These can be used aswel as Synchrone.

              Norm             Send     Receive  Answ.   mode
                              '0'  '1'  '0' '1'  tone
              Bell-103(ORG)  1070 1270 2025 2225 1270   300 FD
              Bell-103(ANS)  2025 2225 1070 1270 2225   300 FD
              Bell-202(main) 2200 1200 2200 1200 2025  1200 HD
              Bell-202(back)  487  387  487  387   -  0-150 HD
              V21 (ORGinate) 1180  980 1850 1650  980   300 FD
              V21 (ANSwer)   1850 1650 1180  980 2100   300 FD
              V23-1 (main)   1700 1300 2100 1300 2100   600 HD
              V23-2 (main)   2100 1300 1700 1300 2100  1200 HD
              V23   (back)    450  390  450  390   -  0-150 HD
              Remark: '0'= "start bit" = "space"
                      '1'= "stop bit"  = "mark"
                      FD = Full Duplex, HD = Half Duplex.

              Important  is,   the  modems  use   filter  technics  to
              seperate the transmit channel and receive channel.  Some
              autoanswering  modems also by filter technics decide  in
              which mode they are called.