SONET Alarms - Optical Communication Networks

SONET Alarms By Sadhish Prabhu SONET frame structure The SONET frame structure has been designed to contain a large amount of overhead information. Functions of overhead It provides a variety of management and other functions: Error performance monitoring pointer adjustment information path status path trace

section trace remote defect, error, and failure indications signal labels new data flag indications data communications channels (DCC) automatic protection switching (APS) control Orderwire and synchronization status message SONET alarms SONET alarms are defined as follows Anomaly: This is the smallest discrepancy that 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. SONET alarms SONET alarms are defined as follows 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. SONET alarms SONET alarms are defined as follows Failure :

This is the inability of a function to perform a required action persisted beyond the maximum time allocated. Various SONET Alarms DESCRIPTION loss of signal (LOS) CRITERIA LOS is raised when the synchronous signal (STSN) level drops below the threshold at which a BER of 1 in 103 is predicted. It could be due to a cut cable, excessive attenuation of the signal, or equipment fault. LOS state clears when two consecutive framing patterns are received and no new LOS condition is detected.

out of frame (OOF) alignment OOF state occurs when four or five consecutive SONET frames are received with invalid (errored) framing patterns (A1 and A2 bytes). The maximum time to detect OOF is 625 microseconds. OOF state clears when two consecutive SONET frames are received with valid framing patterns. Various SONET Alarms DESCRIPTION CRITERIA LOF state occurs when the OOF state exists for a specified time in loss of frame

(LOF) alignment milliseconds. LOF state clears when an in-frame condition exists continuously for a specified time in milliseconds. loss of pointer (LOP) LOP state occurs when N consecutive invalid pointers are received or N consecutive new data flags (NDFs) are received (other than in a concatenation indicator), where N = 8, 9, or 10. LOP state clears when three equal valid pointers or three consecutive AIS indications are received. LOP can also be identified as follows: STS path loss of pointer (SPLOP) VT path loss of pointer (VPLOP) Various SONET Alarms

DESCRIPTION alarm indication signal (AIS) CRITERIA The AIS is an all-ones characteristic or adapted information signal. It is generated to replace the normal traffic signal when it contains a defect condition in order to prevent consequential downstream failures being declared or alarms being raised. AIS can also be identified as follows: line alarm indication signal (AISL) STS path alarm indication signal (SPAIS) VT path alarm indication signal (VPAIS) remote error

indication (REI) This is an indication returned to a transmitting node (source) that an errored block has been detected at the receiving node (sink). This indication was formerly known as far end block error (FEBE). REI can also be identified as the following: line remote error indication (REIL) STS path remote error indication (REIP) VT path remote error indication (REIV) Various SONET Alarms DESCRIPTION remote defect indication (RDI) CRITERIA

This is a signal returned to the transmitting terminating equipment upon detecting a loss of signal, loss of frame, or AIS defect. RDI was previously known as FERF. RDI can also be identified as the following: line remote defect indication (RDIL) STS path remote defect indication (RDIP) VT path remote defect indication (RDIV) remote failure indication (RFI) A failure is a defect that persists beyond the maximum time allocated to the transmission system protection mechanisms. When this situation occurs, an RFI is sent to the far end and will initiate a protection switch if this function has been enabled.

RFI can also be identified as the following: line remote failure indication (RFIL) STS path remote failure indication (RFIP) VT path remote failure indication (RFIV) Various SONET Alarms DESCRIPTION B1 error B2 error B3 error BIP2 error CRITERIA Parity errors evaluated by byte B1 (BIP8) of an STS N are monitored. If any of the eight parity checks fail, the corresponding block is assumed to be in error.

Parity errors evaluated by byte B2 (BIP24 x N) of an STSN are monitored. If any of the N x 24 parity checks fail, the corresponding block is assumed to be in error. Parity errors evaluated by byte B3 (BIP8) of a VTN (N = 3, 4) are monitored. If any of the eight parity checks fail, the corresponding block is assumed to be in error. Parity errors contained in bits 1 and 2 (BIP2: bit interleaved parity 2) of byte V5 of an VTM (M = 11, 12, 2) are monitored. If any of the two parity checks fail, the corresponding block is assumed to be in error. Various SONET Alarms DESCRIPTION CRITERIA loss of Sequence synchronization is considered to be lost and

sequence resynchronization shall be started if the following occur: synchronization Bit error ratio is greater than or equal to 0.20 during an (LSS) integration interval of 1 second. It can be unambiguously identified that the test sequence and the reference sequence are out of phase. Note: One method to recognize the out-of-phase condition is the evaluation of the error pattern resulting from the bit-by-bit comparison. If the error pattern has the same structure as the pseudo-random test sequence, the out-of-phase condition is reached.

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