CA1247206A - Protection switching system for carrier transmission line - Google Patents
Protection switching system for carrier transmission lineInfo
- Publication number
- CA1247206A CA1247206A CA000467452A CA467452A CA1247206A CA 1247206 A CA1247206 A CA 1247206A CA 000467452 A CA000467452 A CA 000467452A CA 467452 A CA467452 A CA 467452A CA 1247206 A CA1247206 A CA 1247206A
- Authority
- CA
- Canada
- Prior art keywords
- line
- stand
- switching
- signal
- station
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
- 230000005540 biological transmission Effects 0.000 title claims abstract description 89
- 230000004224 protection Effects 0.000 title claims abstract description 65
- 238000012423 maintenance Methods 0.000 claims description 3
- 238000012545 processing Methods 0.000 claims description 3
- 238000001514 detection method Methods 0.000 claims 2
- 229910052729 chemical element Inorganic materials 0.000 claims 1
- 230000009850 completed effect Effects 0.000 abstract description 16
- 230000003247 decreasing effect Effects 0.000 abstract description 2
- 238000004891 communication Methods 0.000 description 10
- 230000006854 communication Effects 0.000 description 10
- 238000010586 diagram Methods 0.000 description 10
- 230000015654 memory Effects 0.000 description 7
- 238000000034 method Methods 0.000 description 6
- 230000002159 abnormal effect Effects 0.000 description 5
- 230000008054 signal transmission Effects 0.000 description 4
- 230000008901 benefit Effects 0.000 description 3
- 239000013307 optical fiber Substances 0.000 description 3
- 238000005562 fading Methods 0.000 description 2
- 239000000835 fiber Substances 0.000 description 2
- 238000007667 floating Methods 0.000 description 2
- 241000272470 Circus Species 0.000 description 1
- 208000003251 Pruritus Diseases 0.000 description 1
- 238000013459 approach Methods 0.000 description 1
- 230000015556 catabolic process Effects 0.000 description 1
- 230000007423 decrease Effects 0.000 description 1
- 238000013461 design Methods 0.000 description 1
- 238000005516 engineering process Methods 0.000 description 1
- 238000003780 insertion Methods 0.000 description 1
- 230000037431 insertion Effects 0.000 description 1
- 230000007803 itching Effects 0.000 description 1
- 238000004519 manufacturing process Methods 0.000 description 1
- QSHDDOUJBYECFT-UHFFFAOYSA-N mercury Chemical compound [Hg] QSHDDOUJBYECFT-UHFFFAOYSA-N 0.000 description 1
- 229910052753 mercury Inorganic materials 0.000 description 1
- 238000012544 monitoring process Methods 0.000 description 1
- 230000003287 optical effect Effects 0.000 description 1
- 230000001960 triggered effect Effects 0.000 description 1
Classifications
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
- H04B1/74—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission for increasing reliability, e.g. using redundant or spare channels or apparatus
Abstract
ABSTRACT
This invention provides a line protection switching system of improved reliability and decreased switching time. In order to increase the reliability, the service signals are sent in parallel via several transmission lines, using a service bit area of PCM bit frames. Unlike prior art systems the present system switches a failed transmission line to a stand by lines at a station where a fault is detected before waiting for the switching of the opposite station to be completed. At the same time, the switching command is sent to the opposite station. If the switching of the opposite station fails, the station commands retry of the switching but if it fails again, the station switches the transmission line to the initial state. Since a low priority signal is transmitted through the stand by line, the total system can be economized.
This invention provides a line protection switching system of improved reliability and decreased switching time. In order to increase the reliability, the service signals are sent in parallel via several transmission lines, using a service bit area of PCM bit frames. Unlike prior art systems the present system switches a failed transmission line to a stand by lines at a station where a fault is detected before waiting for the switching of the opposite station to be completed. At the same time, the switching command is sent to the opposite station. If the switching of the opposite station fails, the station commands retry of the switching but if it fails again, the station switches the transmission line to the initial state. Since a low priority signal is transmitted through the stand by line, the total system can be economized.
Description
~7~ 25307-126 The present invention relates to a protection switching system for carrier transmission line, more practically it relates to a method to increase the reliability of the protection system and to utilizing a stand by system as a signal transmission line to economize the carrier transmission system.
It is essential to provide a stand by system to increase the reliability of a carrier transmission system, and when some trouble occurs in the working line, the signals are switched to the stand by system which transmits the signals. Such switching is used very commonly for wireless transmission lines, since fading is inevitable in radio transmission; accordingly, frequency diversity or space diversity systems have been developed. -~ith regard to wired transmission line systems, the problem of fading essentially does not exist and system reliability is increased by increasing the reliability of the equipment or wiring line. How-ever, it is still necessary to provide a stand by system or protection switching system to attain high reliability for such communication systems.
In high reliable communication systems there occurs other problems, one of which is to maintain the reliability of the stand by system or protection switching system itself, otherwise the total reliability of the system can not be attained. One approach has been to provide a parallel system for a signal; or example, at a transmitting terminal the signal is changed to three identical signals and transmitted through three parallel lines, and at a receiving terminal the received three signals are compared to each other and if a diE~erence between them is found, then a major~ty law is adopted. Specifically, if two signals of the three are equal each other then the other is rejected as an error.
Such a system is reliable bu~ expensive, and so it is used only to a limited extent where very high reliability is required. On the other hand the reliability of e~uiprnent in general has improved to a gr2ater extent and it has become prac-tical to provide a single stand by system for several transmission lines. The problem of reliability of the stand by system still remains. Moreover, modern multi-channel transmission systems, such as a light communication system, are expensive and it has become too expensive to leave the stand by system only for the transmission of auxiliary signals which is used for the service or maintenance signals between the stations.
Another problem of the protection swi-tching system is how to reduce the time for switching the signal ~rom a working line to a stand by line when trouble occurs, because the transmis-sion is interrupted during this switchinq time. A "hitless"
system, which means even an instantaneous breakdown of the trans-mission line is not allowed, is a target of modern data transmis-sion lines.
The background of the invention and the invention itself will be described in more detail with reference to the accom-panying drawings, in which:
Figure 1 i5 a block diagram showing a prior art protec-tion switching system for a carrier transmission line;
Figure 2 is a block diagram showing a main part of a prior art swi-tching system;
Figure 3 is a time chart showing the sequence of events in the protection line switching in prior art system, Figure 3a showing a normal sequence of the switching and Figure 3b showing the sequence of the system when the switching has failed by acciden~;
Figure 4 is a block diagram illustrating a portion of a redundant SC signal system of the present invention;
Figure 5 is a chart illustrating a frame structure of a PCM signal;
Figure 6 is a block diagram showing the main part of a second embodiment of the switching system according to the present invention;
Figure 7 is a block diagram of a channel switching controller incorporated in a third embodiment of the present invention; and Figure 8 is a time chart showing the sequence of events in the switching of the invention of Figure 7, Figure 8a showing a ~0 normal sequence of the switching and Figure 8b showing the sequence of the system when the switching has failed by accident.
Before disclosing the present invention, a prior art protection switching system for a carrier transmission line will be described briefly in order to clarify the features and advan-tages of the present invention.
Figure 1 is a block diagram of an exemplary digital multiplex signal transmission line system comprising a protection ~L2g~'7~6 switching system. A multiplexed telephone, television (TV) or data signals SYS-1, SYS-2 and so on a~e transmitted between a terminal station A and a terminal station B (they are called here-inafter STATION A and STATION B respectively) via the transmission lines 11, 12, 13 ---. The transmission line may be a cable, optical fiber cable or any others, and it contains the up (from STATION B to STATION A) and down (from STATION A to STATION B) signal lines. In Figure 1, the signal of SYS-1 is transmitted through a transmission line 12, and the signal of SYS~2 is trans-mitted through the line 13. The switching of these signals isdone in a LP switch (line protection switch) 1 and 2 of each station controlled by channel switching controllers 3 and 4, respectively.
The transmission line 11 is a service line or stand by line, through which is transmitted usually a service signal between the stations, such as the order wire signal for the main-tenance service men between the stations, SC signal (supervisory and control signal) and SV signal (supervisory or surveillance signal) etc. and used for service and order between the stations.
Such signals are called auxiliary signals. Sometimes the system is provided with a redundant SC signal line (not shown) between the switch controllers 3 and 4 of each station.
In the stand by line 11 are inserted in the down word direction (from STATION A to STATION B) a terminal repeater (TR) 7-1-1, repeaters 8-1-1, 9-1-1 and a terminal repeater 10-1-1. And in the up word direction (from STATION B to STATION A) are inserted a terminal repeater 10-1-2, repeaters, 9-1-2, 8-1-2 and a ~ ~ ~t7~ ~ ~
terminal repeater 7-1-2. These components all work altogether for relaying the signals. The configuration of equipment is similar for the working lines 12, 13 and so on. Usually, terminal repeater comprises multip~exer, demultiplexer etc. and sometimes it comprises an electro-optical converter for light transmission systems. Though it is not shown in the figure, the transmission line may further comprise repeaters, amplifiers or repeater stations.
The reference numerals 5 and 6 designate dummy signal generators which send dummy signals to the respective stand by lines. The dummy signal is detected by the terminal repeater of the opposite terminals in order to monitor the transmission char-acteristics of the stand by line. Such dummy signal is especially effective for an optical fiber cable system to maintain the repeaters at their predetermined condition, since the time constant of AGC (automatic gain control) for an optical communica-tion equipment is usually long compared to that of an electronic communication system.
