Network Working Group G. Bernstein
Internet Draft Grotto Networking
Intended status: Standards Track Sugang Xu
NICT
Expires: January 2009 Y.Lee
Huawei
Hiroaki Harai
NICT
D. King
July 7, 2008
Signaling Extensions for Wavelength Switched Optical Networks
draft-bernstein-ccamp-wson-signaling-02.txt
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Abstract
This memo provides extensions to Generalized Multi-Protocol Label
Switching (GMPLS) signaling for control of wavelength switched optical
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networks (WSON). These extensions build on previous work for the
control of G.709 based networks.
Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC-2119.
Table of Contents
1. Introduction...................................................3
2. Terminology....................................................3
3. Requirements for WSON Signaling................................4
3.1.1. WSON Signal Characterization.........................4
3.1.2. Bi-Directional Distributed Wavelength Assignment.....4
3.1.3. Distributed Wavelength Assignment Method Selection...5
3.1.4. Out of Scope.........................................6
4. WSON Signal Types, Forward Error Correction, and Rates.........6
4.1. Traffic Parameters for WSON signals.......................6
5. Bidirectional Lightpath using Same Wavelength on Both Directions7
5.1. Using LSP_ATTRIBUTES Object...............................8
5.2. Bidirectional Lightpath Signaling Procedure...............8
5.3. Backward Compatibility Considerations.....................9
6. Bidirectional Lightpath using Different Wavelengths on Different
Directions.......................................................10
7. RWA Method Related............................................10
7.1. Wavelength Assignment Method Selection...................10
7.2. Supplemental Information for Wavelength Assignment.......10
7.3. Least-Loaded Wavelength Assignment (informational).......12
8. Security Considerations.......................................13
9. IANA Considerations...........................................13
10. Acknowledgments..............................................13
11. References...................................................14
11.1. Normative References....................................14
11.2. Informative References..................................14
Author's Addresses...............................................15
APPENDIX A: Requirement of Bidirectional Lightpath with the Same
Wavelength in Both Directions....................................17
A.1. Introduction.............................................17
A.2. Port-remapping Problem...................................17
A.3. Port-remapping with OXC..................................20
A.4. Avoiding Port-remapping Problem: Bidirectional Lightpath
using Same Wavelength on Both Directions......................21
Intellectual Property Statement..................................22
Disclaimer of Validity...........................................22
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1. Introduction
This memo provides extensions to Generalized Multi-Protocol Label
Switching (GMPLS) signaling for control of wavelength switched
optical networks (WSON). In particular, extensions are given to
characterize optical signal types via traffic parameters, permit
simultaneous bi-directional wavelength assignment, and control the
distributed wavelength assignment process. These extensions build on
previous work for the control of G.709 based networks.
2. Terminology
CWDM: Coarse Wavelength Division Multiplexing.
DWDM: Dense Wavelength Division Multiplexing.
FOADM: Fixed Optical Add/Drop Multiplexer.
ROADM: Reconfigurable Optical Add/Drop Multiplexer. A reduced port
count wavelength selective switching element featuring ingress and
egress line side ports as well as add/drop side ports.
RWA: Routing and Wavelength Assignment.
Wavelength Conversion/Converters: The process of converting an
information bearing optical signal centered at a given wavelength to
one with "equivalent" content centered at a different wavelength.
Wavelength conversion can be implemented via an optical-electronic-
optical (OEO) process or via a strictly optical process.
WDM: Wavelength Division Multiplexing.
Wavelength Switched Optical Networks (WSON): WDM based optical
networks in which switching is performed selectively based on the
center wavelength of an optical signal.
AWG: Arrayed Waveguide Grating.
OXC: Optical Cross Connect.
Optical Transmitter: A device that has both a laser tuned on certain
wavelength and electronic components, which converts electronic
signals into optical signals.
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Optical Responder: A device that has both optical and electronic
components. It detects optical signals and converts optical signals
into electronic signals.
Optical Transponder: A device that has both an optical transmitter
and an optical responder.
Optical End Node: The end of a wavelength (optical lambdas) lightpath
in the data plane. It may be equipped with some optical/electronic
devices such as wavelength multiplexers/demultiplexer (e.g. AWG),
optical transponder, etc., which are employed to transmit/terminate
the optical signals for data transmission.
3. Requirements for WSON Signaling
The following requirements for GMPLS based WSON signaling are in
addition to the functionality already provided by existing GMPLS
signaling mechanisms.
