Uncover why program-controlled switches often fall offline

The program-controlled switch is quite common, So I studied why it is prone to disconnection during the use of the program-controlled switch. I would like to share it with you here, hoping it will be useful to you. At the end of 1992, the tianhushan Mining Bureau introduced the SOPHO-S1000 of Time-controlled user telephone switch produced by Philips in the Netherlands. In the early stage, 304 telephones were installed, and 18 trunk lines were purchased from the post office and put into use at the end of the same year.
 
The performance indicators of the program-controlled switch are normal, but in the recent period, the switch experienced a special fault: an external line, especially a remote call from outside the province, is called internally and the switch is automatically dropped, the probability is about 20%, which greatly reduces the success rate of external dialing.

Fault Analysis

The analysis shows that the call may be dropped because the contact resistance of the relay wires may be too large due to oxidation. The author carefully handles each cable connector of the trunk line. Ensure that the contact resistance is minimized, and the signal of each relay is measured normally at the disconnected block position on the side of the programmable switch, but the disconnection phenomenon has not improved.

I speculate whether it is because the caller uses a portrait phone that can only be used for pulse dialing and cannot send dual-audio signals). However, I carefully analyze and query the local post office that is prone to disconnection, it is found that the other party has adopted the program-controlled exchange, and both of them can send dual-audio signals. There is no horizontal phone number, so this speculation is denied. Next, the author analyzes whether the cause of the fault is in the trunk line itself. After careful analysis of the trunk line characteristics and address table, it is found that all the relay are two-way relay. Two-way relay has a fatal weakness, that is, the priority of the two-way relay is not high. During the traffic peak, some phones will be disconnected due to tight relay.

For example, the peak hours are from to on Monday ~ ) Traffic test showed that the ratio of incoming and outgoing relay traffic was about 52%: 48%, and the total number of existing relay lines was 15, therefore, the author sets the ratio of inbound and outbound relay to, that is, 8 is selected as single-Inbound relay, and 7 are used as single-outbound relay. In this way, the overall traffic volume is balanced, it can also maximize the use of existing trunk line resources.

After the number of incoming relay entries is determined, how can the incoming relay be better connected to the switch? Considering that the contact and communication number previously announced by our Bureau is 3982101, if we change the relay number again, it will cause a lot of inconvenience to users. After comprehensive consideration, the author decided to use the 3989201-centered automatic cycle connection and selection method, that is, 3982101-3982108 trunk lines, automatic cycle connection and selection. When an external line calls 3982101, as long as one of the eight relays is idle, the Programmable switch automatically finds and occupies this relay, which greatly facilitates external users. After the solution is determined, the author goes into one system to determine the specific operation steps:

1) notify the Post Office in writing and apply to change all two-way relay to one-way relay, that is, the single-entry relay is: 3982101 ~ 3982108; single-out relay: 3982109 ~ 3982115; and all inbound relay are required to provide the Automatic loop connection and selection function.

2) After the relay feature of the Post Office is modified, modify the route number of the trunk line accordingly. We define the route number as 03. ** the outbound Route number is 00. ** to analyze and compare the relay feature table, we must set the original 03. ** change the single-out relay route to 00.

In this way, the PE of the program-controlled switch project file is modified, so that the program-controlled switch can normally complete the management of the outgoing and incoming switch trunk signaling. After several days of detailed observation, it was found that the automatic disconnection from the external line had improved, but it still occurred from time to time.

Fault Cause judgment

1) First, the analysis considers that the voltage of the trunk line is unstable and may cause a disconnection. We actually test the voltage of the trunk line. The voltage of the trunk line is 48 volts in no-load status, and the voltage is 15 volts after the server is detached, various voltage indicators are normal. The voltage instability of the trunk line is ruled out.

2) Analysis of the duration of disconnection: the duration of each disconnection is about 1 minute. The analysis shows that the caller is unable to receive the polarity reversal signal from the local post office, and is unable to charge fees. The caller automatically disconnects the cable and stops the call, resulting in a disconnection fault. In order to further confirm the above analysis, we can use the card to pull the phone number from our office at a location that is easily disconnected. After 1 minute of the call, the card will be dropped and the card will be taken out without punching holes. We will insert the card again, the balance displayed on the card machine remains unchanged. Verify that the analysis is correct.

3) in order to analyze and determine whether the fault is on a Programmable switch or on a relay line of the Postal Bureau, we directly connect the telephone to the relay line to cut off the connection between the relay line and the Programmable switch. When the external line is switched to a trunk line, the telephone will directly answer the answer without entering the switch. We have detected the following two situations: 1) when the scoring machine is pulled from the outside line, the system immediately picks up the answer and the call is automatically dropped for 60 s. 2) When the extension is called from the external line, wait a few seconds before answering the call, and the call lasts for a long time.

Let's assume that the disconnection is caused by the disconnection response speed. After careful analysis, the PE of the program-controlled switch project file learned that if the program-controlled switch responds too quickly, it is easy to cause slight distortion of the pulse waveform of the Al signal detected by the caller, mainly because of the variation of the pulse width and height), so as to judge that the polarity reversal signal is not detected, after a delay of 30 s ~ 90 s the time set by various postal bureaus is slightly different) automatically released and released. Because the city calls only go through one switch and directly enter the switch, the caller can still completely receive the waveform with slight distortion, detect the A1 signal, start billing information, start billing, and call normally, so there is no disconnection in the city.

However, the distortion of the pulse waveform of the A1 signal increases with the increase of the transmission distance, and the long-distance telephone is mainly for long distance calls outside the province) must be exchanged by multiple Telecom bureaus, if the transmission distance is long, the pulse waveform distortion is serious. If the distorted A1 signal reaches the caller, the billing information cannot be started. The caller determines that the caller is not responding, and the delay is 30 s ~ Release and split the wire in 90 s. So far, we found the real cause of automatic disconnection, that is, the relay false DDI response time is too short, the caller detected the distorted pulse waveform, unable to start billing information, delay release and disconnection.