面向WSN的能耗均衡非均匀分簇路由协议研究
摘要
随着“普适计算”技术模式的出现,无线传感器网络(WSN)成为对人类生活方式产生重大影响的信息采集技术手段之一。在WSN面临的诸多挑战中,路由协议是其中的一个重要研究课题,按网络结构可分为平面路由协议和分簇路由协议。在分簇路由协议中,簇首一般进行簇内数据融合,大大减少了网络通信量,簇成员功能简单,能耗较少,而且分层结构具有较好的可扩充性,适合大规模网络,因此分簇路由协议是当前研究的热点。但目前提出的分簇路由协议也存在一些问题,例如,簇首的产生开销较大,形成的簇分布不合理,簇间通信也不能很好的实现簇首间的负载均衡,簇间通信没有一个良好的框架可供以后扩展,簇首的失效将导致一定时间内的路由失败,具有“瓶颈”效应,等等。
针对当前存在的这些问题,本文提出了一种能耗均衡非均匀分簇路由协议EBNC。EBNC根据WSN生命周期内各阶段的能量特征和数据传输“多对一”的向心性特征,从簇首的产生、簇间路由两个方面进行改进。EBNC在选举簇首时,动态地根据随机数和剩余能量权衡节点的竞争力,把竞争力转化为不同的时延,由时延决定节点是否成为簇首,簇的大小与节点距离sink节点的远近有关,低于最低能量的簇首没有簇内成员；对于簇间通信,提出一种基于复合势能场的簇间开放式路由框架,并在此框架的基础上实现了能耗均衡的簇间通信。这样产生的簇首同时考虑了节点的剩余能量和随机数,有利于能耗均衡和簇的合理分布,不同的时延减少了成簇时的通信冲突；没有成员的簇首避免了因能量耗尽而形成的“瓶颈”效应；最终达到路由过程中能耗均衡的目的,有效地延长了网络的生命周期,并增强了路由协议面向不同应用的可扩展性。最后用J-SIM仿真平台,对EBNC路由协议进行了仿真实验,通过与LENCH路由协议进行对比研究,证明了改进的算法节省了整个网络的能量消耗,有效地平衡了簇首间的能量消耗,延长了网络的生命周期,改善了网络的性能。
As a new technique of implementing Pervasive Computing,wireless Sensor Network (WSN) is having a profound effect on way of people'life. In many challenges of WSN' related research fields, the routing protocols of WSN are one of the most important issues. How to save energy and extend the network life is a core and basic problem in routing protocol design of WSN. Clustering routing protocols is better than flat routing protocols for its less energy consumption:members of a cluster are simple in function, and all the data of a cluster are merged by cluster-head, which reduce the amount of transfer. So current research is focus on clustering routing protocols. But current clustering routing protocols have some shortcomings such as much energy consumption in cluster forming, not uniform in data transfer, and so on.
With the problems of current cluster-based routing protocols,an energy-balanced routing algorithm for WSN is presented in this paper, according to different energy stages of WSN' life circle and the centripetal characteristics of data transmission. Through balancing a random number and residual energy and change into time delay, the node decide to become a cluster head itself. For routing of cluster heads, this paper proposed an open routing framework based on cluster heads, and realize an energy balance cluster routing algorithm based on this open framework. This algorithm balance energy consumption of cluster heads and cluster members, and extend the network life cycle effectively. Additionally the open framework of cluster-heads extends the scalability for different applications. This paper simulates the algorithm and LENCH routing algorithms and give the analysis of the result. The simulation results show that the algorithm exceeds LENCH in saving energy and prolonging the network's lifetime.
