This parameter is defined as the minimum percentage of tones in the downstream
MEDLEY SET that must be degraded in order to arm the first sub-condition of the
standard SOS triggering criteria (see 13.4.3.2/G.993.2) in the downstream direction.
The parameter SOS-NTONES-ds is defined as a percentage of tones. See G.993.2 for
detailed usage rules. The valid range of values is from 1 to 100 in steps of 1.
Use of the special value 0 is described in 13.4.3.2/G.993.2.