OPTICAL RING PROTECTION APPARATUS AND METHODS

PROCEDES ET DISPOSITIF DE PROTECTION DE BOUCLE OPTIQUE

English Abstract

A wavelength division multiplexed optical communication system protects
against light path failure on a per
wavelength basis. At the source optical node an output device outputs first
and second multiple wavelength signals on respective first
and second light paths. At the sink optical node a first demultiplexer
demultiplexes the first multiple wavelength signal into separate
wavelengths, and a second demultiplexer demultiplexes the second multiple
wavelength signal into separate wavelengths. For each
wavelength signal the sink optical node includes transponders and coupler. The
coupler has inputs connected to the respective outputs
of the first and second transponders, and an output for outputting an optical
signal received at one of the first and second inputs.
A controller determines if the first transponder is outputting the first
optical signal. Inhibiting from outputting the second optical
signal. If not, the second transponder is not inhibited and the coupler
outputs the second optical signal.

French Abstract

L'invention concerne un système de communication optique multiplexé en longueur d'onde offrant une protection contre les erreurs de trajet optique longueur d'onde par longueur d'onde. Au niveau du noeud optique source, un dispositif de sortie produit un premier et un second signal à longueurs d'onde multiples sur deux trajets optiques respectifs. Au niveau du noeud optique de réception, un premier démultiplexeur effectue le démultiplexage du premier signal à longueurs d'onde multiples en longueurs d'onde séparées, un second démultiplexeur assurant le démultiplexage du second signal à longueurs d'onde multiples en longueurs d'onde séparées. Pour chaque signal à longueur d'onde, le noeud optique de réception comprend des transpondeurs et un coupleur. Ce coupleur comporte des entrées connectées à des sorties respectives desdits premier et second transpondeurs, ainsi qu'une sortie destinée à la production d'un signal optique reçu au niveau de la première ou de la seconde entrée. Une unité de commande détermine si le premier transpondeur produit le premier signal optique. Si tel est le cas, le second transpondeur ne peut pas produire le second signal optique. Dans le cas contraire, ledit second transpondeur n'est soumis à aucune restriction, le coupleur produisant alors ce second signal optique.

Claims

23

What is claimed is:


1. In a wavelength division multiplexed communication
system, an apparatus for protection against a lightpath
failure from a source optical node to a sink optical node,
comprising:
said source optical node having first and second outputs
for respectively outputting single wavelength first and
second optical signals;
a first lightpath, with said first lightpath being
coupled to the first output of said source optical node for
receiving the first optical signal;
a second light path, with said second lightpath being
coupled to the second output of said source optical node
for receiving the second optical signal;
said sink optical node having first and second inputs
and first and second outputs, with the first input being
coupled to said first lightpath for receiving the first
optical signal and for outputting the first optical signal
at the first output, with the second input being coupled to
said second lightpath for receiving the second optical
signal and for outputting the second optical signal at the
second output;
a coupler having first and second inputs and an output,
with the first and second inputs being coupled to the first
and second outputs, respectively, of said sink optical
node, and for outputting at the output an optical signal
received at one of the first and second inputs;
an input for inputting a third optical signal to a third
input of said sink optical node, for outputting the third
optical signal at a third output of said sink optical node
for transmission to said source optical node; and
a detector for detecting if said sink optical node is
outputting the third optical signal in addition to the
first optical signal, and if so, inhibiting said sink




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optical node from outputting the second optical signal so
that said coupler outputs the first optical signal, and if
not, not inhibiting said sink optical node from outputting
the second optical signal so that said coupler outputs the
second optical signal.


2. In a wavelength division multiplexed communication
system, a method of protecting against a lightpath failure
from a source optical node to a sink optical node,
comprising:
outputting at said source optical node onto a first
lightpath a single wavelength first optical signal, and
outputting onto a second lightpath a single wavelength
second optical signal;
receiving at said sink optical node from the first light
path at a first input the first optical signal, and
receiving from the second light path at a second input the
second optical signal;
outputting at said sink optical node at a first output
the first optical signal, and outputting at a second output
the second optical signal;
coupling the first and second outputs of said sink
optical node to first and second inputs, respectively, of a
coupler, with the coupler having an output for outputting
an optical signal coupled to one of the first and second
inputs thereof;
inputting a third optical signal to a third input of
said sink optical node, for outputting the third optical
signal at a third output of said sink optical node for
transmission to said source optical node; and
detecting if said sink optical node is outputting the
third optical signal in addition to the first optical
signal, and if so, inhibiting said sink optical node from
outputting the second optical signal so that said coupler
outputs the first optical signal, and if not, not




25

inhibiting said sink optical node from outputting the
second optical signal so that said coupler outputs the
second optical signal.


3. In a wavelength division multiplexed communication
system, apparatus for protecting against a failure in a
light path from a source optical node to a sink optical
node on a per-channel basis, the combination comprising:
an input for inputting a single wavelength optical
signal;
an optical splitter for splitting the input single
wavelength optical signal into first and second optical
signals;
said source optical node comprising:
a multiplexer for separately multiplexing the first and
second optical signals with other optical signals of
different wavelengths for respectively outputting on first
and second light paths first and second multiple wavelength
signals;
said sink optical node comprising:
a demultiplexer for separately demultiplexing the first
and second multiple wavelength signals into separate
wavelengths including the first and second optical signals;
first and second transponders for receiving the first
and second optical signals at respective inputs thereof,
with said first and second transponders each having an
output at which the first and second optical signals are
provided, respectively;
a coupler having first and second inputs connected to
the outputs of said first and second transponders,
respectively, and an output for outputting an optical
signal received at one of the first and second inputs
thereof; and
a detector for detecting if said first transponder is
outputting the first optical signal, and if so, inhibiting





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said second transponder from outputting the second optical
signal so that said coupler outputs the first optical
signal, and if not, not inhibiting said second transponder
so that said coupler outputs the second optical signal.


