SDH Performance Monitoring

SDH performance monitoring

SDH performance monitoring uses overhead bytes to report transmission errors and defects at defined transport layers. ITU-T G.826 defines block-based parameters for digital paths at or above the primary rate and bit or bit error ratio parameters for connections below the primary rate. The recommendation applies to one transmission direction, with path endpoints able to collect the observations. These measurements describe the transport path and do not replace application quality measurements.

The overhead carries error detection codes and defect indications that can be evaluated at section and path boundaries. The network can therefore distinguish a local section event from an error reported for a transported Virtual Container. A bit error can be converted into G.826 events. G.784 also defines performance data collection on demand, at thresholds, and periodically for trend analysis, so counters can support maintenance decisions without being confused with an application service-level result.

Bit Interleaved Parity (BIP) by SDH layer

Bit Interleaved Parity (BIP) is the SDH error detection method placed in the section and path overhead. Each BIP-N value contains one even-parity result for each of N bit positions. A receiver recalculates the value over the defined preceding block and compares it with the value carried by the current frame or Virtual Container. A mismatch identifies an errored block for performance monitoring and helps associate the observation with a transport layer.

BIP calculation and scope

In a BIP-N calculation, the bits in the checked block are assigned to N columns. The transmitter computes even parity for each column and places the N results in the designated overhead field. The receiver applies the same scope to the received block. When the calculated vector differs from the transmitted vector, the controlled block is treated as errored according to the applicable performance definition. The field therefore indicates a discrepancy without identifying the exact bit position or number of wrong bits.

SDH places BIP checks at three transport layers:

• B1 (BIP-8): regenerator section: B1 is carried in the RSOH. The transmitter computes even-parity BIP-8 over all bits of the previous STM-N after scrambling and places the result in B1 of the current STM-N before scrambling. A receiving regenerator can compare the value for the section between adjacent section endpoints, which supports hop by hop fault localization.
• B2 (BIP-N × 24): multiplex section: B2 is carried in the MSOH and is computed over all bits of the previous STM-N except the first three SOH rows. STM-1 uses BIP-24 and STM-4 uses BIP-96. The scope is the multiplex section between its termination points, so the result covers the signal across any intervening regenerator sections.
• B3 (BIP-8): path: B3 is part of the Path Overhead (POH) and is computed over the preceding VC-4 or VC-3. It follows the Virtual Container through the transport network, allowing a path endpoint to evaluate one client payload separately from other services carried by the same STM-N.
• V5 (BIP-2): low-order path: A VC-11 or VC-12 uses two BIP bits in V5 for low-order path monitoring. The remaining V5 fields carry the applicable defect, signal label, and path status information.

G.826 performance events and ratios

G.826 converts errored blocks and defects into one-second events and ratios. Measurements can be reported for 15-minute and 24-hour intervals, while the definitions use the count of available seconds or available blocks in the observation period. The terminology separates an isolated errored block from a severely errored second and records unavailable time separately from available transport time.

• Errored Block (EB) and Errored Second (ES): An EB is a block containing one or more errored bits, as determined by the selected error detection code. An ES is a one-second period containing at least one EB or at least one defect. The event is counted for the direction and path being measured.
• Severely Errored Second (SES) and SESR: For a path, an SES is a one-second period containing at least 30 percent errored blocks or at least one defect. SES is a subset of ES. The Severely Errored Second Ratio is SESR = SES / available seconds for the evaluated period.
• Background Block Error (BBE) and BBER: A BBE is an errored block that is not included in an SES. The Background Block Error Ratio is BBER = BBE / available blocks, where blocks belonging to SES are excluded from the denominator. BBER is defined for a path and expresses the remaining block error activity after severe seconds have been separated.
• Unavailable Second (UAS): Unavailability starts at the onset of ten consecutive SES. Those ten seconds are counted as unavailable, and each following second remains unavailable until the onset of ten consecutive seconds without an SES. The latter ten seconds begin the available period. UAS therefore records the time classification defined by G.826 rather than a separate bit error count.

G.826 gives defect thresholds for SDH path monitoring. For VC-11, VC-12, and VC-2 the threshold in the referenced table is 600 errored blocks per second; for VC-3 and VC-4 it is 2400 errored blocks per second. The threshold must be interpreted together with the monitored path type and the error detection code.

Defects and maintenance signals

Performance events are accompanied by defect indications that describe a loss of optical input, a framing failure, or a defect reported by another network element. The receiving element can replace an unusable signal with a maintenance signal toward downstream equipment and can send a remote defect indication toward the source. These actions preserve the direction of the fault information for section and path maintenance.

Transport defect indications

• Loss of Signal (LOS): The receiver does not detect the optical signal at its input. The condition can result from a fiber interruption, a disconnected path, or a failure in the transmitting equipment, so the element reports the observed absence of signal rather than a unique physical cause.
• Loss of Frame (LOF): The receiver has an input signal but cannot maintain the valid A1 and A2 frame alignment pattern. Frame boundaries are then unavailable to the processing function, and the STM-N signal cannot be interpreted at that layer.

Fault communication signals

AIS and RDI communicate the effect and direction of a detected defect. Their exact coding depends on the SDH layer at which the maintenance function is applied.

• Alarm Indication Signal (AIS): After detecting a defect such as LOS or LOF, an element can send the defined AIS replacement signal toward downstream equipment instead of forwarding the failed client signal. Downstream elements then receive a maintenance indication that the defect is located before the point that generated AIS, which limits repeated local alarms while preserving fault information.
• Remote Defect Indication (RDI): RDI travels toward the source to report a defect detected at the far end. In the multiplex section, MS-RDI uses bits 6 to 8 of K2; at the path layer, the indication is carried by G1. The receiving end can therefore report a reverse-direction defect while the forward direction carries the applicable maintenance signal.