Fault locating arrangement for a two-way repeatered transmission link

A fault locating arrangement for a two-way repeatered transmission link comprising a source in one of the terminals to transmit a unique signal in a first transmission direction from the one of the terminals to the other of the terminals through the repeaters, and a first unique signal detector, a second unique signal detector and logic circuitry disposed in at least the other of the terminals and each of the repeaters. The first unique signal detector is coupled to the first transmission direction, the second unique signal detector is coupled to the opposite transmission direction and the logic circuitry is coupled to the first and second unique signal detectors such that, when the unique signal traveling in the first transmission direction is detected, a loop connection is established for the unique signal from the first transmission direction to the opposite transmission direction and the unique signal is transmitted in the first transmission direction beyond an associated one of the repeaters. The logic circuitry breaks the loop connection when the unique signal is detected traveling in the opposite transmission direction and maintains the loop connection when the unique signal is not traveling in the opposite transmission direction. A phase comparator disposed in the one of the terminals responds to the unique signal transmitted in the first transmission direction and received from the opposite transmission direction to provide an indication of the location of a fault in the transmission link.

BACKGROUND OF THE INVENTION 
The present invention relates to two-way repeatered transmission links and 
more particularly to a fault locating arrangement therefore. 
One known fault locating arrangement has in the past been employed with a 
full duplex (two-way) repeatered wireline transmission link with repeater 
power obtained from a direct current (d.c.) source line feed. The overall 
voltage drop of a looped-back current feed is used as an indication of the 
number of good repeaters in the repeatered transmission link. This is 
facilitated by circuitry in each repeater half section which detects the 
presence or absence of a good data signal and operates a relay which then 
controls the voltage drop in the d.c. current path. 
The above technique for fault location is not applicable to a system which 
has no d.c. current path such as wirelne repeatered transmission links 
which are not d.c. coupled, radio repeater links and optical cable links 
or combinations thereof. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a quick, accurate and 
simple to operate fault locating system for two-way repeatered 
transmission links and in particular fiber optical cable links, radio 
repeater links and wireline links which are not d.c. coupled. 
It should be noted that the fault locating arrangement of the present 
invention can also be employed with wireline transmission links that are 
d.c. coupled, either by itself or in combination with the prior art fault 
locating arrangement of d.c. coupled wireline links. 
A feature of the present invention is the provision of a fault locating 
arrangement for a two-way repeatered transmission link including first and 
second spaced two-way terminals and a plurality of two-way repeaters 
disposed between the first and second terminals comprising: first means 
disposed in one of the first and second terminals to transmit a unique 
signal in a first transmission direction from the one of the first and 
second terminals to the other of the first and second terminals through 
the plurality of repeaters; second means disposed in at least the other of 
the first and second terminals and each of the plurality of repeaters, 
each of the second means being responsive to the unique signal traveling 
in the first transmission direction to establish a loop connection for the 
unique signal from the first transmission direction to a second 
transmission direction opposite to the first transmission direction, to 
enable transmission of the unique signal in the first transmission 
direction beyond an associated one of the plurality of repeaters, to break 
the loop connection when the unique signal is traveling in the second 
transmission direction and to maintain the loop connection when the unique 
signal is not traveling in the second transmission direction; and third 
means disposed in the one of the first and second terminals responsive to 
the unique signal transmitted in the first transmission direction and 
received from the second transmission direction to provide an indication 
of the location of a fault in the transmission link. 
The following description will be directed to a fiber optic two-way 
repeater data transmission link. This description, however, is not 
intended to limit the fault locating arrangement of this invention to just 
this type of transmission link since the fault locating arrangement of the 
present invention can be employed in radio repeatered transmission links 
and wireline repeatered transmission lnks of both the d.c. coupled and not 
d.c. coupled types or combinations thereof.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
In accordance with the principles of the present invention, the location of 
a faulty 8 km (kilometer) section in a two-way repeatered transmission 
link is done in terminal 10 of FIG. 1. The fault locating arrangement 
would be placed in operation when a loss of incoming traffic alarm is set. 
Traffic is then pre-empted and a FSK (frequency shift keyed) 4.608 and 
1.152 MHz (megahertz) digital stream is sent out which is keyed every 
millisecond (msec). This special or unique signal is generated at terminal 
10. The above-identified unique signal is only one example of such a 
unique signal. The unique signal must be unique so as not to be simulated 
by any normal transmission or traffic signal. It may consist of a signal 
which is at a unique frequency or has some other data bit pattern which is 
unique, but which must have a modulation pattern, the parameters of which 
will be dependent upon other parameters including the distance between 
repeaters and total line distance. 
