High-order channel adaptation function block (HPA) and low-order channel terminal function block (LPT)

This block is an important block in the low-order part and is required to handle the vast majority of the low-order overhead bytes, including:

• Low-order channel pointer indication bytes (V1, V2, V3)
• Channel status and signal marker bytes (V5)
• Low-order channel identifier (J2)

The VC-4 signals from the cross-over board are sent to the HPA.The HPA demaps the VC-4 into a VC-12, and each VC-12 pointer is decoded to provide frame bias information in bytes between the VC-4 and the VC-12.

The pointers are required to be constantly adjusted when the node clock at the TU-12 assembly is different from the local reference clock. A positive TU pointer adjustment of TUPJCHIGH and a negative TU pointer adjustment of TUPJCLOW are detected in the downlink signal flow.

In the downstream direction, if a wrong sequence of H4 re-frame bytes is detected, HP_LOM is reported.

If the low-order pointer byte V1 and V2 values are detected to be all "1", the TU_AIS alarm is reported, and if the V1 and V2 values are detected to be illegal, the TU_LOP alarm is reported, and the occurrence of both alarms inserts an all "1 " signal to the next function block.

In addition, if TU_AIS is received, in addition to inserting the AIS signal to the next level function block, the alarm LP_RDI is sent back at the same time, i.e., the b8 bit of the V5 byte is set to "1".

The VC-12 signal flow is sent to the LPT unit for V5 byte processing.The time slot structure of V5 byte is shown in Figure 1.

Bits b5-b7 of the V5 byte are detected in the downstream signal stream and reported as a signal marker. If it is 000, the low-order channel is not loaded with LP_UNEQ. if the signal marker is mismatched, LP_SLM is reported.

The channel RDI information in b8 of the V5 byte will be terminated and the LP_RDI alarm will be reported.

A low-order virtual container unavailability event is considered to have occurred when 10 consecutive Severe Error Seconds SESs are detected in BIP-2 monitoring.

At the same time, the low-order channel identifier J2 byte is also recovered and the value of the received J2 byte is tested to see if the value received is equal to the value expected to be received, and if it is not equal, the LP_TIM alarm is reported.

Low-order channel adaptation function block LPA and asynchronous physical interface module PPI

After the above processing the C-12 data is sent to the LPA where the user data stream is recovered from the container along with the associated clock reference signals and is sent to the PPI as a data and timing reference.

After processing by the LPA, the data and clock are sent to the PPI to form a transmitted 2048 kbit/s signal.