Each receiving terminal repeaters 10-1-1, 10-2-1, 10-3-1, 7-1-2, 7-2-2, t-3-2 and so on is provided with detectors to monitor the input signal level, coding error, bit error or equipment faults, so if trouble or an abnormal situation is detected, the terminal repeater (TR) sends an alarm to respective channel switch controller 3 or 4. Then the channel is switched by the LP switch (line protection switch) 1 or 2 under the control of the channel switch controller 3 or 4. For e~ample if the TR
10-2-1 in the STATION B detects an abnormal, that means a fault ~L;247~
occurring in some part of down signal line 12 (that is a transmis-sion system from TR 7-2-1 through line 12 to T~ l0-1-1), an alarm is sent to the channel switch controller ~CSC) 4. The CSC 4 sends a command to the CSC 3 in STATION A to switch the working line 12 to the stand by line 11. This command is sent through the stand by line 11 or redundant SC signal line (not shown). In some recent systems the command is sent via upward working lines 12, 13 and so on using their idle channel or idle bits.
Receiving the command, the CSC 3 in the STATION A
controls the LP switch 1 to switch the SYS-1 from line 12 to line 11, and when the switching is completed the CSC 3 sends the signal that the switching is completed to the CSC 4, via the redundant SC
signal line (not shown) or idle channel or idle bits of downward working lines (lines 11, 13 and so on). When the signal is received by the CSC 4, it controls the LP switch 2 to switch the SYS-1 from the line 12 to the stand by line 11.
Thus the system switching is Einished. Then the dummy signal 5 and 6 are sent through the line 12 by the command of CSC
3 and 4, and the rehabilitation work of the line 12 is monitored by checking the received dummy signal at the TR 10-2-1 or 7-2-2.
Generally, a single stand by line is enough for several working lines. However, depending on the reliability required on the system, the number of stand by line is increased. Usually, when the fault line (line 12 in this example) is repaired, the system is again switched to the former line 12, and the line 11 is reserved as a stand by line. Such a system is called a fixed protection line system. On the contrary, a system called a 7Z~
floating protection line system keeps the restored line (line 12 in this example) as a new stand by line, and ~ses the former stand by line (line 11) as a working line. Usually, the fixe~ protec-tion line system is used.
As mentioned before, one of the problems of the prior art is the reliability oE the stand by system because, if -the stand by system fails, the system cannot work. Other problems will be described referring to Figures 2 and 3. In these figures like reference numerals and characters designate like or similar parts of Figure 1.
Figure 2 is a block diagram showing a main part of a prior art switching system. The figure shows only the downward system. In practice a similar circu t for the upward line is used forming a pair with the downward line. The explanation will be given assuming a single stand by and fixed protection line system.
l'he switching elements 1~ 2, 1-3 --- and 2-1, 2-2, 2-3 --- are composed of a mercury relay switch for example. R1, R2, R3 ---are resistors having the same resistance to the line impedance (usually 75 Ohms) to terminate the transmission line. In the example of Figure 2, the dummy signal generated by the dummy signal generator 5 is transmitted from the STATION A to STATION B
via the line 11 (stand by line), and terminated by the resistor R1 .
If trouble occurs in the working line 12, the signal o the SYS-1 is switched to the stand by line 11 in the manner described before. In this case, the contact points of the ~.2~
switching elements 1~ 2, 2-1 and 2-2 shift from the position shown by full lines to the position shown by broken lines, following the command of the CSC 3 and 4. Then, as can be seen in the figure, the signals of SYS-1 are switched Erom transmission line 12 to 11, and the dummy signal is switched from the stand by line 11 to the failed line 12 and terminated by the resistor R2.
By analyzing the dummy signal received by TR 10-2-1, the fault point is detected and repaired.
When the trouble is corrected~ the working line is shifted to the line 12 and the line 11 is kept again as a stand by line. In such a system the terminating resistors R1, R2 and so on are wired into the switching elements 2-1, 2-2 and so on. There-fore, the signal arriving at the STATION s via the stand by line 11 cannot be taken out from the switching elements 2. ~ccordingly it is not possible to use the stand by line as a signal transmis-sion line.
Recently, the transmission line systems have become expensive as they are highly multiplexed and complicated. In such a system, the stand by system is almost kept in an idle state, so long as a fault does not occur and a working line is not switched to the stand by line. There arose a desire, therefore, to utilize the stand by line as a working line. Moreover, as can be seen in Figure 2, the switching elements for the STATION A (1-l, 1-2 etc.) must be a different type from that of STATION B (2-1, 2-2 etc~) t it is not desirable to provide and stock different type of parts from the view point of economy.
Another problem of the prior art is the switching time.
~7'Z~
Figure 3 is a time chart showing the sequence of tne protection line switching in the prior art sys-tem. In the figure, (a) shows a normal sequence of the switching and (b) shows the sequence of the system when the switching has failed by accident. In the Figure 3, time flow is shown from up to downward in the figure, and the vertical lines show the se~uence of switching which is performed in STATION A and STATION B respectively.
Referring to Figure 1 and 3, suppose if the terminal re~eater (TR) 7-2-2 in STATION ~ detected an abnormal and sends an alarm to the CSC 3, this triggers the switching of the line 12 to the line 11. As mentioned be~ore, the switching command is sent from the STATION A to the STATION B, and thus takes about 5 ms for the command to reach STATION B. Then the CSC 4 commands the LP
switch 2 to switch the line 12 to the line 11. ~t takes about
It is essential to provide a stand by system to increase the reliability of a carrier transmission system, and when some trouble occurs in the working line, the signals are switched to the stand by system which transmits the signals. Such switching is used very commonly for wireless transmission lines, since fading is inevitable in radio transmission; accordingly, frequency diversity or space diversity systems have been developed. -~ith regard to wired transmission line systems, the problem of fading essentially does not exist and system reliability is increased by increasing the reliability of the equipment or wiring line. How-ever, it is still necessary to provide a stand by system or protection switching system to attain high reliability for such communication systems.
In high reliable communication systems there occurs other problems, one of which is to maintain the reliability of the stand by system or protection switching system itself, otherwise the total reliability of the system can not be attained. One approach has been to provide a parallel system for a signal; or example, at a transmitting terminal the signal is changed to three identical signals and transmitted through three parallel lines, and at a receiving terminal the received three signals are compared to each other and if a diE~erence between them is found, then a major~ty law is adopted. Specifically, if two signals of the three are equal each other then the other is rejected as an error.
Such a system is reliable bu~ expensive, and so it is used only to a limited extent where very high reliability is required. On the other hand the reliability of e~uiprnent in general has improved to a gr2ater extent and it has become prac-tical to provide a single stand by system for several transmission lines. The problem of reliability of the stand by system still remains. Moreover, modern multi-channel transmission systems, such as a light communication system, are expensive and it has become too expensive to leave the stand by system only for the transmission of auxiliary signals which is used for the service or maintenance signals between the stations.
Another problem of the protection swi-tching system is how to reduce the time for switching the signal ~rom a working line to a stand by line when trouble occurs, because the transmis-sion is interrupted during this switchinq time. A "hitless"
system, which means even an instantaneous breakdown of the trans-mission line is not allowed, is a target of modern data transmis-sion lines.
The background of the invention and the invention itself will be described in more detail with reference to the accom-panying drawings, in which:
Figure 1 i5 a block diagram showing a prior art protec-tion switching system for a carrier transmission line;
Figure 2 is a block diagram showing a main part of a prior art swi-tching system;
Figure 3 is a time chart showing the sequence of events in the protection line switching in prior art system, Figure 3a showing a normal sequence of the switching and Figure 3b showing the sequence of the system when the switching has failed by acciden~;
Figure 4 is a block diagram illustrating a portion of a redundant SC signal system of the present invention;
Figure 5 is a chart illustrating a frame structure of a PCM signal;
Figure 6 is a block diagram showing the main part of a second embodiment of the switching system according to the present invention;
Figure 7 is a block diagram of a channel switching controller incorporated in a third embodiment of the present invention; and Figure 8 is a time chart showing the sequence of events in the switching of the invention of Figure 7, Figure 8a showing a ~0 normal sequence of the switching and Figure 8b showing the sequence of the system when the switching has failed by accident.
Before disclosing the present invention, a prior art protection switching system for a carrier transmission line will be described briefly in order to clarify the features and advan-tages of the present invention.
Figure 1 is a block diagram of an exemplary digital multiplex signal transmission line system comprising a protection ~L2g~'7~6 switching system. A multiplexed telephone, television (TV) or data signals SYS-1, SYS-2 and so on a~e transmitted between a terminal station A and a terminal station B (they are called here-inafter STATION A and STATION B respectively) via the transmission lines 11, 12, 13 ---. The transmission line may be a cable, optical fiber cable or any others, and it contains the up (from STATION B to STATION A) and down (from STATION A to STATION B) signal lines. In Figure 1, the signal of SYS-1 is transmitted through a transmission line 12, and the signal of SYS~2 is trans-mitted through the line 13. The switching of these signals isdone in a LP switch (line protection switch) 1 and 2 of each station controlled by channel switching controllers 3 and 4, respectively.
