ISCC C

157 papers

YearTitle / Authors
20122012 IEEE Symposium on Computers and Communications, ISCC 2012, Cappadocia, Turkey, July 1-4, 2012
20123GPP LTE downlink scheduling strategies in vehicle-to-infrastructure communications for traffic safety applications.
Maria Kihl, Kaan Bür, Pradyumna Mahanta, Erik Coelingh
2012A channel availability classification for Cognitive Radio Networks using a monitoring network.
Berk Canberk, Sema Oktug
2012A clustering method for wireless sensors networks.
Said Fouchal, Quentin Monnet, Djamel Mansouri, Lynda Mokdad, Malika Ioualalen
2012A comparative analysis of smith-waterman based partial alignment.
Taner Aruk, Duran Üstek, Olcay Kursun
2012A coverage-based drop-policy in wireless sensor network with disruptive connections.
Daniel Frazão Luiz, Carlos Maurício S. Figueiredo, Eduardo Freire Nakamura
2012A cross-layer goodput enhancement considering CRC coding and ARQ dynamics.
Eser Ustunel, Ibrahim Hökelek, Ömer Ileri
2012A data locality optimization algorithm for large-scale data processing in Hadoop.
Yanrong Zhao, Weiping Wang, Dan Meng, Xiufeng Yang, Shubin Zhang, Jun Li, Gang Guan
2012A fast searching method for the construction of QC-LDPC codes with large girth.
Francis Chung-Ming Lau, Wai Man Tam
2012A framework based on small world features to design HSNs topologies with QoS.
Daniel L. Guidoni, Azzedine Boukerche, Leandro A. Villas, Fernanda S. H. Souza, Raquel A. F. Mini, Antonio Alfredo Ferreira Loureiro
2012A framework for cognitive radio wireless sensor networks.
Luiz Henrique A. Correia, Erasmo E. Oliveira, Daniel F. Macedo, Pedro Moura, Antonio Alfredo Ferreira Loureiro, Jorge M. Sá Silva
2012A hybrid and flexible discovery algorithm for wireless sensor networks with mobile elements.
Koteswararao Kondepu, Francesco Restuccia, Giuseppe Anastasi, Marco Conti
2012A localized de-synchronization algorithm for periodic data reporting in IEEE 802.15.4 WSNs.
Domenico De Guglielmo, Giuseppe Anastasi, Marco Conti
2012A model for contact volume prediction in DTNs.
João Batista Pinto Neto, Edjair Mota, Eduardo Cerqueira, Polianny Almeida, Robson Rojas
2012A modified artificial bee colony algorithm for numerical function optimization.
Bilal Babayigit, Resul Ozdemir
2012A network policy model for virtualized systems.
Hedi Benzina
2012A new 0-1 linear program for QoS and transactional-aware web service composition.
Virginie Gabrel, Maude Manouvrier, Imen Megdiche, Cécile Murat
2012A new adaptive MAC protocol with QoS support for heterogeneous wireless sensor networks.
Marion Souil, Tifenn Rault, Abdelmadjid Bouabdallah
2012A new approach to random interleavers for traditional IDMA systems.
Mehmet Bilim, Nuri Kapucu, Ibrahim Develi
2012A new weighted shortest path tree for convergecast traffic routing in WSN.
Walid Bechkit, Mouloud Koudil, Yacine Challal, Abdelmadjid Bouabdallah, Brahim Souici, Karima Benatchba
2012A novel algorithm for technical articles classification based on gene selection.
Rania Kilany, Reda A. Ammar, Sanguthevar Rajasekaran
2012A novel approach towards modeling TDM-pumped fiber Raman amplifiers.
Vineetha Kalavally, Ivan D. Rukhlenko, Malin Premaratne
2012A prediction-based active queue management for TCP networks.
Mohamed Faten Zhani, Halima Elbiaze, Farouk Kamoun
2012A proposal of cell selection algorithm for LTE handover optimization.
Toshihiko Komine, Toshiaki Yamamoto, Satoshi Konishi
2012A runtime-restricted strategy for highly parallel scheduling human resource in change management.
Wei Dong, Zhiqiang Zhan, Xue-song Qiu
2012A semantic solution for saving energy in wireless sensor networks.
