Method and apparatus for failure recovery in passive optical network

In a passive optical network in which a plurality of subscriber equipments are connected to central office equipment via a star coupler, non-faulty subscriber equipments are quickly recovered from a communication failure caused by a faulty subscriber equipment. A control signal of wavelength .lambda..sub.2 is wavelength-division multiplexed with a main signal of wavelength .lambda..sub.1. Using the control signal of wavelength .lambda..sub.2, the subscriber equipments are selectively deactivated from the central office equipment, and based on the recovery state during the deactivation process, the faulty subscriber equipment is located and only the faulty subscriber equipment is deactivated.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method and apparatus for recovering 
quickly from a failure caused by a faulty subscriber equipment in a 
passive optical network in which a plurality of subscriber equipments are 
connected to central office equipment via an optical coupler (star 
coupler) constructed from a passive device. 
2. Description of the Related Art 
Previously, for a subscriber requiring high-speed, large-capacity 
communications, it was common to lay an optical fiber point to point from 
the central office to serve the subscriber. In recent years, a passive 
optical network has been devised and commercially implemented as a system 
for economically serving subscribers requiring wideband communications. In 
the passive optical network system, an optical coupler constructed from a 
passive device is provided between the central office and the subscribers, 
and a single optical fiber (or two optical fibers to provide redundancy) 
is laid between the central office and the optical redirector coupler, 
while dedicated optical fibers for the individual subscribers are laid 
from the optical coupler to the respective subscribers. 
The optical coupler distributes downstream optical signals from the central 
office to the respective subscribers, and combines upstream optical 
signals being sent from the subscribers to the central office. As a method 
for multiplexing a plurality of subscribers on a single optical 
transmission line, time division multiple access (TDMA) is used for 
upstream transmission, and time division multiplexing (TDM) or time 
compression multiplexing (TCM) is used for downstream signals. 
By sharing the optical transmission line and the optical subscriber unit at 
the central office in this way, the system construction cost can be 
reduced compared with the method that connects each individual subscriber 
to the central office point to point. Furthermore, using a passive device 
for the optical coupler serves to enhance system reliability compared with 
a system designed to multiplex and demultiplex optical signals using an 
active device. 
However, in a passive optical network, sharing the optical transmission 
line gives rise to a problem. That is, a failure caused by a faulty 
subscriber equipment affects communications between the central office and 
other subscriber equipments. For example, when an optical subscriber 
equipment (Optical Network Unit (ONU)) has gone faulty and, because of 
malfunctioning of the unit, has emitted an optical signal in a time slot 
where some other subscriber optical network unit is supposed to emit an 
optical signal, interference is caused to the communication of the optical 
network subscriber using a time slot overlapping that time slot, bringing 
down the communication service. Further, if light is continuously emitted 
because of a failure of laser beam control circuitry or the like in a 
subscriber optical network unit, communications of all the subscribers 
served by the central office through the same optical coupler will be 
brought down. In such cases, it is not possible to locate from the central 
office side the faulty subscriber optical network unit responsible for the 
communication failure. Personnel must be despatched to the premises where 
the optical network units are installed, to examine each terminal and 
optical network unit, and it will take a lot of time and labor to recover 
subscribers' communications from the failure. 
SUMMARY OF THE INVENTION 
In a passive optical network system in which a small number of optical 
transmission lines are shared among a plurality of subscribers, it is an 
object of the present invention to provide a method and apparatus for 
speeding up failure recovery by making it easy to locate a failed point, 
while reducing the degree of propagation of the failure, caused by a 
faulty optical subscriber equipment, to other optical subscriber 
equipments and thereby securing communications between the central office 
and subscribers as much as possible. 
According to the present invention, there is provided, in a passive optical 
network in which a plurality of subscriber equipments are connected to 
central office equipment via an optical redirector coupler, a method of 
recovering non-faulty subscriber equipments from a communication failure 
caused by a faulty subscriber equipment, comprising the steps of: 
automatically locating the faulty subscriber equipment; and deactivating 
only the thus located subscriber equipment. 
According to a first aspect of the present invention, the step of locating 
the faulty subscriber equipment includes the substeps of: selectively 
deactivating the subscriber equipments by sending from the central office 
equipment to each subscriber equipment a deactivation signal at a second 
wavelength different from a first wavelength, which is the wavelength of 
signal light where the communication failure has occurred, by 
wavelength-division multiplexing the deactivation signal with the signal 
light; and locating the faulty subscriber unit, based on the state of the 
communication failure when the subscriber equipments are selectively 
deactivated. 
