Verilink C100 (880-502893-001) Product Manual - Manual (Page 38)

Configuration Using the DIP Switches
3-14 Verilink C100 and C150 T1/FT1 CSU/DSU 6/15/99
must be frequency-locked to the Receive Clock provided by the
C100/C150 T-1 CSU/DSU or DIP Switch pack 2 positions 7 and 8 must
be On.