3.1.1. WSON Signal Characterization
WSON signaling MUST convey sufficient information characterizing the
signal to allow systems along the path to determine compatibility and
perform any required local configuration. Examples of such systems
include intermediate nodes (ROADMs, OXCs, Wavelength converters...),
links (WDM systems) and end systems (detectors, demodulators,
etc...). The details of any local configuration are out of the scope
of this document.
3.1.2. Bi-Directional Distributed Wavelength Assignment
WSON signaling MAY support distributed wavelength assignment
consistent with the wavelength continuity constraint for bi-
directional connections. The following two cases MAY be separately
supported: (a) Where the same wavelength is used for both upstream
and downstream directions, and (b) Where different wavelengths can be
used for both upstream and downstream directions.
The need for the same wavelength on both directions mainly comes from
the color constraint on some edges' hardware. In Appendix section,
two edge relevant scenarios are described, i.e. without and with OXC
at edges. In fact, the edges can be classified into two types, i.e.
without and with the wavelength-port mapping re-configurability.
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Without the mapping re-configurability at edges, the edge nodes must
use the same wavelength in both directions. For example, (1)
transponders are only connected to AWGs (i.e. multiplexer/de-
multiplexer) ports directly and fixedly, or (2) transponders are
connected to the add/drop ports of ROADM and each port is mapped to a
dedicated wavelength fixedly.
On the other hand, with the mapping re-configurability at edges, the
edge nodes can use different wavelengths in different directions. For
example, in edge nodes, transponders are connected to add/drop ports
of colorless ROADM. Thus, the wavelength-port remapping problem can
be solved locally by appropriately configuring the colorless ROADM.
If the colorless ROADM consists of OXC and AWGs, the OXC is
configured appropriately.
The edges of data-plane in WSON can be constructed in different types
based on cost and flexibility concerns. Without re-configurability
we should consider the constraint of the same wavelength usage on
both directions, but have lower costs. While, with re-configurability
we can relax the constraint, but have higher costs.
These two types of edges will co-exist in WSON mesh, till all the
edges are unified by the same type. The existence of the first type
edges presents a requirement of the same wavelength usage on both
directions, which must be supported.
Moreover, if some carriers prefer an easy management lightpath usage,
say use the same wavelength on both directions to reduce the burden
on lightpath management, the same wavelength usage would be
beneficial.
In cases of equipment failure, etc., fast provisioning used in quick
recovery is critical to protect Carriers/Users against system loss.
This requires efficient signaling which supports distributed
wavelength assignment, in particular when the centralized wavelength
assignment capability is not available.
3.1.3. Distributed Wavelength Assignment Method Selection
WSON signaling MAY support the selection of a specific distributed
wavelength assignment method.
As discussed in the [WSON-Frame] a variety of different wavelength
assignment algorithms have been developed. A number of these are
suitable for use in distributed wavelength assignment. This feature
would allow the specification of a particular approach when more than
one are implemented in the systems along the path.
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3.1.4. Out of Scope
This draft does not address signaling information related to optical
impairments.
4. WSON Signal Types, Forward Error Correction, and Rates
Although WSONs are fairly transparent to the signals they carry, to
ensure compatibility amongst various networks devices and end systems
it can be important to include key lightpath characteristics as
traffic parameters in signaling [WSON-Frame].
4.1. Traffic Parameters for WSON signals
As in [RFC4606] and [RFC4328] the following traffic parameters would
become the contents for the RSVP SENDER_TSPEC and FLOWSPEC objects.
The WSON traffic parameters SHOULD be defined as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Mod Type | Mod Params| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BitRate/Analog Bandwidth |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Modulation (Mod) Types:
We have potentially the following:
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Value Type
----- ----
0 Unspecified or Unknown
1 NRZ
2 RZ
Modulation Parameters(Mod Params):
RZ 0 - 33%, 1 - 50%, 2 - 67% duty cycles
See [G.959.1] and [Winzer06].
These are specific to the modulation type employed and may or may not
be used. For example NRZ modulation typically doesn't have extra
parameters, while RZ modulation has a duty cycle parameter.
Bitrate/Analog Bandwidth:
For digital signals this is the bit rate given as a 32 bit IEEE
floating point number.