Means are provided in each repeater half-section (e.g. each direction of 
transmission in a two-way or duplex repeater) of repeaters 1 to N, to 
detect the presence of the unique signal. Means are also provided in each 
of the repeaters 1 to N, such that when the unique signal is detected in a 
first transmission direction from terminal 10 to terminal 11 but not in 
the second transmission direction from terminal 11 to terminal 10, the 
received unique signal will be looped around to the second transmission 
direction. This can be illustrated by referring to FIG. 1. 
A typical fiber optical cable repeatered transmission link is shown 
including two full-duplex terminals 10 and 11 and a number of full-duplex 
repeaters 1 to N. Normal traffic would be transmitted in both directions 
simultaneously, with each direction of transmission in the terminals and 
repeaters being independent. The dark lines indicate the path of the 
unique signal used for the fault locating procedure. 
If repeater N is not functioning (or if there is a cable break between 
repeater (N-1) and repeater N) then repeater (N-1) would receive the 
unique signal in the first transmssion direction only. The internal 
secondary of the repeater then provides a loop connection to loop the 
unique signal back as indicated by the dashed line 3 and the signal is 
transmitted in the second transmission direction to originating terminal 
10. The unique signal is always transmitted in the first transmission 
direction as well as being looped back. This is required because when the 
unique signal is initially applied to the fiber optic cable, each of the 
repeaters 1 to N will initially detect the transmission in the first 
transmission direction only and immediately loop the signal back. That is, 
when the unique signal first arrives at repeater 1 from terminal 10, it 
will loop the unique signal back as well as transmit it to repeater 2. 
When repeater 2 receives the unique signal, it will also loop the unique 
signal back to repeater 1 and continue transmitting the unique signal in 
the first transmission direction. Repeater 1, receiving the unique signal 
from repeater 2, will then break the loop connection and continue acting 
as two independent repeater sections, processing signals from the two 
directions. This action will continue down the link, from repeater to 
repeater, until there is a break in transmission at some pont such as 
illustrated at repeater (N-1) in FIG. 1. At this point, only repeater 
(N-1) will be in the looped-back mode and the unique signal will travel 
the complete distance from terminal 10 to the last working repeater (N-l) 
and back again to terminal 10. 
Terminal 10 includes a means to measure the total transit time of the 
unique signal through the repeaters and back again and a means to display 
the result and measurement. The location of the unique signal interruption 
(failed repeater or broken cable) is calculated by using the propagation 
delay between repeaters and the total propagation delay of the unique 
signal. Two special cases must also be considered: (a) If no unique signal 
is looped back, then the first repeater (repeater 1) has failed or the 
cable section between terminal 10 and repeater 1 has failed. This is an 
acceptable condition since it falls within the overall capability of the 
fault locating arrangement of the present invention to locate a failure 
within one cable link, and (b) if there are no failures in the system, the 
total delay will be equivalent to the transit time from terminal 10 to 
terminal 11 and back to terminal 10. To satisfy this special case, each of 
the terminals 10 and 11 must have the same loop back capability as the 
repeaters 1 to N. 
Once the last repeater that loops back the unique signal is identified 
(repeater (N-1) in the example employed), the transmission link fault is 
known to be in one of the following equipment: (a) one-half of repeater 
(N-1); (b) repeater N; (c) the 8 km section of the cable between repeaters 
(N-1) and N; or (d) a connector in the cable section between repeaters 
(N-1) and N. 
Referring to FIG. 2, there is illustrated therein the circuitry present in 
terminal 10 of FIG. 1 to generate the unique signal employed for fault 
location and the circuitry necessary to provide an indication of the 
location of the fault in the transmission link. As previously mentioned, 
provisions are made in terminal 10 to provide an indication of a loss of 
received traffic at terminal 10. Upon this indication an operator would 
activate a front panel switch to place terminal 10 in a repeater fault 
locate mode. In the fault locate mode, a unique signal is generated and 
sent down the cable in the first transmission direction. Each of the 
repeaters would detect the unique signal and loop it back as described 
hereinabove. The return signal or the signal received from the second 
transmission direction is compared for total phase shift corresponding to 
the delay relative to the transmitted unique signal and an output to a 
front panel indicator would show the total number of good repeaters in the 
transmission line before the first faulty repeater or cable section. 
