The standard and the common malfunctions of PDH optical transceiver

Time : Jun. 01, 2022    View : 95

The standard of PDH optical transceiver

    There are two series of digital optical transceivers in the digital communication transmission system, the plesiochronous digital hierarchy(PDH) optical transceivers and the Synchronous digital hierarchy(SDH) optical transceivers.

    The system using the PDH is to set high-precision clock on every node of digital communication network, and the signals of these clocks have a uniform standard rate. Although it is high-precision clock, there are always some small differences. For ensuring the quality of communication, the difference should be in the specified range. Therefore, this synchronization method is not true synchronous in strictly speaking, so it is called plesiochronous.

    In the past telecommunication network,most use PDH optical transceiver equipment. This series has a better adaptability to traditional point-to-point communication. However, with the rapid development of digital communication, the point-to-point direct communication become less and less. Most of the digital transmission have to be switched, the PDH series can not satisfy the need of modern telecommunication service development and modern telecommunication management. SDH is a transmission system that emerges to meet this new need.

PDH optical transceiver common malfunctions

1. Intermittent malfunction

1)   Individual 2Mb/s port alarms are generated instantaneously due to intermittent faults, and the time for switching relay module DM to identify the alarms is different, so single-ended alarms may occur. It is better to solve the problem by the way of solving permanent malfunction than DDF loop-back way to avoid circuit interruption and influence service.

2) Bulk 2Mb/s port alarm

(1)  The alarm module concentrated on 34Mb/s or 8Mb/s port of multiplexer device etc., it is possible caused by the problem of high digital group or DDF transfer cable or chassis. Detect it according to permanent malfunction if in above situation.

(2)  Several DM alarming in a relay group is possibly caused by switch problem. For example, this kind of problem occurred in our bureau once, and it was found that it is caused by  the heavy work of the multi-frequency register. At that time, our office adopted channel-associated signaling, and the number of dual-tone telephones gradually increased, resulting in heavy work for the multi-frequency register. Later, the No.7 signaling was adopted between offices, and the problem was solved.

3) One kind of special alarm

    Each of 2Mb/s transmission link is normal when they are tested separately, that is , each point on the DDF has no alarm for the exchange loop at both ends and U-Link is also normal when it is measured by universal watch, but there are alarm when U-link is connected to the switch machine, and can’t be used. At this time, making a loop on the transmission side of the low digital group at either end of the A or B, and doing the test at the same position at the another end by transmission analyzer, the ES occurs, and the whole link is in a critical state.

The processing method is to use the meter to test segment by segment until the malfunction point is found and the circuit is restored.

2. Permanent malfunction

    One end adopts loop and the another and the other end can be displayed from the switch or judged segment by segment and layer by layer with a transmission analyzer.

1)Transmission alarm occurs in pair, for example, if a RJA(remote junction alarm) occurs at the A end , and the B end is LFA(loss of frame alignment) or AIS(alarm indication signal), and sometimes both of them are existed. There is another alarm, LS (signal loss)alarm.

The one of reasons of malfunction

    There is problem in the transmission from A end low digital group to B end, causing 2Mb/s signal to be error code or interrupted.

The method of identification

    If there is problem from A to B between the A and B ends low digital group, the B end will present AIS or LFA alarm. If there is the signal interruption from B end low digital group to DM, and the B end indicates LS alarm.


    According to the problem point, re-do the terminal on the DDF(digital distribution frame), or replace the U-Link, or replace the machine disk, to recovery the circuit.

The another of reason of malfunction

    There is a problem of signal sent from the DM at the A end, that is , the 2Mb/s signal sent out is error code or interrupted.

The method of identification

    If the A end has LS or AIS alarm when the DDF at A end to loop back, it is the malfunction point.


    Ask the professional personnel to restart or reload the relay module to clear the alarm.

2)  The transmission alarm do not occur in pair. For example, the AIS and RJA alarm at the A end at the same time, or LFA alarm occurs at the A end, but the opposite end has no alarm. At this time, the call cannot be made (taking the S1240 switch as an example).

     The reason is that the A-end relay module (DM) died. At this time, the DMs at both ends need to be reactivated. Doing so may restore the circuit, or the first case may occur. If the first situation occurs, you can deal with it according to the method described above.


Fei Chang Technology, the manufacturer focusing on voice multiplexer, fiber transceiver, industrial switch, protocol converter, the self-develop and research brand. Welcome to contact us. The hot line :0571-8700-7140.