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What is Wireless Local Area Network (WLAN)?
MilanoSandAの回答
1. 無線LANシステム(WLAN)を利用することで、部屋やビルの中で有線LANでは接続できないラップトップやPDAなどにネットワーク接続範囲を拡げることができる。 2.すべての無線LANは、仮想的にIEEE802規格（802.11a,802.11b,and 802.11gなど）を利用している。これらを総じてWiFiスタンダードと呼んでいる。 3.これらのスタンダードは、イーサネットを使用する場合のデータ転送方法で、イーサネットケーブルの代わりに無線リンクを利用する仕様を定義している。 4.無線LANではアクセスポイントというデバイスを使用する。このアクセスポイントは、一定の距離制限のあるエリア内を無線でカバーするものである。 5.アクセスポイントは、ある範囲内で、既存のイーサネットLANと、無線機能を搭載したコンピュータの間をつなぐブリッジとして機能する。 6.WiFIネットワークでは、データを無線で送受信する際、GPS衛星やデジタル携帯電話ネットワークで利用されているのとよく似たデータエンコーディングスキームを利用している。 7.これらのシステムで使用している無線機能では、無線信号にデータを載せる際、位相シフトキーイング（differential phase shift keying）を利用している。 8. WiFiアクセスポイントのカバー範囲は様々あるが、一個のアクセスポイントのカバー範囲は200-200フィート以内であるのが一般的である。 9.複数のアクセスポイントを配置してLANに接続することで、ビル全体やキャンパス全体をカバーすることも可能である。 10.アクセスポイントは、それぞれ固有の名前をブロードキャストしており、クライアントはこの名前を手がかりに接続するのである。 11. 一個のアクセスポイントは、同時に多数のクライアントと接続することが可能である。 12.個々のクライアントはそれぞれMACアドレスというユニークな識別名を持っており、アクセスポイントはこのMACアドレスを手がかりに、それぞれのクライアントを識別している。 13.クライアントが複数のアクセスポイントの圏内にある場合、クライアントは、信号強度がより強い方を選ぶことができる。 14.無線による接続ではセキュリティが重要であり、無線LANでは認証と暗号化を利用してこのセキュリティを維持している。 15.もっともシンプルで効果的な認証方式では、アクセスポイントは、所定のMACアドレスを持つクライアントだけと接続することになる。 16.MACアドレスはクライアントのアダプターカードやチップセットに、工場出荷時に焼き込まれており、容易に真似することのできないようになっている。 17.さらに複雑な認証スキームでは、ユーザーのアイデンティティを確認する際、スマートカードやバイオメトリックスキャニングを含む「公開鍵システム」を使用している。 18.無線ネットワークでの暗号化システムには２種類ある。古いものでWEPと、新しいものでWPAである。 19.WEPとWPAのどちらも、「ストリーム・サイファー」と呼ばれるRC4暗号化アルゴリズムを利用している。 20.ストリーム・サイファーは、40ビットまたは128ビットのキーから、データの擬似乱数ストリームを生成するという数学的操作を利用して動作している。 21. データを送信する端末は、この擬似乱数ストリームと搬送データとミックスして、暗号化したデータストリームを生成している。
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