Abstract: SDH METRO1000 frame structure has a wealth of overhead bytes, with the help of these overhead bytes to pass the alarm, performance information, so that the SDH system has a strong on-line alarm and BER monitoring capabilities. In this paper, from the point of view of data flow, combined with the single board, detailed description of each single board may appear alarm information and reasons, the single board debugging personnel to understand the way the alarm information is generated and the rapid localization of faults have a certain significance to help.

Line alarm: the signal flow for the SDH interface → cross board → SDH interface on this route alarm information, that is, in the optical board alarms. There are regeneration section, multiplexing section and high-order channel overhead alarm.

The alarm signaling process is as follows:

(1) LOS signal loss (cutoff):

After the STM-N optical signal from the optical path enters the optical receiver module of the optical board, it first undergoes photoelectric conversion and is restored into an electrical signal to be sent to the frame synchronizer and scrambler for processing. During this process, the optical converter module will detect the signal and report LOS (Loss of Signal) alarm if it finds that the input signal has no light, the optical power is too low or too high, and the input signal code type does not match. When the R-LOS alarm occurs, the system will insert a full "1" signal to the next level of circuitry.

(2) OOF frame out-of-step alarm and LOF frame loss alarm (cutoff):

A1 and A2 bytes are used to locate the STM-N signal from the optical/electrical converter module, and at the same time extract the line reference synchronization timing source from it, and send it to the clock board for clock locking. Under normal circumstances, the value of A1 is constant at F6, and the value of A2 is constant at 28; however, if it detects that A1 ≠ F6 or A2 ≠ 28, the OOF alarm (frame out-of-step alarm) will be reported. If the R-OOF alarm lasts for more than 3ms, then LOF (Loss of Frame Alarm) will be reported and a full "1" signal will be inserted. In the LOF state, if the device is in the framing state again for more than 1ms, then the device returns to the normal state.

(3) J0 tracking identifier mismatch alarm:

Check the J0 byte after framing, if the J0 byte of the sender and receiver do not match, then the J0 tracking identifier mismatch alarm will be reported.

(4) B1 error code:

If the B1 byte recovered from the STM-N signal does not match the BIP-8 calculation result in the previous STM-N frame received, a B1 error code is reported.

The above part is the regeneration segment alarm information. The following section describes the multiplexing segment alarm. The regenerative segment overhead bytes mainly handled in this part that are related to alarms and performance are: automatic protection reversal path bytes (K1, K2), and multiplexing segment error code monitoring byte (B2).

(5) MS-AIS, MS-RDI alarm:

If K2 (b6-b8) = 111 is detected, the MS-AIS alarm is reported and the full "1" signal is inserted. If K2 (b6-b8) = 110 is detected, the MS-RDI alarm is reported.

(6) B2 Error.

If the B2 byte recovered from the STM-N signal does not agree with the BIP-24 calculation of the previous STM-N frame (all bits except the regeneration segment overhead), a B2 error code is reported. and the MS-REI alarm is reported via the M1 byte (note that this alarm is a returned alarm message).

The following section describes the pointer and higher-order channel overhead alarms. This part mainly deals with high-order pointer adjustment and high-order channel overhead, the bytes related to pointer adjustment are H1, H2, H3, while the bytes related to alarm and error code are high-order channel tracking byte (J1), signal marking byte (C2), high-order channel error code monitoring byte (B3), channel status byte (G1), and reframing position indication byte (H4). (Note: Some systems have higher order channel overheads handled on the tributary board)

(7) AU-AIS management unit-alarm indication signals

If all H1 and H2 bytes are "1", the AU-AIS alarm is reported and the all "1" signal is inserted.

(8) AU-LOP (management unit - loss of pointer)

If the value of the pointer represented by the H1 and H2 bytes is illegal (not within the normal range of 0 to 782), and illegal pointers are received for 8 consecutive frames, an alarm is reported and an all "1" signal is inserted.

After processing the pointer, the optical board processes the higher order channel overhead and generates the corresponding alarm message.

(9) J1 byte and the predetermined value is different, then report the HP-TIM alarm and insert the full "1" signal.

(10) Higher-order channel not loaded, higher-order channel signal marker mismatch.

If byte C2 = 00 is detected, the HP-UNEQ (high order channel not loaded) alarm is reported and a full "1" signal is inserted. If byte C2 is detected to be different from the predefined value, report HP-SLM (Higher-order channel signal mark mismatch) alarm and insert all "1" signals.

(12) B3 Error Code.

If the calculation result of the B3 byte recovered from the HPOH is inconsistent with the BIP-8 calculation of the VC-4 signal of the previous frame, the B3 error code is reported and the full "1" signal is interpolated.

If the result of B3 byte recovered from HPOH and the result of BIP-8 calculation of the previous VC-4 signal are inconsistent, B3 error code is reported.

(13) HP-RDI and HP-REI:

If byte G1 (bit5) = 1 is detected, the HP-RDI alarm is reported, depending on whether byte G1 (bit1-bit4)

(bit1-bit4) value is 1-8 or not, it will be judged whether to report HP-REI alarm or not.

(14) LOM (Loss of Multi-frame) Alarm

If illegal H4 byte is detected, LOM (Loss of Multi-frame) alarm is reported and all "1" signals are inserted.

The line alarms have been introduced, and the following describes the tributary board alarms. The tributary boards include E1T1, E3T3, 140M, and Ethernet boards. The low-order channel overhead alarms are mainly considered. The service flow direction is branch board → cross board → optical board.

The VC-4 signal from the crossover board is sent to the tributary board, and then the VC-4 is demapped to VC-12.

(15) If the H4 compound frame byte sequence error is detected, the HP-LOM is reported.(Same as 14)

(16) TU-AIS alarm, TU-LOP alarm, LP-RDI

If the low-order pointer byte V1 and V2 values are detected as all "1s", the TU-AIS alarm is reported, and if the V1 and V2 values are detected as illegitimate, the TU-LOP alarm is reported, and the occurrence of both alarms inserts the all "1s" signal into the next function block. " signal.

In addition, if TU-AIS is received, in addition to inserting the AIS signal in the downward data, it is also necessary to report back to LP-RDI, i.e., set the b8 bit of the V5 byte byte to "1".

(17) Low-order channel unloaded LP-UNEQ, Signal marker mismatch LP-SLM

Bits b5-b7 of the V5 byte are detected and reported as a signal marker. If 000, the low-order channel is not loaded with LP-UNEQ and an AIS signal is inserted to the next level of circuitry. If the signal marker is mismatched, the LP-SLM is reported and the AIS signal is inserted toward the next level of circuitry. The channel RDI message in b8 of the V5 byte is terminated and the Remote Alarm Indication is reported.

(18) LPBBE, LPFEBBE:

Detect the BER monitoring bits b1 and b2 of V5 byte, calculate BIP-2 for VC-12, compare the BIP-2 value calculated for the current frame with b1 and b2 of V5 recovered in the next frame, and report LPBBE if there is inconsistency; at the same time, bit b3 of V5 byte is recovered, and if "1", it indicates that far-end alarm indication is detected. ", it indicates that a BIP-2 error is detected at the far end and is reported as LPFEBBE.

(19) Low-order channel tracking identifier mismatch alarm (LP-TIM):

Detects whether the value of the J2 byte received is equal to the value expected to be received, and if it is not, reports a low-order channel tracking identifier mismatch alarm (LP-TIM). At the same time, the low-order channel identifier J2 byte is recovered and the value of the received J2 byte is tested to see if it is equal to the expected value, and if it is not equal, a low-order channel tracking identifier mismatch alarm (LP-TIM) is reported.