When the front panel switch is placed in the fault locate mode, a repeater 
fault locate signal RFL would have a high value which is coupled through 
inverter 12 to deactivate AND gate 13 to prevent data input to gate 13 
from being transmitted through OR gate 14 to bit stream generator 15. The 
inverted signal RFL is also coupled through inverter 16 and AND gate 17 
for enablng this gate so that a 500 Hz square wave is passed through gate 
17 to gate 14 and, hence, to bit stream generator 15. In the fault locate 
mode bit stream generator 15 would produce a 4.608 MHz squarewave and a 
1.152 MHz squarewave alternating at a 1 msec rate. This resulting unique 
signal is then coupled to electrical to optical transmitter 18 and then to 
the fiber optic cable. The unique signal then propagates down the cable 
and is looped back at the most distant good repeater. The total delay per 
good repeater and cable section is 80 microseconds (.mu. secs) plus a few 
.mu. secs for circuit delays. This is based upon 200,000 km/s propagation 
speed for the optical signal in the cable and a total of 16 km round-trip 
8 km cable section. 
The total delay of one to seven times 80 .mu. secs is measured by 
generating a pulse with the phase comparator flip flop 19 which receives 
the looped-back unique signal from optical to electrical receiver 20 
coupled to the fiber optic cable. The output from the phase comparator is 
provided by setting flip flop 19 with the leading edge of the 500 Hz 
square wave used to generate the unique signal, and resetting flip flop 19 
with the received data 500 Hz envelope. The pulses, occurring at a 500 Hz 
repetition rate, at the output of flip flop 19 are coupled to buffer 
amplifier 21 and then integrated in integrator 22 and then amplified in 
amplifier 23 prior to being coupled to a front panel indicator. The output 
from amplifier 23 will be scaled to provide a full scale indication for 
the longest delay measured, up to seven good repeaters. 
Transmitter 18 may be any known type of electrical to optical converter, 
such as a gas laser and optical modulator, laser diode or light emitting 
diode and receiver 20 may be any well-known optical to electrical 
converter, such as a photodetector diode or avalanche photodiode. 
FIG. 3 illustrates the transmitted unique signal envelope and the two 
extremes of the received unique signal envelope utilized to operate phase 
comparator flip flop 19. 
FIG. 4 illustrates one possible embodiment of bit stream generator 15 of 
FIG. 2. Generator 15 could include a 4.608 MHz pulse generator 24 and a 
1.152 MHz pulse generator 24a for use in generating the unique signal and 
a rate controlled bit stream generator 25 for generating the bit streams 
used to convey the data when data is coupled from gate 14 to generator 15. 
The output from generator 24 is coupled to a first input of AND gate 24b 
having a second input coupled to the output of gate 14. The output of 
generator 24a is coupled to a first input of AND gate 24c having an 
inverter input coupled to the output of gate 14. Gate 24b will be enabled 
and gate 24c will be disabled. When the 500 Hz square wave is high and 
gate 24b will be disabledand gate 24c will be enabled when the 500 Hz 
square wave is low resulting in the desired 4.608 and 1.152 MHz digital 
stream keyed every 1 msec at the output of OR gate 24d. The output from 
gate 24 is coupled to a first input of AND gate 26 having a second input 
coupled to the output of gate 14 and a third input coupled to the output 
of inverter 12 of FIG. 2 which provides the signal RFL. The output of 
generator 25 is coupled to a first input of AND gate 27 which has its 
second input coupled to gate 14 and an inverter input coupled to inverter 
12 of FIG. 2. When signal RFL is low and data is applied from gate 14, 
gate 27 will provide a bit stream output modulated by the data which will 
be coupled through OR gate 28 to transmitter 18. When signal RFL is high 
indicating that the front panel switch has been moved to the repeater 
fault locate mode, gate 27 will be inhibited and gate 26 will be enabled 
when the 500 Hz square wave is high and disabled when the 500 Hz square 
wave is low resulting in the desired 4.608 and 1.152 MHz bit stream 
modulated for 1 msec intervals with the resultant unique signal being 
coupled through gate 28 to transmitter 18. 
FIG. 5 is a block diagram of a data rate transparent repeater employed in 
the transmission link of FIG. 1 incorporating the fault locating circuitry 
in accordance with the principles of the present invention. The repeater 
includes an optical to electrical receiver 27, such as an avalanche 
photodiode (APD), a pulse regenerator 28 to regenerate the signal 
amplitude of the data signals transmitted from terminal 10, AND gate 29, 
NOR gate 30 and electrical to optical transmitter 31, such as a laser 
diode, in the first transmission direction and an identical optical to 
electrical receiver 32, a pulse regenerator 33, a NAND gate 34, a NOR gate 
35 and an electrical to optical transmitter 36 in the second transmission 
direction. The fault locating circuitry includes unique signal detector 37 
coupled to the first direction of transmission, such as at the output of 
regenerator 28, unique signal detector 38 coupled to the second 
transmission direction, such as at the output of regenerator 33, NAND 
gates 39 and 40 and control logic 41. 
