---------------   ------------------   ------------------   ------------------
                                R G                  R G     
---------------                 E R                  E R     FTAM       X.400
                                D E                  D E    ------------------ 
---------------                   Y                    Y     PRES        PRES
                    X29        -----     X29        -----   ------------------ 
---------------                                              SES        SES 
                             BLUE                  BLUE     ------------------ 
---------------            ---------                         TRAN        TRAN 
                              YB                     
---------------    -----------------    ------------------  ------------------
                          X25 (1980)            X25 (1984)          X25 (1984)
---------------    -----------------    ------------------  ------------------
                        LAP-B                 LAP-B               LRP-B
---------------    -----------------    ------------------  ------------------
                        PHY                   PHY                 PYY
---------------    -----------------     -----------------   -----------------
                       Current            After network      Replacement of
                                          layer transition   upper level
                                                             protocols