Method and system for hybrid protection in optical networks
Field of the Invention
The invention relates to a technique for protection of data traffic in
optical networks, including two types of traffic protection
Background of the invention
Protection of traffic in optical networks can generally be classified
under two types The first type of protection, being a concept of line
protection, is a so-called Optical Multiplex Section (OMS) protection
used preferably in multi-channel multi-section ring networks. The ring
network is formed by at least two concentric optical fiber rings capable of
carrying data traffic in two opposite directions (clockwise and
counterclockwise) between network elements NE in the nng The two
concentric fiber rings usually serve as a mam loute and a protection route
for all optical channels of the network, although sometimes the mam and
the protection routes are arranged within the same fiber. The OMS
protection ensures that in case of a fault of the traffic via a multiplex
section on the mam route (say, in a section between two specific network
elements, that may include multiple optical channels), the protection route
can be used to redirect the required data traffic from the mam route, and
thus to bring the traffic to its destination network element using the
opposite direction. In this case, the redirected traffic might pass a longer
distance than it would pass via the main route The OMS protection is
usually utilized in SDH and SONET optical networks, where it is
respectively named MSSPRING (Multiplex Section Shared Protection
Ring) and BLSR (Bi-directional Line Shared Protection).

The second known type of traffic protection in optical networks is
a so-called OCH protection (Optical Channel protection) The OCH
protection is intended for protecting a specific optical channel and by
default can be implemented by providing simultaneous transmission of
data traffic of that specific optical channel along both the main route and
the protection route In case of a fault in one of the routes, the other one
will safely bring the traffic to its destination point.
Some attempts to combine the OCH and the OMS protection
concepts are known in the art.
An article "WDM architectures and economics in Metropolitan
areas" to RH. Cardwell et al in the Optical Networks Magazine July
2000 describes a number of possible architectures combining both the
OMS and the OCH protection principles, though it does not describe
methods of how they are used together.
ITU-T standard recommendations G798 (sections 10 4 1,
12.1 1.1) and G.841 (section 8.4) describe a possibility of implementing
both of the above-mentioned protection concepts, and define so-called
switch initiation criteria. These criteria are necessary when protection
means are activated, for selecting a signal to be further used in the ring
network The switch initiation criteria are based on analyzing presence of
various alarm indications in the signals to be compared in their binary
electrical form. In addition, complex timing issues are considered for
adequate selection criteria.
US patent 6,317,426 describes a hybrid protection circuit for
selectively protecting data streams according to one of the above-
mentioned methods. Namely, the hybrid protection circuit can be
electrically configured to one of various industry-standard protection
techniques, including a bi-directional line-switched ring (BLSR)

protection, a unidirectional path switched ring (UPSR) protection, and
one-plus-one (1+1 or OCH) line protection. It should be emphasized that
the solution of the US patent 6,317,426 allows using different protection
schemes separately.
Other methods exist in the prior art, for example a method
described in an article "Optical Network Architectures: Routing and
Protection" by Antonio J. Ramos (CPRM-Marconi)
http.//www.eurescom de/public-
seminars/1998/)ADM/Proceeedings/Paperl 6.html. The article proposes
using, in ling networks, OMS protection with OCH protection, wherein
the latter is performed by electrical digital cross-connects In case of a
network fault, the traffic in the OCH level is rerouted electrically via
existing connections by SDH equipment.
To the best of the Applicants' knowledge, the relevant prior art
does not describe a method by which a hybrid OMS and OCH protection
schemes could be combined in a fully optical manner, at any desired
extent (for any number of optical channels as needed), without additional
optical network components such as optical cross-connects, and without
any form of wavelength conversion
Summary of the invention
It is therefore the object of the piesent invention to propose a
technology (a method and a system) for combined OMS and OCH
protection of data traffic in telecommunication optical networks, which
would allow simple selection of a correct and stable signal of the OCH
protected channel in any situation in the network, and overcome a
technical difficulty to distinguish signals passing via the mam and the
protection rings, and assist in solving a known problem of wavelength

