Tektronix CTS 850, User Manual

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Functional Overview
CTS850 SDH/PDH Test Set User Manual
2–29
Anomaly, Defect, Failure, Alarm
Anomaly – The smallest discrepancy which can be observed
between the actual and desired characteristics of an item. The
occurrence of a single anomaly does not constitute an interruption in
the ability to perform a required function.
Defect – The density of anomalies has reached a level where the
ability to perform a required function has been interrupted. Defects
are used as input for performance monitoring, the control of
consequent actions, and the determination of fault cause.
Failure – The inability of a function to perform a required action
persisted beyond a maximum time allocated.
Alarm – The maintenance signal used in the digital network to alert
downstream equipment that a defect or equipment failure has been
detected.
SDH Multiplexing Principles
The multiplexing principles of SDH follow:
H
Mapping – A process used when tributaries are adapted into
Virtual Containers (VCs) by adding justification bits and Path
Overhead (POH) information.
H
Aligning – This process takes place when a pointer is included in
a Tributary Unit (TU) or an Administrative Unit (AU), to allow
the first byte of the Virtual Container to be located.
H
Multiplexing – This process is used when multiple lower–order
path layer signals are adapted into a higher–order path signal, or
when the higher–order path signals are adapted into a Multiplex
Section.
H
Stuffing – SDH has the ability to handle various input tributary
rates from PDH. As the tributary signals are multiplexed and
aligned, some spare capacity has been designed into the SDH
frame to provide enough space for all these various tributary
rates. Therefore, at certain points in the multiplexing hierarchy,
this space capacity is filled with ”fixed stuffing” bits that carry no
information, but are required to fill up the particular frame.