4. The wavelength division multiplexed communication system
according to claim 3, wherein the first and second optical
signals have a same wavelength.


5. The wavelength division multiplexed communication system
according to claim 3, wherein at least one of said first
and second lightpaths includes at least one intermediate
optical node.


6. The wavelength division multiplexed communication system
according to claim 5, wherein said at least one
intermediate optical node comprises one of an optical line
terminal and an add/drop multiplexer.


7. The wavelength division multiplexed communication system
according to claim 3, wherein said first and second light
paths are part of an optical network.


8. The wavelength division multiplexed communication system
according to claim 3, wherein said coupler is a passive
coupler.


9. The wavelength division multiplexed communication system
according to claim 3, wherein said sink optical node
further comprises:
an input for inputting a third optical signal at a given
wavelength to another input of said first transponder for
outputting at another output of said first transponder for
transmission to said source optical node,
with said detector further detecting if said first



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transponder is outputting the third optical signal in
addition to the first optical signal, and if so, inhibiting
said second transponder from outputting the second optical
signal so that said coupler outputs the first optical
signal, and if not, not inhibiting said second transponder
so that said coupler outputs the second optical signal.


10. In a wavelength division multiplexed communication
system, apparatus for protecting against a failure in a
light path from a source optical node to a sink optical
node, including at least one intermediate optical node,
comprising:
an output at said source optical node for outputting
first and second multiple wavelength signals on respective
first and second light paths;
said at least one intermediate node including an
add/drop multiplexer for adding/dropping at least one
wavelength to/from the first and second multiple wavelength
signals;
said sink optical node comprising:
a first demultiplexer for demultiplexing the first
multiple wavelength signal into separate wavelengths;
a second demultiplexer for demultiplexing the second
multiple wavelength signal into separate wavelengths;
said sink optical node further comprising, for each
separate wavelength:
a first transponder for receiving at an input a given
one of the separate wavelengths demultiplexed by said first
demultiplexer, and for outputting a first optical signal at
an output;
a second transponder for receiving at an input a given
one of the separate wavelengths demultiplexed by said
second demultiplexer, and for outputting a second optical
signal at an output;
a coupler having first and second inputs connected to




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the outputs of said first and second transponders,
respectively, and an output for outputting an optical
signal received at one of the first and second inputs
thereof; and
a detector for detecting if said first transponder is
outputting the first optical signal, and if so, inhibiting
said second transponder from outputting the second optical
signal so that said coupler outputs the first optical
signal, and if not, not inhibiting said second transponder
so that said coupler outputs the second optical signal.


11. The wavelength division multiplexed communication
system according to claim 10, wherein the first and second
optical signals have a same wavelength.


12. The wavelength division multiplexed communication
system according to claim 10, wherein said coupler is a
passive coupler.


13. The combination claimed in claim 10, wherein said sink
optical node further comprises:
an input for inputting a third optical signal at the
given wavelength to another input of said first transponder
for outputting at another output of said first transponder
for transmission to said source optical node,
with said detector further detecting if said first
transponder is outputting the third optical signal in
addition to the first optical signal, and if so, inhibiting
said second transponder from outputting the second optical
signal so that said coupler outputs the first optical
signal, and if not, not inhibiting said second transponder
so that said coupler outputs the second optical signal.


14. A wavelength division multiplexed communication system,
comprising:




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a source optical node having first and second outputs
through which respective first and second optical signals
are outputted;
a sink optical node having first and second inputs and
first and second outputs;
a first lightpath coupled between the first output of
said source optical node and the first input of said sink
optical node;
a second light path coupled between the second output of
said source optical node and the second input of said sink
optical node; and
a passive optical coupler, having a first input coupled
to the first output of said sink optical node, a second
input coupled to the second output of said sink optical
node, and an output coupled through the passive optical
coupler to the first and second inputs of said passive
optical coupler,
wherein at least one of said source optical node and
said sink optical node further comprises a controller,
wherein at least one of said source optical node and said
sink optical node further comprises a third signal input,
arranged to input a third optical signal into the at least
one of said source optical node and said sink optical node,
and a third signal output, arranged to output the third
optical signal from the at least one of said source optical
node and said sink optical node, and wherein said
controller determines if the first optical signal and the
third optical signal are present in said at least one of
said source optical node and said sink optical node, and if
so, said controller inhibits the second optical signal from
being provided to the second input of said passive optical
coupler, and permits the first optical signal to be
provided to the first input of said passive optical
coupler, and if not, said controller inhibits the first
optical signal from being provided to the first input of




30



said passive optical coupler and permits the second optical
signal to be provided to the second input of said passive
optical coupler.