Kalil A. Bispo, Nelson S. Rosa, Paulo Roberto Freire Cunha
2012A semi and fully distributed handover algorithm for heterogeneous networks using MIIS.
Muhammad Qasim Khan, Steinar Hidle Andresen
2012A topology inference algorithm for wireless sensor networks.
Theofanis Kontos, George Alyfantis, Yiannis Angelopoulos, Stathes Hadjiefthymiades
2012A weighted-graph based approach for solving the cold start problem in collaborative recommender systems.
Nassira Chekkai, Salim Chikhi, Hamamache Kheddouci
2012Accuracy-preserving measurement collection for realistic wireless simulations.
Mustafa Al-Bado, Cigdem Sengul, Ruben Merz
2012Accurate BEP of Adaptive Demodulate-and-Forward relaying over Rayleigh fading channels.
Wided Hadj Alouane, Noureddine Hamdi, Soumaya Meherzi
2012Admission control for QoS provision in wireless mesh networks.
Eduardo Pompeo da Silva Mineiro, Débora Christina Muchaluat-Saade
2012An ISP and end-user cooperative intradomain routing algorithm.
Ali Norouzi, Michael P. Howarth, Ning Wang
2012An adaptive data gathering strategy for target tracking in cluster-based wireless sensor networks.
Juan Feng, Zhonglei Wang, Jörg Henkel
2012An adaptive data gathering strategy for target tracking in cluster-based wireless sensor networks.
Juan Feng, Zhonglei Wang, Jörg Henkel
2012An efficient certificate revocation validation scheme with k-means clustering for vehicular ad hoc networks.
Qingwei Zhang, Mohammed Almulla, Yonglin Ren, Azzedine Boukerche
2012An improved energy detection scheme for cognitive radio networks in low SNR region.
Ahmed S. B. Kozal, Madjid Merabti, Faycal Bouhafs
2012An operating system runtime reprogramming infrastructure for WSN.
Rodrigo Vieira Steiner, Giovani Gracioli, Rita de Cassia Cazu Soldi, Antônio Augusto Fröhlich
2012An optimized Markov model to predict link quality in mobile wireless sensor networks.
Gustavo Medeiros de Araújo, Jörg Kaiser, Leandro Buss Becker
2012Analyzing application private information leaks with privacy Petri Net.
Lejun Fan, Yuanzhuo Wang, Xueqi Cheng, Jinming Li
2012Automatic QoS evaluation method for web services.
Emra Askaroglu, Pinar Senkul
2012Automatic classification of cross-site scripting in web pages using document-based and URL-based features.
Angelo Eduardo Nunan, Eduardo Souto, Eulanda Miranda dos Santos, Eduardo Feitosa
2012Autonomous intruder detection system using wireless networked mobile robots.
Gurkan Tuna, Coskun Tasdemir, Kayhan Gulez, Tarik Veli Mumcu, Vehbi Cagri Gungor
2012Camera selection without location information: A topological approach.
Marzieh Varposhti, Mehdi Dehghan, Reza Safabakhsh
2012CastFlow: Clean-slate multicast approach using in-advance path processing in programmable networks.
Cesar Augusto Cavalheiro Marcondes, Tiago Santos, Arthur Pedro Godoy, Caio César Viel, César A. C. Teixeira
2012Classification of HTTP traffic based on C5.0 Machine Learning Algorithm.
Tomasz Bujlow, Tahir M. Riaz, Jens Myrup Pedersen
2012Cloud-Vision: Real-time face recognition using a mobile-cloudlet-cloud acceleration architecture.
Tolga Soyata, Rajani Muraleedharan, Colin Funai, Minseok Kwon, Wendi B. Heinzelman
2012Convexity analysis for cyclostationary detection in cognitive radios.
Sana Tariq, Abdul Ghafoor, Salma Zainab Farooq
2012Cooperative sensing analysis under imperfect reporting channel.
Can Altay, H. Birkan Yilmaz, Tuna Tugcu
2012DBA-VM: Dynamic bandwidth allocator for virtual machines.
Ahmed Amamou, Manel Bourguiba, Kamel Haddadou, Guy Pujolle
2012DDS-enabled Cloud management support for fast task offloading.
Antonio Corradi, Luca Foschini, Javier Povedano-Molina, Juan M. López-Soler
2012Database security management for healthcare SaaS in the Amazon AWS Cloud.