For analog signals or when modulation type is given as 0
(unspecified), this is the bandwidth of the signal around the center
frequency (c/lambda) and not the bit/byte rate. This is given as a 32
bit IEEE floating point number that represents the bandwidth in
Hertz. The exact definition of bandwidth, e.g., 3dB power bandwidth,
etc. is TBD and may be network specific.
5. Bidirectional Lightpath using Same Wavelength on Both Directions
With the wavelength continuity constraint in CI-incapable [RFC3471]
WSONs, where the nodes in the networks cannot support wavelength
conversion, the same wavelength on each link along a unidirectional
lightpath should be reserved. Per the definition in [RFC3471], a
bidirectional lightpath can be seen as a pair of unidirectional
lightpaths, which are provisioned along the same route simultaneously
by the RSVP-TE signaling with Upstream Label and Label Set Objects in
the messages [RFC3473]. This does not necessarily require the same
wavelength in both directions.
In addition to the wavelength continuity constraint, requirement
3.1.2(a) gives us another constraint on wavelength usage in data
plane, in particular, it requires the same wavelength to be used in
both directions.
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The simplest and efficient way is to only define an extension to the
processing of Label Set [RFC3473], and leave the other processes
untouched. The issues related to this new functionality including an
LSP_ATTRIBUTES object defined in [RFC4420] and the new procedure are
described in the following sections. This approach would have a lower
blocking probability and a shorter provisioning time. In cases of
equipment failure, etc., fast provisioning used in quick recovery is
critical to protect Carriers/Users against system loss.
5.1. Using LSP_ATTRIBUTES Object
To trigger the new functionality at each GMPLS node, it is necessary
to notify the receiver the new type lightpath request. One multi-
purpose flag/attribute parameter container object called
LSP_ATTRIBUTES object and related mechanism defined in [RFC4420] meet
this requirement. One bit in Attributes Flags TLV which indicates the
new type lightpath, say, the bidirectional same wavelength lightpath
will be present in an LSP_ATTRIBUTES object. Please refer to
[RFC4420] for detailed descriptions of the Flag and related issues.
5.2. Bidirectional Lightpath Signaling Procedure
Considering the system configuration mentioned above, it is needed to
add a new function into RSVP-TE to support bidirectional lightpath
with same wavelength on both directions.
The lightpath setup procedure is described below:
1. Ingress node adds the new type lightpath indication in an
LSP_ATTRIBUTES object. It is propagated in the Path message in
the same way as that of a Label Set object for downstream;
2. On reception of a Path message containing both the new type
lightpath indication in an LSP_ATTRIBUTES object and Label Set
object, the receiver of message along the path checks the local
LSP database to see if the Label Set TLVs are acceptable on both
directions jointly. If there are acceptable wavelengths, then
copy the values of them into new Label Set TLVs, and forward the
Path message to the downstream node. Otherwise the Path message
will be terminated, and a PathErr message with a "Routing
problem/Label Set" indication will be generated;
3. On reception of a Path message containing both such a new type
lightpath indication in an LSP_ATTRIBUTES object and an Upstream
Label object, the receiver MUST terminate the Path message using
a PathErr message with Error Code "Unknown Attributes TLV" and
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Error Value set to the value of the new type lightpath TLV type
code;
4. On reception of a Path message containing both the new type
lightpath indication in an LSP_ATTRIBUTES object and Label Set
object, the egress node verifies whether the Label Set TLVs are
acceptable, if one or more wavelengths are available on both
directions, then any one available wavelength could be selected.
A Resv message is generated and propagated to upstream node;
5. When a Resv message is received at an intermediate node, if it is
a new type lightpath, the intermediate node allocates the label
to interfaces on both directions and update internal database for
this bidirectional same wavelength lightpath, then configures the
local ROADM or OXC on both directions.
Except the procedure related to Label Set object, the other processes
will be left untouched.
5.3. Backward Compatibility Considerations
Due to the introduction of new processing on Label Set object, it is
required that each node in the lightpath is able to recognize the new
type lightpath indication Flag carried by an LSP_ATTRIBUTES object,
and deal with the new Label Set operation correctly. It is noted
that this new extension is not backward compatible.
According to the descriptions in [RFC4420], an LSR that does not
recognize a TLV type code carried in this object MUST reject the Path
message using a PathErr message with Error Code "Unknown Attributes
TLV" and Error Value set to the value of the Attributes Flags TLV
type code.