routing ambiguities (the matter relevant to the presently known combined
OMS and OCH protection schemes).
The concept of the invention is creating such a technique of
combined OMS protection and OCH protection of optical traffic in an
optical communications network, which would utilize switching functions
of OMS protection activated in case of a fault in a network section for
selecting a suitable optical signal as a working signal to be finally used in
an OCH protected optical channel.
The above object can be achieved by providing a method of
carrying out protection of data traffic in a multi-channel multi-section
optical communications network by simultaneously using Optical
Multiplexed Section (OMS) protection and Optical Channel (OCH)
protection, the method comprises a step of selecting an optical signal (as a
working signal) in an OCH protected optical channel, in case of a fault in
a section of said network, by relying on an indication associated with
OMS switching functionality required to overcome said fault.
More particularly, said netwoik is a multi-channel multi-section
optical communications network comprising network elements connected
in two ring-like optical fiber configurations (contours) wherein one of
said contours is considered a main contour and the other is consideied a
protecting contour, and the method comprises.
- providing the OMS protection for at least one section of the
network by ensuring, in case of a failure in said section, switching
of the data traffic from one of said contouis to another of said
contours at each of two ends of said section,
- providing the OCH protection for at least one optical channel in the
network, by transmitting data related to said optical channel from
one of said network elements being a source point of said channel

to another of said network elements being a destination point of
said channel by means of two optical signals carrying identical
information, wherein one of said optical signals is transmitted via
the main contour and another of said optical signals is transmitted
via the protect contour,
- rejecting such one of the two optical signals of said OCH protected
channel, that has undergone switching from one of said contours to
another of said contours at least at one end of said section, to allow
using at the destination point only the other one of said optical
signals that has not undergone said switching, as the working
signal.
The concept of rejecting the optical signal of OCH protected
channel if switched by means of OMS protection to a bypass route
(and thus being either unsuitable or less preferable) differs from any
other method described in the prior art The method is advantageous in
that it simplifies the measures of preventing ambiguities in routing
optical channels in optical networks. It can practically be performed in
a number of ways, some of which will be described and illustrated
later on
It should be first clarified what are the features of the OMS
protection and OCH protection which allow using them together in a
cooperative way, according to the above-defined invention
The step of providing OMS protection to at least one section of the
network should be understood as enabling circulation of the data
traffic in said network in case of a fault in said section. It is
conventionally performed in two stages 1) at one end of the faulty
section, redirecting (switching) traffic of all optical channels, which
are to pass via the main contour of the section, to the protect contour

of the network in order to bring the data traffic up to another end of
the faulty section using the opposite direction, 2) redirecting the traffic
from the protect contour back to the main route The OMS protection
of a particular network section is usually implemented by two
switching means situated at the ends of the section, each enabling
switching of the data traffic from one of said contours to another of
said contours (i.e, to the alternative contour)
Therefore, one preferred version of rejecting the "switched" signal
is by rejecting the optical signal that carries any indication (physical
trace, imprint, artifact) of having been switched by said (OMS)
switching means.
Detecting the fact of switching the signal by the switching means
can be performed, for example, by detecting a momentary loss
inherently created in the signal while switching it in the switching
means.
Another example could be by detecting any indication in the form
of a signaling artifact upon intentionally adding it to the signal that has
been switched in the switching means. Such a signaling artifact can be
an intentionally created/amplified momentary loss, a predetermined
modulation tone, imprinting a certain known signature, etc
As has been noted above, the step of providing the OCH protection
at a particular optical channel in the network should be understood as
transmitting the data of that channel from its source point to its
destination point via both the main and the protect contours of the
network Usually, it is provided either by splitting the traffic data of
the particular optical channel, or by using two transmitters of the
particular traffic data, thereby obtaining two identical sources of the
traffic data. For example, they can be located at the network element

being the source point of the channel In the present invention,
providing the OCH protection preferably comprises providing OADM
(Optical Add Drop Multiplexer) at the souice point and at the
destination point of the OCH protected channel, both on the main
route and on the protect route of the source point and the destination
point.
The step of rejecting can be based on a different principle, for
example on the fact that a redirected optical signal just added to the
tiaffic may be dropped at the same node if its wavelength remains the
same (a so-called self-feeding). Alternatively, or in addition to the
described options, the rejecting step may comprise preliminarily
applying said indication in the form of an additional pilot tone for
transmitting data from the source point of the OCH protected channel,
and further detecting at the destination point of said OCH protected
channel an optical signal not comprising said additional pilot tone
Such a detected signal is further rejected, in order to select (as the
working signal) the optical signal having said additional pilot tone.
The OCH protection can be arranged for a unidirectional channel,
but usually it is provided in both directions (for a bi-directional
channel), so that each of the source point and the destination point
serves as a source/destination point
The OCH protection of one bi-directional channel can be
performed using either different wavelengths, or one and the same
optical carrier wavelength at its two source/destination points. When
no faults exist in the network, two optical signals launched due to the
OCH protection arrive to a source/destination point of the channel and
by default, the optical signal from the main contour is selected for
further use (considered to be the working signal).