According to a second aspect of the present invention, in the step of 
locating the faulty subscriber equipment the faulty subscriber equipment 
is located by detecting within each subscriber equipment an abnormality of 
an optical signal being sent to the central office equipment. 
According to the present invention, there is provided, in a passive optical 
network in which a plurality of subscriber equipments are connected to 
central office equipment via an optical coupler, an apparatus for 
recovering non-faulty subscriber equipments from a communication failure 
caused by a faulty subscriber equipment, comprising: means for 
automatically locating the faulty subscriber equipment; and means for 
deactivating only the thus located subscriber equipment. 
According to a first aspect of the present invention, the faulty subscriber 
equipment locating means includes: means for selectively deactivating the 
subscriber equipments by sending from the central office equipment to each 
subscriber equipment a deactivation signal at a second wavelength 
different from a first wavelength, which is the wavelength of signal light 
where the communication failure has occurred, by wavelength-division 
multiplexing the deactivation signal with the signal light; and means for 
locating the faulty subscriber unit, based on the state of the 
communication failure when the subscriber equipments are selectively 
deactivated. 
According to a second aspect of the present invention, the faulty 
subscriber equipment locating means locates the faulty subscriber 
equipment by detecting within each subscriber equipment abnormality of an 
optical signal being sent to the central office equipment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 is a diagram showing the configuration of a passive optical network. 
As shown in FIG. 1, in the passive optical network system, an optical 
coupler 14 constructed from a passive device is provided between central 
office equipment 10 and subscriber equipment 12, and a single optical 
fiber (or two optical fibers to provide redundancy) is laid between the 
central office equipment and the optical coupler 14, while a dedicated 
optical fiber for each individual subscriber is laid between the optical 
coupler 14 and the subscriber equipment 12. In FIG. 1, reference numeral 
16 is an exchange, 18 is an optical subscriber unit (OSU) which terminates 
the optical transmission line at the central office end, and 20 are 
optical network units (ONUs) which terminate the optical transmission line 
at the subscriber end. 
The optical coupler 14 distributes downstream optical signals from the 
central office to the respective subscribers, and combines upstream 
optical signals being sent from the subscribers to the central office. As 
a method for multiplexing a plurality of subscribers on a single optical 
transmission line, time division multiple access (TDMA), where the 
subscribers transmit signals using different time slots, is used for 
upstream transmission, and time division multiplexing (TDM) or time 
compression multiplexing (TCM) is used for downstream signals, as shown in 
FIG. 2. 
The passive optical network, however, involves the problem that a failure 
caused by a faulty ONU affects the communications between the central 
office and other subscribers. For example, as shown in FIG. 3, when a 
certain ONU (ONU 2) has gone faulty and, because of malfunctioning of the 
unit, has emitted an optical signal in time slot 24, not time slot 22 
assigned to it, the communication of the optical subscriber (ONU 1) using 
a time slot overlapping that time slot is interfered with as shown by 
hatching in FIG. 3, bringing down the communication service. Further, if 
light is continuously emitted because of a failure of laser beam control 
circuitry or the like in the ONU, communications of all the subscribers 
served by the central office through the same optical coupler will be 
brought down. 
FIG. 4 is a diagram showing a first embodiment of the present invention. 
The first embodiment of the present invention is particularly suitable for 
a system where a signal of wavelength .lambda..sub.1 and a signal of 
wavelength .lambda..sub.2 are transmitted between the OSU 18 and the ONUs 
20 by wavelength division multiplexing (WDM). 
Here, consider a situation where interference has occurred to 
communications at the wavelength .lambda..sub.1 due to an illegal optical 
signal generation because of faulty optical transmission circuitry in a 
certain ONU, and communications at the wavelength .lambda..sub.1 have been 
brought down for all or some of the subscribers. 
Using a downstream signal at the wavelength .lambda..sub.2 not affected by 
the interference, the OSU 18 sends a control signal to each ONU 20 in 
sequence, causing the respective ONUs 20 to stop outputting optical 
signals. In response to the control signal, each ONU 20 shuts down its 
upstream optical signal transmission going from the ONU 20 to the OSU 18. 
In the process of shutting down the ONUs 20 in sequence, communications at 
the wavelength .lambda..sub.1 will be restored when the faulty ONU is shut 
down. The ONU responsible for the failure can thus be located. When the 
faulty ONU has been located, the sequential ONU shutdown process is 
terminated, and the central office again sends a control signal to cause 
the ONUs, excluding the faulty ONU, to resume optical signal transmission. 
In this way, communications at the wavelength .lambda..sub.1 are secured, 
excluding the faulty ONU. 