The transmission line 11 is a service line or stand by line, through which is transmitted usually a service signal between the stations, such as the order wire signal for the main-tenance service men between the stations, SC signal (supervisory and control signal) and SV signal (supervisory or surveillance signal) etc. and used for service and order between the stations.
Such signals are called auxiliary signals. Sometimes the system is provided with a redundant SC signal line (not shown) between the switch controllers 3 and 4 of each station.
In the stand by line 11 are inserted in the down word direction (from STATION A to STATION B) a terminal repeater (TR) 7-1-1, repeaters 8-1-1, 9-1-1 and a terminal repeater 10-1-1. And in the up word direction (from STATION B to STATION A) are inserted a terminal repeater 10-1-2, repeaters, 9-1-2, 8-1-2 and a ~ ~ ~t7~ ~ ~
terminal repeater 7-1-2. These components all work altogether for relaying the signals. The configuration of equipment is similar for the working lines 12, 13 and so on. Usually, terminal repeater comprises multip~exer, demultiplexer etc. and sometimes it comprises an electro-optical converter for light transmission systems. Though it is not shown in the figure, the transmission line may further comprise repeaters, amplifiers or repeater stations.
The reference numerals 5 and 6 designate dummy signal generators which send dummy signals to the respective stand by lines. The dummy signal is detected by the terminal repeater of the opposite terminals in order to monitor the transmission char-acteristics of the stand by line. Such dummy signal is especially effective for an optical fiber cable system to maintain the repeaters at their predetermined condition, since the time constant of AGC (automatic gain control) for an optical communica-tion equipment is usually long compared to that of an electronic communication system.
Each receiving terminal repeaters 10-1-1, 10-2-1, 10-3-1, 7-1-2, 7-2-2, t-3-2 and so on is provided with detectors to monitor the input signal level, coding error, bit error or equipment faults, so if trouble or an abnormal situation is detected, the terminal repeater (TR) sends an alarm to respective channel switch controller 3 or 4. Then the channel is switched by the LP switch (line protection switch) 1 or 2 under the control of the channel switch controller 3 or 4. For e~ample if the TR
10-2-1 in the STATION B detects an abnormal, that means a fault ~L;247~
occurring in some part of down signal line 12 (that is a transmis-sion system from TR 7-2-1 through line 12 to T~ l0-1-1), an alarm is sent to the channel switch controller ~CSC) 4. The CSC 4 sends a command to the CSC 3 in STATION A to switch the working line 12 to the stand by line 11. This command is sent through the stand by line 11 or redundant SC signal line (not shown). In some recent systems the command is sent via upward working lines 12, 13 and so on using their idle channel or idle bits.
Receiving the command, the CSC 3 in the STATION A
controls the LP switch 1 to switch the SYS-1 from line 12 to line 11, and when the switching is completed the CSC 3 sends the signal that the switching is completed to the CSC 4, via the redundant SC
signal line (not shown) or idle channel or idle bits of downward working lines (lines 11, 13 and so on). When the signal is received by the CSC 4, it controls the LP switch 2 to switch the SYS-1 from the line 12 to the stand by line 11.
Thus the system switching is Einished. Then the dummy signal 5 and 6 are sent through the line 12 by the command of CSC
3 and 4, and the rehabilitation work of the line 12 is monitored by checking the received dummy signal at the TR 10-2-1 or 7-2-2.
Generally, a single stand by line is enough for several working lines. However, depending on the reliability required on the system, the number of stand by line is increased. Usually, when the fault line (line 12 in this example) is repaired, the system is again switched to the former line 12, and the line 11 is reserved as a stand by line. Such a system is called a fixed protection line system. On the contrary, a system called a 7Z~
floating protection line system keeps the restored line (line 12 in this example) as a new stand by line, and ~ses the former stand by line (line 11) as a working line. Usually, the fixe~ protec-tion line system is used.
As mentioned before, one of the problems of the prior art is the reliability oE the stand by system because, if -the stand by system fails, the system cannot work. Other problems will be described referring to Figures 2 and 3. In these figures like reference numerals and characters designate like or similar parts of Figure 1.
Figure 2 is a block diagram showing a main part of a prior art switching system. The figure shows only the downward system. In practice a similar circu t for the upward line is used forming a pair with the downward line. The explanation will be given assuming a single stand by and fixed protection line system.
l'he switching elements 1~ 2, 1-3 --- and 2-1, 2-2, 2-3 --- are composed of a mercury relay switch for example. R1, R2, R3 ---are resistors having the same resistance to the line impedance (usually 75 Ohms) to terminate the transmission line. In the example of Figure 2, the dummy signal generated by the dummy signal generator 5 is transmitted from the STATION A to STATION B
via the line 11 (stand by line), and terminated by the resistor R1 .
If trouble occurs in the working line 12, the signal o the SYS-1 is switched to the stand by line 11 in the manner described before. In this case, the contact points of the ~.2~
switching elements 1~ 2, 2-1 and 2-2 shift from the position shown by full lines to the position shown by broken lines, following the command of the CSC 3 and 4. Then, as can be seen in the figure, the signals of SYS-1 are switched Erom transmission line 12 to 11, and the dummy signal is switched from the stand by line 11 to the failed line 12 and terminated by the resistor R2.
By analyzing the dummy signal received by TR 10-2-1, the fault point is detected and repaired.
When the trouble is corrected~ the working line is shifted to the line 12 and the line 11 is kept again as a stand by line. In such a system the terminating resistors R1, R2 and so on are wired into the switching elements 2-1, 2-2 and so on. There-fore, the signal arriving at the STATION s via the stand by line 11 cannot be taken out from the switching elements 2. ~ccordingly it is not possible to use the stand by line as a signal transmis-sion line.
Recently, the transmission line systems have become expensive as they are highly multiplexed and complicated. In such a system, the stand by system is almost kept in an idle state, so long as a fault does not occur and a working line is not switched to the stand by line. There arose a desire, therefore, to utilize the stand by line as a working line. Moreover, as can be seen in Figure 2, the switching elements for the STATION A (1-l, 1-2 etc.) must be a different type from that of STATION B (2-1, 2-2 etc~) t it is not desirable to provide and stock different type of parts from the view point of economy.
Another problem of the prior art is the switching time.
~7'Z~
Figure 3 is a time chart showing the sequence of tne protection line switching in the prior art sys-tem. In the figure, (a) shows a normal sequence of the switching and (b) shows the sequence of the system when the switching has failed by accident. In the Figure 3, time flow is shown from up to downward in the figure, and the vertical lines show the se~uence of switching which is performed in STATION A and STATION B respectively.
Referring to Figure 1 and 3, suppose if the terminal re~eater (TR) 7-2-2 in STATION ~ detected an abnormal and sends an alarm to the CSC 3, this triggers the switching of the line 12 to the line 11. As mentioned be~ore, the switching command is sent from the STATION A to the STATION B, and thus takes about 5 ms for the command to reach STATION B. Then the CSC 4 commands the LP
switch 2 to switch the line 12 to the line 11. ~t takes about
2 ms ~or this switching. When the switching in STATION ~ is complet~d, the CSC 4 sends a signal to the STATION A that the switching is completed. It takes again about 5 ms for the signal to reach STATION A. After receiving this signal, the CSC 3 commands the lP switch 1 to switch the signal SYS-1 from the line 12 to line 11, thus completing the system switching. But it takes again about 2 ms for this switching in the STATION A. Thus, it takes altogether approximately 14 ms for the system to switch from a working line (line 12 in this example) to the stand by line 11.
In a rare case the STATI~N B fails to switch the line.
Figure 3(b) shows the sequence of events in such case. The trig-gering of the switching is similar to that of Figure 3(a), but in STATION B when the CSC 4 is signaled by the LP switch 2 that the z~
switching has failed by some failure of equipment or parts, the CSC 4 sends a signal that the switching has faile~ to the STATIOI~
A, the signal indicating the s~itching situation. Then the CSC 3 sends a command to retry the switching to the STATION ~. By this command the STATION B tries again the switching. If the switching is successful, the following steps are the same as in Figure 3(a), and in this case it will be understood that it takes about 26 ms for the switching. However, if the switching has failed again, the CSC 4 sends a signal that the switching has failed, and when this signal is received, the CSC 3 stops the switching and sends an alarm to the operator the situation as a matter of importance.
As has been described, it is important to reduce the switching time to reduce the down time of the system.
An object of the present invention, therefore, is to increase the reliability of a communication system by increasing the reliability of the protection switching system.
Another object of the present invention is to economize the transmlssion system by utilizing a stand by system as a trans-mission line.
~0 Further object of the present invention i5 to reduce the switching time of a communication system from a working line to a stand by line.
In order to increase the reliability o~ the stand by system, the SC signal (supervisory and control signal) of the system is transmitted not only through the stand by line, but also through the working lines in parallel, so that, even if the stand by line has failed, the SC signal is transmitted so long as a z~i single working lines is alive, therefore, the system control is never lost.
According to the present invention, the stand by system is not kept in an idle state but it is used for a transmission of lower priority signals. At the same time the switching elements for the channel switching is standardized which is economical both rom the view points of manufacturing and stocking oE the elements.
In order to reduce the switching time, the system of the present invention switches the line at the station where the failure is detected, without waiting for the switching of the opposite station, and at the same time it commands the opposite station to switch the line. Therefore, the switching time is reduced to about a half of that of the prior ar~ systems.
