Method for transmitting LCAS protocol via an optical communication line
Field of the invention
The invention relates to transmission of LCAS protocol by SDH, SONET or
OTN frames in corresponding communication networks.
Background of the invention
The nse of Link Capacity Adjustment Scheme (LCAS) for virtual
concatenated signals is described in the ITU-T Standard Recommendation G.7042.
The Recommendation specifies a link capacity adjustment scheme that should be used
to increase or decrease the capacity of a container that is transported in an SDH/OTN
network using Virtual Concatenation. The scheme is applicable to every member of
the Virtual Concatenation group.
This Recommendation defines the required states at the source and at the sink
side of the link as well as the control information exchanged between both the source
and the sink side of the link to enable the flexible resizing of this Virtual
Concatenated signal. The actual information fields used to convey the control
information through the transport network are defined in their respective
Recommendations, which are ITU-T Recs G.707/Y.1322 and G.783 for SDH and
ITU-T Recs G.709/Y.1331 and G.798 for OTN which are also called OTH - Optical
Transport Hierarchy.
For example, in SDH/SONET frames, control information concerning LCAS
protocol is transmitted by means of POH (Path Overhead) bytes of a standard
SONET/SDH transport module. In OTH, such LCAS control information is normally
transmitted in OPU-k virtual concatenation overhead (VCOH 1/2/3).
LCAS, in the virtual concatenation source and sink adaptation functions,
provides a control mechanism to smoothly increase or decrease the capacity of a link
to meet the bandwidth needs of the application. It also provides the capability of
temporarily removing member links that have experienced a failure. The LCAS
assumes that in cases of capacity initiation, increase or decrease, the construction or
destruction of the end-to-end path of each individual member is the responsibility of
the Network and Element Management Systems.

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Synchronization of changes in the capacity of the transmitter (So) and the
receiver (Sk) shall be achieved by a control packet. Each control packet describes the
state of the link during the next control packet. Changes are sent in advance so that the
receiver can switch to the new configuration as soon as it arrives.
It should be noted that LCAS protocol is transmitted between a source and a
destination via the transport network as control information. Since transmission via
the communication link is usually bi-directional, a so-called hand-shaking procedure
is required to maintain the transmission. Therefore, the control information
concerning LCAS protocol is transmitted by means of overhead bytes (such as POH
bytes of SDH data frame), and in both directions of any bi-directional link.
In cases of substantially unidirectional data traffic, data flow in the opposite
direction of a bi-directional link is almost negligible. Such situations are typical for
video on demand applications, when, according to a customer's order, massive data
flows carry the ordered video information in one transmission direction via the
communication link. Another example of a unidirectional data traffic is any kind of e-
learning process when heavy data traffic is transferred to a client (in one transmission
direction) upon his/her single momentary request (in the opposite direction). In these
cases, the need of transmitting LCAS in the mentioned opposite direction results in
forwarding almost or completely empty SDH/SONET or OTN transport modules just
for the purpose of transporting some informative overhead bytes thereof.
Consequently, bandwidth capacity of the • communication link is utilized non-
effectively in such cases.
Summary of the invention
It is therefore the object of the invention to provide a method of effective
utilizing of bandwidth capacity in optical communication lines when LCAS protocol
is used.
Other objects of the invention will become apparent as the description of the
Invention proceeds.
In all presently known systems, LCAS protocol is transmitted using overhead
bytes in SDH/SONET or OTN frames (virtual containers of concatenated groups)
traveling in both transmission directions of a bi-directional optical line connecting two
communicating network elements.