An LSR that does not recognize a bit set in the Attributes Flags TLV
MUST reject the Path message using a PathErr message with Error Code
"Unknown Attributes Bit" and Error Value set to the bit number of the
new type lightpath Flag in the Attributes Flags.The reader is
referred to the detailed backward compatibility considerations
expressed in [RFC4420].
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6. Bidirectional Lightpath using Different Wavelengths on Different
Directions
TBD
7. RWA Method Related
7.1. Wavelength Assignment Method Selection
As discussed in [HZang00] a number of different wavelength assignment
algorithms maybe employed. In addition as discussed in [WSON-Frame]
the wavelength assignment can be either for a unidirectional
lightpath or for a bidirectional lightpath constrained to use the
same lambda in both directions. A simple TLV could be used to
indication wavelength assignment directionality and wavelength
assignment method. This would be placed in an LSP_REQUIRED_ATTRIBUTES
object per [RFC4420]. The use of a TLV in the LSP required attributes
object was pointed out in [Xu].
[TO DO: The directionality stuff needs to be reconciled with the
earlier material]
Directionality: 0 unidirectional, 1 bidirectional
Wavelength Assignment Method: 0 unspecified (any), 1 First-Fit, 2
Random, 3 Least-Loaded (multi-fiber). Others TBD.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Direction | WA Method | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
7.2. Supplemental Information for Wavelength Assignment
Distributed wavelength assignment makes extensive use of the label
set object/TLV of [RFC3471]. Some higher performance algorithms
suitable for multi-fiber networks such as Least-Loaded assignment
require supplemental information concerning the potential lambdas to
be used. An ordered set of TLVs in correspondence with the group of
one or more label set TLVs can be used to convey this information in
the form of a general wavelength "acceptability" metric.
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Note that the label set syntax of [RFC3471] allows group of
wavelengths into ranges. For the purpose of supplementing this
information with wavelength count only those wavelengths with the
same counts could be grouped.
The general format for supplemental wavelength selection information
could be as follows:
The information carried in a Wavelength Set Metric TLV is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Info Type | Metric Size | Num Metrics |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Wavelength Metric Info |
| From lowest to highest frequency if more that one value |
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Info Type: 8 bits
0 - Single Value
The enclosed single value for the wavelength metric is given to all
wavelengths in the corresponding wavelength set.
1 - List
The enclosed list gets applied in a one-to-one fashion to each
wavelength in the corresponding wavelength set. An error occurs if
the number of metrics in this list and the number of wavelengths in
the wavelength set is not equal.
Metric Size:
Indicates the size of the wavelength metric information as follows
0 - 8 bits
1 - 16 bits
2 - 32 bits
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Number 0f Metrics: 24 bits
Wavelength Metric: (1, 2, or 4 octets)
The wavelength metric represents in some fashion the
desirability or lack thereof to use this wavelength over another
available wavelength. Different wavelength assignment algorithms may
use this information differently.
7.3. Least-Loaded Wavelength Assignment (informational)
The Least-Loaded wavelength assignment algorithm [HZang00] can be
implemented in a distributed fashion via signaling with the addition
of channel count metric information. Least-loaded assignment applies
to multi-fiber links hence the supplemental information pertains to
the number of available channels at a particular wavelength. Hence
the subchannel metric of section 7.2. would simple be the channel
count of a particular wavelength.
The per node processing to implement the least-loaded assignment
algorithm consists of receiving the label set and supplementary
information TLVs (wavelengths and their channel counts) and taking
the minimum of the received channel counts and the egress channel
counts on a per wavelength basis. Where wavelengths with zero
available channels will be discarded from the label set. The
resulting channel counts and wavelength set will then be forwarded on
to the next node for processing. For more details on least loaded
wavelength assignment see [WSON-Frame] and [HZang00].
Example of Wavelength set and wavelength channel count metric.
Suppose that in a 40 channel multi-fiber system and that the
wavelengths (frequencies) have the following number of channels (this
is a multi-fiber system) available:
Frequency(THz) channels available
-----------------------------------------
192.0 3
192.5 2
193.1 1
193.9 2
194.0 2
195.2 1
195.8 1
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We can then represent this list of available frequencies using the
standard label set inclusive list. The wavelength metric list
corresponding to this wavelength set would be given by:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Info Type=1 | M.Size = 0 | Num Metrics = 7 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 3 | 2 | 1 | 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 2 | 1 | 1 | Padded to 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
8. Security Considerations
This document has no requirement for a change to the security models
within GMPLS and associated protocols. That is the OSPF-TE, RSVP-TE,
and PCEP security models could be operated unchanged.