If any section of the network is faulty, and if different carrier
wavelengths are used for OCH protection in a bi-directional optical
channel, each of the source/destination points receive two optical
signals at a wavelength different from the wavelength at which it
transmits data to the other source/destination point. At each of the
source/destination points, the undesired optical signal can be rejected
upon detecting the fact of switching thereof, and the "desired" receiver
can be then electrically selected as an output
When one and the same earner wavelength is used for OCH
protection of a bi-directional channel, one of the optical signals just
added to the main and the protect contours at a source/destination
point node will immediately be rejected by a suitable OADM when
the signal passes the same node after switching the contour In other
words, the above-mentioned problem of self-feeding is not prevented,
but intentionally created and used heie for rejecting a non-desired
switched optical signal The same situation happens at the second
source/destination point Each of the source/destination points thus
drop, at one and the same carrier wavelength, one irrelevant (self-fed)
optical signal and one relevant optical signal arriving from the second
source/destination point To select the signal required for further use
at a particular source/destination point, one should apply some type of
detection.
The detection can be performed as described above, i.e. by
detecting a characteristic imprint or an artificial signaling artifact
generated by an optical switch or by any device in association with the
optical switch responsible for the OMS protection mechanism.
Howevei, the detection can be performed by another manner, for
example by using two different pilot tones, in addition to a carrier

wavelength, for tiansmitting data from two respective
source/destination points.
Selection of the required (working) optical signal at a particular
source/destination point will be performed by* rejecting the signal having
the pilot tone which is used for transmitting data of the OCH protected
channel from that same particular source/destination point, thereby
selecting the signal having the second pilot tone (which is used for
transmitting data from the other source/destination point of the OCH
protected bi-directional channel).
By selecting the signal that has not overcome switching to an
alternative route, the network avoids reaching undesirable states such as
selecting a channel that originates from a wrong location The proposed
solution inherently assures that all optical channel designations reach the
proper (target) receiving end. Signals that, due to a fault in the network,
erroneously arrive to an OCH slot from wrong locations - will be rejected
at the stage of the OCH channel selection.
As has been noticed before, the OCH protection, which is usually
expensive when applied to each optical channel in the network, can be
selectively provided only to important optical channels, by using the
proposed invention.
The combination of the OMS and OCH protection allows
protecting some specific (for example, highly important) optical channels
more reliably At the same time, presence of the OCH protection does
not affect functions of the OMS protection with respect to other (non
OCH protected) optical channels of the network, and vice versa.
It should be noted that in OMS protected networks, OADMs (or
other optical filtering means) are customarily located only on the main
contour at nodes (network elements) where traffic data is to be added

to/dropped from the optical network According to the present
invention, OADMs (or the like) are located also on the protect
contour, but only at the nodes bemg either a source node or a
destination node of the OCH protected optical channels Owing to
that, the present invention proposes a low cost and scalable solution to
protecting optical networks in a manner such that the network operator
may select the level of protection desired for a specific channel.
Namely, for low cost protection against OMS failures such as fiber
cuts, OMS protection could suffice; howevei, for high priority
signals/channels which secure protection is mandatory, individual
OCH protection is offered to be used simultaneously with the OMS
protection The cost function foi protection of such a multi-channel
network is linear; as opposed to other protection schemes where the
addition of one OCH channel requires a large amount of equipment to
set-up, in the described solution the user pays an incremental amount
for each OCH channel he/she wishes to add to the system.
According to another aspect of the invention, there is also provided
a system capable of smiultaneously performing OMS protection and OCH
protection of optical traffic in a multi-channel multi-section optical
communications network, the system being operative to utilize switching
functions of OMS protection for simplifying implementation of the OCH
protection functions, namely for simplifying selection of a suitable and
stable optical signal of an OCH protected optical channel in case of a
fault in at least one of the sections.
In other words, the object can be achieved by a system capable of
smiultaneously carrying out Optical Multiplex Section (OMS) protection
and Optical Channel (OCH) protection of optical traffic in a multi-

