes. Since both the links would be in transmit mode and as the antennas are not common i.e., the links are not part of a P-to-mP link set which would mean that they cannot work jointly. If the link is X=>B it interferes. Since the node B is in reception phase and, hence would be receiving traffic from its child as well. We have shown this in above figure. If the link is B=> X it doesn't interfere. Since, as stated above node B is in receive phase and hence, cannot transmit. For link C=> D which does not fulfill any of the above stated conditions, it interferes A=> B. Here X, C and D denotes any arbitrary node. Dependence of deployment cost on tower height The cost of the deployment depends on the heights of the towers. Keeping in mind that the tower costs don't grow linearly with tower heights this is because its design also changes with height. However, there are two major problems that need to considered, before expressing an algorithm to assign cost optimal heights to the towers. They are as follows: The Obstruction height assignment As the links are long-distance, possibly few tens of kilometres apart, we must know the exact height of the maximum obstruction, and its exact location, beforehand we can go on and allocate tower heights to clear it. This is crucial as if substantial obstructions are present in between the two antennas at two ends the signal would get significantly reduced in power due to diffraction caused by the obstruction [10]. This can be well understood by the figure shown below. 2016, IJARCSSE All Rights Reserved Page 293

4 We now express the optimum height assignment problem. Let s assume that we are having two nodes 1 and 2, say node 1 is the leaf node and 2 the non-leaf node. where, h 1, h 2 = heights of the two towers, D = separation between the two points L = height of the obstruction d = separation of the obstruction from location 1 Providing we need to confirm LOS between the two nodes, the heights of two towers placed at points 1 and 2 is related as follows, h 1 *(D-d) + h 2 * d L*D This can be simply derived by trigonometry. So, for a set of n nodes with n-1 edges between them the formulation becomes, We can derive similar equations for all the n-1 edges in a n node tree. Let, h thre = upper bound of mast heights. Ai is the cost coefficient of the tower at point i, which for root and non-leaf nodes, would be that of a tower and for leaf nodes would be that of a mast. Also, Assuming the obstruction height and the distance of separation is the same for all the points. Henceforth, the formulation turns into, Minimize ( A i * hi), Subject to, for all nodes (i,j) which have an edge between them, h i *(D -d) + h j *d L* D, where h i is the child of h j h k h thres ; k is a leaf node h thres h k, k is a non-leaf node Approach to compute obstruction heights One of the key issues in determining the height of towers raised to achieve LOS is the understanding of the obstructions present along the straight line. Defining the same by manual survey would be a tiresome process as this would involve covering few tens of kilometers. In this section we term a mechanism to compute the same. The Shuttle Radar Topography Mission (SRTM), an international project headed by the National Geo-Spatial Agency (NGA) and National Aeronautical and Space Administration (NASA) [3], have made topography elevation data of the entire world easily available on their website. This data which has an resolution of 3 Arc seconds (which translates to height measurements on a grid of dimensions 90 meters), can be used to nearly infer the terrain height values between any two end points. Also given the aerial separation between two points, we can find the radius of the Fresnel curve as well by using the following formula, R(n) = M = 17.3, R (n) in meter and D1, D2 in Km = 72.1, R (n) in feet and D1, D2 in statue mile as stated earlier wireless link formation clearance of (60% for 2.4GHZ) first Fresnel zone is required. We make a simplifying postulation that, the angle of elevation of the line segment linking the two antennas with the horizontal axis, would be small as the difference between the tower heights at both ends is in order of meters while the link length would be in order of kilometers [5]. This basically means that we can adopt the Fresnel curve to be horizontal, i.e., the radius of the curve can be assumed to vertical. Given the earlier calculated height values of the intermediary points and the Fresnel radius at those points we can simply calculate the point of maximum obstruction. This is the point which if cleared would result in clearance of all the other points. The main concern with this technique would be the (lack of) accuracy of the SRTM3 data. Henceforth, an independent study was undertaken to find out the accuracy of the theoretical readings. The main goal of the study was to find if estimate of tower heights can be carried out based on the available satellite data. We found the actual feasible tower height values are within a 2 meters of their predicted values, henceforth leading providing confidence to the method. The comprehensive methodology of extraction of height values form the SRTM3 format and calculation of tower heights as well as the evaluation can be found in [11]. Heuristic algorithm for antenna assignment Here we mention an heuristic algorithm with (n 2 ) time complexity, where n is number of child nodes. In the heuristic algorithm we emphasis on reducing the interference between the various nodes instead of reducing the cost which is not a dominant factor as pointed out previously. The heuristic algorithm INPUT: Co-ordinates of the parent node and the child set. Radially organized child set. 2016, IJARCSSE All Rights Reserved Page 294