15. An optical communication system, comprising:
plural optical communication paths arranged to propagate
corresponding optical signals;
a passive optical coupler having a plurality of inputs
coupled through said passive optical coupler to an output
of said passive optical coupler, each of the inputs being
coupled in a corresponding one of said optical
communication paths; and
at least one optical node interposed in the plural
optical communication paths, each at least one optical node
comprising at least one sub-node arranged to perform
detecting of at least one predetermined condition
indicating that there has been a failure in at least one of
the plural optical communication paths, and being
responsive to detecting the at least one predetermined
condition by preventing an optical signal from continuing
to propagate in the at least one optical communication path
towards a corresponding one of the inputs of said passive
optical coupler coupled in that at least one optical
communication path, while permitting at least one other
optical signal to propagate towards at least one other
corresponding input of said passive optical coupler through
at least one other corresponding optical communication
path, wherein each of the optical signals has a same
wavelength, and
wherein each at least one sub-node comprises:
at least one first transponder coupled in the at least
one optical communication path; and
a first controller coupled to said at least one first
transponder, said first controller arranged to perform the
detecting of the at least one predetermined condition, and




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being responsive to detecting the at least one
predetermined condition for controlling said at least one
first transponder so as to substantially prevent said at
least one first transponder from propagating the optical
signal in the at least one optical communication path.


16. The optical communication system according to claim 15,
further comprising at least one of an intermediate optical
node and an optical network coupled in said plural optical
communication paths.


17. The optical communication system according to claim 15,
wherein the at least one sub-node includes a first sub-node
and a second sub-node, the first sub-node comprising said
at least one first transponder and said first controller,
the second sub-node comprising:
at least one second transponder coupled in the at least
one other corresponding optical communication path, and
at least one second controller coupled to said at least
one second transponder, said at least one second controller
arranged to detect the at least one predetermined condition
indicating that there has been a failure in the at least
one other corresponding optical communication path, and
being responsive thereto by controlling said at least one
second transponder so as to substantially prevent said at
least one second transponder from propagating the at least
one other optical signal in the at least one other
corresponding optical communication path, and, if the at
least one predetermined condition is not detected by said
at least one second controller, said at least one second
controller does not prevent the at least one second
transponder from propagating the at least one other optical
signal in the at least one other corresponding optical
communication path.





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18. The optical communication system according to claim 17,
wherein each of the optical signals has substantially a
same wavelength.


19. The optical communication system according to claim 18,
further comprising at least one further optical node
interposed in the plural optical communication paths, each
at least one further optical node comprising:
a third sub-node comprising:
at least one third transponder coupled in the at least
one optical communication path, and
at least one third controller coupled to said at least
one third transponder, said at least one third controller
arranged to detect the at least one predetermined condition
indicating that there has been a failure in the at least
one optical communication path, and being responsive
thereto by controlling said at least one third transponder
so as to substantially prevent said at least one third
transponder from propagating the optical signal in the at
least one optical communication path, and, if the at least
one predetermined condition is not detected by said at
least one third controller, said at least one third
controller does not prevent the at least one third
transponder from propagating the optical signal in the at
least one optical communication path; and
a fourth sub-node comprising:
at least one fourth transponder coupled in the at least
one other corresponding optical communication path, and at
least one fourth controller coupled to said at least one
fourth transponder, said at least one fourth controller
arranged to detect the at least one predetermined condition
indicating that there has been a failure in the at least
one other corresponding optical communication path, and
being responsive thereto by controlling said at least one
fourth transponder so as to substantially prevent said at




33



least one fourth transponder from propagating the at least
one other optical signal in the at least one other
corresponding optical communication path, and, if the at
least one predetermined condition is not detected by said
at least one fourth controller, said at least one fourth
controller does not prevent the at least one fourth
transponder from propagating the at least one other optical
signal in the at least one other corresponding optical
communication path.


20. The optical communication system according to claim 19,
wherein said first sub-node further comprises at least one
first multiplexer having at least one input and an output
coupled in the at least one optical communication path,
after the at least one first transponder, said second sub-
node further comprises at least one second multiplexer
having at least one input and an output coupled in the at
least one other corresponding optical communication path,
after the at least one second transponder, the third sub-
node further comprises at least one first demultiplexer
having an input and at least one output coupled in the at
least one optical communication path, before the at least
one third transponder, and the fourth sub-node further
comprises at least one second demultiplexer having an input
and at least one output coupled in the at least one other
corresponding optical communication path, before the at
least one fourth transponder.


21. An optical communication system, comprising:
plural optical communication paths;
a passive optical coupler having a plurality of inputs
coupled through said passive optical coupler to an output
of said passive optical coupler, each of the inputs also
being coupled in a corresponding one of said optical
communication paths; and




34



at least one optical node, each comprising:
plural optical line terminals, each of said plural
optical line terminals being interposed in a corresponding
at least one of said plural optical communication paths,
each of said plural optical line terminals being operable
to either (a) substantially prevent an optical signal
propagating in that corresponding at least one optical
communication path from reaching a corresponding one of the
inputs of said passive optical coupler through the at least
one optical communication path, or (b) permit the optical
signal to propagate in that corresponding at least one
optical communication path towards the corresponding one of
the inputs of said passive optical coupler,
wherein each of said optical line terminals comprises:
at least one transponder coupled in the corresponding at
least one of the optical communication paths in which the
optical line terminal is interposed; and
a controller coupled to said at least one transponder,
said controller being operable to control said at least one
transponder to cause said at least one transponder to
either substantially prevent the optical signal propagating
in the corresponding at least one optical communication
path from reaching the corresponding one of the inputs of
said passive optical coupler, or propagate the optical
signal in the corresponding at least one optical
communication path towards the corresponding one of the
inputs of said passive optical coupler.


