V.23 modem modulation Explained

Summary of the differences between Bell 202T modem and V23 modem

The V.23 specification is for a Dial or leased line modem operating at 600/1200 bps over 2 wires in one direction. It has a backward channel of up to 75 bps for error control.

Bell 202T specification is a 0 to 1200/1800 bps modem for 2 or 4 wire private line modem. It is capable of operating on multi-drop circuits for polling applications.

Specification	V.23	Bell 202T
Dialup	Yes	No
Leased Line	Yes	Yes
FSK	Yes	Yes
1200 Hz Mark Frequency	1300 Hz	1200 Hz
1200 Hz Space Frequency	2100 Hz	2200 Hz
Back Channel Mark Freq.	390 Hz	1200 Hz
Back Channel Space Freq.	450 Hz	2200 Hz
Up to 600 Hz Mark Frequency	1300 Hz	1200 Hz
Up to 600 Hz Space Frequency	1700 Hz	2200 Hz
1200 to 1800 Hz	No	Same as above

The mark space frequencies are close enough that many 202T modems can communicate with V.23 modems. Many times V.23 is mistakenly used to identify a 0/1200 bps modem in applications outside the U.S. V.23 is NOT 1200 bps in both directions.

The 202T will work perfectly in a V.23 application since it is transparent to the Data Terminal Equipment (DTE) speed, including /75/600/1200 bps in both directions.

The only time a V.23 modem is needed is to connect to an existing true V.23 modem,

Or an application where the same modem type must do both dial and leased line operation. There are very few TRUE V.23 modems.