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X.25の物理層がX.21とありますが | 戸根勤のネットワーク情報
X.21 - Wikipedia
通信基本用語 - X.21 | LINEEYE
- X.21 (sometimes referred to as X21) is an interface specification for differential communications introduced in the mid-1970s by the ITU-T. X.21 was first introduced as a means to provide a digital signaling interface for telecommunications between carriers and customers' equipment.en.wikipedia.org/wiki/X.21
X.25 - Wikipedia
The X.21 Interface a Tutorial - ARC Elect
ITU-T X.25 ‐ 通信用語の基礎知識
The X21 Protocol - Ring of Saturn
【超図解】OSI参照モデルの役割とプロトコル一覧を丁寧に見て …
X.25 ネットワーク - IBM