What does the TNC expect when using SSB.

The TNC expects to receive the same tones it transmits. A TNC usually has a
bargraph of LEDs that light in a certain manner when the signal is properly
tuned. While this accomplishes the task, additional information may help us
eliminate the noise that is also received with a normal SSB signal. (By
convention, digital modes use LSB on all bands.)

Digital information is sent as a signal that changes between two tones. The
"standard" tones are 2125 Hz and 2295 Hz; however the European standard is
1275 and 1445. If we do the math, the difference between 2125 and 2295 is
170 Hz. The difference between 1275 and 1445 is also 170 Hz. Therefore, a
signal sent with tones of 1275 and 1445 can be received as tones at 2125
and 2295 and carry the same digital information. The difference between the
tones (or the shift) is the important factor, not the specific transmitted
tones.

You do not need to be concerned with which tones another station transmits to
you. When a radio receives a signal in its passband, the radio produces tones
that you hear from the radio's speaker. As you move the tuning dial, the
frequencies of the tones change because their location in the passband
changes. More technically, early in the radio's receive circuits the radio
mixes the receive signal with another signal based on the dial setting
(VFO). This mixing results in a frequency the circuits of the radio are
designed to handle.  As you move the tuning dial, the VFO input to the mixer
changes, therefore changing the output (tone). The TNC's bargraph is an aid
for you to tune the radio to the tones expected by the TNC.

Besides the two tones needed for decoding the digital message, the radio is
also receiving other noise that is within the passband of the radio. A noisy
signal will be harder for the TNC to copy.  Knowing the bandwidth of a signal
helps you choose the right filters to receive a signal with the least noise.