An ITU standard (1964) for asynchronous 0-300 bps full-duplex analog modems for use on dial-up lines. It uses FSK modulation.
"V.21." YourDictionary, n.d. Web. 21 November 2018. <https://www.yourdictionary.com/v-21>.
V.21. (n.d.). Retrieved November 21st, 2018, from https://www.yourdictionary.com/v-21
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