Fabio Bracci, Antonio Corradi, Luca Foschini
2012Design and implementation of an OFDM-based voice transmission system for mobile Underwater Vehicles.
Seyyed Mahmood Jafari Sadeghi, Mostafa Derakhtian, Mohammad Ali Masnadi-Shirazi
2012Distributed relay node positioning for connectivity restoration in partitioned Wireless Sensor Networks.
Izzet F. Senturk, Kemal Akkaya, Sabri Yilmaz
2012EBRAR: Energy-balanced rate allocation and routing protocol for body area networks.
Nedal Ababneh, Nicholas Timmons, Jim Morrison
2012Energy-efficient P2P file sharing for residential BitTorrent users.
Ilaria Giannetti, Giuseppe Anastasi, Marco Conti
2012Energy-efficient RF source power control for opportunistic distributed sensing in wireless passive sensor networks.
Ahmet Ozan Biçen, Özgür B. Akan
2012Energy-efficient RF source power control for opportunistic distributed sensing in wireless passive sensor networks.
Ahmet Ozan Biçen, Özgür B. Akan
2012Enhancing reliable mobile multicasting with RaptorQ FEC.
Christos Bouras, Nikolaos Kanakis, Vasileios Kokkinos, Andreas Papazois
2012Evaluating IPv6 in peer-to-peer Gigabit Ethernet for UDP using modern operating systems.
Burjiz K. Soorty, Nurul I. Sarkar
2012Event detection framework for wireless sensor networks considering data anomaly.
Leticia Decker de Sousa, Alejandro César Frery, Eduardo Freire Nakamura, Antonio Alfredo Ferreira Loureiro
2012Expectation-Maximization (EM) based channel estimation and interpolation in OFDM systems.
Eser Ustunel
2012Exploring playback continuity and delay trade-off in peer-to-peer streaming.
Samaneh Heidari, Nazanin Dehghani, Mohammad Sadegh Talebi, Ahmad Khonsari
2012FOMA: Flexible overlay multi-path data aggregation in wireless sensor networks.
Majid Ashouri, Hamed Yousefi, Ali Mohammad Afshin Hemmatyar, Ali Movaghar
2012FPC: A self-organized greedy routing in scale-free networks.
Yonggong Wang, Gaogang Xie, Mohamed Ali Kâafar
2012Fast GPU algorithms for endmember extraction from hyperspectral images.
Mahmoud Maghrbay, Reda A. Ammar, Sanguthevar Rajasekaran
2012Fast forced symbol method for distributed space-time turbo codes using complex orthogonal design.
Zeeshan Sattar, Youssouf Ould-Cheikh-Mouhamedou, Abdulhameed M. Alsanie, Hatim M. Behairy
2012Figures of merit for the placement (in) efficiency of interconnected CDNs.
Vitor Jesus, Rui L. Aguiar
2012Flow-based routing architecture for Valiant Load-Balanced networks.
Imad Khazali, Anjali Agarwal
2012Formulations for the RWA problem with traffic grooming, protection and QoS in WDM optical networks.
Fernanda S. H. Souza, Daniel L. Guidoni, Geraldo Robson Mateus
2012Greening the multi-granular optical transport network design under the optical reach constraint.
Nabil Naas, Burak Kantarci, Hussein T. Mouftah
2012Guaranteed-delivery in arbitrary dimensional Wireless Sensor Networks by means of recursive virtual coordinates.
Stefano Chessa, Soledad Escolar, Susanna Pelagatti, Paolo Baronti, Jesús Carretero
2012Half cycling dynamic bandwidth allocation with prediction on EPON.
Özgür Can Turna, M. Ali Aydin, Abdül Halim Zaim, Tülin Atmaca
2012Home-Box-assisted content delivery network for Internet Video-on-Demand services.
Soraya Ait Chellouche, Daniel Négru, Yiping Chen, Mamadou Sidibé
2012Honeyguide: A VM migration-aware network topology for saving energy consumption in data center networks.
Hiroki Shirayanagi, Hiroshi Yamada, Kenji Kono
2012How a structured testbed enables the rapid development and deployment of cloud services: The VISION Cloud use case.
Massimo Villari, Francesco Longo, Maurizio Paone, Francesco Tusa, Michael C. Jaeger
2012Impact of FTTH deployment on live streaming delivery systems.