channel multi-section optical communications network, the system being
operative to select an optical signal in an OCH protected optical channel in case of a fault in a section of said network by relying on an indication
associated with OMS switching functionality required to overcome said
fault.
More particularly, the network comprises network elements
connected in at least two nng-like optical fiber configurations (contours)
wherein one of said contours is considered a main contour and the other is
considered a protect contour, and the system comprises:
an OMS protection arrangement for at least one section of said network,
including switching means at both ends of said section,
an OCH protection arrangement for at least one optical channel formed in said network between a source point and a destination point, the OCH
protection arrangement including OADM (or other optical filtering
means) in a protect contour at the destination point of said optical
channel,
means for rejecting use at the destination point of an optical signal related
to the OCH protected optical channel, that has been switched at least once
from one of said contours of the network to another by the OMS
protection mechanism.
The rejecting means may be situated at the destination point of said
optical channel and compnse a detector of said indication (an imprint or
an aihfact) associated with the switching of said optical signal in the
switching means and a comparator or selector for further selecting an
optical signal not comprising said indication. The rejecting means may
further compnse an amplifier or a generator of said indication, associated
with (for example, situated within or nearby) any one of the OMS
switches in the network.

Alternatively, or in addition, the rejecting means may comprise a
modulator at the source point of the OCH protected optical channel for
applying a specific pilot tone to data transmitted via said channel, and a
tone detector (selector, filter) at the destination point of said optical
channel for further detecting and rejecting any optical signal havmg a
different pilot tone. In this embodiment, the additional pilot tone serves
said the above-mentioned indication, the selection of the suitable signal is
performed based on the presence of such an indication while rejection of
unsuitable signal is carried out in response to the absence of the
indication.
The system is preferably adapted to bi-directional OCH protected
optical channels According to one embodiment of the system, one and
the same carrier wavelength is used for transmitting data in said OCH
protected optical channel in both said directions.
The proposed method and the system can be used both for ring-
like optical networks and for mesh optical networks.
Further details of the invention will become more clear as the
description proceeds.
Brief description of the drawings.
The present invention will be further described and illustrated with
reference to the followmg non-limiting drawings, in which.
Fig. 1 illustrates one example of an optical network implementing
both the OMS and OCH protection systems of data traffic
Fig. 2 illustrates another example of an optical network
implementing the OMS and OCH data protection

Detailed description of the preferred embodiment.
Fig. 1 illustrates an exemplary optical network, in this particular
case a ring-like network 10 It comprises four network nodes 12, 14, 16
and 18 with optical amplifiers depicted as triangles, and four links formed
between the nodes. Each of the links is bi-directional and comprises two
segments of optical fibers for transmitting data traffic in two opposite
directions. Therefore, the rmg network comprises one ring formed by
optical fibers transmitting data in the counterclockwise direction (let it be
the outer ring called main ring and marked M) and one protect
(protection) ring transmitting traffic between the nodes in the clockwise
direction (the inner ring called protection ring and marked P). It should be
noted that the rings' functions may change dynamically: in the course of
operation the main ring may become a protection rmg and vice versa.
Two sections in the rmg-like network (section 11 between nodes 12 and
18, and section 13 between nodes 12 and 14) are OMS protected, say by
means of a OMSPRmg system (optical multiplexed section protected
rmg). Alternatively, the OMS protection may be implemented by BLSR
protection, or the like Optionally, the other two sections of the network
10 can also be protected in a similar way by OMS protection but, without
loss of generality, are shown as unprotected in this example To this end,
each protected section is provided with a 2x2 switch (switches 40,41, 42,
43 are shown), each allowing redirection of traffic from the main ring to
the protection rmg and vise versa if a fault is detected in some section of
the ring and a faulty section is to be isolated. Every switch has for I/O
ports 1, 2, 3,4 and operates as follows When the network is in its normal
regime, the traffic is transmitted through each specific switch directly,
along the contour to which the traffic was launched (i.e., between ports 1
and 2, and between ports 3 and 4) The internal connections marked m