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There is presently proposed a method of exchanging LCAS control
information between a first network element and a second network element
interconnected - by a bi-directional optical communication line, while conveying
substantially unidirectional data traffic in the form of a virtual concatenated group
(VCG) binary stream transmitted in a first direction via said bi-directional optical
communication line,
the method comprises the following arrangement for exchanging the LCAS
control information:
a) in the first direction, transmitting the LCAS control information via said bi-
directional optical communication line in said VCG binary stream, by
placing the LCAS control information in one or more overhead bytes
conventionally specified for said VCG binary stream,
b) in a second direction being opposite to said first direction, transmitting the
LCAS control information in an existing binary stream non-related to said
unidirectional data traffic and passing in the second direction via a
communication trail connecting said first network element and said second
network element, by placing said LCAS control information in one or more
overhead bytes of said existing binary stream.
In the first direction, the LCAS control information is transmitted according to
presently effective standards, for example - in each member stream path overhead
(POH).in SDH/SONET networks (or in an equivalent portion of standard frames in
OTH network).
Preferably, the one or more overhead bytes of said existing binary stream are
not the one or more bytes conventionally specified for said VCG binary stream. Most
preferably, the one or more overhead bytes of said existing binary stream are
overhead bytes not specified for transporting LCAS control information in any VCG
binary stream.
In the frame of the present description, LCAS control information is to be
understood as a succession of LCAS control packets.
For the purpose of our description, the term VCG binary stream should be
understood as a group of member streams, wherein the group comprises one binary
stream or two or more binary streams, and wherein each of said member streams is
respectively formed by a succession of equal transport modules (virtual containers);
the member streams being transmitted via parallel routes from one of said first

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network element (NE1) and second network element (NE2) to the other of said two
network elements (NE). The VCG binary stream is understood as formed of transport
modules suitable for a communication network selected from a non-exhaustive list
comprising SDH, SONET, OTH networks.
It should be noted that each of said communication line and the
coimiunication trail may comprise one or more communication links, i.e. each of
them may include additional network elements NE between the first network element
NE1 and the second network element NE2.
For SDH communication network, an example of a VCG binary stream is a
plurality of one or more virtual containers (transport modules) VC-n (n=3, 4, 12),
which are carried by standard frames STM-k (k=l, 4, 16, 64,...), for example as
follows:
STM- 4 may comprise up to four concatenated VC-4; '
STM-16 may comprise up to 16 concatenated VC-4;
STM-64 may comprise up to 64 concatenated VC-4.
For SONET networks, equivalent transport modules and VCG groups exist,
known to those skilled in the art, and thus can be not referred to in the present
description.
For OTH, there are VCG data streams composed from transport modules
called OPU-k, were the index "k" represents bit rate (k =1 represents bit rate of about
2.5 Gb/sec, k==2 represents bit rate of about lOGb/sec, k=3 represents bit rate of about
40Gb/sec according to standard Recommendations G.709/Y.1331 and G.798).
The term "conventionally specified overhead bytes" should be understood as
bytes specified by one or another above-mentioned standard recommendation
(concerning SDH, SONET or OTH) for carrying the LCAS control information.
In SDH, the conventionally specified overhead byte is, for example, byte H-4 '
in a Path Overhead (POH) portion of the virtual container VC-4, according to the
ITU-T Standard Recommendation G.707.
In OTH, the conventionally specified bytes are bytes in VCOH 1/2/3
according to the ITU-T Standard Recommendation G.709.
In the above definition of the proposed method, the mentioned communication
trail may be either a part of said bi-directional communication line, or a separate
communication trail (link, path) existing in the network, connecting the first network
element and the second network element, and carrying traffic not related to the

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mentioned VCG stream of the first direction. According to the invention, the role of
such a communication trail is in that it already carries any suitable traffic (i.e., said
pre-existing binary stream) which may "give a lift" to the LCAS control information
and thus to transport it in the required second direction. For the VCG binary stream
that has been initially transmitted via the bi-directional line, the function of LCAS
will thereby be fulfilled in both directions.
The binary stream existing between the first and the second network elements
in the second direction but not related to the unidirectional traffic may be, for
example:
a) a VCG stream of the type similar or different from the VCG binary
stream transmitted in the first direction,
b) a single binary stream, acceptable in the communication network
comprising said bi-directional communication line, wherein said
single stream is not a member of any VCG.
As has been noted above, transmitting the LCAS control information in the'
existing binary stream should preferably be performed using overhead bytes, being
different from the conventionally specified bytes. This approach is especially useful
for the case where the communication trail just forms part of the bi-directional
communication line for transmitting traffic in the second direction.
Such "different" overhead bytes may be unused bytes of so-called Section
Overhead (SOH) portion of the standard STM-n frame of SDH network. For
example, there are bytes called MS-DCC, RS-DCC bytes in the SOH overhead
portion, which can be utilized for transportation of LCAS control information. MS-
DCC and RS-DCC are respectively positioned in Multiplexed Section Overhead
(MSOH) and Regeneration Section Overhead (ROH) being parts of the SOH section
of the SDH/SONET standard transport module.
For OTH transport modules, such reserve overhead bytes, for example, are
located in row 2 intersections with, columns 1, 2, 3; in row 4 intersections with
columns 9 to 14. Said reserve overhead bytes or other unused overhead bytes in the
OTH transport modules can be used for the purpose of transporting "the foreign"
LCAS control information.
Owing to the proposed arrangement, the second direction of the bi-directional
communication line (the first direction of which is occupied by the unidirectional
VCG binary stream) is freed from allocating special transport modules just for
5