Bertrand Mathieu, Yann Levene
2012Impact of mobility and topology on information diffusion in MANETs.
Dimitri Perrin, Hiroyuki Ohsaki
2012Implementing OMA RESTful location services in wireless sensor environments.
Md. Asadul Islam, Fatna Belqasmi, Roch H. Glitho, Ferhat Khendek
2012Improved approximate decoding based on position information matrix.
Minhae Kwon, Hyunggon Park, Pascal Frossard
2012Improving the accuracy of boolean tomography by exploiting path congestion degrees.
Zhiyong Zhang, Gaolei Fei, Fucai Yu, Guangmin Hu
2012Input-evaluation: A new mechanism for collecting data using games with a purpose.
Adem Efe Gencer, Tunga Gungor, Asli Gurer, A. Sumru Özsoy
2012Integration of CLEVER clouds with third party software systems through a REST web service interface.
Antonio Celesti, Francesco Tusa, Massimo Villari, Antonio Puliafito
2012Intrusion detection framework of cluster-based wireless sensor network.
Hichem Sedjelmaci, Sidi-Mohammed Senouci, Mohammed Feham
2012Intrusion detection framework of cluster-based wireless sensor network.
Hichem Sedjelmaci, Sidi-Mohammed Senouci, Mohammed Feham
2012Inverse multiplexing gain considering physical layer impairments in mixed line rate networks.
Haydar Çukurtepe, Aysegül Gençata Yayimli, Biswanath Mukherjee
2012Investment optimization methodology applied to investments on non-technical losses reduction actions.
Sergio Santiago Ribeiro, Thiago Cazes, Rui Figueira Mano, Denis Maia
2012Key Update Assistant for resource-constrained networks.
Ender Yüksel, Hanne Riis Nielson, Flemming Nielson
2012LBTool: A stochastic toolkit for leave-based key updates.
Ender Yüksel, Hanne Riis Nielson, Flemming Nielson, Bo Friis Nielsen
2012LIAITHON: A location-aware multipath video streaming scheme for urban vehicular networks.
Renfei Wang, Cristiano G. Rezende, Heitor S. Ramos, Richard W. Pazzi, Azzedine Boukerche, Antonio Alfredo Ferreira Loureiro
2012Latency analysis of ratio-counter based dynamic bandwidth allocation algorithm for passive optical networks.
Syed Ali Haider, M. Yasin Akhtar Raja
2012Learning interactions among objects, tools and machines for planning.
Mustafa Ersen, Sanem Sariel Talay
2012Link-level reliability control for wireless electrocardiogram monitoring in indoor hospital.
Juyoung Park, Jaemyoun Lee, Kyungtae Kang
2012Load balancing with minimal flow remapping for network processors.
Imad Khazali, Anjali Agarwal
2012Maestro: A cloud computing framework with automated locking.
Murat Demirbas, Serafettin Tasci, Sandeep S. Kulkarni
2012Managing secure authentication for standard mobile medical networks.
Alexandru Egner, Alexandru Soceanu, Florica Moldoveanu
2012Minimizing the provisioning delay in the cloud network: Benefits, overheads and challenges.
Burak Kantarci, Hussein T. Mouftah
2012Mission-aware placement of RF-based power transmitters in wireless sensor networks.
Melike Erol-Kantarci, Hussein T. Mouftah
2012Mobility-aware P/I-CSCF: A solution for achieving seamless handover in IMS.
Julien Arnaud, Daniel Négru
2012Motion compensation for video codec based on disparity estimation.
Osama Hosam
2012Multi-antenna spectrum sensing for cognitive radio under Rayleigh channel.
Alphan Salarvan, Gunes Karabulut-Kurt
2012NAVI: Novel authentication with visual information.
Emmanouil Georgakakis, Nikos Komninos, Christos Douligeris
2012Network availability modeling of VMIMO link in multi-hop wireless network.
Mohamed Escheikh, Kamel Barkaoui
2012Network loss tomography using link independence.
Yan Qiao, Guanjue Wang, Xuesong Qiu, Ran Gu
2012Network-resource isolation for virtualization nodes.
Yasusi Kanada, Kei Shiraishi, Akihiro Nakao
2012Neural network equalization for frequency selective nonlinear MIMO channels.
