Reseaux Wifi Essay

2678 words - 11 pages


Un réseau sans fil est un réseau dans lequel au moins deux terminaux sont capables de communiquer entre eux grâce à des signaux radioélectriques. Ces réseaux dits « sans-fil » sont de plusieurs sortes : WiFi (Wireless Fidelity), BlueTooth, BLR (Boucle Locale Radio), UMTS (Universal Mobile Telecommunications System), etc. Les technologies « sans fil », la norme 802.11 en particulier, facilitent et réduisent le coût de connexion pour les réseaux de grande taille. Ces technologies peuvent être classées en quatre parties : Les réseaux personnels sans fil : Wireless Personal Area Network (WPAN) , Les réseaux locaux sans fil : Wireless Local Area Network (WLAN) , Les ...view middle of the document...

Voici les principaux avantages et inconvénients à déployer un réseau sans fil WiFi :
Avantages : Mobilité : les utilisateurs sont généralement satisfaits des libertés offertes par un réseau sans fil et de fait sont plus enclin à utiliser le matériel informatique. Facilité et souplesse : un réseau sans fil peut être utilisé dans des endroits temporaires, couvrir des zones difficiles d’accès aux câbles, et relier des bâtiments distants. Coût : si leur installation est parfois un peu plus coûteuse qu’un réseau filaire, les réseaux sans fil ont des coûts de maintenance très réduits ; sur le moyen terme, l’investissement est facilement rentabilisé. Evolutivité : les réseaux sans fil peuvent être dimensionnés au plus juste et suivre simplement l’évolution des besoins.
Inconvénients : Qualité et continuité du signal : ces notions ne sont pas garanties du fait des problèmes pouvant venir des interférences, du matériel et de l’environnement. Sécurité : la sécurité des réseaux sans fil n’est pas encore tout à fait fiable du fait que cette technologie est novatrice.
Interoperabilité du matériel : C’est la capacité à communiquer et/ou travailler avec des équipements identiques ou radicalement différents. Le standard 802.11b/g permet l’interopérabilité des différents équipements. Mais l'arrivé de norme propriétaire ayant comme but le doublement du débit, comme la norme 802.11g+ (SuperG) qui offre 108 Mb/s théorique bride le matériel à la vitesse nominale définie par la norme 802.11g (54 Mb/s) en cas de mélange des marques sur le réseau.
Points d’accès (composés en général d’une carte WiFi et d’une antenne) qui permettent de donner un accès au réseau filaire – auquel il est raccordé – aux différentes stations avoisinantes équipées de cartes WiFi. Cette sorte de concentrateur est l'élément nécessaire pour déployer un réseau centralisé (mode infrastructure).
Il y a deux types de points d’accès : Le point d’accès simple qui n’a qu’une fonction de lien entre le réseau filaire et le réseau sans fil. Le point d’accès routeur qui permet de connecter un modem ADSL Ethernet afin de partager une connexion Internet sur un réseau sans fil. Ils peuvent intégrer un concentrateur offrant de connecter d’autres appareils sur un réseau sans fil.
Antennes : Les antennes omnidirectionnelles qui ont un gain variant entre 1 et 15 dBi et qui offrent un rayonnement sur 360°. Elles s’installent généralement sur le point d’accès relié au réseau voire sur les cartes PCI. Les antennes directionnelles ont un gain allant de 5 à 24 dBi avec un rayonnement directif. Elles permettent d’établir des liaisons point à point mais également de couvrir une zone limitée dans le cas d’une antenne à angle d’ouverture important. Elles sont de plusieurs types comme par exemple les antennes paraboles ou encore les antennes panneaux.
WEP : le standard...

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