针对当前存在的这些问题,本文提出了一种能耗均衡非均匀分簇路由协议EBNC。EBNC根据WSN生命周期内各阶段的能量特征和数据传输“多对一”的向心性特征,从簇首的产生、簇间路由两个方面进行改进。EBNC在选举簇首时,动态地根据随机数和剩余能量权衡节点的竞争力,把竞争力转化为不同的时延,由时延决定节点是否成为簇首,簇的大小与节点距离sink节点的远近有关,低于最低能量的簇首没有簇内成员；对于簇间通信,提出一种基于复合势能场的簇间开放式路由框架,并在此框架的基础上实现了能耗均衡的簇间通信。这样产生的簇首同时考虑了节点的剩余能量和随机数,有利于能耗均衡和簇的合理分布,不同的时延减少了成簇时的通信冲突；没有成员的簇首避免了因能量耗尽而形成的“瓶颈”效应；最终达到路由过程中能耗均衡的目的,有效地延长了网络的生命周期,并增强了路由协议面向不同应用的可扩展性。最后用J-SIM仿真平台,对EBNC路由协议进行了仿真实验,通过与LENCH路由协议进行对比研究,证明了改进的算法节省了整个网络的能量消耗,有效地平衡了簇首间的能量消耗,延长了网络的生命周期,改善了网络的性能。
As a new technique of implementing Pervasive Computing,wireless Sensor Network (WSN) is having a profound effect on way of people'life. In many challenges of WSN' related research fields, the routing protocols of WSN are one of the most important issues. How to save energy and extend the network life is a core and basic problem in routing protocol design of WSN. Clustering routing protocols is better than flat routing protocols for its less energy consumption:members of a cluster are simple in function, and all the data of a cluster are merged by cluster-head, which reduce the amount of transfer. So current research is focus on clustering routing protocols. But current clustering routing protocols have some shortcomings such as much energy consumption in cluster forming, not uniform in data transfer, and so on.
With the problems of current cluster-based routing protocols,an energy-balanced routing algorithm for WSN is presented in this paper, according to different energy stages of WSN' life circle and the centripetal characteristics of data transmission. Through balancing a random number and residual energy and change into time delay, the node decide to become a cluster head itself. For routing of cluster heads, this paper proposed an open routing framework based on cluster heads, and realize an energy balance cluster routing algorithm based on this open framework. This algorithm balance energy consumption of cluster heads and cluster members, and extend the network life cycle effectively. Additionally the open framework of cluster-heads extends the scalability for different applications. This paper simulates the algorithm and LENCH routing algorithms and give the analysis of the result. The simulation results show that the algorithm exceeds LENCH in saving energy and prolonging the network's lifetime.
引文
[1]李腊元,李春林。计算机网络技术(第二版)[M].北京：国防工业出版社,2004：1～125
[2]孙雨耕,张静,孙永进。无线自组传感器网络[J].传感技术学报,2004,6(2)：331～335
[3]Ten emerging technologies that will change the world. Technology Review[J].Feb 2003Vol.106, No.1:22-49
[4]孙利民,李建中,陈渝等编著.无线传感器网络[M].北京：清华大学出版社,2005：3-96
[5]Rabaey J, Ammer J, Da S.J. J. L, et al. Pico Radio:Ad Hoc Wireless Networking of Ubiquitous Low Energy Sensor Monitor Nodes[C].In:V LSI, eds. Proceedings of the IEEE Computer Society Annual Workshop. Washington:IEEE Computer Society, 2000/9-12
[6]Shah R C, Rabaey J M. Energy aware routing for lower energy ad hoc sensor networks[C]. In:Proceeding of Wireless Communication and Networking Conference. WCNC2002.2002 IEEE,2002.350-355
[7]李晓维.无线传感器网络技术.北京：北京理工大学出版社,2007
[8]宇宏毅,李鸥,张效义.无线传感器网络理论、技术与实现.北京：国防工业出版社,2008
[9]Intanagonwiwat C, Govindan R, Estrin D, Heidemann J. Directed diffusion for wireless sensor networking. IEEE/ACM Trans.on Networking,2003,11(1):2-16
[10]Heinzelman WR, Kulik J, Balakrishnan H. Adaptive protocol for information. dissemination in wireless sensor networls. In:Proc. of the ACM MobiCom'99.Seattle: ACM Press,1999/174-185.http://nms.lcs.mit.edu/papers/spin-mobicom99.html