22. The optical communication system according to claim 21,
wherein each of the optical signals has a same wavelength.

23. The optical communication system according to claim 21,
wherein in a case where a first one of said optical line
terminals operates to substantially prevent an optical
signal of a predetermined wavelength from reaching said




35



passive optical coupler, a second one of said optical line
terminals operates to permit an optical signal of the
predetermined wavelength to propagate towards said passive
optical coupler.


24. The optical communication system according to claim 21,
wherein said controller has an input and is operable in
response to information being applied to the input.


25. The optical communication system according to claim 24,
wherein if the information is indicative of a failure in
the corresponding at least one optical communication path,
said controller of a corresponding one of said optical line
terminals controls said at least one transponder of a same
one of said optical line terminals to substantially prevent
the optical signal propagating in the corresponding at
least one corresponding optical communication path from
reaching the corresponding input of said passive optical
coupler.


26. The optical communication system according to claim 21,
wherein each of said optical line terminals is operable to
substantially prevent the optical signal from reaching the
corresponding one of the inputs of said passive optical
coupler through the at least one optical communication
path, in response to receiving a predetermined message
forwarded from another one of said optical line terminals.

27. The optical communication system according to claim 26,
wherein the predetermined message indicates that a
communication path failure has been recognized by the one
of said optical line terminals which forwarded the
predetermined message.


28. A wavelength division multiplexed communication system


36
for protecting against a failure in a light path, the
communication system comprising:
an optical splitter, arranged to split an applied single
wavelength optical signal into first and second optical
signals;
a source optical node comprising:
at least one multiplexer, coupled to the first and
second optical signals, and arranged to separately
multiplex the first and second optical signals with other
optical signals of different wavelengths to respectively
output on first and second light paths first and second
multiple wavelength signals including the first and second
optical signals, respectively;
a sink optical node comprising:
at least one demultiplexer, coupled to the first and
second multiple wavelength signals through the first and
second light paths, respectively, and arranged to
separately demultiplex the first and second multiple
wavelength signals, which include at least the first and
second optical signals, respectively, into separate
wavelengths, and
first and second transponders having inputs arranged to
receive the first and second optical signals, respectively,
from said at least one demultiplexer, said first and second
transponders each having an output at which the first and
second optical signals, respectively, are outputted; and
a coupler having a first input coupled to the output of
the first transponder, and a second input coupled to the
output of the second transponder, said coupler also having
a corresponding output for outputting an optical signal
applied to at least one of the first and second inputs
thereof,
wherein said sink optical node further comprises a
controller which is operable to either (a) control the
second transponder to substantially prevent the second


37
transponder from outputting the second optical signal,
while permitting the first transponder to output the first
optical signal towards the first input of said coupler, or
(b) control the first transponder to substantially prevent
the first transponder from outputting the first optical
signal, while permitting the second transponder to output
the second optical signal towards the second input of said
coupler.

29. A wavelength division multiplexed communication system
for protecting against a failure in a light path, the
communication system comprising:
a source optical node, arranged to output first and
second multiple wavelength signals on respective first and
second light paths;
at least one intermediate node including an add/drop
multiplexer, arranged to add/drop at least one wavelength
to/from the first and second multiple wavelength signals;
a sink optical node comprising:
a first demultiplexer coupled to the first multiple
wavelength signal through the first light path, and
arranged to multiplex the first multiple wavelength signal
into separate wavelengths,
a second demultiplexer coupled to the second multiple
wavelength signal through the second light path, and
arranged to demultiplex the second multiple wavelength
signal into separate wavelengths,
for each separate wavelength separated by said first
demultiplexer, a first transponder having an input which
receives the separate wavelength, and an output which
outputs a corresponding first signal,
for each separate wavelength separated by said second
demultiplexer, a second transponder which has an input
which receives the separate wavelength from said second
demultiplexer, and an output which outputs a corresponding


38
second signal; and
at least one coupler having first and second inputs
connected to the outputs of the first and second
transponders, respectively, and an output for outputting an
optical signal applied to at least one of the first and
second inputs thereof,
wherein said sink optical node further comprises a
controller which is operable to perform either (a)
controlling the second transponder to substantially prevent
the second transponder from outputting the second signal,
while permitting the first transponder to output the first
signal towards the first input of said coupler, or (b)
controlling the first transponder to substantially prevent
the first transponder from outputting the first signal,
while permitting the second transponder to output the
second signal towards the second input of said coupler.

30. A wavelength division multiplexed communication system
as set forth in claim 29, wherein said controller performs
at least one of (a) and (b) based on whether said
controller receives information indicative of a failure
condition.

31. An optical communication system, comprising:
plural optical communication paths;
a passive optical coupler having a plurality of inputs
coupled through said passive optical coupler to an output
of said passive optical coupler, each of the inputs also
being coupled in a corresponding one of said optical
communication paths; and
at least one optical node, each comprising:
plural optical line terminals, each of said plural
optical line terminals being interposed in a corresponding
at least one of said plural optical communication paths,
each of said plural optical line terminals being operable


39
to either (a) substantially prevent an optical signal
propagating in that corresponding at least one optical
communication path from reaching a corresponding one of the
inputs of said passive optical coupler through the at least
one optical communication path, or (b) permit the optical
signal to propagate in that corresponding at least one
optical communication path towards the corresponding one of
the inputs of said passive optical coupler,
wherein each of said optical line terminals is operable
to substantially prevent the optical signal from reaching
the corresponding one of the inputs of said passive optical
coupler through the at least one optical communication
path, in response to receiving a predetermined message
forwarded from another one of said optical line terminals.
32. The optical communication system according to claim 31,
wherein each of the optical signals has a same wavelength.
33. The optical communication system according to claim 31,
wherein in a case where a first one of said optical line
terminals operates to substantially prevent an optical
signal of a predetermined wavelength from reaching said
passive optical coupler, a second one of said optical line
terminals operates to permit an optical signal of the
predetermined wavelength to propagate towards said passive
optical coupler.

