Chapter 3. Configuration3-4 MX2800 STS-1 User Manual 61200659L1-1signal or reception of AIS will cause the unit to enter hold-overmode. The STS-1 receive signal must be valid for at least 10seconds for the unit to exit hold-over mode and restore loop timing.When the unit is configured for F REE -RUN timing mode, timing isderived from a +/-20 ppm internal reference providing a SONETMinimum Clock (SMC).Setting the unit to EXTERNAL timing mode configures the unit toderive clocking from one of the two external sources selected in theP RI E XT . C LOCK and S EC E XT . C LOCK options. These two entriesselect which of the 28 active T1/E1 ports will be used as theexternal clock source. The external clock source may be disabled (ifonly one source exists, or no external clock sources are desired). Avalid clock source is one on which neither LOS or AIS conditionsexist. The selected T1/E1 may be a data carrying tributary. Whenboth sources are configured, failure of one source will cause theunit to switch to the other source (if it is a valid source). Failure ofboth sources will cause the unit to enter hold-over mode. At leastone external clock source must be valid for at least 10 seconds forthe unit to exit hold-over mode and restore external clock timing.XCV ThresholdThe XCV T HRESHOLD (excessive code violations threshold) sets alimit on code violations accepted by the unit before it switchescontroller cards. If set to D ISABLED, code violations will not causethe unit to switch controller cards. The threshold limits aredescribed in the following chart:Switching between PRImary and SECondary external clock sourcesis non-reverting.