[11]Braginsky D,Estrin D. Rumor routing algorithm for sensor networks. In:Proc. of the 1st Workshop on Sensor Networks and Applications. Atlanta:ACM Press,2002.22-31
[12]Sohrabi K,Gao J, Ailawadhi V, Pottie GJ, Protocols for self-orgnization of a wireless sensor network. IEEE Personal Commnications,2000,7(5):16-27
[13]Heinzelman W,Chandrakasan A, Balakrishnan H. Energy-Efficient communication protocol for wireless microsensor networks. In:Proc. of the 33rd Annual Hawaii Int'l Conf. on System Siences.Maui:IEEE Computer Society,2000.3005-3014
[14]Lindsey S,Raghavendra CS.PEGASIS:Power-Efficient gathering in sensor information systems. In:Proc.of the IEEE Aerospace Conf. Montana:IEEE Aerospace and Electronic Systems Society,2002.1125-1130
[15]Manjeshwar A, Grawal DP. TEEN:Aprotocol for enhanced efficiency in wireless sensor networks. In:Proc. of the 15th Parallel and Distributed Processing Symp.San Francisco: IEEE Computer Society,2001.2009-2015
[16]Younis O, Fahmy S.Heed:A hybrid, energy-efficient, distributed clustering approach for ad-hoc sensor networks. IEEE Trans, on Mobile Computing,2004,3(4):660-669
[17]Wireless Ad Hoc Networking.1 edition. Wiley-IEEE Press.2004
[18]Akildiz I. A Survey on Sensor Networks[C].IEEE Communication Magzines Aug. 2002,40(8):102-114
[19]William Stalling.无线通信与网络.何军等译[M].北京：清华大学出版社,2005：1-16
[20]Akyilsiz I, Su W, Cayirci E. Wireless sensor network[J]. Computer Networks.2002, vol.38:393-422
[21]Edgar H.Callaway. Wireless sensor networks:Architectures and protocols[M].Boca Raton:CRC Press, Jan.2004
[22]Holger Karl, Andreas Willig. Protocols and Architectures for Wireless Sensor Networks. Wiley Intersicence,2007
[23]Paolo Santi, Topology Control in Wireless Ad Hoc and Sensor Networks. Wiley,2005
[24]Elson J, Romer K. Wireless sensor networks:a new regime for time synchronization. ACM SIGCOMM Computer Communication Review,2003,33(1):149-154
[25]N.Bulusu,J. Heidemann, D.Estrin. Density Adaptive Algorithms for Beacon Placement in Wireless Sensor Networks. Technical Report ULCA-CS-010013,2001
[26]Hall D L,Llinas J. Handbook of Multisensor Data Fusion. CRC Press,2001
[27]Szewczyk R,Ferencz A. Energy implications of network sensor designs. http://www.cs.berkeley.edu/-szewczyk/cs252/paper.pdf
[28]http://www.tinyos.net/tynios-1.x/doc
[29]A.Arora, P. Dutta,S.Bapat. Line in the sand:A wireless sensor network for targer detection, classification and tracking[C].In:Special Issue of Elsevier Computer Networks on Future Advances in Military Wireless Communications. December 2004. 605-634
[30]B.Blum, P.Nagaraddi, A.Wood. An entity maintenance and connection service for sensor networks[C].In:proc 1st International Conference on Mobile Systems, Applications and Services(MOBISYS'03),San Francisco, California May 2003.202-214
[31]Szewczyk R,Mainwaring A,Polast re J. An Analysis of a Large Scale Habitat Monitoring Application [J].A CM Sen Sys,2004,11:325
[32]Ye W, Heideman J, Estrin D.An energy-dfficient MAC protocol for wireless sensor network. In:Proceedings of the INFOCOM 2002.San Francisco:IEEE Computer Society,2002
[33]Anna Hac. Wireless Sensor Network Designs.Hohn Wiley & Sons,2003
[34]Jamal N.AL-Kariki. Ahmed E.Kamal.Routing Technologies in wireless sensor networks:a survey. IEEE Wireless Communications.2004:6-28
[35]唐勇,周明天,张欣.无线传感器网络中路由协议研究进展[J].软件学报,2006,17(3)：410-421
[36]Yu HB, Zeng P, Wang ZF, Liang Y, Shang ZJ, Study of communication protocol of distributed sensor network. Journal of China Institute of Communications,2004,25(10): 102-110(in Chinese with English abstract)