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transmitting the LCAS control information. In other words, there is no need in
spending considerable bandwidth for especially transmitting a succession of transport
modules being almost empty. Due to this saving, bandwidth available on the bi-
directional communication line can be spent more judiciously, for example be'
assigned in advance to other users. If needed, traffic of such users (non-related to the
unidirectional traffic) may be successfully used as the existing binary stream for
lifting some additional auxiliary information - namely the LCAS control information.
As has been mentioned, the LCAS control information can be transmitted in
the second direction by utilizing any other, be it called alternative or additional
communication trail connecting the first network element and the second network
element and by using any traffic (binary stream) existing in the required, second
direction on this communication trail. Such existing traffic is definitely not related to
our "problematic" unidirectional traffic.
As mentioned above, the LCAS control information can be placed in overhead
bytes (preferably, reserve overhead bytes of the pre-existing traffic).
However, other overhead bytes (even the conventionally specified ones) can
be used for transporting LCAS control information over an existing binary stream (not
related to the unidirectional VCG) via the alternative communication trail. In this
case, the existing binary stream is preferably a single binary stream or a number of
single binary streams not being member(s) of any VCG.
The proposed method is initiated and performed under supervision of a
Network Management System (NMS) of the network. The NMS is aware of the
network topology and is therefore capable of allocating an alternative communication
trail (path) in the communication network to which the NE1 and NE2 belong. NMS
may be adapted to allocate a suitable communication trail via the mentioned
communication network, and sometimes even via additional neighboring network.
The condition for selecting such a trail would be that the trail connects the NE1 and
NE2 and conducts any existing traffic in the second direction. Also, during
transmitting the existing traffic via any number of network elements, the trail in the
required second direction must transparently convey the LCAS control information
inserted at one (say, 1st) network element while passing the whole communication
trail up to the other (say, 2nd) network element.
There is also provided an Element Management System (EMS), possibly
acting in cooperation with the above NMS. The EMS should allow (a user') to select

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position where to insert LCAS control information into the existing binary stream in
the second direction, should assist the insertion and should allow recovering LCAS
control information upon being received from said second direction.
According to a further aspect of the invention, there is provided a
communication network, capable of performing the above-described method.
The network is preferably equipped with the NMS and/or EMS, capable of
controlling network nodes of the network for performing the above-described method.
According to still a further aspect of the present invention, there is provided a
program product, preferably, constituting at least part of the NMS and/or EMS
software system, and comprising software instructions which, being run on a
computer, are capable of perfonning the above-described method at network nodes of
a communication network.
Brief description of the drawings:
Fig. 1A (prior art) illustrates a schematic block diagram of the standardized
transmission of LCAS control data within a Virtual Concatenated Group binary
.stream via a bi-directional communication link;
Fig. IB (prior art) schematically illustrates a standard SDH data frame of a
transport module of a VCG binary stream, and a standardized position in the overhead
portion of the frame, where the LCAS control information may be inserted;
Fig. 2 is a block diagram schematically illustrating one version of the method
according to the invention for exchanging LCAS control information in a case of
unidirectional traffic, by using a communication trail between a source network
element and a sink network element, carrying existing traffic urn-elated to the
unidirectional traffic;
Fig. 3 schematically illustrates another version of the proposed method, where
the LCAS control information is transmitted over an existing binary stream, passing
via the same bi-directional line between the source and the sink network elements and
not related to the unidirectional traffic transmitted via the bi-directional line;
Fig. 4 schematically illustrates an SDH frame format of the existing binary
stream, with reserve overhead bytes carrying the LCAS control information in the
second, direction of the transmission.
Fig. 5 schematically illustrates an OTH transport module, with reserve bytes
of the GCC channel which can be used for transmitting control information on LCAS.

