А часом вот про энтот сигнал не забыли?..
Цитата
Master Clock. The master clock (1.544 MHz for T1, 2.048 MHz for E1)
input must be independent, free-running, continuously active and jitter free
for receiver operation. Since the transceivers derive their RCLK timing from
the MCLK input on Loss of Signal (LOS), MCLK cannot be derived from
RCLK.
З.Ы. А... тремя постами выше уже говорили про него, сорри...