34. The optical communication system according to claim 31,
wherein each of said optical line terminals comprises:
at least one transponder coupled in the corresponding at
least one of the optical communication paths in which the
optical line terminal is interposed; and
a controller coupled to said at least one transponder,
said controller being operable to control said at least one
transponder to cause said at least one transponder to


40
either substantially prevent the optical signal propagating
in the corresponding at least one optical communication
path from reaching the corresponding one of the inputs of
said passive optical coupler, or propagate the optical
signal in the corresponding at least one optical
communication path towards the corresponding one of the
inputs of said passive optical coupler.

35. The optical communication system according to claim 34,
wherein said controller has an input and is operable in
response to information being applied to the input.

36. The optical communication system according to claim 35,
wherein if the information is indicative of a failure in
the corresponding at least one optical communication path,
said controller of a corresponding one of said optical line
terminals controls said at least one transponder of a same
one of said optical line terminals to substantially prevent
the optical signal propagating in the corresponding at
least one corresponding optical communication path from
reaching the corresponding input of said passive optical
coupler.

37. The optical communication system according to claim 31,
wherein the predetermined message indicates that a
communication path failure has been recognized by the one
of said optical line terminals which forwarded the
predetermined message.

Description

CA 02394163 2002-06-11

WO 01/45311 PCTIUSOO/33720
TITLE
OPTICAL RING PROTECTION APPARATUS AND METHODS

10

Field of the Invention
The invention is in the field of optical
communications, and more particularly, pertains to a
Wavelength Division Multiplexed (WDM) optical
communication system in which light path failure is

protected against on a per-channel wavelength basis
using passive optics such as splitters and couplers.
Background Of The Invention

In WDM links protection against light-path failure is
typically at a facility level utilizing active optics
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such as optical switches. For example, as shown in
Fig. 1, a double light path connection for a multiple
wavelength signal, i.e. a light path connection having
a working optical fiber 2 and a redundant optical fiber

4 is connected between optical nodes 6 and 8 in an
optical telecommunication system.

At the node 8 of the receive side, the two optical
fibers 2 and 4 are combined by an optical switch 10 via
which the working optical fiber 2 is connected to the

node 8 of the receive side during normal operation,
when a demultiplexer 3 demultiplexes the multiple
wavelength into separate individual wavelengths. If a
break occurs in the working optical fiber 2, which can
be identified at the switch 10 on the basis of the

outage of the light transmitted over the working
optical fiber 2 , the switch 10 automatically switches,
so that the redundant optical fiber 4 is now connected
to the node 8 of the receive side instead of the

working optical fiber 2.

At the node 6 of the transmission side a multiplexer 5
multiplexes a plurality of input separate wavelengths
into an optical multiple wavelength facility signal to
be transmitted, which is split onto the working fiber 2

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and the redundant optical fiber 4 by an optical
splitter 12. Two optical switches 14 and 16, both of
which are closed in normal operation, are now inserted
between the optical splitter 12 and the optical fibers
2 and 4. In case of an alternate circuiting, i.e.

given a switching at the receive side from the working
optical fiber 2 onto the redundant fiber 4, let the
node 6 of the transmission side receive a message
during the course of a corresponding protocol. In

response thereto that optical switch 16 of the two
switches 14 and 16, which is inserted between the
redundant optical fiber 4 and the optical splitter 12,
continues to remain closed. In contrast, optical
switch 14, which is inserted between the optical

splitter 12 and the working optical fiber 2, is opened.
Thus, by way of the switching at the receive side, the
interrupt time associated with the alternate circuiting
continues to be kept short, on the one hand, and, on

the other hand, it is assured that shortly after the
interruption that the broken fiber no longer carries
the optical multiple wavelength facility signal.

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A problem with WDM system protecting against light path
failure at the facility level is that such protection
protects only against breaks in the optical fiber
carrying the multiple wavelength facility signal, and

does not protect against failures on a per channel
basis in the optical fiber, and at the respective
optical nodes.

Summary of the Invention

In view of the above, it is an aspect of the invention
to protect against light path failures on a per-channel
basis using passive optics such as splitters and
couplers.

It is another aspect of the invention to protect
against light path failure from a source optical node
to a sink optical node via at least one intermediate
optical node on a per wavelength basis. At the source
optical node an output means outputs first and second

multiple wavelength signals on respective first and
second light paths. The intermediate node is situated
in at least one of the first and second light paths and
includes an add/drop multiplexer for adding/dropping at
least one wavelength to/from the first and second

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multiple wavelength signals. At the sink optical node
a first demultiplexer demultiplexes the first multiple
wavelength signal into separate wavelengths, and a
second demultiplexer demultiplexes the second multiple

wavelength signal into separate wavelengths. For each
demultiplexed separate wavelength signal the sink
optical node further includes first and second
transponders and a coupler. The first transponder
receives a given one of the separate wavelengths

demultiplexed by the first demultiplexer, and outputs a
first optical signal at an output. The second
transponder receives a given one of the separate
wavelengths demultiplexed by the second demultiplexer,
and outputs a second optical signal at an output. The

coupler has first and second inputs connected to the
respective outputs of the first and second
transponders, and an output for outputting an optical
signal received at one of the first and second inputs
thereof. A determining means determines if the first

transponder is outputting the first optical signal. If
so, the second transponder is inhibited from outputting
the second optical signal so that the coupler outputs
the first optical signal. If not, the second

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transponder is not inhibited and the coupler outputs
the second optical signal.