Oussama Ben Haj Belkacem, Rafik Zayani, Mohamed Lassaad Ammari, Ridha Bouallegue, Daniel Roviras
2012Non-stationary link inference and localization in communication networks.
Ran Gu, Yan Qiao, Xuesong Qiu
2012Novel interference alignment in multi-secondary users cognitive radio system.
Bassant Abdelhamid, Maha Elsabrouty, Salwa El-Ramly
2012Novel mechanism for bandwidth reuse in network virtualization.
Dongdong Chen, Zhaowei Qu, Shunli Zhang, Xuesong Qiu, Ao Xiong
2012On performance evaluation of brain based learning processes using neural networks.
Hassan M. Mustafa
2012On performance evaluation of brain based learning processes using neural networks.
Hassan M. Mustafa
2012On resource allocation strategies in managed peerassisted VOD streaming systems.
Thibaud Rohmer, Abdelhamid Nafaa, Amir Nakib
2012On static RWA in translucent optical networks.
Quazi Rahman, Subir Bandyopadhyay, Yash P. Aneja
2012Ontology-based routing of web services in distributed service discovery system containing domain specific nodes.
Deniz Canturk, Pinar Senkul
2012Optimal roadside units placement in urban areas for vehicular networks.
Baber Aslam, M. Faisal Amjad, Cliff Changchun Zou
2012Overcoming the energy versus delay trade-off in cloud network reconfiguration.
Burak Kantarci, Hussein T. Mouftah
2012PADP: Prediction assisted dynamic surface gateway placement for mobile underwater networks.
Jun Liu, Xu Han, Manal Al-Bzoor, Michael Zuba, Jun-Hong Cui, Reda A. Ammar, Sanguthevar Rajasekaran
2012PC-TRIO: An indexed TCAM architecture for packet classifiers.
Tania Banerjee-Mishra, Sartaj Sahni, Gunasekaran S. Seetharaman
2012Parallel decoding of LDPC convolutional codes using OpenMP and GPU.
Chi Ho Chan, Francis Chung-Ming Lau
2012Performance analysis of H.264/AVC, H.264/SVC, and VP8 over IEEE 802.11 wireless networks.
Yohaan Yoon, Myungchul Kim, Sooyong Lee, Ben Lee, Soon Joo Hyun, KyungHee Lee
2012Performance evaluation of differential evolution based topology control method for autonomous MANET nodes.
Stephen Gundry, Janusz Kusyk, Jianmin Zou, Cem Safak Sahin, M. Ümit Uyar
2012Performance evaluation of feasible and holistic CSH-MU handoff solution for seamless emergency service provisioning.
Thang Tran, Maike Kuhnert, Christian Wietfeld
2012Performance evaluation of process-oriented wireless relay deployment in emergency scenarios.
Andreas Wolff, Mohamad Sbeiti, Christian Wietfeld
2012Performance of Zero-forcing and Block-Diagonalization methods for cooperative MIMO Downlink base stations in the presence of channel estimation errors.
Souhaila Fki, Guillaume Ferré, Fatma Abdelkefi, Mohamed Siala
2012Predicting peer interactions for opportunistic information dissemination protocols.
Marco Marchini, Mauro Tortonesi, Giacomo Benincasa, Niranjan Suri, Cesare Stefanelli
2012Protection from unresponsive flows with geometric CHOKe.
Addisu Eshete, Yuming Jiang
2012Public watermarking scheme for 3D laser scanned archeological models.
Osama Hosam, Walaa M. Sheta, Bayumy B. A. Youssef, M. A. Abdou
2012QoM: A new quality of experience framework for multimedia services.
Khalil ur Rehman Laghari, Thanh-Tung Pham, Hang Nguyen, Noël Crespi
2012QoS-HAN: QoS provisioning in Home Automated Networks over IEEE 802.11n.
Indira Paudel, Fatma Outay, Badii Jouaber
2012QoS-aware elastic cloud brokering for IMS infrastructures.
Paolo Bellavista, Giuseppe Carella, Luca Foschini, Thomas Magedanz, Florian Schreiner, Konrad Campowsky
2012Radio resource sharing in multi-agency TEDS networks.
Salman Ali AlQahtani
2012Resilient packet rings with heterogeneous links.
Mete Yilmaz, Nirwan Ansari
2012Resource allocation in the network operator's cloud: A virtualization approach.