[37]Dongkyun Kim, J.J Garcia-Luna-Aceves. Routing mechanisms for mobile ad hoc networks based on the energy drain rate. IEEE Transactions on Mobile Computing,2003:161-173
[38]Dongkyun Kim J.J Garcia-Luna-Aceves,Obraczka K, Cano J, and Manzoni P. Power-aware routing based on the energy drain rate for mobile ad hoc networks.In: Proceedings of IEEE ICCCN2002.Miami, Florida,2002:565-569
[39]Hedetniemi W,Liestman A. A Survey of Gossiping and Broadcasting in Communication Networks[J].1998,18(4):319-349
[40]Soro S, Heinzelman W.Prolonging the lifetime of wireless sensor networks via unequal clustering [A].Proceedings of the 5th International Workshop on algorithms for Wireless,Mobile,Ad Hoc and Sensor Networks[C],Denver,CO,2005
[41]李成法,陈贵海等.一种基于非均匀分簇的无线传感器网络路由协议[J].计算学报,2007,30(1)：27-36
[42]张擎,柴乔林,刘玲.无线传感器网络不等规模节能分簇路由算法[J].计算机工程,2008,34(23)
[43]W Heinzelman,A ChandraKasan,H. Balarkrishman, "Energy efficient communication protocol for wireless sensor networks."Proceedings of the Hawaii international Conference on System Sciences.IEEE,2000,Vol(2):10
[44]Heinzelmam W, Chandrakasan A, Balkrishnan H. An application-specific protocol architecture for wireless microsensor networks[J].IEEE Trans on Wireless Commmunications,2002,1(4):660-670
[45]Heinzelman W. Application-Specific prorocol architectures for wireless networks[Ph.D.Thesis].Boston:Massachusetts Institute of Technology,2000
[46]Krishnamachari B,Estrin D,wicker S.Modelling data-centric routing in wireless sensor networks. In:Proc IEEE INFOCOM,2002
[47]吴小兵,陈贵海.无线传感器网络中节点非均匀分布的能量空洞问题[J].计算机学报,2008,31(2)：253-261WU Xiaobing, CHEN Guihai. The energy hole problem of nonuniform node distribution in wireless sensor networks[J].Chinese Journal of Computers,2008,32(2):253-261
[48]OLARIU S,STOJMENOVIC I.Design guidelines for maximizing lifetime and avoiding energy holes in sensor networks with uniform distribution and uniform reporting[C]// INFOCOM 2006.New York, USA:IEEE Communications Society,2006:1-12
[49]何国权基于能量有效性的无线传感器网络路由协议研究[D].西安：西安电子科技大学,2008.23-42
[50]BASU A, LIN A, RAMANATHAN S.Routing using potentials:A dynamic traffic-aware routing algorithm[C]//Proceedings of Conference on Applications,Technologies, Architectures,and Protocols for Computer Communication(SIGCOMM'03),Aug 25-29, 2003,Karlsruhe, Germany. New York, NY,USA:ACM,2003:37-48
[51]E. Y. K wang,C.C.Youn. Analytic Hierarchy Process Approach for Identigying Relative Importance of Factors to Improve Passenger Security Check at Airports. Air Transport Management,2006,12(3):135-142
[52]L. Bajaj,M. Takai, R. Ahuja, et al.GloMoSim:Ascalable network simulation environment. Technical Report, Computer Science Department, University of California, LosAngeles, May 1999:99-270
[53]OPNET. Heep://www.opnet.com
[54]S.Baiaj,L. Breslau, D.Estrin, et al.Improving simulation for network research. Technical Report, University of Southern California.1999:99-702
[55]J-Sim:http://www.j-sim.org/
[2]孙雨耕,张静,孙永进。无线自组传感器网络[J].传感技术学报,2004,6(2)：331～335
[3]Ten emerging technologies that will change the world. Technology Review[J].Feb 2003Vol.106, No.1:22-49
[4]孙利民,李建中,陈渝等编著.无线传感器网络[M].北京：清华大学出版社,2005：3-96
[5]Rabaey J, Ammer J, Da S.J. J. L, et al. Pico Radio:Ad Hoc Wireless Networking of Ubiquitous Low Energy Sensor Monitor Nodes[C].In:V LSI, eds. Proceedings of the IEEE Computer Society Annual Workshop. Washington:IEEE Computer Society, 2000/9-12
[6]Shah R C, Rabaey J M. Energy aware routing for lower energy ad hoc sensor networks[C]. In:Proceeding of Wireless Communication and Networking Conference. WCNC2002.2002 IEEE,2002.350-355