5 Cost price of the different kinds of antennas. Tables which given the angle with the main axis of the antenna gives the gain of the antenna in that direction for all the antennas. ALGORITHM: Reorganize the child set so that the points with maximum angular separation are the ones which utmost apart. Recursively do the following Allocate an antenna which can cover all the points within this set and set cost equal to cost of this antenna. Find the adjoining points which have the maximum angular separation, and along this point divide into two sets and call the algorithm on these two points, return the configuration from the previous run or this run based on which one is the lower costing one. Termination (of recursive function): If the set of points have an angular spread (the angular gap between the farthest two child points), less than that of a directional antenna return the cost of the same. The working of the algorithm can be easily understood from Figure shown below. Radial ordering of the points is mandatory to ensure that we can choose the point of maximum angular separation by likening the points with their neighbors only [6]. The goal for finding the maximum angular separation point between the points is interference minimization, as incrementing angular separation confirms the main sections of the two antennas are also separated. The power equations are framed universally, and cracked as an LP. This confirm that all the links (as determined by MAC) can function all together. We use the notation A=> B to designate the fact that node A is transmitting to node B. Hence, we are afraid about the interference at the antenna at node B. Let, P ij represent the power transmitted by the antenna placed at i labelled for j towards j. G ijk denote the gain of an antenna located at i labelled for j in the direction of k. and, PL ij indicate the path loss suffered by the signal while passing through from node i to j. In case two links i => j and k => l obstructs with each other, the strength of interference at node j is calculated as, Interference (I) = P kl * G jik * G klj This is demonstrated by above figure. Henceforth, the total interference at a node j, is the summation of all the interference at the node from all the interfering links. 2016, IJARCSSE All Rights Reserved Page 295

6 We now express the set of linear equations for power assignment problem. We do not consider the set of fixed losses due to RF cables and connectors in our scheme, but these are small and also they can easily be substituted by a constant upper bound (so that the losses are never larger than this value). For a given link i => j, the subsequent three conditions should be fulfilled, a. The transmitted power by i, P ij G ijj should be lesser than a maximum value P max, this is the constraint placed by spectrum regulatory authority of the country (like FCC in USA and WPC in India) in EIRP [1]. This can be shown as, P ij * G ijk P max b. The received signal strength by j, (P ij *G ijj *G jii )/PL ij should be higher than the receiver sensitivity P min. (P ij *G ijj *G jii )/PL ij P min. c. The ratio between the signal strength and the noise strength at node j should be greater than a bound SIR reqd. (P ij *G ijj *G jii )/PL ij SIR rerd * (<k,l> <k,l> R) (P kl *G jik *G klj )/PL kj. Where, R is the set of all links which interfere with the link i => j. The above set of equations constitutes ab LP, with the objective of minimization of being the sum of all the power values, radiated over all the links. The value of SIR reqd. is 10 db. Where, the noise strength is calculated as the summation of the noise signal strength of all the interfering links at the receiver. The nodes that are interfering are selected based on the MAC protocol. IV. CONCLUSIONS Cost optimization is a crucial criterion in the field of technology deployment especially in developing domains. IEEE is a set of media access control (MAC) and physical layer (PHY) specifications for implementing wireless local area network (WLAN) computer communication in the 900 MHz and 2.4, 3.6, 5, and 60 GHz frequency bands. It helps in proving a cost effective option to connect remote villages. In this paper we have tried to study the problem of topology planning considering long-distance based countryside networks. First we tried to formulate this problem in terms of its restrains and optimization metric. Being combinatorial in nature it is not possible to scale it up for larger sets of inputs. So we break the problem into various constituent sub-parts followed by several searching and trimming approaches for generating an optimal solution. Finally, we presented the results generated by the algorithm and able to reduce the cost of transmission by factors of tens. By improving the process of assigning the tower height the transmission cost can be reduced to 15 times. While planning the network we gave special attention to the relationship between the Medium Access Layer (MAC) technology and the throughput requirements of the nodes. REFERENCES [1] Effective isotropically-radiated power-wikipedia, the free encyclopedia. wiki/effective_isotropically-radiated_power.. [2] Media labs asia, iit madras. [3] Soekris Engineering. [4] Srtm repository. ftp://e0srp01u.ecs.nasa.gov. [5] Madhuresh Agrawal. Optimum tower height assignments for long distance links. Technical report, cs397 Report,Indian Institue of Technology, Kanpur, [6] Daniel Aguayo, John Bicket, Sanjit Biswas, Glenn Judd, and Robert Morris. Link-level Measurements from an b Mesh Network. In SIGCOMM, Aug [7] Pravin Bhagwat, Bhaskaran Raman, and Dheeraj Sanghi. Turning Inside-Out. In HotNets-II, Nov [8] S.A. Borbash and E.H. Jennings. Distributed topology control algorithm for multihop wireless networks. In IJCNN, [9] Eric Brewer, Michael Demmer, Bowei Du, Kevin Fall, Melissa Ho, Matthew Kam, Sergiu Nedevschi, Joyojeet Pal, Rabin Patra, and Sonesh Surana. The Case for Technology for Developing Regions. IEEE Computer, 38(6):25{38, June [10] Zhuochuan Huang, Chien-Chung Shen, C. Srisathapornphat, and C.Jaikaeo. Topology control for ad hoc networks with directional antennas. In ICCN, [11] Paul Ipe. Power allocation issues in a wireless mesh network. Technical report, BTech Project Re-port,Indian Institue of Technology, Kanpur, [12] Ashok Jhunjhunwala and Sangamitra Ramchander. Role of wireless technologies in connecting rural india. In Indian Journal of Radio & Space Physics, pages 363{372, [13] Bhaskaran Raman and Kameswari Chebrolu. Design and evaluation of a new mac protocol for long-distance mesh networks. In MOBICOM, , IJARCSSE All Rights Reserved Page 296