Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
IEEE 802.1aq wikipedia , lookup
Computer network wikipedia , lookup
Policies promoting wireless broadband in the United States wikipedia , lookup
Airborne Networking wikipedia , lookup
Wireless security wikipedia , lookup
Cracking of wireless networks wikipedia , lookup
JOURNAL OF INFORMATION, KNOWLEDGE AND RESEARCH IN COMPUTER ENGINEERING ROLE OF VIRTUAL CARRIER SENSING ON FAIRNESS ISSUES IN IEEE 802.11 BASED AD HOC NETWORKS 1 NIMAVAT ASHISH V, 2 TEJAS M. VASAVADA , 3 MAYUR M. VEGAD 1 Dept. of Computer Engineering , Birla Vishvakarma Mahavidhyalaya , Vallabh Vidhyanagar, India 2 Dept. of Information Technology, A.D. Patel Institute of Technology, New Vallabh Vidhyanagar, India 3 Dept. of Computer Engineering, Birla Vishvakarma Mahavidhyalaya, Vallabh Vidhyanagar, India [email protected] , [email protected], [email protected] ABSTRACT—IEEE 802.11 based multi hop ad hoc networks(MANETs) suffers from hidden terminal problem and exposed terminal problem, more than in the single hop cases due to large interference range and the large carrier sensing range. Unfairness is the major issue where one flow is getting channel utilization and others are starving. Ideally virtual carrier sensing (also known as RTS/CTS handshake) can eliminate interference. From our literature study we believe that less attention is given to fairness issue. In this paper we study the effectiveness of the virtual Carrier sensing mechanism on IEEE 802.11 based Ad Hoc Networks on fairness issue. From simulation study carried out on two topologies Cross and Grid in Network Simulator – 2, we observed that use of virtual carrier sensing increases the fairness in network at some cost of increased overhead. Keywords: Wireless Ad Hoc Networks; Virtual Carrier Sensing; Interference Range; Hidden Terminals; Fairness I. INTRODUCTION A wireless ad hoc network is a collection of mobile nodes equipped with wireless transceivers that can work without any fixed networking infrastructure [1]. A node can transmit data packets to other nodes who are within its radio coverage range directly, and who are outside the range via multi-hop store-and-forward relay. In Wireless medium all the station in the range share the medium at the same time. So for fair medium access, medium access control mechanism is required. Therefore medium access control protocols were introduced like IEEE 802.11, HIPERLAN ½, HomeRF, Bluetooth, [9] etc. Among all these standards, IEEE 802.11 is the most popular medium access control protocol for wireless LANs and Mobile Ad hoc networks. IEEE 802.11 DCF is the most popular MAC layer protocol for both the wireless LAN’s and MANET. Hidden terminal problem and exposed terminal problem are due to large interference range in wireless networks [5]. So to reduce interference at the time of receiving a data frame, virtual Carrier Sensing mechanism is used with MAC protocol [2]. It is also known as RTS/CTS handshake. Rest of the paper is organized as follows. Section II describes well known hidden and exposed terminals problems. Section III discusses the effectiveness of virtual carrier sensing. In Section IV we study the impact of a large interfere range that affects the success of a reception of a frame followed by simulation study in Section V. In Section VI result analysis is discussed and final section concludes the paper. II. HIDDEN TERMINAL AND EXPOSED TERMINAL PROBLEM Due to limited transmission range of mobile stations, multiple transmitters within range of the same receiver may not know one another’s transmissions, and thus are in effect “hidden” from one another [1]. When these transmitters transmit to the same receiver at around the same time, they do not realize that their transmissions collide at the receiver. This is the socalled “hidden terminal” problem which is known to degrade throughput significantly. Hidden terminal problem is given in Figure 1 where node A and node C are hidden from each other. ISSN: 0975 – 6760| NOV 10 TO OCT 11 | VOLUME – 01, ISSUE - 02 Page 34 JOURNAL OF INFORMATION, KNOWLEDGE AND RESEARCH IN COMPUTER ENGINEERING Figure 1 Hidden Terminal Problem Exposed terminal problem is complementary to hidden terminal problem. For example, in Fig. 