Unique signal detectors 37 and 38 each include a frequency discriminator or 
detector 39 and a 500 Hz tone detector 41. The function of detectors 37 
and 38 is to detect the 4.608 and 1.152 MHz bit stream when modulated at 
500 Hz. When the unique signal in the first transmission direction is 
detected by detector 37, control logic 41 controls gate 29 to route the 
unique signal through gates 29 and 30 and transmitter 31 to the fiber 
optical cable and controls gate 39 to provide the loop connection for the 
unique signal when no unique signal is detected in the second direction of 
transmission by detector 38. When the unique signal is detected by 
detector 38, control logic 41 breaks the loop connection by inhibiting 
gate 39 and the signals pass through the repeater in the normal manner. 
The inverse action occurs if the repeater is tested for a fault from 
terminal 11 rather than terminal 10 with control logic 41 controlling NAND 
gate 40 to provide the loop connection. 
Referring to FIG. 6, there is illustrated therein a block diagram of the 
circuitry of each of regenerators 28 and 33 of the repeater of FIG. 5. 
Each of regenerators 28 and 33 include a transimpedance bipolar transistor 
front end 42, an AGC controlled wideband amplifier 43, a filter 44, a 
wideband amplifier 45, a balanced clamp and high signal comparator 46, an 
AGC amplifier 47 and a receiver power supply 48. The signal detected by an 
APD receiver 27 has maximum sensitivity and is applied as an input to 
front end 42 which provides high performance and yet high stability and 
low power. The output of front end 42 is coupled to amplifier 43 then to 
filter 44 and finally a high level wideband amplifier 45. Filter 44 
further limits the bandwidth and shapes the frequency response of the 
regenerator to reduce noise and minimize time jitter. The filter could 
have been placed at any pont in the signal path before the input to 
comparator 46, however, there are distinct advantages to the position 
illustrated. For example, if filter 44 were located directly after front 
end 42, adjustment of filter 44 would effect the load impedance of front 
end 42. The reverse is also true. On the other hand, if filter 44 were 
placed after amplifier 45 and before comparator 46, filter 44 would be on 
a balanced signal path requiring a more complex and balanced filter. In 
the locaton shown, filter 44 is isolated from loading effects by 
amplifiers 43 and 45. 
Amplifier 45 serves two functions. First amplifier 45 provides additional 
linear amplification to the incoming signal, and secondly, it provides a 
balanced signal to the clamp and comparator 46. Automatic gain control is 
not applied to amplifier 45 because it operates at a higher signal level 
than amplifier 43. Automatic gain control over large signal levels is 
considerably more difficult than control over lower level signals, since 
distortion is more likely to occur as the gain of a high signal level 
stage is changed. If considerable dynamic range was required, then 
automatic gain control of amplifier 45 would also be required. In this 
instance without automatic gain control, the high signal levels would 
cause distortion. 
The nature of the level adaptive balanced clamp circuit of clamp and 
comparator 46 requires converting the ground referenced signal to a 
balanced line signal. The signal could be processed from amplifier 43 in a 
balanced configuration. That would have reduced the shielding requirement 
and reduced the effects of ground loops. Such a configuration would, 
however, be considerably more complex and would not conserve power. 
The signal provided to the balanced clamp of clamp and comparator 46 is of 
sufficient amplitude for presentation to the comparator of clamp and 
comparator 46, however, there was no d.c. ground reference with which to 
compare it. That is, the binary "0" state of either line is the lowest or 
highest voltage to which that line is driven under any given signal level. 
This minimum voltage is defined not only by the amplitude of the incoming 
signal, but also by the amplifier quiescent bias. Both the bias of 
amplifier 45 and the signal level were variables to some extent and, 
therefore, the common practice of comparing the incoming signal 
(single-ended) to a fixed voltage reference would not have been 
satisfactory. The effects of bias changes could have been eliminated by 
subsequent peak detection. However, since the voltage drops across the 
clamp and peak detector diodes would be significant compared to the signal 
level, the dynamic range with this approach would be limited. Greater 
dynamic range than that possible with clamping and peak detection is 
achieved with balanced clamping. For this approach, the two incoming lines 
are clamped at the peak excursions, and then compared to each other. 
Control logic 41 of FIG. 5 can be any appropriate combination of logic 
gates that will respond to the output of detectors 37 and 38 to control 
gates 29, 34, 39 and 40 to carry out the fault locating function in 
accordance with the principles of the present invention. One particular 
combination of gates that can be used in control logic 41 is illustrated 
in Block form in FIG. 7 and includes an EXCLUSIVE-OR gate 52 and a NOT 
gate 53. 
While we have described above the principles of our invention in connection 
with specific apparatus it is to be clearly understood that this 
description is made only by way of example and not as a limitation to the 
scope of our invention as set forth in the objects thereof and in the 
accompanying claims.