João Soares, Jorge Carapinha, Márcio Melo, Romeu Monteiro, Susana Sargento
2012Routing in DS-TE networks with an opportunistic bandwidth allocation model.
Rafael F. Reale, Walter da Costa Pinto Neto, Joberto Sérgio Barbosa Martins
2012Scheduler simulation using iSPD, an iconic-based computer grid simulator.
Denison Menezes, Aleardo Manacero, Renata Spolon Lobato, Diogo T. da Silva, Roberta Spolon Ulson
2012Services collaboration in Wireless Sensor and Actuator Networks: Orchestration versus Choreography.
Sylvain Cherrier, Yacine Ghamri-Doudane, Stéphane Lohier, Gilles Roussel
2012Single attribute Join Queries within latest sampling periods in sensor networks.
Shangfeng Mo, Hong Chen, Yinglong Li
2012Slotted contention-based energy-efficient MAC protocols in delay-sensitive wireless sensor networks.
Messaoud Doudou, Djamel Djenouri, Nadjib Badache, Abdelmadjid Bouabdallah
2012Split protocol client/server architecture.
Bharat S. Rawal, Ramesh K. Karne, Alexander L. Wijesinha
2012Synchronous distributed wireless network emulator for high-speed mobility: Implementation and evaluation.
Minoru Koizumi, Tomoichi Ebata, Tomoaki Tsutsumi, Kohta Ohshima, Matsuaki Terada
2012The contract net based task allocation algorithm for wireless sensor network.
Lin Chen, Xuesong Qiu, Yang Yang, Zhipeng Gao, Zheng Qu
2012Time-efficient algorithms for the outdegree limited bluetooth scatternet formation problem.
Ahmed Jedda, Guy-Vincent Jourdan, Hussein T. Mouftah
2012Towards a verification approach for reconfigurable embedded systems.
Fatma Krichen, Amal Gassara, Bechir Zalila, Mohamed Jmaiel
2012Trellis-coded-modulation-OFDMA for spectrum sharing in cognitive environment.
Nader Mokari, Keivan Navaie, Hamid Saeedi
2012Trustworthiness monitoring and prediction of composite services.
Hisain Elshaafi, Jimmy McGibney, Dmitri Botvich
2012Understanding the basic principles implied in green traffic routing.
Yunlong Song, Min Liu
2012Using Bittorrent and SVC for efficient video sharing and streaming.
Abdelhalim Amer, Ahmed Touflk, Walid-Khaled Hidouci, Satoshi Matsuoka
2012VCARP: Vehicular Ad-hoc Networks context-aware routing protocol.
Rodrigo B. Soares, Eduardo Freire Nakamura, Carlos Maurício S. Figueiredo, Antonio Alfredo Ferreira Loureiro
2012Virtual distance: A generalized metric for overlay tree construction.
Suat Mercan, Murat Yuksel
2012Vision-based user tracking for outdoor augmented reality.
Erkan Bostanci, Adrian F. Clark, Nadia Kanwal
2012WDM optical packet ring performance insights: Scheduling and capacity.
Bogdan Uscumlic, Petar Matavulj, Annie Gravey, Philippe Gravey, Michel Morvan
2012WPA2 security-bandwith trade-off in 802.11n peer-peer WLAN for IPv4 and IPv6 using Windows XP and Windows 7 operating systems.
Samad Salehi Kolahi, Peng Li, Mulugeta Argawe, Mustafa Safdari
2012Wavelet transform and Non Local Means based super resolution.
Muhammad Zafar Iqbal, Abdul Ghafoor, Adil Masood Siddiqui
2012Wideband spectrum sensing by compressed measurements.
Davood Mardani Najafabadi, Aliakbar Tadaion, Masoud Reza Aghabozorgi Sahaf
2012Zone-based spectrum sensing in cognitive radio.
Bilal Acar, Mehmet Akif Ersoy, H. Birkan Yilmaz, Salim Eryigit, Tuna Tugcu
2012socP2P: P2P content discovery enhancement by considering social networks characteristics.
Reza Farahbakhsh, Noël Crespi, Ángel Cuevas, Sraddha Adhikari, Mehdi Mani, Teerapat Sanguankotchakorn
2012αΩHighway interconnection network architecture for high performance computing.
Plamenka Borovska, Dragi Kimovski