[7]李晓维.无线传感器网络技术.北京：北京理工大学出版社,2007
[8]宇宏毅,李鸥,张效义.无线传感器网络理论、技术与实现.北京：国防工业出版社,2008
[9]Intanagonwiwat C, Govindan R, Estrin D, Heidemann J. Directed diffusion for wireless sensor networking. IEEE/ACM Trans.on Networking,2003,11(1):2-16
[10]Heinzelman WR, Kulik J, Balakrishnan H. Adaptive protocol for information. dissemination in wireless sensor networls. In:Proc. of the ACM MobiCom'99.Seattle: ACM Press,1999/174-185.http://nms.lcs.mit.edu/papers/spin-mobicom99.html
[11]Braginsky D,Estrin D. Rumor routing algorithm for sensor networks. In:Proc. of the 1st Workshop on Sensor Networks and Applications. Atlanta:ACM Press,2002.22-31
[12]Sohrabi K,Gao J, Ailawadhi V, Pottie GJ, Protocols for self-orgnization of a wireless sensor network. IEEE Personal Commnications,2000,7(5):16-27
[13]Heinzelman W,Chandrakasan A, Balakrishnan H. Energy-Efficient communication protocol for wireless microsensor networks. In:Proc. of the 33rd Annual Hawaii Int'l Conf. on System Siences.Maui:IEEE Computer Society,2000.3005-3014
[14]Lindsey S,Raghavendra CS.PEGASIS:Power-Efficient gathering in sensor information systems. In:Proc.of the IEEE Aerospace Conf. Montana:IEEE Aerospace and Electronic Systems Society,2002.1125-1130
[15]Manjeshwar A, Grawal DP. TEEN:Aprotocol for enhanced efficiency in wireless sensor networks. In:Proc. of the 15th Parallel and Distributed Processing Symp.San Francisco: IEEE Computer Society,2001.2009-2015
[16]Younis O, Fahmy S.Heed:A hybrid, energy-efficient, distributed clustering approach for ad-hoc sensor networks. IEEE Trans, on Mobile Computing,2004,3(4):660-669
[17]Wireless Ad Hoc Networking.1 edition. Wiley-IEEE Press.2004
[18]Akildiz I. A Survey on Sensor Networks[C].IEEE Communication Magzines Aug. 2002,40(8):102-114
[19]William Stalling.无线通信与网络.何军等译[M].北京：清华大学出版社,2005：1-16
[20]Akyilsiz I, Su W, Cayirci E. Wireless sensor network[J]. Computer Networks.2002, vol.38:393-422
[21]Edgar H.Callaway. Wireless sensor networks:Architectures and protocols[M].Boca Raton:CRC Press, Jan.2004
[22]Holger Karl, Andreas Willig. Protocols and Architectures for Wireless Sensor Networks. Wiley Intersicence,2007
[23]Paolo Santi, Topology Control in Wireless Ad Hoc and Sensor Networks. Wiley,2005
[24]Elson J, Romer K. Wireless sensor networks:a new regime for time synchronization. ACM SIGCOMM Computer Communication Review,2003,33(1):149-154
[25]N.Bulusu,J. Heidemann, D.Estrin. Density Adaptive Algorithms for Beacon Placement in Wireless Sensor Networks. Technical Report ULCA-CS-010013,2001
[26]Hall D L,Llinas J. Handbook of Multisensor Data Fusion. CRC Press,2001
[27]Szewczyk R,Ferencz A. Energy implications of network sensor designs. http://www.cs.berkeley.edu/-szewczyk/cs252/paper.pdf
[28]http://www.tinyos.net/tynios-1.x/doc
[29]A.Arora, P. Dutta,S.Bapat. Line in the sand:A wireless sensor network for targer detection, classification and tracking[C].In:Special Issue of Elsevier Computer Networks on Future Advances in Military Wireless Communications. December 2004. 605-634
[30]B.Blum, P.Nagaraddi, A.Wood. An entity maintenance and connection service for sensor networks[C].In:proc 1st International Conference on Mobile Systems, Applications and Services(MOBISYS'03),San Francisco, California May 2003.202-214
[31]Szewczyk R,Mainwaring A,Polast re J. An Analysis of a Large Scale Habitat Monitoring Application [J].A CM Sen Sys,2004,11:325