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Detailed description of the invention
Fig. 1A (prior art) illustrates a conventional method of transmitting LCAS.
control information in a Virtual Concatenated Group binary stream between a first
network element NE1 and a second network element - NE2 in an optical network 10,
LCAS device in NE1 is marked 12, the analogous LCAS device in NE2 is marked 14.
Let the two network elements are connected by a bi-directional communication line
16 which consists of two pluralities of links or paths: one plurality (15) comprises one
or more links (paths) capable of conveying traffic in a first direction, let it be from
NE1 to NE2, and the other plurality (17) comprises links or paths for carrying traffic
in the second direction, from N2 to Nl. The meaning of the term "path" is in that the
network elements NE1 and NE2 may be interconnected not directly, but via other
network elements which are not shown in the figure.
For the sake of simplicity, only one sub-link (sub-path) 15 is shown, that
conveys traffic in the first direction, and one sub-link 17 is illustrated as carrying
traffic in the second direction. Suppose that the traffic via the line 16 is substantially
bi-directional.
Fig. 1A schematically illustrates a Virtual Concatenated Group (VCG) binary
stream marked 18, consisting of four member streams formed by virtual containers
VC-4 (20) that are transmitted in parallel via four sub-paths (only one of them 15 is
shown). LCAS control information concerning the line status is inserted in each VC-4
Path Overhead (POH) portion H4 byte and is marked with an asterisk.
Fig. 1A also illustrates traffic in the second direction from NE2 to NE1, in the
form of a similarly shaped Virtual Concatenated Group binary stream marked 22
consisting of member streams 24. In this example, the stream 22 in the second
direction is shown identical to the stream 18 in the first direction.
However, another type of a VCG binary stream can be used in the second
direction say, formed from transport modules VC-3 can be used for the purpose of
transmitting LCAS control information in the second direction. It should be noted that
each sub-link of the bi-directional line 16 may constitute a physically separate link
(path).
If we now suppose that the traffic 18 in the first direction is unidirectional
(say, it carries video programs), then traffic 22 in the second direction will be
practically absent. It then becomes clear that, only for transmitting LCAS control
information in the second direction, at least one VC-4n stream must be formed and

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transmitted, even if payload of the VC-4m (m may be equal to or different from n) or
remains empty (idle).
Fig. IB (prior art) schematically illustrates a standard SDH frame 30,
representing one transport module of a VCG flow (say, one VC-4 per STM frame).
The figure schematically illustrates that the VCG stream comprises a number of
' member streams transmitted in parallel. The frame comprises a predetermined number
of bytes arranged in rows and columns, and is divided into standard sections: Payload
32 carrying data information, and a number of overhead bytes: POH (path overhead)
bytes 34, RSOH 36, MSOH 38 and AU pointers 39. There is byte H4 (marked 35) in
the POH section 34, which is conventionally specified (specified by standard) for
transmitting control information concerning LCAS.
Fig. 2 illustrates one arrangement for transmitting LCAS control information
according to the invention. Elements, similar to those illustrated in Fig. la, are marked
with similar numbers. In an optical communication network 40, provided with a
common Network Management System NMS 41 and EMS 43 for a number of
network elements, a bi-directional optical line 16 is provided between a network
element NE1 and a network element NE2.
As in Fig. la, a VCG binary stream 42 is transmitted via the line 16 in the first
direction, from NE1 to NE2. For the sake of example only, this stream is formed
from virtual containers VC-4, As in Fig. la, in the first direction of the line 16, the
LCAS control information is incorporated in overhead bytes of the POH portion,
specified by standard, of a transport module 44 of the VCG 42. Let us suppose that
the mentioned traffic is unidirectional, i.e., the second direction of the line 16 (though
required for a number of purposes, including transmission of LCAS control
information) is not used for transmitting data related to the binary stream VCG 42.
Contrary to the conventional arrangement shown in Fig. 1A, the LCAS control
information in the opposite, second direction (from NE2 to NE1) is not necessary
transmitted via the same bi-directional link 16. In an example of this figure it is
explicitly not; under supervision of NMS 41, the LCAS control information in the
second direction is sent via an alternative communication trail 46, being separate from
the bi-directional communication line 16. Any traffic, existing in the trail 46, passing
from NE2 to NE1 in the network 40 and not related to the traffic 42 via the line 16,
can be used for transporting LCAS control information concerning the line 16. For
example, in the SDH network 40, while VC-4 containers of the VCG stream are