However satisfying the requirements for RWA using the existing
protocols may significantly affect the loading of those protocols.
This makes the operation of the network more vulnerable to denial of
service attacks. Therefore additional care maybe required to ensure
that the protocols are secure in the WSON environment.
Furthermore the additional information distributed in order to
address the RWA problem represents a disclosure of network
capabilities that an operator may wish to keep private. Consideration
should be given to securing this information.
9. IANA Considerations
TBD. Once finalized in our approach we will need identifiers for such
things and modulation types, modulation parameters, wavelength
assignment methods, etc...
10. Acknowledgments
This document was prepared using 2-Word-v2.0.template.dot.
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11. References
11.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3471] Berger, L., "Generalized Multi-Protocol Label Switching
(GMPLS) Signaling Functional Description", RFC 3471,
January 2003.
[RFC3473] Berger, L., Ed., "Generalized Multi-Protocol Label
Switching (GMPLS) Signaling Resource ReserVation Protocol-
Traffic Engineering (RSVP-TE) Extensions", RFC 3473,
January 2003.
[RFC4328] Papadimitriou, D., "Generalized Multi-Protocol Label
Switching (GMPLS) Signaling Extensions for G.709 Optical
Transport Networks Control", RFC 4328, January 2006.
[RFC4420] Farrel, A., Ed., Papadimitriou, D., Vasseur, J.-P., and A.
Ayyangar, "Encoding of Attributes for Multiprotocol Label
Switching (MPLS) Label Switched Path (LSP) Establishment
Using Resource ReserVation Protocol-Traffic Engineering
(RSVP-TE)", RFC 4420, February 2006.
[RFC4606] Mannie, E. and D. Papadimitriou, "Generalized Multi-
Protocol Label Switching (GMPLS) Extensions for Synchronous
Optical Network (SONET) and Synchronous Digital Hierarchy
(SDH) Control", RFC 4606, August 2006.
11.2. Informative References
[WSON-Frame] G. Bernstein, Y. Lee, W. Imajuku, "Framework for GMPLS
and PCE Control of Wavelength Switched Optical Networks",
work in progress: draft-bernstein-ccamp-wavelength-
switched-03.txt, February 2008.
[HZang00] H. Zang, J. Jue and B. Mukherjeee, "A review of routing and
wavelength assignment approaches for wavelength-routed
optical WDM networks", Optical Networks Magazine, January
2000.
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[Otani] T. Otani, H. Guo, K. Miyazaki, D. Caviglia, "Generalized
Labels for G.694 Lambda-Switching Capable Label Switching
Routers (LSR)", work in progress: draft-ietf-ccamp-gmpls-g-
694-lambda-labels-01.txt, May 2008.
[Xu] S. Xu, H. Harai, and D. King, "Extensions to GMPLS RSVP-TE
for Bidirectional Lightpath the Same Wavelength", work in
progress: draft-xu-rsvpte-bidir-wave-01, November 2007.
[Winzer06] Peter J. Winzer and Rene-Jean Essiambre, "Advanced
Optical Modulation Formats", Proceedings of the IEEE, vol.
94, no. 5, pp. 952-985, May 2006.
[G.959.1] ITU-T Recommendation G.959.1, Optical Transport Network
Physical Layer Interfaces, March 2006.
[G.694.1] ITU-T Recommendation G.694.1, Spectral grids for WDM
applications: DWDM frequency grid, June 2002.
[G.694.2] ITU-T Recommendation G.694.2, Spectral grids for WDM
applications: CWDM wavelength grid, December 2003.
[G.Sup43] ITU-T Series G Supplement 43, Transport of IEEE 10G base-R
in optical transport networks (OTN), November 2006.
Author's Addresses
Greg Bernstein
Grotto Networking
Fremont, CA, USA
Phone: (510) 573-2237
Email: gregb@grotto-networking.com
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Hiroaki Harai
National Institute of Information and Communications Technology
4-2-1 Nukui-Kitamachi, Koganei,
Tokyo, 184-8795 Japan
Phone: +81 42-327-5418
Email: harai@nict.go.jp
Daniel King
Aria Networks
44/45 Market Place,
Chippenham, SN15 3HU, United Kingdom
Phone: +44 7790 775187
Email: daniel.king@aria-networks.com
Young Lee (ed.)