Brief Description of The Drawings

Fig. 1 is a schematic diagram of a prior art scheme for
protecting against light path failure at a multiple
wavelength facility level;

Figs. 2A and 2B when taken together as shown in Fig. 2
form a block diagram of a WDM optical communication
system in which light path failure is protected against
on a per-channel basis; and

Figs. 3A, 3B and 3C when taken together as shown in
Fig. 3 form a flow chart of the control mechanism for
protecting against light path failure on a per-channel
basis.

Detailed Description

Refer now to Fig. 2 which is a block diagram of a WDM
optical communication system 50 for protecting against
light path failure on a per-channel basis when
transmitting optical signals from a source optical node
51 to a sink optical node 52 via an optical network 76.

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In the description that follows system operation is
described for a single given wavelength in a plurality
of wavelengths that are propagated from the source
optical node 51 to the sink optical node 52 as a

multiple wavelength facility signal, and that at least
one intermediate node may be included in the light path
which includes an add/drop multiplexer for
adding/dropping wavelengths to/from the multiple
wavelength facility signal. It is to be appreciated

that the remaining ones of the plurality of wavelengths
are propagated in a like manner. Likewise, it is
understood that the plurality of wavelengths are
propagated in the reverse direction from the sink
optical node 52 to the source optical node 51 in a

similar manner.

Referring to Fig. 2A, the WDM system 50 includes the
source node 51 having a client equipment 53 which
outputs an optical signal at a given wavelength to an

optical splitter 54 which splits the input optical
signal into first and second optical signals of the
given wavelength which are input to Optical Line
Terminals (OLT's) 56 and 58, respectively. In
practice, there are other client equipments (not shown)

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which input other wavelengths to other optical
splitters (not shown), which split the other
wavelengths into respective first and second optical
signals for input to OLT's 56 and 58, respectively.

The client equipment 53 may be any one of a computer, a
SONET terminal, a telephone switch, a central office
switch for telephones, a digital cross-connect switch,
an end device such as a terminal, or the like.

OLT 56 includes a transponder 60, a processor 62 and a
multiplexer 64. It is understood that in practice, OLT
56 also includes a demultiplexer (not shown) for
propagating optical signals received in the opposite
direction from the sink node 52 to the source node 51.

It is also understood that OLT 56 includes other
transponders (not shown) for receiving other first
optical signals at different wavelengths from the other
optical splitters (not shown).

The processor 62 receives system protocols and
Identification Codes (ID's) from a system manager
computer (not shown) on line 66, and reports back
status and the like on that line to the system manager

computer. The processor 62 controls and exchanges
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information with transponder 60 via line 68a, and
exchanges information with the other transponders (not
shown), via lines 68b-68n, which provide remaining ones
of the plurality of wavelengths to the multiplexer 64

on lines 70b-70n. The processor 62 communicates with a
corresponding processor 80 in OLT 58 via line 72.

The first optical signal at the given wavelength is
received at a Portside Input (PI) interface for

transponder 60 (T1), which interface is termed PI(TI)
and is output at a Lineside Output (LO) interface for
transponder 60 (T1), which interface is termed LO(T1).
PI(T1) and LO(T1) serve as test points to test for the
presence of the first optical signal at the input and

output, respectively, of transponder 60. The first
optical signal output from transponder 60 on line 70a
is multiplexed with the other wavelengths on lines 70b-
70n by multiplexer 64 to form a first multiple
wavelength optical facility signal which is output on

optical fiber 74 to the network 76.

A third optical signal at the given wavelength is
received at a lineside input (LI) interface for
transponder 60 (T1), which interface is termed LI(T1).

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The third optical signal is then provided to client
equipment 52 via a coupler (not shown). The third
optical signal is demultiplexed from a third multiple
wavelength facility signal which is provided to a

demultiplexer (not shown) in OLT 56 from interface
LO(T1') of a transponder 99 in an OLT 94 at sink node
52 via the network 76. This is described in more
detail with respect to Fig. 2B.

OLT 58 includes a transponder 78, a processor 80 and a
multiplexer 82. It is understood that in practice, OLT
58 also includes a demultiplexer (not shown) for
propagating optical signals received in the opposite
direction from the sink node 53 to the source node 51.

It is understood that OLT 58 includes other
transponders (not shown) for receiving other second
optical signals at different wavelengths from the other
optical splitters (not shown).

The processor 80 receives system protocols and ID codes
from the system manager (not shown) on line 82 and
reports back status and the like on that line to the
system manager. The processor 80 controls and
exchanges information with transponder 78 via line 84a,

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and exchanges information with the other transponders
(not shown), via lines 84b-84n, which provide remaining
ones of the plurality of wavelengths to the multiplexer
82 on lines 86b-86n. The processor 80 communicates

with processor 62 of OLT 56 via the line 72.