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transmitted in the first direction of the bi-directional communication line 16, a single
stream 47 of VC-3 virtual containers exists, transmitted for carrying foreign data
traffic in the second direction via a separate communication trail 46. This stream 47 •
can be used for carrying "the second direction" LCAS control information in respect
of the line 16. For this purpose, one or more overhead bytes in the existing virtual
.containers 47 (either the conventionally specified bytes in the POH portion, or reserve
bytes 48 in SOH portion) can be used.
Trail 46 may bypass via a number of additional network elements in said
network, and even cross a section of a different communication network 49. The
utility- condition is that the LCAS information in the POH of VC-3 containers 47 is
preserved or restored when the foreign existing traffic leaves the different
communication network 49 and returns to the optical communication network 40.
Fig. 3 illustrates yet another version of the method according to the invention.
In the network 40, the similar NE1 and NE2 are interconnected by a similar bi- •
directional line 16 (compare to Fig. la). Let us suppose that in the first direction
served by a plurality of paths 15 (from NE1 to NE2), a VCG stream 42 is formed
from a number of VC-4 transport modules 44 transmitted in parallel via four sub-
paths 15. LCAS control information concerning status of each sub-path 15 of the line
16 is transmitted in their POH portion to NE2. Suppose that traffic 19 not related to
the traffic 42 exists in the second direction 17 of the bi-directional link 16. The
existing binary stream 19 may comprise virtual containers 43 of the same type, or of
another type compared to those transmitted in the first direction. At least a single
binary stream (not being a member of any VCG stream) is already sufficient for
transmitting the LCAS control information about status of sub-paths 15 of the line 16
in the second direction. As mentioned above, bytes which can be used for the.
purpose, may be any preliminarily agreed overhead bytes. However, if the traffic in
the second direction is a VCQ stream, the overhead bytes should preferably be not the
conventionally specified bytes, in order not to mix LCAS control information of
different traffic streams about link 16.
The advantage of using existing traffic for assisting the LCAS communication
in cases of unidirectional data transmission is in that there will be no more need in
creating and allocating (from the point of bandwidth) separate transport modules for
transporting negligible by volume control information in the second direction of the
unidirectional traffic. The control information can be now transported by overhead

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bytes of so-called "foreigner" traffic already existing in the second direction over the
bi-directional communication line or over a separate alternative communication trail.
The foreigner traffic will therefore give a "lift" to the LCAS control information
concerning the bi-directional line.
Fig. 4 illustrates a frame of an existing traffic module 52. The figure again
utilizes an example of a standard SDH frame. However, analogous illustrations could
be found for the SONET and the OTH cases. Reserve bytes 57 are located, in this
embodiment, in sections RS (56) and MS (58) of the overhead portion of the frame
52. Alternatively, bytes called MS-DCC and/or RS-DCC, respectively located in the
MSOH and RSOH sections of TOH (if not in use for their initial purpose) can be used
for transmitting LCAS control information in the second direction.
Fig. 5 schematically illustrates an example of illustrates an OTH frame 60.
The OPUk overhead portion is also called VCOH portion of the frame and is
somehow analogous to POH portion in SDH/SONET standard frames.
In some other portions of the frame a single asterisk symbolizes bytes that can
be used for carrying LCAS control information according to the invention, and they
can be called reserved bytes.
Bytes marked by double asterisk can be used for transporting LCAS control
information as proposed in the invention, if not used for their direct purpose.
It should be appreciated that, though examples in the detailed description
show only SDH frames, bytes and SDH-related VCG streams, the principle of the
invention applies to the SONET and OTH transport modules, frames, bytes and
virtual concatenated groups mutatis mutandis.

