問52
頻出度★★★
通信事業者が自社のWANを利用して，顧客の遠く離れた複数拠点のLAN同士を，ルータを使用せずに直接相互接続させるサービスはどれか。
平成26年度 春期 ITパスポート試験 問52
テクノロジ系ネットワーク › ネットワーク方式
This question's explanation and your answers so far.
解説
問 52
Incorrectア
ISDN is a communication service that digitizes the telephone network and is centered on circuit switching; it merely provides a “line” that carries voice and data in an integrated way. It is not a service that directly joins LANs while keeping them as Ethernet, and connecting sites by IP requires routers such as dial-up routers. So it does not meet the condition “directly interconnected without using routers”.
Incorrectイ
PoE (Power over Ethernet) is a technology that supplies electric power together with communication over the LAN cable (Ethernet wiring), and is used to power wireless LAN access points and IP phones. It is a means of supplying power inside a LAN, not a site-to-site connection service offered by a telecom carrier, and it has nothing to do with linking distant sites over a WAN.
Incorrectウ
The Internet is a public network that interconnects networks all over the world at the IP layer (Layer 3); it is not “a closed service that a telecom carrier provides over its own WAN”. If the sites are linked via the Internet, a router (such as a broadband router) must always be placed at each site to perform IP routing and address translation, which contradicts the condition that no router is needed.
- エ
正解 The correct answer is “エ Wide-area Ethernet”. In wide-area Ethernet, a telecom carrier provides its own WAN as an Ethernet (Layer 2 = data link layer) service, so the LANs at distant sites are directly interconnected as if they were one single LAN joined by one switch. Because frames are relayed inside the carrier's network, the user side does not need to place Layer 3 routers at the sites to do IP routing; Layer 2 switches alone are enough to connect the sites. Wide-area Ethernet is the only choice that satisfies all three conditions in the question: “uses its own WAN”, “between the LANs of geographically distant sites”, and “directly interconnected without using routers”.
ここがポイント
- Wide-area Ethernet = a service in which a telecom carrier provides its WAN as Layer 2 Ethernet. Its biggest feature is that the LANs of distant sites can be directly joined as if they were one LAN, and no router is needed on the site side (Layer 2 switches are enough).
- Distinguish WAN connection services by the layer at which they are provided: wide-area Ethernet is provided at Layer 2, while the Internet and IP-VPN connect at Layer 3 with IP. PoE is a LAN power-supply technology and ISDN is a circuit-switching service; neither is a service that directly joins the LANs of different sites.
用語