3Com 56k User's Manual And Reference page 63

&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.
Technical Reference
&N>0
Attempts a connection at the speed
defined by &Nn.
Connects at any
speed between
&Nn. and &Un.
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