The GAO V.21 modem software implements the ITU V.21 recommendation for speeds up to 300bps for use in the GSTN. GAO's implementation for this modem software uses frequency shift modulation with the symbol rate equal to the bit rate. The mean frequencies for channel 1 and 2 are 1080Hz and 1750Hz respectively. This modem software allows both synchronous and asynchronous procedures.
For examples of GAO's Modem Software Architecture, click GAO's Modem Software Architecture.
For a complete list of GAO's Modem Software products, please see GAO's Softmodem List.