the switches by curved lines can be established in case of a fault in the
ring, and only in those switches surrounding the faulty section of the ring.
If a fault occurs in any OMS protected section of the network (say, in
section 11, the fault is marked with a cross), each switch adjoining the
faulty section redirects the OMS protected multi-channel signals so as to
slap the faulty section by using an internal loop in the switch (namely, the
switches 42 and 43 now form internal connections between ports 1 and 3,
and between ports 2 and 4) The multi-channel traffic thus passes through
the protection ring, reaches the second looped switch adjoining the faulty
section, where the traffic is redirected again to the main ring and finally
arrives to its destination point(s)
The network 10 is provided with so-called DCF units (Dispersion
Compensating Fiber umts). DCF units are inserted to compensate effects
of optical chiomatic dispeision accumulated in the transmission fibers
The example further shows that in addition to OMS protection
intended for traffic of all optical channels using the protected ring
sections, one specific optical channel (added at node 18) is individually
protected by an OCH system. To this end, transmitter 20 of this channel
is connected both to OADM 22 of the main ring, and to OADM 24 which
should be placed for tins purpose on the protection ring at node 18. The
data traffic of that individually protected channel may originate from one
and the same transmitter 20 and be simply split into two portions to be
added to the two contours, or it may be transmitted from two separate
transmitters to ensure the equipment protection. The data traffic of the
discussed channel is therefore transmitted from the source node 18 (say,
using wavelength XI) via both of the rings to the destination point, being

in this example node 12, where this channel is dropped by internal
OADMs 26,28 of the node 12, at the wavelength XI.
In this example, both signals comprising data traffic of the
individually OCH protected channel from node 18 are received at
receivers 30 and 32 and are continuously compared (a comparator scheme
is marked 31) The comparator can be placed before or after the receivers
If neither of the signals is found to be switched by the switching
mechanism of OMSSpring (i.e., no faults were detected so no redirection
was provided), the system selects the signal arriving from the main ring -
more exactly, from the ring which is currently considered to be main.
But if, for example, link 11 has been, switches 42 and 43 are
switched to redirect the OMS signal (the multi-channel signal) from the
main ring to the protection ring. In this case, all OMS protected channels
that are not OCH protected, are transparently redirected through the
protection ring to D-tour the faulty link and return to the main ring. In
addition to OMS protection, the signal originating from node 18 is also
OCH protected. Smce the signal portion carried on the protection fiber is
not affected by the fault on link 11, this signal is unchanged and the
receiver 32 contmues to be accepting the transmission data. Contrary to
that, receiver 30, which receives the redirected signal from the main ring
via OADM 26, must recognize it as the OMS-switched signal which is to
be rejected according to the invention (say, the receiver 32 can be
switched by the comparator 31 to accept the transmitted data).
In general, the switched signal can acquire any characteristic
"imprint" of the switches 42 and 43 currently in the OMS protection
mode, that affects all channels passing through the switches. The imprint
can then be detected by the comparator 31 The switched signal can be

determined, for example, by detecting a momentary loss, or a particular
modulation applied in the signal upon passing through switch 43 and/or
42
If any of the signals comprises the mentioned impnnt/artifact, the
alternative signal is selected for further use, since it is considered a more
reliable signal.
If the discussed OCH protected channel is bi-directional, a
symmetric equipment set can be provided to perform the similar
operations when data traffic is transmitted from node 12 to node 18
Namely, transmitter 33 may transmit the data traffic to the two rings via
the OADMs 26 and 28, and two receivers 21 and 23 may receive and
compare the two signals at the node 18 from the two OADMs 22 and 24.
The transmitter 33 may transmit the data at a different wavelength (say,
X2\ and the OADMs 22 and 24 (and then receivers 21 and 23) will drop
the signals at A.2
However, and preferably, the transmitter 33 may use the same
wavelength XI as the tiansmitter 20, and this situation (being problematic
for many known combined protection schemes) can successfully be used
in the frame of the proposed concept.
The proposed invention enables combining the two types of
protection without a need in wavelength conversion. Continuing the
previous example where link 11 was faulty, and for a specific case where
the channel transmitted on the protect ring is of the same wavelength as
the channel transmitted on the main ring, the transmitter 33 transmits at
the same wavelength as the transmitter 20 In this case the OADM 26 gets
its looped signal directly from OADM 28 through switch 42, so the signal
being dropped to receiver 30 is actually the same signal as that being

