問77
頻出度★★★
VoIP の説明として, 適切なものはどれか。
平成23年度 特別試験 ITパスポート試験 問77
テクノロジ系ネットワーク › ネットワーク応用
This question's explanation and your answers so far.
解説
問 77
Incorrectア
“A method of using applications and data located on the Internet from a PC” is a description of cloud computing. Users do not install software or store data on the PC at hand; instead they use a provider's resources on the far side of the Internet as a service, taking only as much as they need. It is classified into SaaS, PaaS, and IaaS according to the scope of what is provided. What it deals with is applications and data in general, not voice calls, and it does not include packetization or real-time transmission, which are the core elements of VoIP, so it is wrong.
- イ
正解 The correct answer is イ. VoIP (Voice over Internet Protocol) is a technology for carrying voice over an IP network. It works in roughly three stages: first, the analog voice waveform picked up by the microphone is encoded and converted into digital data; next, that data is divided into pieces of a fixed length and packed into IP packets; finally, the packets are sent to the other party over the IP network, and the receiving side rearranges them into their original order and restores the voice. The key point here is that a traditional fixed-line telephone occupies one circuit for the whole duration of a call, which is circuit switching, whereas VoIP sends packets only when they are needed, which is packet switching; the line can therefore be shared with other communication, and communication costs can be greatly reduced. Another key point is real-time behavior: with voice, any break or delay is immediately heard as a drop in quality, so the design uses UDP, which does not retransmit and tolerates some loss in order to give priority to timeliness (low delay), together with protocols for real-time transfer such as RTP (Real-time Transport Protocol), in order to keep delay low. The telephone service provided on top of VoIP is IP telephony, which is widely used for integrating company telephone extensions, for Internet telephone calls, and for the audio part of Web conferences. The statement in イ, “A technology that packetizes voice data and transmits and receives it in real time”, expresses exactly the essence of VoIP — encoding, packetization, and real-time transmission — with nothing missing and nothing extra, so it is appropriate.
Incorrectウ
“A technology that ensures security in communication between sites connected over a network by means of authentication, encryption, and decryption” is a description of a VPN (Virtual Private Network). It builds an encrypted communication path (a tunnel) on top of a public network such as the Internet, safely linking sites such as a head office and a branch office as if they were connected by a dedicated line. Its purpose is solely to ensure the confidentiality and legitimacy of communication; it is not a technology for packetizing voice and delivering it. VoIP traffic is sometimes carried inside a VPN, but the two are different technologies with different roles.
Incorrectエ
“A system that manages the mapping between the host names of computers connected to a network and their IP addresses” is a description of DNS (Domain Name System). It is the name-resolution mechanism that converts between domain names and host names, which are easy for people to remember, and the IP addresses actually needed for communication, and it is the foundation of using the Internet. It only takes charge of managing this mapping; it does not handle voice data and has nothing to do with real-time transmission, so it is wrong.
ここがポイント
- The definition of VoIP — VoIP (Voice over Internet Protocol) is a technology that encodes voice, splits it into IP packets, and transmits and receives them in real time over an IP network. It uses packet switching rather than circuit switching, which occupies a line, so the line can be shared and communication costs can be lowered. Because it is sensitive to delay, UDP and RTP are used; the service that provides this as a telephone service is IP telephony.
- Telling confusing network terms apart — Cloud computing is a method of using resources on the Internet as a service; a VPN is a technology that sets up an encrypted tunnel over a public network to protect communication between sites; DNS is the name-resolution mechanism that maps host names to IP addresses. For questions about abbreviations, tell them apart by “what they act on, and what they do”.
用語