Huawei Technologies
1700 Alma Drive, Suite 100
Plano, TX 75075
USA
Phone: (972) 509-5599 (x2240)
Email: ylee@huawei.com
Sugang Xu
National Institute of Information and Communications Technology
4-2-1 Nukui-Kitamachi, Koganei,
Tokyo, 184-8795 Japan
Phone: +81 42-327-6927
Email: xsg@nict.go.jp
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APPENDIX A: Requirement of Bidirectional Lightpath with the Same
Wavelength in Both Directions
A.1. Introduction
With the Lambda Switch (LSC) support defined in GMPLS [RFC3471] and
RSVP-TE signaling [RFC3473], by properly configuring the wavelength
selective switching elements such as ROADMs or OXCs at the transit
nodes, both unidirectional and bidirectional wavelength (optical
lambdas) lightpaths can be established in a wavelength switched
optical network (WSON).
With the wavelength continuity constraint in CI-incapable [RFC3471]
WSONs, where the nodes in the networks cannot support wavelength
conversion, the same wavelength on each link along a unidirectional
lightpath should be reserved. Per the definition in [RFC3471], a
bidirectional lightpath can be seen as a pair of unidirectional
lightpaths, which are provisioned along the same route simultaneously
by the RSVP-TE signaling with Upstream Label and Label Set Objects in
the messages [RFC3473]. This does not necessarily require the same
wavelength in both directions.
In addition to the wavelength continuity constraint, there is another
constraint on wavelength usage, say, require the same wavelength on
both directions. This constraint might be introduced by carriers for
a simplified management to reduce the OPEX. Moreover, according to
some network hardware configurations, users' bidirectional lightpath
has to use the same wavelength in both directions. For example, only
a specific wavelength among the multiplexed wavelengths could be
added/dropped to an optical end node. Some type of ROADMs may
add/drop the same wavelength simultaneously. In particular, with
some WSONs, if different wavelengths in two inverse directions are
used, this brings a port-remapping problem, which is stated as
follows.
A.2. Port-remapping Problem
This problem occurs in the following situations:
(1) Fixed wavelength multiplexer/demultiplexer like AWGs may be
employed in data plane at each node. Each incoming and outgoing
wavelength is with a dedicated fixed port of AWG. For example,
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wavelength lambda 1 is on port 1, and wavelength lambda 2 is on port
2, and so on. See Fig.2.1.
+--+
| |---> lambda 1: port 1
-->| |---> lambda 2: port 2
| |---> lambda 3: port 3
+--+
A. AWG Demultiplexer case.
+--+
| |<--- lambda 1: port 1
<--| |<--- lambda 2: port 2
| |<--- lambda 3: port 3
+--+
B. AWG Multiplexer case.
Fig.2.1. The fixed wavelength-port mapping of AWG
Multiplexer/Demultiplexer.
(2) Compared to a wavelength-tunable optical transponder array, low
cost fixed-tuned optical transponder array may be employed at the
edge node. In an optical transponder, the optical responder is bound
with the transmitter. Each of the optical transmitters and
responders are physically connected to one port of AWG or OXC
according to the hardware configuration. See Fig.2.2.
+--+ +----+
| |<---lambda 1---| T1 |
<--| |<---lambda 2---| T2 |
| |<---lambda 3---| T3 |
+--+ +----+
AWG Multiplexer optical transmitter array
A. The configuration with the optical transmitters connecting AWG.
+--+ +----+
| |---lambda 1--->| R1 |
-->| |---lambda 2--->| R2 |
| |---lambda 3--->| R3 |
+--+ +----+
AWG Demultiplexer optical responder array
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B. One possible configuration with the optical responders connecting
AWG.
+--+ +-----+ +----+
| |--->| |--->| R1 |
-->| |--->| OXC |--->| R2 |
| |--->| |--->| R3 |
+--+ +-----+ +----+
AWG Demultiplexer optical responder array
C. One possible configuration with the optical responders connecting
OXC.
Fig.2.2. The fixed optical transmitter/responder- AGW/OXC port
mapping at the optical end nodes.
Consider a bidirectional lightpath with different wavelengths on two
directions. The optical transmitter of which output wavelength is the
same as the outgoing-wavelength (say lambda 1) is chosen first for
using the lightpath. Then, the optical responder attached to that
transmitter should be selected for receiving the incoming wavelength
(say lambda 2). The responder generally can receive any of different
wavelengths. Therefore, if another bidirectional lightpath is
assigned the same outgoing wavelength (lambda 1) but with a different
incoming wavelength (say lambda 3), the same transmitter and
responder pair is selected. See Fig.2.3.
