US Robotics 56K Series Manual page 80

&Un When set above 0, the value chosen from the table sets the floor connect
speed (the lowest acceptable connect speed). If a connection cannot be made
at or above this speed, the modem will hang up. This command can also be
used in conjunction with &Nn. Note: &U17 through &U39 apply only to
V.90 products.
&N=0
&U=0 Connects at best
possible speed
between your modem
and the remote modem.
Note: These factory default settings should be sufficient for most users.
&U>0 Connects at any speed
faster than the value
&Un.
74
&N>0
Attempts to connect at the speed
defined by &Nn.
Connects at any
speed between
&Nn. and &Un.