As mentioned before it is important to attain a high reliability of the service or control signal system of a communi-cation system, otherwise the system will lose control and a high reliability of the total system can not be attained. In other words, the system reliability cannot be higher than the relia-~0 bility of the service and control line system.
In order to attain high reliability of the service andcontrol line, the present invention intends to transmit an auxi-liary signal or the service signals, that is the order wire signals, SC signals (supervisory and control signals) and S~
signals (supervisory or surveillance signals) both through the 7;~36 standby line and working lines in parallel; therefore, so long as at least one line is working normally, the system never loses control. By sending the service signals through the working line, each working line loses its channels or bit numbers, but as will be described later, the loss of the channel or bit is very small, or is not lost at all. ~oreover, the present invention uses the stand by line for transmitting a signal of lower priority, so the total channel or bits transmitted through the system is much larger than the prior art system, in which the stand by line was kept idle except transmitting the service signals.
The invention may be summarized as a protection switch-ing system for a digital carrier transmission line transmitting pulse code modulation bit frames having an auxiliary bit area, comprising; terminal stations, each including a selector for receiving auxiliary signals and means for detecting trouble on the digital carrier transmission line; at least one main trans-mission line, operatively connected to said terminal stations, for transmitting main signals between said terminal stations;
means for detecting trouble on said at least one main transmis-sion line; and at least one stand by line, operatively connected to said terminal stations, for transmitting the main signal when said means detects trouble on said at least one main transmis-sion line, the auxiliary signals between said terminal stations being transmitted in parallel using at least two lines among said at least one main transmission line and said at least one stand by line, utilizing the auxiliary bit area of the puIse code modu-~2~7,'~6~6 -12a- 23507-126 lation bit frames, and the selector selecting the auxiliary signals received over one of said stand by line and said rnain transmission line which is operating normally.
Another advantage of the present invention is -that the switching time of the system from a working line to a stand by line when trouble occurs is reduced. These features and advan-tages will be disclosed in referring to some preferred embodi-ments.
A first embodiment involves sending the service signals both through the stand by line and the working lines. The block diagram of the total system is similar to that of Figure 1 but the channel switching controller (CSC) is improved. Figure 4 shows the CSC of the present invention illustrating the structure of the portion relevant to the invention. The figure shows only the portion concerning the down signal line~ as similar equipment is used for the up signal line forming a pair system.
Figure ~ is a part of a system equivalent to that of Figure 1. The syste~ links the STATION A and STATION B with transmission lines 11, 12 --- and ln, altogether n transmission iL7 2,53 ~;
lines. Each line includes repeaters, terminal repeaters (TR), cables or optical fiber cables and 50 on in a manner similar to that of Figure 1. In this example, the transmission line ln is used as a stand by ]ine. Each station A and B is provided with a LP switch 1 and 2 respectively, and each transmission line is terminated respectively to TRs 7-1-1, 7-1-2 --- of STATION A and TRs 10-1-1, 10-1-2 --- of STATION B. These configurations are all similar to that o~ Figure 1. If necessary, the stand by line ln may be used as a working line to transmit signals, though the system reliability decreases because there is no stand by line.
Unlike the system of Figure 1, the transmission lines 11, 12 --- carry the service signals and, as shown in Figure 4, the service signals are fed in parallel to all transmission lines through the service signal lines 21-1, 21-2, --- 21-n in LP switch (line protection switch) 1. The method oE inserting the auxiliary or service signal into or separate from the main signals of each line will be disclosed later. At the receiving station (STATION B
in the example of Figure 4) these auxiliary or service signals are separated and sent respectively to the terminals of a selection ~0 switch SW of a selector 33, via the service signal lines 23-1, 23-1 --- 23-n. As mentioned with respect to Figure 1, each TR of both stations is provided with detectors to watch the input signal level, coding error, bit error or equipment faults, and if a fault or abnormal situation is detected, the TR sends an alarm to a selector controller 34 via the alarm signal lines 22-1, 22-1, ---22-n. The selector 33 under the control of the selector controller 34, selects the contact o~ the selection switch SW
72~6 where there is no alarm, so the SC signal can be transmi-tted to STATION B so long as at least one transmission line is alive.
The system of Figure 4 is provided with further means to increase the reliability of the service signal. As shown in the figure, the SC signal is further precisely monitored by an error checker 35, which is o~erated automatically or manually, and though there is no alarm signal, the SV signals received by each TR are compared, and the ~elector is controlled to connect to the best transmission line. Therefore, the STATION B always receives the service signal at the best condition of the transmission line.
The switching action of the system from a working line to the stand by line when a fault or abnormal situation occurs, is quite similar to that of the prior art system described with respect to Figure 1.
The manner in which the service signals (denoted as SC
signals hereinafter) are inserted into or dropped from the main signals in each TR is a well known and widely used procedure in the art of communication, and so the circuit and the method of ~0 drop/insertion will not he described herein. Howe~er, this tech-nique is disclosed in detail in for example, "405 Mb/s SINGLE-MODE
FIBER TRANSMISSION SYSTEM", 1984 IEEE International Conference on Communications; or "DC-400M PROTECTION SWITCHING E~UIPMENT USING
MICROPROCESSOR", FSTJ. Vol. 16, No. 4, 1980 Dec. to which refer-ence may be made, if desired.
Figure 5 shows the frame structure of a PCM (pulse code modulation) signal. The ~igure shows a bit train of a 405 Mbit ~Z~7~~$
PCM signal as an example. The coded input signal is arranged in a frame of 4,608 bits as shown in the figure, and transmitted with a rate of ~8,000 frames per second. ~ach frame is further divided into eight sub-frames of 576 bits. At the head of each frame and sub-frame are provided respectively frame bits F or service bits S1, S2 --- as shown in the figure. Bit numbers of the frame bit and service bit depends on the system design; usually 9 bits are allotted. The frame bit F is used to indicate the beginning of the frame, and sometimes an inverse frame bit F (each bit of which is inverse to that of F) is inserted as shown in Figure 5, to check an error of the frame bit itself. The service bits are used to transmit signals between stations to consult each other, or remote checking of the conditions of repeaters or equipment of each station.
Consider as an example, a signal transmitted by a bit of the frame shown in Figure 5. It corresponds to ~8 Kbit/sec which is equivalent to more than a channel of a telephone signal, so it is enough to allot few bits per frame for the SC signals. Usually one of the service bits denoted H in the figure is used for trans-~0 mission of the SC signal. Therefore, as mentioned before, thechannels of bits in the main signal are never lost by transmitting the SC signal in parallel through the working lines. In the rare case which has no service bit allotted in the frame, the allott-ment of few bits in each frame means loss of less than one percent (for the case of Figure 5, it is 9/4,608).
In a second embodiment of the inventlon the line switching equipment is improved to take out a signal from a term-~ ~7~
inating resistor, in order to ~ansmit not only the S~ signal but also the main signal through a stand by line. Figure 6 shows a main par~ of the switching equipment in an LP switch improved by the present invention~ The figure shows only the downward line.
Like the case of Figure 2, a similar circuit for the upward line is used forming a pair with those of the downward line. The dis-closure will be done assuming a single stand by and a fixed pro-tection line system.
In Figure 6, like or similar reference characters or numerals designate same or corresponding parts of Figure 2. Un-like the prior art switch shown in Figure 2, the terminating resistors R1,R2--- are taken out from the switching equipment and a termination resistor Ro is connected to an output terminal B of the switching element 2'-1 of the stand by line 11. The numeral 2' designates a switching equipment of the receiving side, which is identical to that used in the sending side swi~ch. Therefore, the signal of the stand by line 11 can be taken out from the out-put terminal ~.
In Figure 6, a dummy signal generated by a dummy signal generator 5 is transmitted through the stand by line 11 and term-inated by the terminal resistor Ro. If a fault occurs in the transmission line 12, for example, the contact points of the switching elements 1'-1, 1'-2, ---, and 2'-l, 2'-2, --- are shifted from the position of the solid line to that of the broken line. Thus in the STATION A the signal of S~S-l is switched from the working line 12 to the stand by line 11, and the dummy signal is switched from the stand by line 11 to the failed working line 7'~
12. At the sam~ time in the STP~TION B, the signal received by the terminal repeater (TR) 10-l-l is switched to the output of the SYS-l, and the dummy signal received by TR 10-2-l is terminated by the termination resis~or Ro. This switchiny is done under the control of the channel switching controller (not shown) in the same way as described with respect to Figure 1.
The failure point is detected by analyzing the dummy signal received at SrrATION B, and the rehabilitation oE the failed line 12 is performed by monitoring the dummy signal. When the failure is repaired, the system is switched again to transmit the signal of SYS-l through the line 12, and the line 11 is kept as a stand by line.
As mentioned before, the stand by line 11 may be used for transmitting a signal having a lower priority, such as a time-signal, weather forecast etc. which are transmitted at a fixed time. The priority of the signal is determined beforehand and, at the transmission stations these low priority signals are switched manually or automatically under the command of the channel switching controller and are transmitted through the stand by ~0 line. In STATION ~ the low priority signal is fed to the input terminal A, and taken out from the output terminal B in sTArrIoN
B.
In such a manner, the stand by line can be used not only for the SC signals but also for some low priority signal trans-mission. Thus the utilization rate of the system is increased.
Moreover, since the switching elements for both upward and down-ward LP switches are standardized it is economical for manu-k~,';'2~
facturing and stocking the switching element.
