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What happens to SONET errors?

I'm not particularly strong with SONET and only have a small SONET network so any help with this question would be greatly appreciated.  When line terminating equipment (SONET node) detects a section error what does it do with that frame when it switches it to the next section?  Somehow it will not appear as an error on the next section and my group has some differing opinions on how this is accomplished.  Any opinion and associated documentation you could provide would be great.

Everyone's tags (4)
Cisco Employee

Re: What happens to SONET errors?


B1 byte in a section overhead used for section errors monitoring. B1 is a section bit interleaved parity code (BIP-8) byte - This is a parity code (even parity) used to check for transmission errors over a regenerator section. Its value is calculated over all bits of the previous STS-N frame after scrambling, then placed in the B1 byte of STS-1 before scrambling.

Hence nothing would be done with a frame itself on a SONET node as an error for a frame can be detected in a next frame only.




Re: What happens to SONET errors?

SONET node) detects a section error what does it do with that frame when it switches it to the next section.

There  are several BIP for a SONET frame. When the Node detects section error  an alarm is raised. Then that node calaculates the parity again and  write the value for next node.Now in next section there was no error so  it will calculate the same value which earlier node wrote. Process will  keep repeating at every node where it compares the value it counted with  the value earlier node wrote.

All next node will not have any error but the E1/T1 for which this parity doenot match will have an error till the end.

Cisco Employee

Re: What happens to SONET errors?


SONET uses the layer model.

The overhead and transport functions are broken into layers: Physical, Section, Line and Path.  The layers have a hierarchical relationship and are viewed from the top down.

The lowest layer is the physical layer, which represents the transmission medium. This is usually a fiber link.  No overhead is associated with the physical layer.  The main function of this layer is conversion between internal STS-N signals and external optical or electrical SONET signals

The Section layer deals with the transport of an STS-N frame across the physical medium.  Functions of this layer include framing, scrambling, Section error monitoring and Section level communications overhead The Section overhead is interpreted and modified or created by Section Terminating Equipment.

The line layer is the part of the SONET link between multiplexers.  The line overhead is used for the line layer.  This layer provides synchronization and multiplexing functions for the Path layer.  The overhead associated with these functions includes overhead for maintenance and line protection purposes

The Path Layer is the part of the SONET network from where the asynchronous digital signals enter and to where these signals exit the SONET network.  (end-to-end).  The Path layer maps the payloads into the format required by the Line layer. In addition, this layer communicates end-to-end via the Path Overhead. The POH is interpreted and modified or created by Path Terminating Equipment.

Each layer has its own set of overhead bytes that it uses for communication and monitoring purposes.

The information in the overhead bytes is interpreted and re-created at the corresponding terminating layer.   For example,  section overhead uses the B1 parity bit to monitor for section errors .  This information is propagated  separately for each section start/end point.  When section terminating node sees B1 errors, it declares a section alarm or threshold crossing for that segment.  The Section overhead and parity is then recalculated for the next segment of section overhead.  Hence you will not see a section alarm or error propagated from segment to segment.

GR-253-CORE is the official industry standard for SONET transport systems.  It is published by Telcorida Technologies.  It described in-depth the generic criteria that all telecommunications equipment are supposed to conform to.  Probably not the best place to start but it is an excelelnt source on SONET protocols.

There are some good on-line SONET tutorials that you can find using a GOOGLE search.  Here are a few good links.


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