generated by transmitter 33. This ambiguity involving proper recognition
of signals on the main and protect rings is not harmful since it is solved
by the fact that receiver 30 will any way detect a characteristic imprint or
a predetermined artifact in the dropped signal, while receiver 32 detects a
"clean" signal As a result, the correct signal will be selected as a working
signal for further use at the node 12 The similar situation will take place
at node 18 Owing to the original method of selecting only signals
without OMS switching traces, the task of proper recognition of OCH
protected signals and the problem of ambiguous routing in combined
OMS and OCH protected systems are resolved.
Fig. 2 illustrates an example, where a network 100 composes
modified nodes 112, 114, 116 and 118, and where switches 40, 41, 42
and 43 (see Fig. 1) are replaced, for example, with 1X2 switches 50, 51,
52 and 53 respectively coopeiating with lxl optical switches 60 and 61
integrated with optical power splitters 54, 55, 56 and 57 The OMS
protection shown in Fig 2 serves the links 111 and 113, but is slightly
different than in Fig. 1. The exemplary implementation shown in Fig. 2 is
arranged in such a way that by using less complex equipment it allows
avoiding a single point of failure (which in Fig. 1 is introduced by using
switches 40, 41, 42 and 43) Sphtteis, being passive devices, are less
subjected to failure, also, separation of components belonging to the main
and the protect rings reduces chances of disabling both of the rings
simultaneously. One skilled in the art may see that the configuration of
Fig 2 is able to provide similar results configuration of Fig 1 using the
2x2 switches.
Optionally, the couplers in Fig 2 can be modified For example,
instead of being wide band couplers (coupling all the OMS protected
channels), they may be replaced with narrowband filters that transfer only

selected channels for OMS protection. Such a scheme may be useful if it
is required to protect only particular channels by the OMS protection
scheme
Hie system of OCH protection is similar to that in Fig. 1. It can be
arranged for one or moie individual optical channels, either for one or for
both directions of transmission The drawing shows an arrangement for
OCH protection of a bi-directional optical channel added to the network
at node 118.
Let us suppose that data is transmitted via the OCH protected
channel in both directions at the same wavelength XI. Let us also suppose
that the network 100 is not provided with detectors of any previously
discussed OMS switch imprmt in the optical signals that have overcome
ledirection via the OMS switching means (m this case, by the splitters
and couplers) In order to recognize the optical signals and select propei
ones foi further use in the OCH protected channels, pilot tones PI and P2
are respectively added to the transmitted signals (PI at transmitter 120
and P2 at transmitter 133) In case of failure in section 111, the optical
signal at XI modulated by PI added to OADM 122 of the main ring will
be redirected to OADM 124 of the protect ring and will be immediately
dropped (as being carried at A.1) to receiver 123 Smce the receiver 123 is
adapted to receive an OCH protected signal from node 112 at the
wavelength XI modulated with pilot tone P2, the signal will be rejected
Receiver 121 will receive from OADM 122 a signal that was added to
OADM 126 by the transmitter 133 (XI and P2), which did not change its
route and is inherently stable Comparator 125 in this case may comprise
a P2 filter, winch will reject the wrong signal transmitted from tins same

node 118 and select for further use (as a working signal) the correct stable
signal from node 112.
It will be appreciated by a person skilled in the art that the present
invention is not limited by what has thus far been described with respect
to specific embodiments. Other embodiments of equipment and
configurations of networks could be proposed for implementing the
concept of the invention Rather, the present invention is limited only by
the claims which follow When used in the following claims, the terms
"comprises", "comprising", "includes", "including" or the like mean
"including but not limited to"

WE CLAIM
1 A method of carrying out protection of data traffic in a multi-
channel multi-section ring-like optical communications network by simultaneously
using Optical Multiplexed Section (OMS) protection and Optical Channel(OCH)
protection, the method comprises a step of selecting an optical signal in an OCH
protected optical channel, in case of a fault in a section of said network, by
relying on an indication associated with OMS switching functionality required to
overcome said fault, characterized in that the communication network consisting
of no more than two ring-like configurations respectively formed by no more
than two optical fibers connecting network elements, one of said configurations
is considered a main configuration and the other is considered a protecting
configuration,the method comprising .
providing the OMS protection for at least one section of the network by
ensuring, in case of a failure in said section, switching of the data traffic
from one of said configurations to another of said configurations at each of two
ends of said section;
providing the OCH protection for at least one optical channel in the network, by
transmitting data related to said optical channel from one of said network
elements being a source point of said channel to another of said network

elements being a destination point of said channel by means of two optical
signals, wherein one of said optical signals is transmitted via the main
configuration and another of said optical signals is transmitted via the protecting
configuration;
said selecting being performed by rejecting the one of the two optical signals of
said OCH protected channel, that has undergone said switching from one of said
configurations to another of said configurations at least once, by relying on said
indication.
2 The method as claimed in claim 1, wherein said indication being created in
the optical signal while undergoing said switching.
3 The method as claimed in Claim 2, comprising providing switching means for
the OMS protection and wherein the step of rejecting comprises detecting said
indication in the optical signal having been switched by said switching means.
4 The method as claimed in Claim 3, wherein said indication is a
momentary loss created in said signal while switching it in the switching
means.

