Bridging
2-80 SONET Port Configuration
With a SONET link, there are three levels of error conditionsÑanomalies, defects,
and failures.
¥Anomalies are small discrepancies between a desired and actual characteristic
of an item, which when occurring singly will not interrupt the ability of the
SONET network elements to perform their required functions.
¥Defects indicate that anomalies have reached a level where the ability of the
SONET network elements to perform their required functions has been
interrupted. Defects are used in performance monitoring and in determining
the faultÕs cause, and have impact on consequent actions on the network.
¥Failures indicate that a network element has been unable to perform its
required functions beyond a maximum time allocated to a given error
condition.
These errors can occur in any of the four optical layers of a SONET network,
which are (in order from lowest to highest layer in the hierarchy) the physical
Medium, Section, Line, and Path layers.
¥ The Medium layer is the Photonic layer that physically converts electrical
signals to optical signals.
¥ The Section layer deals with the transport of frames across the optical
medium, including framing and scrambling data for transmission, the error
monitoring and maintenance between section-layer elements (such as signal
regenerators/repeaters), and orderwire (provisioning channels).
¥ The Line layer is responsible for reliably transporting the higher-level Path
layer payload and overhead across the physical medium. It is responsible for
synchronizing (clocking) the data transmission, multiplexing signals into a
single channel, error monitoring and maintenance between line-layer
elements (such as Add/Drop Multiplexers), and switching to secondary data
paths should the primary path experience failure.
¥ The Path layer transports services between path-terminating equipment.
It maps signals into a format required by the line layer, and reads, interprets,
and modiÞes path overhead for performance monitoring and automatic
protection switching.
Error reporting occurs at the Section, Line, and Path layers, and is carried within
the corresponding SONET overhead. In terms of the SONET protocol stack, the
three layers with overhead are mapped to the SONET link as shown in the
following diagram.