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Meanings and definitions for X21
What does X21 stand for? What does X21 mean?
- 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.
- Northrop X-21, an experimental aircraft
- SA-X-21 or S-400 (missile), a Russian anti-aircraft weapon system
- X.21 (sometimes referred to as X21), an interface specification for differential communications
- X21 (band) (次世代ユニット), a Japanese girl idol group
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