[32]Ye W, Heideman J, Estrin D.An energy-dfficient MAC protocol for wireless sensor network. In:Proceedings of the INFOCOM 2002.San Francisco:IEEE Computer Society,2002
[33]Anna Hac. Wireless Sensor Network Designs.Hohn Wiley & Sons,2003
[34]Jamal N.AL-Kariki. Ahmed E.Kamal.Routing Technologies in wireless sensor networks:a survey. IEEE Wireless Communications.2004:6-28
[35]唐勇,周明天,张欣.无线传感器网络中路由协议研究进展[J].软件学报,2006,17(3)：410-421
[36]Yu HB, Zeng P, Wang ZF, Liang Y, Shang ZJ, Study of communication protocol of distributed sensor network. Journal of China Institute of Communications,2004,25(10): 102-110(in Chinese with English abstract)
[37]Dongkyun Kim, J.J Garcia-Luna-Aceves. Routing mechanisms for mobile ad hoc networks based on the energy drain rate. IEEE Transactions on Mobile Computing,2003:161-173
[38]Dongkyun Kim J.J Garcia-Luna-Aceves,Obraczka K, Cano J, and Manzoni P. Power-aware routing based on the energy drain rate for mobile ad hoc networks.In: Proceedings of IEEE ICCCN2002.Miami, Florida,2002:565-569
[39]Hedetniemi W,Liestman A. A Survey of Gossiping and Broadcasting in Communication Networks[J].1998,18(4):319-349
[40]Soro S, Heinzelman W.Prolonging the lifetime of wireless sensor networks via unequal clustering [A].Proceedings of the 5th International Workshop on algorithms for Wireless,Mobile,Ad Hoc and Sensor Networks[C],Denver,CO,2005
[41]李成法,陈贵海等.一种基于非均匀分簇的无线传感器网络路由协议[J].计算学报,2007,30(1)：27-36
[42]张擎,柴乔林,刘玲.无线传感器网络不等规模节能分簇路由算法[J].计算机工程,2008,34(23)
[43]W Heinzelman,A ChandraKasan,H. Balarkrishman, "Energy efficient communication protocol for wireless sensor networks."Proceedings of the Hawaii international Conference on System Sciences.IEEE,2000,Vol(2):10
[44]Heinzelmam W, Chandrakasan A, Balkrishnan H. An application-specific protocol architecture for wireless microsensor networks[J].IEEE Trans on Wireless Commmunications,2002,1(4):660-670
[45]Heinzelman W. Application-Specific prorocol architectures for wireless networks[Ph.D.Thesis].Boston:Massachusetts Institute of Technology,2000
[46]Krishnamachari B,Estrin D,wicker S.Modelling data-centric routing in wireless sensor networks. In:Proc IEEE INFOCOM,2002
[47]吴小兵,陈贵海.无线传感器网络中节点非均匀分布的能量空洞问题[J].计算机学报,2008,31(2)：253-261WU Xiaobing, CHEN Guihai. The energy hole problem of nonuniform node distribution in wireless sensor networks[J].Chinese Journal of Computers,2008,32(2):253-261
[48]OLARIU S,STOJMENOVIC I.Design guidelines for maximizing lifetime and avoiding energy holes in sensor networks with uniform distribution and uniform reporting[C]// INFOCOM 2006.New York, USA:IEEE Communications Society,2006:1-12
[49]何国权基于能量有效性的无线传感器网络路由协议研究[D].西安：西安电子科技大学,2008.23-42
[50]BASU A, LIN A, RAMANATHAN S.Routing using potentials:A dynamic traffic-aware routing algorithm[C]//Proceedings of Conference on Applications,Technologies, Architectures,and Protocols for Computer Communication(SIGCOMM'03),Aug 25-29, 2003,Karlsruhe, Germany. New York, NY,USA:ACM,2003:37-48
[51]E. Y. K wang,C.C.Youn. Analytic Hierarchy Process Approach for Identigying Relative Importance of Factors to Improve Passenger Security Check at Airports. Air Transport Management,2006,12(3):135-142
[52]L. Bajaj,M. Takai, R. Ahuja, et al.GloMoSim:Ascalable network simulation environment. Technical Report, Computer Science Department, University of California, LosAngeles, May 1999:99-270
[53]OPNET. Heep://www.opnet.com
[54]S.Baiaj,L. Breslau, D.Estrin, et al.Improving simulation for network research. Technical Report, University of Southern California.1999:99-702
[55]J-Sim:http://www.j-sim.org/