5. The method as claimed in Claim 3, wherein said indication is a
signaling artifact added to the signal that has been switched in the
switching means.
6 The method as claimed in claim 5, wherein said signaling artifact is
a modulating tone applied to the signal having been switched by said
switching means.
7 The method as claimed in any one of Claims 2 to 6, wherein the method
comprises preliminarily applying said indication in the form of a specific
additional pilot tone for transmitting data from the source point of the OCH
protected channel via a specific one of said configurations, and further detecting
at the destination point of said OCH protected channel an optical signal not
comprising said specific additional pilot tone, for further rejection.
8 The method as claimed in any one of Claims 2 to 7, wherein the step of
providing the OCH protection comprises providing an Optical Add Drop
Multiplexer (OADM) at each of the source point and the destination point of the
OCH protected channel, both in the main configuration and in the protecting
configuration thereof.

9 The method as claimed in any one of Claims 2 to 8, wherein the step of
providing the OCH protection comprises arrangement thereof for a bi-directional
optical channel, wherein each of the source point and the destination point
serves as a source/destination point
10. The method as claimed in Claim 9, wherein one and the same carrier
wavelength is used for transmitting data along said bi-directional OCH protected
optical channel.
11 A system capable of simultaneously performing Optical Multiplex Section
(OMS) protection and Optical Channel (OCH) protection of optical traffic in a
multi-channel multi-section optical communications network, the system being
operative to select an optical signal in an OCH protected optical channel, in case
of a fault in a section of said network, by relying on an indication associated with
OMS switching functionality required to overcome said fault,
said system is characterized in that the communication network consisting of no
more than two ring-like configurations respectively formed by no more than two
optical fibers connecting network elements, one of said configurations is
considered a main configuration and the other is considered a protecting
configuration, the system comprising:

an OMS protection arrangement for at least one section of the network, including
switching means at both ends of said section,
an OCH protection for at least one optical channel formed in said network
between a source point and a destination point of said channel, the OCH
protection arrangement including Optical Add Drop Multiplexer (OADM) in a
protecting configuration at the destination point of said optical channel,
means for rejecting use at said destination point of such an optical signal related
to the OCH protected optical channel, that has been switched from one of said
configurations to another at least once, by relying on said indication.
12. The system as claimed in Claim 11, wherein said means for rejecting, rely on
said indication being created in said optical signal while undergoing said
switching.
13. The system as claimed in Claim 12, wherein said means for rejecting are
situated at the destination point of said optical channel and comprise.
a detector of said indication associated with the switching of said optical signal in
the switching means and
a selector for selecting for further use an optical signal not comprising said
indication

14 The system as claimed in Claim 13, comprising a generator of said indication,
associated with the switching means
15. The system as claimed in Claim 11, wherein said means for rejecting
comprise a modulator at the source point of the OCH protected optical channel
for preliminarily applying said indication in the form of a specific pilot tone to
data transmitted via said channel through a specific one of said configurations,
and a tone detecting means at the destination point of said optical channel for
further detecting and rejecting any optical signal not comprising said pilot tone.
16 The system as claimed in any one of Claims 11 to 15, wherein said OCH
protected optical channel is bi-directional.
17. The system as claimed in Claim 16, wherein one and the same carrier
wavelength is used for transmitting data in said OCH protected optical channel in
both said directions.

ABSTRACT

TITLE " A method of and a system for carrying out protection of data traffic in a
multichannel multi-section ring-like optical communications network by simultaneously
using Optical Multiplexed Section (OMS) protection and Optical Channnel (OCH)
protection"
A technique for carrying out protection of data traffic in a multi-channel multi-
section optical communications network by simultaneously using Optical
Multiplexed Section (OMS) protection and Optical Channel (OCH) protection, the
technique comprises selection of a working optical signal in an OCH protected
optical channel, in case of a fault in a section of the network, by relying on an
indication associated with OMS switching functionality that is required to
overcome the mentioned fault.