The second optical signal at the given wavelength is
received at a portside input interface PI(T2) for
transponder 78 and is output at a lineside output

interface LO(T2). PI(T2) and LO(T2) serve as test
points to test for the presence of the second optical
signal at the input and output, respectively, of
transponder 78. The second optical signal output from
transponder 78 on line 86a is multiplexed with the

other wavelengths on lines 86b-86n by multiplexer 82 to
form a second multiple wavelength optical facility
signal which is output on optical fiber 82 to the
network 76.

A fourth optical signal at the given wavelength is
received at a lineside input (LI) interface for
transponder 78 (T2), which interface is termed LI(T2).
The fourth optical signal is then provided to client
equipment 53 via a coupler (not shown). The fourth

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optical signal is demultiplxed from a fourth multiple
wavelength facility signal which is provided to a
demultiplexer (not shown) in OLT 58 from interface
LO(T2') of a transponder 112 in an OLT 96 at sink node

52 via the network 76. This is described in more
detail with respect to Fig. 2B.

The network 76, for example, may be a point-to-point
link, a point-to-multi-point link, a ring, a mesh or
any other network configuration including intermediate

optical nodes such as OLTs or Optical Add/Drop
Multiplexers.

The network 76 then outputs the first and second

multiple wavelength facility signals on optical fibers
90 and 92, respectively, to the sink node 52.
Referring to Fig. 2B, the sink node 52 includes OLT's
94 and 96 and a client equipment 98. The client

equipment may be any one of a computer, a SONET
terminal, a telephone switch, a central office switch
for telephones, a digital cross-connect switch, an end
device such as a terminal, or the like.

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OLT 94 includes a demultiplexer 97, a transponder 99
and a processor 100. It is understood that in
practice, OLT 94 also includes a multiplexer (not
shown) for propagating optical signals in the opposite

direction from the sink node 52 to the source node 51.
The processor 100 receives system protocols and IDs
from the system manager computer (not shown) on line
102 and reports back status and the like on that line

to the system manager computer. The processor 100
controls and exchanges information with transponder 99
via line 102a, and exchanges information with other
transponders (not shown), via lines 102b-102n, which
receive remaining ones of the plurality of wavelengths

from the demultiplexer 97 on lines 104b-104n. The
processor 100 communicates with a corresponding
processor in OLT 96 via line 106.

The first optical signal demultiplexed from the first
facility signal by demultiplexer 97 is provided on line
104a to lineside input interface LI(T1') of transponder
99 and is output at portside output.interface PO(T1')
to an optical coupler 108.

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For propagation of an optical signal at the given
wavelength in the opposite direction from the sink node
53 to the source node 51, the client equipment 98
provides an optical signal at the given wavelength to

an optical splitter 109 which splits that signal into
third and fourth optical signals at the given
wavelength for provision to OLT's 94 and 96,
respectively.

The third optical signal at the given wavelength is
received at a portside input port interface PI(TI') of
transponder 99 and is output at a lineside output port
interface LO(T1') thereof for provision via the network
76 to a multiplexer (not shown) in OLT 94 which

generates a third multiple wavelength facility signal
for provision to interface LI(Tl) of OLT 56 of source
node 51 via the network 76.

Operability of transponder 99 is determined by testing
for the presence of the first optical signal at

interf ace LI(T1') and PO(T1'), and by testing for the
presence of the third optical signal at interface
PI(T1') and LO(T1'). This is explained in more detail
with respect to Fig. 3.

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OLT 96 includes a demultiplexer 110, a transponder 112
and a processor 114. It is understood that in
practice, OLT 96 also includes a multiplexer (not
shown) for propagating optical signals in the opposite

direction from the sink node 52 to the source node 51.
The processor 114 receives system protocols and ID's
from the system manager computer (not shown) on line
116 and reports back status and the like on that line

to the system manager computer. The processor 114
controls and exchanges information with transponder 112
via line 118a, and exchanges information with other
transponders (not shown), via lines 118b-118n, which
receive remaining ones of the plurality of wavelengths

from the demultiplexer 110 on lines 120b-120n. The
processor 114 communicates with processor 100 in OLT 94
via the line 106.

The second optical signal demultiplexed from the second
facility signal by demultiplexer 110 is provided on
line 120a to lineside input interface LI(T2') of
transponder 112 and is output at portside output
interface PO(T2') to the output coupler 108.

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The fourth optical signal at the given wavelength is
received from splitter 108 at a portside input
interface PI(T2') of transponder 112 and is output at a
lineside output port interface LO(T2') thereof for

provision via network 76 to a multiplexer (not shown)
in OLT 96 which generates a fourth multiple wavelength
facility signal for provision to interface LI(T2) of
OLT 58 of source node 51 via the network 76.

As discussed above, the coupler 108 is connected to
interface PO(T1') of transponder 99 of OLT 94 and
interface PO(T2') of transponder 112 of OLT 96 for
receiving either the first optical signal or the second

optical signal as controlled by processor 100 of OLT 94
according to the control flow chart of Fig. 3 for
outputting the received optical signal to client
equipment 98. If it is determined that transponder 99
is transmitting the first and third optical signals,
transponder 112 of OLT 96 is inhibited from outputting

the second optical signal and coupler 108 only receives
the first optical signal from transponder 99 of OLT 94,
which in turn is provided to client equipment 98. On
the other hand, if it is determined that transponder 99
is not transmitting either one of the first and third

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optical signals, transponder 99 in OLT 94 is inhibited
from outputting the first optical signal and the
coupler 108 only receives the second optical signal
from transponder 112 OF OLT 96, which in turn is

provided to client equipment 98. This is explained in
more detail below with respect to Fig. 3.