An operational sequence for locating and isolating a faulty ONU will be 
described in detail with reference to FIG. 5. Consider the case where ONUs 
20-1 to 20-16 for subscribers #1 to #16 are connected to the OSU 18, and 
communications are down for subscribers #1, #2, and #3 with the unit of #2 
responsible for the failure, as shown in part (a). The OSU 18 first sends 
an optical signal stopping control signal to the subscriber #2 (part (b)). 
Since communications for the subscribers #2 and #3 are not restored when 
the optical output of the subscriber #1 is stopped, the OSU 18 then causes 
the subscriber #2 to stop outputting optical signals (part (c)). If 
communications for the subscriber #3 are restored when the optical signal 
output of the subscriber #2 is stopped, the OSU 18 determines that the ONU 
20-2 is faulty. Since the faulty ONU has been located, the OSU 18 does not 
deactivate the ONU 20-3, but reactivates the optical output of the normal 
ONU 20-1 (part (d)). With this sequence, the determination and isolation 
of the faulty ONU is completed. The OSU 18 then transfers the ID of the 
thus located ONU to a supervisory control unit 26 controlling the OSU 18, 
and the control unit 26 indicates the faulty ONU to maintenance personnel. 
The network maintenance personnel perform maintenance work on the 
equipment of the thus located subscriber ID. 
FIG. 6 shows an example of the detailed configuration of the OSU and ONU in 
the first embodiment of the present invention. In this example, control 
signals for deactivating and reactivating each individual ONU are sent out 
using the wavelength of a wideband video signal of 1.5-.mu.m wavelength 
which is broadcast from the central office to all the subscribers by being 
wavelength-division multiplexed with a bidirectional main signal of 
1.3-.mu.m wavelength. 
Each ONU is assigned a unique ID which is used to identify each individual 
entity. Reference numeral 28 is a main-signal multiplexer/demultiplexer 
which performs assembly and disassembly of subscriber burst 
transmit/receive signal frames. Reference numerals 30 and 32 are 
respectively an E/O and an O/E for use at 1.3 .mu.m, which respectively 
perform an electrical to optical conversion and an optical to electrical 
conversion on the main signal. Reference numeral 34 is a directional 
optical coupler which distributes and combines downstream and upstream 
signals. Reference numeral 36 is a controller which monitors upstream 
frames from the subscribers and, in the event of a fault detection, 
initiates a failure recovery sequence. The control signal from the 
controller 36 and the wideband video signal to be broadcast to the 
subscribers are combined in a combiner 38 and converted by a 1.5-.mu.m E/O 
40 into an optical signal. Reference numeral 42 is a WDM filter having a 
light-wavelength splitting/combining function, which combines the 
1.5-.mu.m optical signal from the E/O 40 and the 1.3-m optical signal from 
the optical coupler 34. Reference numeral 14 is an optical star coupler, 
which distributes downstream optical signals from the central office to 
the respective subscribers and combines upstream optical signals being 
sent from the subscribers to the central office. A single optical fiber is 
laid from the central office to the optical star coupler, and the optical 
star coupler is connected to each ONU 20 by a single optical fiber. Each 
ONU 20 includes a lightwavelength splitting/combining filter 44 which 
couples the 1.3-.mu.m optical signal and 1.5-.mu.m optical signal to 
respective optical/electrical or electrical/optical converters 48, 50, and 
52. A controller 54 monitors the control signal on the 1.5-.mu.m 
downstream signal addressed to its own ONU and controls the optical output 
of the electrical/optical converter 50. 
When a fault condition is detected in upstream frames by the 
multiplexer/demultiplexer 28 in the OSU 18, the controller 36 sends a 
control signal to each subscriber via the 1.5-.mu.m E/O 40 in accordance 
with the degree of the failure (all communications down, or communications 
down only for particular subscribers), thereby causing the affected ONUs 
to stop outputting optical signals. As a method to send the control signal 
to the subscribers, the OSU 18 employs frequency-division multiplexing 
where a carrier of a frequency different from that of the wideband video 
signal is phase-shift keying (PSK) modulated by the control signal and 
added to the video signal, as shown in FIG. 7, for example. FIG. 8 shows 
an example of the frame format of the control signal. Each ONU that 
receives the optical signal stopping control signal shuts down its optical 
signal output regardless of whether the OSU is responsible for the 
failure. One method of shutdown is to cut off the laser device current to 
the electrical/optical converter 50 by using the control signal from the 
controller. 