The above description has been carried out with respectto a fixed protection line system, but it will be easy for those skilled in the art to apply this to a floating protection line system.
A third embodiment involves decreasing the switching time to switch a main signal from a working line to a stand by line when a fault occurs in the working line. To do so, the system switches the main signal at first in a station where the fault is detected, without waiting for the completion of switching in the remote station. At the same time a switching command is sent to the remote station, so that the switching time is reduced to about half of the prior art switching time.
Figure 7 is a block diagram of a channel switching con-troller of the present invention, which corresponds to that des-ignated by reference numerals 3 or 4 in Figure 1. The block dia-gram of Figure 7 shows the equipment for the STATION A. As a practical matter, the STATION B is provided with a similar equip-ment and they work as a pair. The system configuration is similar ~0 tG that of Figure 1, except for the channel switching controllers.
The sequence of switching will be described mainly with respect to the operations in the STATION A referring to Figures. 1 and 7. In a manner similar to the foregoing examples, the main signals SYS-l, SYS-2 --- are transmitted through the transmission lines 12, 13 --- respectively, and the line 11 is a stand by line.
In Figure 7, reference numeral 110 designates a CPU
(central processing unit) which controls the channel switching ., ~.2~7Z~ç~
controller, 111 is a RO~ (read only memory) whi.ch stores the pro-grams for switching, and 112 is a decoder which assigns the add-resses corresponding to the signal sent from the CPU 110 via an address bus 123. The CPU 110 sends a command to the decoder 112 to assign the address of each buffer memories 115 and 116 to read out the memories stored in them in proper timing, so that the memories are taken out successively to the data bus 124. When the system is operating in a normal state, the memory of ~he bufer memories 114 and 115 are indicating tha~ the state is in normal state, and the CPU 110 recognizes this fact.
A trigger detector 120 collects a trigger signal from each TR (terminal repeater). Suppose that the terminal repeater (TR) 7-2-2 in STATION A has detected a fault and sends a trigger signal, the trigger detector 120 of the channel switching con-troller 3 discrimlnates which TR is sending the trigger signal, and stores the data in the buffer circuit 116. The CPU 110 reads this signal via the buffer amplifier 11~ and a data bus 124.
Recognizing that a fault has occurred, the CPU 110 takes out a switching program stored in the ROM 111. The switching program 2n comprises a program for its own station (STATION A) and for the opposite station (STATION B), and the former is latched in a latch circuit 114 and the latter is latched in a latch circuit 113.
The latched program in the latch circuit 114 i5 sent to the LP switch 1 of the STATION A, and commands it to switch the signal of SYS-l from the transmission line 12 to a stand by line 11. When the switching is completed, the status monitor 119 sends a signal that the transmission line is switched and by this signal 7~
the m~mory in the buffer circult 115 is altered, and the CPU 110 recognizes it through the data bus 124.
Meanwhile, the swi-tching program latched in the latch circuit 113 is converted to a serial signal by a P/S converter (parallel to serial converter) 117 and sent out to the channel switching controller 4 of the opposite station (STATION B) by a line driver 121 via a SC signal line (which is the stand by line 11 in this example).
In the opposite station (STATION B), the sa~e equipment as shown in Fig. 7 is provided, and so the description hereinafter will refer to the equipment shown in Fig. 7, though the description concerns STATION B. The switching command from the STATION A is received by a line receiver 122 and converted to a parallel signal by a S/P converter (serial to parallel converter) 118 and sent to the CPU 110 via the data bus 124.
CPU 110 of the STATION B takes out a switching program stored in the ROM 111 and latches it in the latch circuit 114, and commands the LP switch 4 of STATION B to switch the S~S-l from the line 12 to the stand by line 11. When the switching is completed, ~0 the CPU 110 recognizes this fact through the status monitor 119 and a bufEer memory 115, and takes out a signal stored in the ROM
111 indicating that the switching is completed. It should be pointed out that at this instant the SYS-l is switched from the failed transmission line 12 to a stand by line 11, and the trans-mission of the main signal is recovered. The following sequences are only to certificate between each STATIONS A and B that the switching is completed. This signal is latched by the latch - 2~ -~2~72~3~
circuit 113, and sent to the STATION A by the line driver 121 via SC signal line, after being converted to a serial signal by the P/S converter 117. Though it is not shown in the figure, the SC
signal line is kept always in good condition applying the system described with respect to the embodiments 1 or 2 of the present invention. In STATION A, the signal from STATION B indica-ting that the switching is completed is received by the line receiver 122, and converted to a parallel signal by the S/P converter 118 and sent to the CPU 110 via the data bus 124. The se~uence is completed by recognition of the state by CPU 110.
How the time for protection line switching is reduced by the above sequence will be explained by referring to Figure 8, which shows the time sequence of the switching. The time flow is shown from up to downward in the figure, and vertical lines in the figure show the sequence which is performed in STATION A and STATION B respectively.
Figure 8(a) shows the case wherein the switching is com-pleted successfully. Suppose that in the STATION A the triggering of the switching has occurred. The CPU 110 of the STATION A com-mands the LP switch 1 of own station to switch the SYS-l to the stand by line, and at the same time the command is sent to the STATION B, it will take approximately 5 ms for the command signal to reach STATION B and the switching in each station takes approx-imately 2 ms. As soon as the switching in the STATIOW A is com-pleted, the main signal is sent to the STATION ~ via the stand by line (shown by the sloping broken line)~ In STATION B the switching is finished in approximately 2 ms, so the STATION B is ~Z~'7;2~
ready to receive the main signal when it arrives, and so the total switching time is approximately 7 ms. The following sequence is to certify between each station that the switching is completed successfully. The STATION B sends a signal to STATION A that the switching is completed. It takes a further 5 ms for STATION A to recognize the switching is performed correctly. Compared to Figure 3(a), the switching time is reduced effectively to about half of the prior system.
Figure 8(b) shows the sequence oE a very rare case wherein the switching has failed due to a failure of equipment or parts of the system. In the STATION A the switching is trig-gered,and in the way as described with respect to Figure 8(a), STATION A switches its LP switch and at the same time a switching command is sent to the STATION B. Receiving this command, the CPU
110 of the STATION B commands the switching of the LP switch.
However, if the switching has not been done correctly, due to the failure of equipment, the status monitor 119 detects it and sends a fail signal to the buffer circuit 115, and the CPU 110 recog-niæes that the switching was not done correctly. Then the CPU 110 selects a fail signal out of the ROM 111. This fail signal is sent to the STATION A via the latch circuit 113, the P/S converter 117, the line driver 121, and the SC signal line.
Meanwhile, the main sigr~al is sent from the STATION A to the STATION B, but the STATION B cannot receive the signal. In the STATION A the fail si~nal is received by the line receiver 122 and sent to the CPU via the S/P converter 118 and the data bus 124. The CPU 110 of the STATION A recognizes that the switching ~%~.7~2~6 has not been completed in a correct manner, and then the CPU
selects a retry command out of the ROM 111, and sends it to the STATION B through the latch circuit 113, the P/S converter 117, and the line driver 121 via the SC signal line. ~eceiving this signal, the CPU of the 5TATION B selects a retry pro~ram out of the ROM 111, and commands the switching of the LP switch again like a similar manner described above.
When the switching is completed correctly, the following sequences are the same as that of the Figure 8 (A), but if the switching has failed again, the CPU of the STAT~ON B recognizes it, and sends a fail signal again to the CPU of the STATION A in a similar manner as described above. The CPU of the STATION A rec-ognizes that the switching has failed again and it then selects a pro~ram to go back to the initial state from the ROM 111, and commands the LP switch to go back to the initial state. The system then goes back to the initial state and stops and at the same time, the CPU sends an alarm to the operator.
As has been described above, the present invention provides a hi~her reliability of the protection switching system and reduces 2~ the switching time for the protection line. Moreover, it eco-nomizes the transmission system utilizing a stand by system for transmission of main signals.
The invention has been described above essentially in terms of the process sequences. All the equipment and components referred to are all conventional for transmission technology, and so the structural and performance details of this equipment have omitted for simplicity.
~ ~ Z~7z~6 It will be clear that the present invention can be applied not only for cable transmission lines or opt.ical fiber cable lines but it can be applied to radio transmission lines.
In a rare case the STATI~N B fails to switch the line.
Figure 3(b) shows the sequence of events in such case. The trig-gering of the switching is similar to that of Figure 3(a), but in STATION B when the CSC 4 is signaled by the LP switch 2 that the z~
switching has failed by some failure of equipment or parts, the CSC 4 sends a signal that the switching has faile~ to the STATIOI~
A, the signal indicating the s~itching situation. Then the CSC 3 sends a command to retry the switching to the STATION ~. By this command the STATION B tries again the switching. If the switching is successful, the following steps are the same as in Figure 3(a), and in this case it will be understood that it takes about 26 ms for the switching. However, if the switching has failed again, the CSC 4 sends a signal that the switching has failed, and when this signal is received, the CSC 3 stops the switching and sends an alarm to the operator the situation as a matter of importance.
As has been described, it is important to reduce the switching time to reduce the down time of the system.
An object of the present invention, therefore, is to increase the reliability of a communication system by increasing the reliability of the protection switching system.
Another object of the present invention is to economize the transmlssion system by utilizing a stand by system as a trans-mission line.