+----+
<-lambda 1---| T1 |
+----+
A. Optical transmitter T1 sends optical signals on lambda 1.
+----+
-lambda 2--->| R1 |
+----+
B. Optical responder R1 receives optical signals on lambda 2 for one
bidirectional lightpath.
+----+
-lambda 3--->| R1 |
+----+
C. Optical responder R1 can receive optical signals on lambda 3 for
another bidirectional lightpath.
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Fig.2.3. Transmitter sends optical signals on the fixed-tuned
wavelength; the responder can receive data on different wavelengths.
However, the communication using the transponder and the
bidirectional lightpath with different wavelengths will not succeed
under the situations (1) and (2) mentioned above. Remember the fixed
port mapping that each incoming wavelength is fixed on a unique port
of AWG due to the situation (1), and the optical responder is also
fixedly connected to a unique port of AWG or OXC due to the situation
(2). Conversely, the incoming wavelength may change every lightpath
(see lambda 2 and lambda 3 in the above case) for the same outgoing
wavelength (lambda 1). The current incoming wavelength (lambda 3) is
not on the port of AWG to which the optical responder connects
originally (lambda 2), see Fig. 2.4. To connect the optical
responder to the proper port on which the incoming wavelength is,
even in different outgoing wavelengths, a port-remapping process
between the optical responder and AWG ports may be required.
+--+ +----+
| |<---lambda 1---| T1 |
<--| |<---lambda 2---| T2 |
| |<---lambda 3---| T3 |
+--+ +----+
AWG Multiplexer
A. Optical transmitter T1 sends optical signals on lambda 1.
+--+
| |-- +----+
-->| |--lambda2---->| R1 |
| |--lambda3-X +----+
+--+
AWG Demultiplexer
B. Optical responder R1 cannot receive optical signals on lambda 3
due to the fixed port mapping, in case of that R1 is physically
connected to the port 2 of lambda 2 on AWG.
Fig.2.4. Port-remapping problem occurs due to the fixed port-mapping
between the optical responder and AWG port.
A.3. Port-remapping with OXC
The port-remapping capability depends on the system configurations at
users' optical end nodes. For example, an OXC may be employed to switch
the incoming wavelength from the port of AWG to the port which the
optical responder is connected physically, see Fig. 3.1.
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However, equipping users' optical end nodes with OXCs introduces
extra costs. There exists a trade-off between port-remapping
capability and cost/system complexity.
+--+ +-------+ +----+
| |-lambda 1-->| /--|--->| R1 |
-->| |-lambda 2-->|---/ |--->| R2 |
| |-lambda 3-->| OXC |--->| R3 |
+--+ +-------+ +----+
AWG Demultiplexer
A. The optical responder R1 can receive the optical signals on lambda
2.
+--+ +-------+ +----+
| |-lambda 1-->| /---|--->| R1 |
-->| |-lambda 2-->| / |--->| R2 |
| |-lambda 3-->|-/ OXC |--->| R3 |
+--+ +-------+ +----+
AWG Demultiplexer
B. The optical responder R1 can receive the optical signals on lambda
3.
Fig.3.1. The port-remapping capability provided by OXC.
Users have various types of optical end node configurations to choose
from. Some configurations such as those equipped with OXCs might
provide flexibility but could be costly and potentially complicated.
Equally, while other configurations without OXCs might lack the
flexibility they may be inexpensive and easy to use and maintain.
A.4. Avoiding Port-remapping Problem: Bidirectional Lightpath using
Same Wavelength on Both Directions
Which solution will be employed depends on the considerations of the
flexibility and cost/complexity trade-off. If users do not have
port-remapping capability at optical end nodes, then it is necessary
to avoid the port-remapping, and find a feasible approach to provide
users full-duplex transmission capability with bidirectional
lightpath.
A feasible approach is to establish a bidirectional lightpath with
the same wavelength on both directions. At the optical end node,
fixed-tuned transponder array is connected to the proper ports of AWG
according to the wavelength. Optical transmitter and responder pair
connecting the selected outgoing and incoming wavelength ports of AWG
will be assigned to the bidirectional lightpath. In this situation,
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the bidirectional lightpath with the same wavelength on both
directions is required.
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