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Claims
1. A method for bidirectional exchange of LCAS control information for
controlling a virtual concatenated group (VCG) binary stream transmitted between a
first network element and a second network element via a bi-directional optical
communication line interconnecting said two network elements, while said VCG
binary stream forms substantially unidirectional data traffic transmitted in a first
direction via said bi-directional optical communication line, with none or negligible
data traffic transmitted in a second direction being opposite to said first direction; the
method comprises:
a) in the first direction, transmitting said LCAS control information via said
bi-directional optical communication line in said VCG binary stream, by
placing the LCAS control information in one or more overhead bytes
conventionally specified for said VCG binary stream,
b) in the second direction, transmitting said LCAS control information by
placing it in one or more overhead bytes of an existing binary stream,
wherein said existing binary stream
- having no relation to said VCG binary stream to be controlled by
said LCAS control information,
not provisioned specifically for transmitting said LCAS control
information, and
- passing in said second direction via a communication trail
connecting said first network element and said second network
element.
2. The method according to Claim 1, wherein said one or more overhead bytes of
said existing binary stream are not the one or more bytes conventionally specified for
said VCG binary stream.
3. The method according to Claim 1, wherein said VCG binary stream is formed
of transport modules suitable for a communication network selected from among
SDH, SONET and OTH networks.
4. The method according to any one of the preceding claims, wherein the
communication trail is apart of said bi-directional communication line.

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5. The method according to any one of the preceding claims, wherein the
communication trail is an additional communication trail connecting the first network
element and the second network element, not forming part of said bi-directional
communication line and carrying traffic not related to said VCG binary stream.
6. The method according to Claim 2, wherein said overhead bytes in the existing
binary stream are unused reserve bytes.
7. The method according to Claim 1, wherein said overhead bytes in the Existing
binary stream are MS-DCC and/or RS-DCC bytes of an SDH/SONET standard frame.
8. The method according to Claim 5, wherein said overhead bytes in the existing
binary stream are the conventionally specified bytes of the existing binary stream
being a binary stream not forming a member of any VCG.
9. A Network Management System (NMS) of a communication network, adapted
to perform the method according to any one of Claims 1 to 8 and capable of allocating
the communication trail between the first network element and the second network
element.
10. An Element Management System (EMS), adapted to perform the method
according to any one of Claims 1 to 8, said EMS being operative to provide placing
the LCAS control information in the existing binary stream in the second direction,
and to ensure recovering the LCAS control information upon being received from said
second direction.
11. A communication network, equipped with NMS according to Claim 9, and/or
with EMS according to Claim 10, thereby capable of performing the method
according to any one of Claims 1 to 8.
12.. A program product comprising software instructions which, being run on a
computer, are capable of performing the method according to any one of Claims 1 to
8 at network elements of a communication network.
ECIP/F065/WO amended

A technique for exchanging LCAS control information between a first network element
and a second network element interconnected by a bi-directional optical communication line, while conveying substantially unidirectional data traffic in the form of a virtual
concatenated group (VCG) binary stream transmitted in a first direction via the bi-
directional optical communication line, the technique comprising :-in the first direction
transmitting the LCAS control information via the bi-directional optical communication
line in the VCG binary stream, by placing the information in one or more overhead bytes
conventionally specified for the VCG binary stream,-in a second (opposite) direction,
transmitting the LCAS control information in an existing binary stream non-related to
the above-mentioned unidirectional data traffic; the existing binary stream should pass
via a communication trail connecting the first network element and the second network
element but not obligatory coinciding with the above-mentioned line; the discussed
information should be placed in one or more overhead bytes of the existing binary
stream.