~0 Further object of the present invention i5 to reduce the switching time of a communication system from a working line to a stand by line.
In order to increase the reliability o~ the stand by system, the SC signal (supervisory and control signal) of the system is transmitted not only through the stand by line, but also through the working lines in parallel, so that, even if the stand by line has failed, the SC signal is transmitted so long as a z~i single working lines is alive, therefore, the system control is never lost.
According to the present invention, the stand by system is not kept in an idle state but it is used for a transmission of lower priority signals. At the same time the switching elements for the channel switching is standardized which is economical both rom the view points of manufacturing and stocking oE the elements.
In order to reduce the switching time, the system of the present invention switches the line at the station where the failure is detected, without waiting for the switching of the opposite station, and at the same time it commands the opposite station to switch the line. Therefore, the switching time is reduced to about a half of that of the prior ar~ systems.
As mentioned before it is important to attain a high reliability of the service or control signal system of a communi-cation system, otherwise the system will lose control and a high reliability of the total system can not be attained. In other words, the system reliability cannot be higher than the relia-~0 bility of the service and control line system.
In order to attain high reliability of the service andcontrol line, the present invention intends to transmit an auxi-liary signal or the service signals, that is the order wire signals, SC signals (supervisory and control signals) and S~
signals (supervisory or surveillance signals) both through the 7;~36 standby line and working lines in parallel; therefore, so long as at least one line is working normally, the system never loses control. By sending the service signals through the working line, each working line loses its channels or bit numbers, but as will be described later, the loss of the channel or bit is very small, or is not lost at all. ~oreover, the present invention uses the stand by line for transmitting a signal of lower priority, so the total channel or bits transmitted through the system is much larger than the prior art system, in which the stand by line was kept idle except transmitting the service signals.
The invention may be summarized as a protection switch-ing system for a digital carrier transmission line transmitting pulse code modulation bit frames having an auxiliary bit area, comprising; terminal stations, each including a selector for receiving auxiliary signals and means for detecting trouble on the digital carrier transmission line; at least one main trans-mission line, operatively connected to said terminal stations, for transmitting main signals between said terminal stations;
means for detecting trouble on said at least one main transmis-sion line; and at least one stand by line, operatively connected to said terminal stations, for transmitting the main signal when said means detects trouble on said at least one main transmis-sion line, the auxiliary signals between said terminal stations being transmitted in parallel using at least two lines among said at least one main transmission line and said at least one stand by line, utilizing the auxiliary bit area of the puIse code modu-~2~7,'~6~6 -12a- 23507-126 lation bit frames, and the selector selecting the auxiliary signals received over one of said stand by line and said rnain transmission line which is operating normally.
Another advantage of the present invention is -that the switching time of the system from a working line to a stand by line when trouble occurs is reduced. These features and advan-tages will be disclosed in referring to some preferred embodi-ments.
A first embodiment involves sending the service signals both through the stand by line and the working lines. The block diagram of the total system is similar to that of Figure 1 but the channel switching controller (CSC) is improved. Figure 4 shows the CSC of the present invention illustrating the structure of the portion relevant to the invention. The figure shows only the portion concerning the down signal line~ as similar equipment is used for the up signal line forming a pair system.
Figure ~ is a part of a system equivalent to that of Figure 1. The syste~ links the STATION A and STATION B with transmission lines 11, 12 --- and ln, altogether n transmission iL7 2,53 ~;
lines. Each line includes repeaters, terminal repeaters (TR), cables or optical fiber cables and 50 on in a manner similar to that of Figure 1. In this example, the transmission line ln is used as a stand by ]ine. Each station A and B is provided with a LP switch 1 and 2 respectively, and each transmission line is terminated respectively to TRs 7-1-1, 7-1-2 --- of STATION A and TRs 10-1-1, 10-1-2 --- of STATION B. These configurations are all similar to that o~ Figure 1. If necessary, the stand by line ln may be used as a working line to transmit signals, though the system reliability decreases because there is no stand by line.
Unlike the system of Figure 1, the transmission lines 11, 12 --- carry the service signals and, as shown in Figure 4, the service signals are fed in parallel to all transmission lines through the service signal lines 21-1, 21-2, --- 21-n in LP switch (line protection switch) 1. The method oE inserting the auxiliary or service signal into or separate from the main signals of each line will be disclosed later. At the receiving station (STATION B
in the example of Figure 4) these auxiliary or service signals are separated and sent respectively to the terminals of a selection ~0 switch SW of a selector 33, via the service signal lines 23-1, 23-1 --- 23-n. As mentioned with respect to Figure 1, each TR of both stations is provided with detectors to watch the input signal level, coding error, bit error or equipment faults, and if a fault or abnormal situation is detected, the TR sends an alarm to a selector controller 34 via the alarm signal lines 22-1, 22-1, ---22-n. The selector 33 under the control of the selector controller 34, selects the contact o~ the selection switch SW
72~6 where there is no alarm, so the SC signal can be transmi-tted to STATION B so long as at least one transmission line is alive.
The system of Figure 4 is provided with further means to increase the reliability of the service signal. As shown in the figure, the SC signal is further precisely monitored by an error checker 35, which is o~erated automatically or manually, and though there is no alarm signal, the SV signals received by each TR are compared, and the ~elector is controlled to connect to the best transmission line. Therefore, the STATION B always receives the service signal at the best condition of the transmission line.
The switching action of the system from a working line to the stand by line when a fault or abnormal situation occurs, is quite similar to that of the prior art system described with respect to Figure 1.
The manner in which the service signals (denoted as SC
signals hereinafter) are inserted into or dropped from the main signals in each TR is a well known and widely used procedure in the art of communication, and so the circuit and the method of ~0 drop/insertion will not he described herein. Howe~er, this tech-nique is disclosed in detail in for example, "405 Mb/s SINGLE-MODE
FIBER TRANSMISSION SYSTEM", 1984 IEEE International Conference on Communications; or "DC-400M PROTECTION SWITCHING E~UIPMENT USING
MICROPROCESSOR", FSTJ. Vol. 16, No. 4, 1980 Dec. to which refer-ence may be made, if desired.
Figure 5 shows the frame structure of a PCM (pulse code modulation) signal. The ~igure shows a bit train of a 405 Mbit ~Z~7~~$
PCM signal as an example. The coded input signal is arranged in a frame of 4,608 bits as shown in the figure, and transmitted with a rate of ~8,000 frames per second. ~ach frame is further divided into eight sub-frames of 576 bits. At the head of each frame and sub-frame are provided respectively frame bits F or service bits S1, S2 --- as shown in the figure. Bit numbers of the frame bit and service bit depends on the system design; usually 9 bits are allotted. The frame bit F is used to indicate the beginning of the frame, and sometimes an inverse frame bit F (each bit of which is inverse to that of F) is inserted as shown in Figure 5, to check an error of the frame bit itself. The service bits are used to transmit signals between stations to consult each other, or remote checking of the conditions of repeaters or equipment of each station.
Consider as an example, a signal transmitted by a bit of the frame shown in Figure 5. It corresponds to ~8 Kbit/sec which is equivalent to more than a channel of a telephone signal, so it is enough to allot few bits per frame for the SC signals. Usually one of the service bits denoted H in the figure is used for trans-~0 mission of the SC signal. Therefore, as mentioned before, thechannels of bits in the main signal are never lost by transmitting the SC signal in parallel through the working lines. In the rare case which has no service bit allotted in the frame, the allott-ment of few bits in each frame means loss of less than one percent (for the case of Figure 5, it is 9/4,608).
In a second embodiment of the inventlon the line switching equipment is improved to take out a signal from a term-~ ~7~
inating resistor, in order to ~ansmit not only the S~ signal but also the main signal through a stand by line. Figure 6 shows a main par~ of the switching equipment in an LP switch improved by the present invention~ The figure shows only the downward line.
Like the case of Figure 2, a similar circuit for the upward line is used forming a pair with those of the downward line. The dis-closure will be done assuming a single stand by and a fixed pro-tection line system.
In Figure 6, like or similar reference characters or numerals designate same or corresponding parts of Figure 2. Un-like the prior art switch shown in Figure 2, the terminating resistors R1,R2--- are taken out from the switching equipment and a termination resistor Ro is connected to an output terminal B of the switching element 2'-1 of the stand by line 11. The numeral 2' designates a switching equipment of the receiving side, which is identical to that used in the sending side swi~ch. Therefore, the signal of the stand by line 11 can be taken out from the out-put terminal ~.
In Figure 6, a dummy signal generated by a dummy signal generator 5 is transmitted through the stand by line 11 and term-inated by the terminal resistor Ro. If a fault occurs in the transmission line 12, for example, the contact points of the switching elements 1'-1, 1'-2, ---, and 2'-l, 2'-2, --- are shifted from the position of the solid line to that of the broken line. Thus in the STATION A the signal of S~S-l is switched from the working line 12 to the stand by line 11, and the dummy signal is switched from the stand by line 11 to the failed working line 7'~
12. At the sam~ time in the STP~TION B, the signal received by the terminal repeater (TR) 10-l-l is switched to the output of the SYS-l, and the dummy signal received by TR 10-2-l is terminated by the termination resis~or Ro. This switchiny is done under the control of the channel switching controller (not shown) in the same way as described with respect to Figure 1.