In practice, there are other couplers (not shown), each
receiving other first and second optical signals from
the other transponders (not shown) for outputting one

of the other first and optical signals to other client
equipment (not shown). Likewise, couplers (not shown)
are used to source node 51 for coupling respective
wavelengths received from sink node 52 via the network

76 to other client equipment.

Also in practice, there are other splitters (now shown)
for coupling other individual wavelengths from other
client equipment (not shown) to the other transponders

(not shown) in OLT's 94 and 96.

Figure 3 is a block diagram of the control protocol for
protecting against light path failures on a per-channel
basis. For purposes of explanation, the control

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protocol is described as being run on processor 100 of
OLT 94. However, it is to be appreciated that a like
protocol is run on processor 114 of OLT 96, and is also
run on processor 62 of OLT 56 and processor 80 of OLT

58.

Referring to Fig. 3A, at step S1 processor 100 of OLT
94 gets an ID of 100 from the system manager computer
(not shown) via the system management interface, such

as CMIPT, SNMP, or TL1, and likewise the processor 114
of OLT 96 gets an ID of 20. Likewise, the system
management interface provides IDs for processors 62 and
82 of OLTs 56 and 58, respectively. These ID's are
unique, and for purposes of explanation the ID of OLT

94 (OLT 1') is 100 and the ID of OLT 96 (OLT 2') is 20.
Therefore, the ID of OLT 1' > ID of OLT 2'. The ID of
OLT 56 (OLT 1) is 10 and the ID of OLT 58 (OLT 2) is 2.
Therefore, the ID of OLT 1 > ID of OLT 2.

The following steps determine the operability of
transponder (Ti') 99 of OLT 94 based on the failure to
detect light at the respective interfaces of
transponder 99. At step S2, if transponder (T1') 99 is
OFF, it is turned ON. At step S3 a determination is

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made as to whether or not there has been a failure to
detect light at interface PI(Tl'). If light is
detected at step S3, at step S4 a determination is made
as to whether or not interface PI(T1') doesn't receive

a SONET frame. If PI(T1') does receive a SONET frame,
at step S5 a determination is made as to whether or not
transponder (T1') 99 has failed. If transponder 99
hasn't failed, control proceeds to step S7 (Fig. 3B).

If the answer is Yes at any one of steps S3, S4 or S5,
at step S6 a SONET Alarm Indication Signal (AIS) is
transmitted from interface LO(T1') of transponder 99 of
OLT 94 at sink node 52 to transponder 60 of OLT 56 of
source node 51 via the network 76, and control proceeds
to step S7 (Fig. 3B).

Referring to Fig. 3B, at step S7 a determination is
made as to whether or not there has been a failure to
detect light at interface LI(T1'). If light is

detected, a determination is made at step S8 as to
whether or not interface LI(T1') doesn't receive a
SONET frame. If LI(Tl') does receive a SONET frame,
at step S9 a determination is made as to whether a
SONET AIS frame is received at interface LI(T1') from

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source node 51, which is indication of a failure at
the source node 51. If not, control proceeds to step
S11 (Fig. 3C).

If the answer is YES at any one steps S7, S8 or S9, at
step S10 the transponder (T1') 99 of OLT 94 is turned
OFF and a return is made to step S2 of Fig. 3A via Al.
Since only the transponder (T2') 112 of OLT 96 is ON,
coupler 108 provides the second optical signal to

client equipment 98.

Referring to Fig. 3C, at step S11 a determination is
made as to whether or not transponder 99 (T1') of OLT
94 has failed. If it has failed, a return is made to

step S9 of Fig. 3B via t'~13, and transponder 99 (T1') of
OLT 94 is turned OFF. I` transponder 99 (T1') hasn't
failed, at step S12 processor 100 of OLT 94 sends a
message via line 106 to processor 114 of OLT 96 to turn
OFF transponder 112 (T2'), so that coupler 108 only

receives the first optical signal which is then
received by client equipment 98. At step S13 a
determination is made as to whether or not processor
100 in OLT 94 (OLT 1') is receiving a message from
processor 114 in OLT 96 ;oLT2') and if the ID of the

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received message from processor 114 of OLT 96 is
greater than the ID of processor 100 of OLT 94. That
is, is the ID (OLT 2') > ID (OLT 1'). If the answer is
YES a return is made to step S10 of Fig. 3B via A4 and

transponder 99 (Tl') of OLT 94 is turned OFF. If the
answer is NO a return is made to S2 via A2 and the
procedure is repeated. In this instance, since the
answer is No, the return is made to S2 via A2.

Thus, according to the control protocol, protection is
provided against lightpath failure, and coupler 108
only receives either the first or second optical signal
at any given time for provision to client equipment 98.

In summary, in the apparatus of the present invention
in a WDM optical communication system, light path
failure is protected against on a per-channel
wavelength basis using passive optics such as splitters
and couplers which are less susceptible to failure than

active devices such as switches.

Although certain embodiments of the invention have been
described and illustrated herein, it will be readily
apparent to those of ordinary skill in the art that a
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number of modifications and substitutions can be made
to the preferred example methods and apparatus
disclosed and described herein without departing from
the true spirit and scope of the invention.


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Representative Drawing