問68
頻出度★★★
パケット交換方式に関する記述のうち, 適切なものだけを全て挙げたものはどれか。 a インターネットにおける通信で使われている方式である。 b 通信相手との通信経路を占有するので, 帯域保証が必要な通信サービスに向いている。 c 通信量は, 実際に送受信したパケットの数やそのサイズを基にして算出される。 d パケットのサイズを超える動画などの大容量データ通信には利用できない。
平成25年度 秋期 ITパスポート試験 問68
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 68
Incorrectア
Statements a and c are correct, but b is wrong, so this option is not appropriate. The statement b, “because it occupies the communication path with the communication partner, it is suitable for communication services that require bandwidth guarantees”, describes circuit switching. Circuit switching secures a path to the other party before communication starts and occupies that path during communication, so it can easily guarantee a certain quality and bandwidth. Packet switching, by contrast, shares the path with other communications, so delay and jitter occur when the network is congested, and in principle the bandwidth is not guaranteed. Therefore ア, which includes b, cannot be chosen.
Incorrectイ
Statement a is correct, but the two statements b and d are both wrong. As described above, b is a characteristic of circuit switching. The statement d, “it cannot be used for large-capacity data that exceeds the packet size”, is also wrong: data that exceeds the packet size is divided into multiple packets by the sender and reassembled in the correct order by the receiver, and this is the basic operation of packet switching. Video streaming and the transfer of large files are also carried out routinely on the Internet through this division and reassembly.
- ウ
正解 The correct answer is ウ “a, c”. Packet switching is a method in which the data to be sent is divided into packets (small blocks) of a fixed size, and control information such as the destination is attached to each packet before it is sent out. Switching devices (routers and the like) read the destination and relay the packets one after another, so a single line can be shared by many users on a time-division basis, giving high line utilization efficiency. As for a, the Internet uses TCP/IP to divide data into packets and transfer them, which is packet switching itself, so it is appropriate. As for c, because sending and receiving can be counted packet by packet, the traffic volume can be calculated by adding up the number and the size of the packets actually exchanged, and this is the basis of pay-per-volume charging (packet communication fees) in mobile communication and the like, so it is appropriate. Therefore only the two statements a and c are appropriate, and ウ, which lists that combination, is correct.
Incorrectエ
Both b and d are wrong, and both of the correct statements a and c are missing, so this is the least appropriate combination. Statement b describes circuit switching, which occupies the path, and does not apply to packet switching, which shares the path. Statement d overlooks the mechanism of division into packets and reassembly; large-capacity data is precisely what gets divided into multiple packets to be sent. Because it contains not a single appropriate statement, it is wrong.
ここがポイント
- Comparison of packet switching and circuit switching: packet switching divides data into packets, attaches destination information, and relays them while many users share the line, so line utilization efficiency is high; on the other hand, delay can occur when the network is congested, so in principle bandwidth cannot be guaranteed. Circuit switching, in contrast, occupies the path during communication, so bandwidth and quality are easy to guarantee, but the line is occupied even while nothing is being sent, so efficiency is low. The Internet (TCP/IP) uses packet switching.
- Division into packets and the idea behind charging: data that exceeds the packet size is divided into multiple packets by the sender and reassembled in the original order by the receiver, so large-capacity data such as video can also be sent by packet switching. In addition, because the number and the size of the packets exchanged can be measured, pay-per-volume charging based on the traffic volume rather than on the connection time (packet communication fees) becomes possible.
用語