The failure point is detected by analyzing the dummy signal received at SrrATION B, and the rehabilitation oE the failed line 12 is performed by monitoring the dummy signal. When the failure is repaired, the system is switched again to transmit the signal of SYS-l through the line 12, and the line 11 is kept as a stand by line.
As mentioned before, the stand by line 11 may be used for transmitting a signal having a lower priority, such as a time-signal, weather forecast etc. which are transmitted at a fixed time. The priority of the signal is determined beforehand and, at the transmission stations these low priority signals are switched manually or automatically under the command of the channel switching controller and are transmitted through the stand by ~0 line. In STATION ~ the low priority signal is fed to the input terminal A, and taken out from the output terminal B in sTArrIoN
B.
In such a manner, the stand by line can be used not only for the SC signals but also for some low priority signal trans-mission. Thus the utilization rate of the system is increased.
Moreover, since the switching elements for both upward and down-ward LP switches are standardized it is economical for manu-k~,';'2~
facturing and stocking the switching element.
The above description has been carried out with respectto a fixed protection line system, but it will be easy for those skilled in the art to apply this to a floating protection line system.
A third embodiment involves decreasing the switching time to switch a main signal from a working line to a stand by line when a fault occurs in the working line. To do so, the system switches the main signal at first in a station where the fault is detected, without waiting for the completion of switching in the remote station. At the same time a switching command is sent to the remote station, so that the switching time is reduced to about half of the prior art switching time.
Figure 7 is a block diagram of a channel switching con-troller of the present invention, which corresponds to that des-ignated by reference numerals 3 or 4 in Figure 1. The block dia-gram of Figure 7 shows the equipment for the STATION A. As a practical matter, the STATION B is provided with a similar equip-ment and they work as a pair. The system configuration is similar ~0 tG that of Figure 1, except for the channel switching controllers.
The sequence of switching will be described mainly with respect to the operations in the STATION A referring to Figures. 1 and 7. In a manner similar to the foregoing examples, the main signals SYS-l, SYS-2 --- are transmitted through the transmission lines 12, 13 --- respectively, and the line 11 is a stand by line.
In Figure 7, reference numeral 110 designates a CPU
(central processing unit) which controls the channel switching ., ~.2~7Z~ç~
controller, 111 is a RO~ (read only memory) whi.ch stores the pro-grams for switching, and 112 is a decoder which assigns the add-resses corresponding to the signal sent from the CPU 110 via an address bus 123. The CPU 110 sends a command to the decoder 112 to assign the address of each buffer memories 115 and 116 to read out the memories stored in them in proper timing, so that the memories are taken out successively to the data bus 124. When the system is operating in a normal state, the memory of ~he bufer memories 114 and 115 are indicating tha~ the state is in normal state, and the CPU 110 recognizes this fact.
A trigger detector 120 collects a trigger signal from each TR (terminal repeater). Suppose that the terminal repeater (TR) 7-2-2 in STATION A has detected a fault and sends a trigger signal, the trigger detector 120 of the channel switching con-troller 3 discrimlnates which TR is sending the trigger signal, and stores the data in the buffer circuit 116. The CPU 110 reads this signal via the buffer amplifier 11~ and a data bus 124.
Recognizing that a fault has occurred, the CPU 110 takes out a switching program stored in the ROM 111. The switching program 2n comprises a program for its own station (STATION A) and for the opposite station (STATION B), and the former is latched in a latch circuit 114 and the latter is latched in a latch circuit 113.
The latched program in the latch circuit 114 i5 sent to the LP switch 1 of the STATION A, and commands it to switch the signal of SYS-l from the transmission line 12 to a stand by line 11. When the switching is completed, the status monitor 119 sends a signal that the transmission line is switched and by this signal 7~
the m~mory in the buffer circult 115 is altered, and the CPU 110 recognizes it through the data bus 124.
Meanwhile, the swi-tching program latched in the latch circuit 113 is converted to a serial signal by a P/S converter (parallel to serial converter) 117 and sent out to the channel switching controller 4 of the opposite station (STATION B) by a line driver 121 via a SC signal line (which is the stand by line 11 in this example).
In the opposite station (STATION B), the sa~e equipment as shown in Fig. 7 is provided, and so the description hereinafter will refer to the equipment shown in Fig. 7, though the description concerns STATION B. The switching command from the STATION A is received by a line receiver 122 and converted to a parallel signal by a S/P converter (serial to parallel converter) 118 and sent to the CPU 110 via the data bus 124.
CPU 110 of the STATION B takes out a switching program stored in the ROM 111 and latches it in the latch circuit 114, and commands the LP switch 4 of STATION B to switch the S~S-l from the line 12 to the stand by line 11. When the switching is completed, ~0 the CPU 110 recognizes this fact through the status monitor 119 and a bufEer memory 115, and takes out a signal stored in the ROM
111 indicating that the switching is completed. It should be pointed out that at this instant the SYS-l is switched from the failed transmission line 12 to a stand by line 11, and the trans-mission of the main signal is recovered. The following sequences are only to certificate between each STATIONS A and B that the switching is completed. This signal is latched by the latch - 2~ -~2~72~3~
circuit 113, and sent to the STATION A by the line driver 121 via SC signal line, after being converted to a serial signal by the P/S converter 117. Though it is not shown in the figure, the SC
signal line is kept always in good condition applying the system described with respect to the embodiments 1 or 2 of the present invention. In STATION A, the signal from STATION B indica-ting that the switching is completed is received by the line receiver 122, and converted to a parallel signal by the S/P converter 118 and sent to the CPU 110 via the data bus 124. The se~uence is completed by recognition of the state by CPU 110.
How the time for protection line switching is reduced by the above sequence will be explained by referring to Figure 8, which shows the time sequence of the switching. The time flow is shown from up to downward in the figure, and vertical lines in the figure show the sequence which is performed in STATION A and STATION B respectively.
Figure 8(a) shows the case wherein the switching is com-pleted successfully. Suppose that in the STATION A the triggering of the switching has occurred. The CPU 110 of the STATION A com-mands the LP switch 1 of own station to switch the SYS-l to the stand by line, and at the same time the command is sent to the STATION B, it will take approximately 5 ms for the command signal to reach STATION B and the switching in each station takes approx-imately 2 ms. As soon as the switching in the STATIOW A is com-pleted, the main signal is sent to the STATION ~ via the stand by line (shown by the sloping broken line)~ In STATION B the switching is finished in approximately 2 ms, so the STATION B is ~Z~'7;2~
ready to receive the main signal when it arrives, and so the total switching time is approximately 7 ms. The following sequence is to certify between each station that the switching is completed successfully. The STATION B sends a signal to STATION A that the switching is completed. It takes a further 5 ms for STATION A to recognize the switching is performed correctly. Compared to Figure 3(a), the switching time is reduced effectively to about half of the prior system.
Figure 8(b) shows the sequence oE a very rare case wherein the switching has failed due to a failure of equipment or parts of the system. In the STATION A the switching is trig-gered,and in the way as described with respect to Figure 8(a), STATION A switches its LP switch and at the same time a switching command is sent to the STATION B. Receiving this command, the CPU
110 of the STATION B commands the switching of the LP switch.
However, if the switching has not been done correctly, due to the failure of equipment, the status monitor 119 detects it and sends a fail signal to the buffer circuit 115, and the CPU 110 recog-niæes that the switching was not done correctly. Then the CPU 110 selects a fail signal out of the ROM 111. This fail signal is sent to the STATION A via the latch circuit 113, the P/S converter 117, the line driver 121, and the SC signal line.
Meanwhile, the main sigr~al is sent from the STATION A to the STATION B, but the STATION B cannot receive the signal. In the STATION A the fail si~nal is received by the line receiver 122 and sent to the CPU via the S/P converter 118 and the data bus 124. The CPU 110 of the STATION A recognizes that the switching ~%~.7~2~6 has not been completed in a correct manner, and then the CPU
selects a retry command out of the ROM 111, and sends it to the STATION B through the latch circuit 113, the P/S converter 117, and the line driver 121 via the SC signal line. ~eceiving this signal, the CPU of the 5TATION B selects a retry pro~ram out of the ROM 111, and commands the switching of the LP switch again like a similar manner described above.
When the switching is completed correctly, the following sequences are the same as that of the Figure 8 (A), but if the switching has failed again, the CPU of the STAT~ON B recognizes it, and sends a fail signal again to the CPU of the STATION A in a similar manner as described above. The CPU of the STATION A rec-ognizes that the switching has failed again and it then selects a pro~ram to go back to the initial state from the ROM 111, and commands the LP switch to go back to the initial state. The system then goes back to the initial state and stops and at the same time, the CPU sends an alarm to the operator.
As has been described above, the present invention provides a hi~her reliability of the protection switching system and reduces 2~ the switching time for the protection line. Moreover, it eco-nomizes the transmission system utilizing a stand by system for transmission of main signals.
The invention has been described above essentially in terms of the process sequences. All the equipment and components referred to are all conventional for transmission technology, and so the structural and performance details of this equipment have omitted for simplicity.
~ ~ Z~7z~6 It will be clear that the present invention can be applied not only for cable transmission lines or opt.ical fiber cable lines but it can be applied to radio transmission lines.