1, station B is transmitting to station C. Station A being in carrier sensing range of station B, senses the channel busy. Consequently, it may defer its own transmission to some other station D. If station A is out of interfering range from station C then this deferring of A is indeed unnecessary, losing an opportunity of transmission. Clearly, exposed terminals problem reduces the spatial reuse. III. EFFECTIVENESS OF VIRTUAL CARRIER SENSING The main objective of MAC protocol is to avoid collisions due to shared channel allocation. To this end IEEE 802.11 has incorporated virtual carrier sensing [8] besides the physical carrier sensing in DCF. It is a four way handshake protocol with two additional frames known as request-to-send (RTS) and clear-to-send (CTS). Transmitter initially transmits the RTS frame instead of data frame. If the RTS is received successfully the receiver responds with CTS after SIFS period. All other nodes in the transmission range of sender or receiver will hear RTS or CTS or both the frames and load set their virtual carrier sense indicators known as network allocation vector (NAV) with the duration declared in the RTS/CTS. This information will be used along with the physical carrier sensing to sense the availability of the medium two-hops far. Thus the probability of interfering from the nodes in transmission range of sender and receiver is reduced. Effectively, the virtual carrier sensing shortens the duration of collisions due to the hidden terminals. So this mechanism increases throughput and hence performance of the network. Virtual carrier sensing mechanism is illustrated in Figure 2. However, in scenarios where number of competing flows is less or when the size of the data frame is small, the use of RTS/CTS proves to be a substantial amount of overhead affecting the performance negatively as compared to the basic access scheme without RTS/CTS. Figure 2. RTS/CTS handshake IV. LARGE INTERFERENCE RANGE Virtual carrier sensing can eliminate most of the interference from the other nodes. But nodes those are out of transmission range of both receiver and transmitter can also interfere to receiver. This situation happens due to large interference range. In Wireless LANs there is less possibility of occurrence of this type of situation, but in mobile ad hoc networks where nodes are spread in large area this situation becomes serious. RTS-CTS cannot function well if distance between transmitter and receiver is larger than the 0.56 * Rtx (Transmitter range) [5]. But from our simulation study we found RTS-CTS useful in our topologies. This is as shown in Figure 3. In general path loss of a signal is modeled by two way ground model [1] having distance d between transmitter and receiver. So from the equation (1) we can calculate power of signal at receiver that is Pr . Here, transmission power is Pt. Pr =Pt Gt Gr (ht2hr2/d4) (1) Gt and Gr are the antenna gains for transmitter and receiver respectively. ht and hr are the heights of antenna of transmitter and receiver. d is the distance between transmitter and receiver. Signal at the receiver is considered valid if Signal to noise ratio is above certain threshold. SNR ratio is given as equation (2). SNR = Pr / Pi (2) SNR = (r/d) 4 >= TSNR (3) Where TSNR is the SNR threshold. r= (4) In general SNR threshold is set to 10. So putting 10 in equation 4 we get, Ri = 1.78 * d (5) Where Ri refers to the interference range of receiver. This is shown in Figure 3. ISSN: 0975 – 6760| NOV 10 TO OCT 11 | VOLUME – 01, ISSUE - 02 Page 35 JOURNAL OF INFORMATION, KNOWLEDGE AND RESEARCH IN COMPUTER ENGINEERING Figure 3 Interference Range V. SIMULATION STUDY All the simulation is done in the network simulator ns-2 (ver. 2.33) [4]. We have considered topologies like cross and grid which are having competing flows. Some simulation parameters are given below. Table 1 Parameters used during simulation Parameter Transmitter range Simulation time Number of nodes Traffic type Application Packet size total simulation Time Channel type Propagation model MAC protocol Type Max packet in queue Inter nodal distance Value 250 m 100 sec 9 and 25 Tcp ftp 1500 Bytes 100 sec Channel/Wireless channel TwoRayGround Mac/802_11 50 130-170-210-250 Figure 4 Grid and Cross