FIG. 9 shows an example of the failure recovery sequence performed by the 
controller 36. In FIG. 9, when the occurrence of a failure is detected 
(step 1000), a list 100 of brought down ONUS, such as the one shown in the 
figure, is created (step 1002). Next, 1 as an initial value is substituted 
for variable N (step 1004), and in step 1006, a deactivation signal is 
sent to the ONU corresponding to the value of N to deactivate that ONU. In 
this condition, it is checked to see whether the other ONUs brought down 
but not deactivated yet have been recovered from the failure (step 1008); 
if not recovered, N is incremented by 1 (step 1010), and it is determined 
whether N has reached the number, n, of brought down ONUs (step 1012). If 
N has yet to reach n, the process returns to step 1006 to repeat the 
processing in steps 1006 and 1008. If it is determined in step 1008 that 
the brought down ONUs have recovered from the failure, then the 
immediately previous, deactivated ONU corresponding to the value of N at 
that time is determined as the faulty ONU (step 1014), and the other ONUs 
are reactivated (step 1016), upon which the sequence is terminated. In 
step 1012, if N has reached n, failure recovery is determined as 
impossible, and the sequence is terminated. 
FIG. 10 shows the configuration of the ONU according to a second embodiment 
of the present invention. In this embodiment, each subscriber equipment 
monitors optical output of its own device, and stops its optical output by 
itself in the event of an illegal optical signal output such as a 
continuous light output condition. Reference numeral 46 is a directional 
coupler which redirects and combines a downstream signal from the central 
office to the subscriber and an upstream signal from the subscriber to the 
central office, for coupling into an optical/electrical converter 48 and 
an electrical/optical converter 58. Reference numeral 56 is a 
multiplexer/demultiplexer which performs assembly and disassembly of 
upstream/downstream signal frames. 
In the electrical/optical converter 58, a photodiode 62 is provided in 
close proximity to a laser device 60 that produces an optical output, to 
perform backward monitoring of the intensity of the laser light being 
emitted. This monitoring device can be constructed to also serve as a 
device for controlling the current to the light producing laser device. 
The monitor output is converted to a voltage, which is then amplified by 
an amplifier 64 to a suitable level. This signal is integrated by an 
integrator 60 having an integration time determined by a light output 
stopping reference time constant, and compared in a comparator 68 with a 
reference voltage 70 that gives a level by which light stopping is judged. 
The integration time of the light output monitor voltage is set equal to 
the length of an optical signal reference frame .tau..sub.O as shown in 
FIG. 11. The light output stopping reference voltage 70 is determined from 
an integrated value when the light output is on (mark) in any upstream 
frame period .tau..sub.S assigned to the subscriber within the reference 
frame time. In normal operating conditions, the upstream signal from the 
ONU emits a light pulse whose period is substantially shorter than the 
reference frame length, and this signal is usually coded so that the mark 
space ratio is about 50%. Using this method, abnormality of the ONU 
optical output can be effectively detected, as shown in FIG. 12. 
The signal indicating the ONU light output abnormality detected by the 
comparator 68 is transferred to a controller 72, which in response latches 
and outputs the light output stopping control signal to the 
electrical/optical converter 58 until an appropriate action is taken to 
cope with the failure, for example, until the equipment is reset. Further, 
during the period when the equipment is placed in the light stopping 
state, the controller 72 outputs an equipment alarm indication, allowing 
the subscriber to know the failure of his equipment. In response to the 
light output stopping signal, a laser current controller 74 in the 
electrical/optical converter 58 cuts off the current to the laser device 
60, thereby shutting down the light output of the equipment. 
Upon stopping the light output of the faulty ONU, communications are 
restored for the other subscribers whose communications were brought down 
because of the light output abnormality of the faulty ONU. In the second 
embodiment of the present invention, since a fault is detected only at the 
ONU end to effect the shutdown of the ONU, the faulty ONU cannot be 
identified from the OSU end. To address this, the OSU sends an activation 
request to quiescent ONUs 20, as shown in FIG. 13, and identifies a 
non-responding ONU to determine the faulty ONU. If such activation is 
initiated from the OSU side to verify the normal operating state of ONUs 
when the line is not used, since a fault can be located before the line is 
used, a failure can be isolated before communications are brought down by 
the failure. 
In the prior art, when an abnormality has occurred to an output optical 
signal due to malfunctioning of an optical network unit at a certain 
subscriber, communications of the other subscribers connected via the same 
optical redirector coupler are interrupted, in the worst case bringing 
down the communication services for all the subscribers. On the other 
hand, in a system where the present invention is applied, the time during 
which communications of non-faulty subscriber units are interrupted is 
drastically reduced. Furthermore, since the faulty optical network unit 
can be located from the central office, which was not possible with the 
prior art, work in the field can be drastically reduced.