Claims (19)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A protection switching system for a digital carrier transmission line transmitting pulse code modulation bit frames having an auxiliary bit area, comprising; terminal stations, each including a selector for receiving auxiliary signals and means for detecting trouble on the digital carrier transmission line; at least one main transmission line, operatively connected to said terminal stations, for transmitting main signals between said terminal stations, means for detecting trouble on said at least one main transmission line; and at least one stand by line, opera-tively connected to said terminal stations, for transmitting the main signal when said means detects trouble on said at least one main transmission line, the auxiliary signals between said ter-minal stations being transmitted in parallel using at least two lines among said at least one main transmission line and said at least one stand by line, utilizing the auxiliary bit area of the pulse code modulation bit frames, and the selector selecting the auxiliary signals received over one of said stand by line and said main transmission line which is operating normally.
2. A protection switching system according to claim 1, wherein the auxiliary signals between said terminal stations include a control signal.
3. A protection switching system according to claim 1, wherein the auxiliary signals between said terminal stations in-clude a supervisory signal.
4. A protection switching system according to claim 1, wherein the auxiliary signals between said terminal stations in-clude an order wire telephone maintenance signal.
5. A protection switching system according to claim 1, wherein the auxiliary signals between said terminal stations are transmitted in parallel using said at least one main transmission line and said at least one stand by line.
6. A protection switching system according to claim 1, wherein each of said terminal stations further comprises an error checker, operatively connected to said main transmission and stand by lines, for comparing the auxiliary signals and selecting a best signal.
7. A protection switching system according to claim 1, further comprising means for transmitting a low priority signal among the main signals over said stand by line.
8. A protection switching system according to claim 1, further comprising means for transmitting low priority signals among the main signals over said stand by line when one of said at least one main transmission line is in trouble; and wherein each of said terminal stations further comprises a line protec-tion switch, operatively connected to said main transmission and stand by lines, for switching said main transmission and stand by lines, said line protection switch having a terminal for out-putting the signals received by said terminal stations over said stand by line; and a dummy signal generator, operatively con-nected to said line protection switch, for providing said stand by line with a dummy signal to be transmitted with the low priority signals.
9. A protection switching system according to claim 8, wherein each of said line protection switches comprises switch-ing elements, operatively connected to said main transmission and stand by lines, which are identical for both the sending and receiving performed in said terminal stations.
10. A protection switching system according to claim 8, wherein each of said terminal stations further comprises a ter-mination resistor operatively connected to the terminal which outputs the signals sent over said stand by line.
11. A protection switching system according to claim 1 wherein a commanding station among said terminal stations is arranged to switch the failed one of said main transmission lines to said stand by line and simultaneously command an opposite terminal station to perform a switching operation of the failed one of said main transmission lines and said stand by line, the opposite terminal station sending a status signal to the command-ing station which is commanding the switching operation, the commanding station reversing the switching of the failed one of said main transmission lines and said stand by line, when the status signal is received with an indication that the opposite terminal station did not switch the failed one of said main transmission lines and said stand by line.
12. A protection switching system according to claim 11, wherein if the switching operation fails, the one of said ter-minal stations commanding the switching operation commands the opposite one of said terminal stations, to retry the switching operation, and if the switching operation fails twice consecu-tively, the one of said terminal stations commanding the switch-ing operation stops the switching operation, then reverses the switching of the failed one of said main transmission lines and said stand by line.
13. A protection switching system according to claim 1, wherein each terminal station comprises: a termination resistor for terminating a first dummy signal generated by a remote ter-minal station; a dummy signal generator for generating a second dummy signal; line protection switch means, operatively connected to said dummy signal generator, for switching the main signal onto a main one of the transmission lines and the auxiliary signal onto at least two of the transmission lines including the main one and the stand by line, comprising: identical switching ele-ments, operatively connected to said transmission lines, for switching the main and auxiliary signals being transmitted and received, and a terminal, operatively connected to said termina-tion resistor for outputting the first dummy signal to said ter-mination resistor; and channel switching control means for controlling the switching of said line protection switch means.
14. A protection switching system according to claim 13, wherein said line protection switch means further comprises a selector, operatively connected to the transmission lines and said channel switching control means, for selecting one of the auxiliary signals to be supplied to said channel switching con-trol means.
15. A protection switching system according to claim 14, further comprising error check means for checking the auxiliary signal for errors and for controlling said selector.
16. A protection switching system according to claim 13 wherein the remote terminal station includes remote line protec-tion switch means, and wherein said channel switching control means comprises: failure detection means for detecting a failure in a failed one of the transmission lines; local line protection switch command means, for controlling switching of said line protection switch means to switch the failed one of the trans-mission lines and the stand by line; remote line protection switch command means, for controlling switching of the remote line pro-tection switch means to switch the failed one of the transmission lines and the stand by line; status means for receiving local and remote status signals indicating whether the switching of the failed one of said transmission lines and the stand by line in said line protection switch means and the remote line protection switch means was successful; and processing means, operatively connected to said failure detection means, said local and remote line protection switch command means and said status means, for controlling the local and remote line protection switch command means to simultaneously command said line protection switch means and the remote line protection switch means to switch the failed one of the transmission lines and the stand by line and for reversing the switching of the failed one of the transmission lines and the stand by line when one of the local and remote status signals indicates that the switching was unsuccessful.
17. A terminal station according to claim 16 wherein said processing means controls the remote line protection switch com-mand means to issue a retry command to the remote line protection switch means if the remote status signal indicates that the remote line protection switch means failed to switch the failed one of the transmission lines and the stand by lines.
18. A terminal station according to claim 16, wherein the main signals include a low priority signal, and wherein said ter-minal station further comprises means for transmitting the low priority signal together with the auxiliary signal over the stand by line.
19. A terminal station according to claim 13, wherein the main signals include a low priority signal, and wherein said ter-minal station further comprises means for transmitting the low priority signal together with the auxiliary signal over the stand by line.
Applications Claiming Priority (6)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP58212030A JPS60103836A (en) | 1983-11-11 | 1983-11-11 | Multiplexing system of supervisory signal |
| JP212030/83 | 1983-11-11 | ||
| JP21740283A JPS60109932A (en) | 1983-11-18 | 1983-11-18 | Line switching system |
| JP217402/83 | 1983-11-18 | ||
| JP1125684A JPS60154720A (en) | 1984-01-25 | 1984-01-25 | Line switching control system |
| JP011256/84 | 1984-01-25 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1247206A true CA1247206A (en) | 1988-12-20 |
Family
ID=27279349
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000467452A Expired CA1247206A (en) | 1983-11-11 | 1984-11-09 | Protection switching system for carrier transmission line |
Country Status (7)
| Country | Link |
|---|---|
| US (1) | US4680776A (en) |
| EP (1) | EP0142138B1 (en) |
| AU (1) | AU551278B2 (en) |
| CA (1) | CA1247206A (en) |
| DE (1) | DE3474893D1 (en) |
| HK (1) | HK70890A (en) |
| SG (1) | SG9490G (en) |
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| US5175639A (en) * | 1986-11-21 | 1992-12-29 | Hitachi, Ltd. | Optical subscriber network transmission system |
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| US5029158A (en) * | 1987-07-23 | 1991-07-02 | Northern Telecom Limited | Protection switching in a multi-channel communications system |
| JPS6460026A (en) * | 1987-08-31 | 1989-03-07 | Fujitsu Ltd | Transmission line switching device for communication equipment |
| DE3850934T2 (en) * | 1987-11-10 | 1994-12-01 | Nec Corp | Channel switching device. |
| CA1321001C (en) * | 1988-02-04 | 1993-08-03 | Nec Corporation | Transmission line switching system |
| JPH01284035A (en) * | 1988-05-10 | 1989-11-15 | Toshiba Corp | Data transmission equipment |
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| JPH0279542A (en) * | 1988-09-14 | 1990-03-20 | Fujitsu Ltd | Subsignal transmission system |
| JP2591128B2 (en) * | 1988-12-20 | 1997-03-19 | 日本電気株式会社 | Communication line switching method |
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| JP3268460B2 (en) * | 1991-07-30 | 2002-03-25 | 株式会社日立製作所 | Transmission line switching control method and device |
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| US3111624A (en) * | 1960-01-04 | 1963-11-19 | Bell Telephone Labor Inc | Automatic system for selectively substituting spare channels for failed working channels in a multichannel multilink communication system |
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-
1984
- 1984-11-09 DE DE8484113522T patent/DE3474893D1/en not_active Expired
- 1984-11-09 EP EP84113522A patent/EP0142138B1/en not_active Expired
- 1984-11-09 CA CA000467452A patent/CA1247206A/en not_active Expired
- 1984-11-09 US US06/669,836 patent/US4680776A/en not_active Expired - Lifetime
- 1984-11-12 AU AU35311/84A patent/AU551278B2/en not_active Ceased
-
1990
- 1990-02-13 SG SG94/90A patent/SG9490G/en unknown
- 1990-09-06 HK HK708/90A patent/HK70890A/en unknown
Also Published As
| Publication number | Publication date |
|---|---|
| AU551278B2 (en) | 1986-04-24 |
| EP0142138B1 (en) | 1988-10-26 |
| US4680776A (en) | 1987-07-14 |
| DE3474893D1 (en) | 1988-12-01 |
| EP0142138A2 (en) | 1985-05-22 |
| AU3531184A (en) | 1985-05-16 |
| HK70890A (en) | 1990-09-14 |
| EP0142138A3 (en) | 1986-03-05 |
| SG9490G (en) | 1990-08-31 |
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