In the digital communication transmission system, there are two types of digital fiber multiplexers, one is called “Plesiochronous Digital Hierarchy“, referred to as PDH MUX; the other is called “Synchronous Digital Hierarchy” (Synchronous Digital Hierarchy), referred to as SDH MUX.
The system using the Pseudo-Synchronous Digital Hierarchy (PDH) is to set high-precision clocks on each node of the digital communication network, and the signals of these clocks have a uniform standard rate. Although the accuracy of each clock is high, there are always some small differences. In order to ensure the quality of communication, it is required that the difference between these clocks cannot exceed the specified range. Therefore, this synchronization method is not strictly speaking true synchronization, so it is called “quasi-synchronization”.
In the past telecommunication network, most use PDH MUX equipment. This series has better adaptability to traditional point-to-point communication. With the rapid development of digital communication, there are fewer and fewer direct point-to-point transmissions, and most of the digital transmissions have to be switched. Therefore, the PDH series cannot meet the needs of the development of modern telecommunication services and the management of modern telecommunication networks. need. SDH is a transmission system that emerges to meet this new need.
Common faults of PDH MUX
1. Intermittent failure
①. The alarms of individual 2Mb/s ports are generated instantaneously due to intermittent faults, and the time for the switch relay module DM to recognize the alarms is different, so a single-ended alarm may occur. At this time, the method of dealing with a permanent fault can be used to solve it. But it is best not to use the DDF loopback method, so as not to interrupt the circuit and affect the business.
②. Batch 2Mb/s port alarm
(1) The alarm module is concentrated on the 34Mb/s port or 8Mb/s port of the multiplexing device, etc. This may be a problem with the high-order group, a problem with the DDF adapter, or a problem with the machine disk. At this time, the permanent fault method can be detected.
(2) Several DM alarms for a trunk group, this may be a switch problem. For example, this kind of problem occurred in our bureau once, and it was found that it was 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. A special alarm It is normal to test each 2Mb/s transmission link separately, that is, there is no alarm for each point on the DDF to the switching loop at both ends, and the U-Link is also normal when measured with a multimeter, but the U-Link When the link is connected to the switch, an alarm occurs and cannot be used. At this time, a loop is made on the transmission side of the low-order group end at either end of A and B, and a transmission analyzer is used to test at the same position at the other end. It is found that ES appears, and the entire link is in a critical state.
The processing method is to use the meter to test segment by segment until the obstacle point is found and the circuit is restored.
2. Permanent faults
One end of the loop can be used, 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 alarms appear in pairs. For example, if an RJA (remote alarm) occurs at the A end, the B end may be an LFA (frame out-of-sync alarm) or an AIS (receive alarm), and sometimes both alarms exist; there is also an LS ( signal loss) alarm.
Reason 1: A problem occurs in the transmission of the low-order group from the A end to the B end, resulting in an error or interruption of the 2Mb/s signal.
Judgment method: If there is a problem from A to B between the low-order groups at both ends of A and B, the AIS or LFA alarm will be displayed at the B end. If the low-order group to DM signal from the B side is interrupted, the B side displays an LS alarm.
Solution: According to the detected obstacles, re-do the head on the DDF, replace the U-Link, or replace the machine disk, in order to achieve the purpose of restoring the circuit.
Reason 2: There is a problem with the signal sent by the DM at the A-end, that is, the 2Mb/s signal sent out is erroneous or interrupted.
Judgment method: Use the DDF frame of the A-end to loop back the A-end. If there is an LS or AIS alarm on the A-end, it is a fault.
Solution: Ask the computer room personnel to restart or reload the relay module to clear the alarm.
②. Transmission alarms do not appear in pairs. For example, AIS, RJA alarms, or LFA alarms appear on the A side at the same time, but there is no alarm on the opposite side, and the call cannot be made at this time (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, it can be handled according to the method described above.
Well, the above is the introduction of some related knowledge of PDH. If you want to know more information about PDH, welcome to consult.