Topologies. Fairness is an important performance criterion in all resource allocation schemes, including those in distributed computer systems. Fairness is an important consideration in most performance studies [10]. Particularly in distributed systems, where a set of resources is to be shared by a number of users, fair allocation is important. If goal is to provide the same throughput to all nodes, the fairness index would be Fairness (throughput) = Where T is the throughput of a particular flow. n is the no of throughput in the system. VI. RESULT ANALYSIS In above topologies cross and 5x5 grid (fig 5 and 6) fairness is the most important performance metrics. As we can see from the results, use of RTS/CTS increases overhead in network but it increases fairness in the network. Topologies are as below: Figure 5 Inter Nodal Distance V/s Throughput for Cross Topology ISSN: 0975 – 6760| NOV 10 TO OCT 11 | VOLUME – 01, ISSUE - 02 Page 36 JOURNAL OF INFORMATION, KNOWLEDGE AND RESEARCH IN COMPUTER ENGINEERING As we can see from Figure 7 & 8 fairness index is better as distance between transmitter and receiver increases. From equation 5 as distance (d) increases interference range also increases and hence nodes will face more interference from other nodes. In this type of situation virtual carrier sensing mechanism is efficient. Figure 6 Inter Nodal Distance V/s Throughput for Grid Topology Figure 7 Inter Nodal Distance V/s Fairness for Cross topology Figure 8 Inter Nodal Distance V/s Fairness for grid Topology VII CONCLUSION AND FUTURE WORK In this paper we have examined effectiveness of virtual carrier sensing in multi hop mobile ad hoc network scenarios having competing tcp flows. We simulated grid and cross topology in both with and without virtual carrier sensing having performance matrices like aggregate throughput and fairness index. As a part of contribution of this paper we can say that virtual carrier sensing mechanism plays vital role in reducing significant amount of interference in network as distance increases and hence increases fairness. Virtual carrier sensing mechanism increases fewer overhead in network but it is negligible as compared to its good performance in increasing fairness. As a part of future work some improvement in virtual carrier sensing mechanism can be done for better performance of network and also with some random topologies its performance can be evaluated. VIII REFERENCES [1] T. Rappaport, “Wireless Communications: Principles and Practice,” Prentice Hall, New Jersey, 1996. [2] J. Weinmiller, H. Woesner, J. Ebert, and A. Wolisz, “Analyzing the RTS/CTS Mechanism in the DFWMAC Media Access Protocol for Wireless LAN's,” In Proceedings of IFIP TC6 Workshop Personal Wireless Communications (Wireless Local Access), Prague, Czech Republic, April 1995. [3] Xu, S.Saadawi, T. “Does the IEEE 802.11 MAC Protocol Work Well in Multihop Wireless Ad Hoc Networks?” IEEE Communications Magazine, Volume: 39 Issue: 6, pp. 130–137, June 2001. [4] VINT Group, UCB/LBNL/VINT network simulator-ns (version 2), http://www.isi.edu/nsnam/ns. [5] Kaixin Xu, Mario Gerla, Sang Bae, “How Effective is the IEEE 802.11 RTS/CTS Handshake in Ad Hoc Networks?,” Globecom 2002 – IEEE Global Telecommunications Conference, no. 1, pp. 72–77, November 2002. [6] Jeffrey P. Monks, Vaduvur Bharghavan, and Wen-mei W. Hwu, “A Power Controlled Multiple Access Protocol for Wireless Packet Networks,” IEEE INFOCOM 2001, Vol. 1, pp.219 – 228, 2001. [7] G. Berger-Sabbatel, A. Duda, O. Gaudoin, M. Heusse, F. Rousseau, “Fairness and Its Impact on Delay in 802.11 Networks”, In Proc. of IEEE Globecom, Dallas, Texas , USA, 2004, Vol. 5, pp. 2967- 2973. [8] IEEE Std. 802.11, “Wireless LAN Media Access Control (MAC) and Physical Layer (PHY) Specifications,” 1999. Downloadable at http://standards.ieee.org/getieee802/. [9] Tzu - Chieh Tsai, Chein – Ming Tu, “Improving IEEE 802.11 RTS/CTS Handshake in Wireless Ad Hoc Networks Considering Large Interference Range”. [10] Rajendra K. Jain, Dah-nimg w. chiu, William R.Have, DEC-TR-301 “A quantitative measure of fairness and discrimination for resource allocation in shared computer system”, Eastern Research Lab. ISSN: 0975 – 6760| NOV 10 TO OCT 11 | VOLUME – 01, ISSUE - 02 Page 37