Network Working Group Fatai Zhang
Internet-Draft Xiaobing Zi
Intended status: Standards Track Huawei
O. Gonzalez de Dios
Telefonica
Ramon Casellas
CTTC
Expires: April 27, 2012 October 27, 2011
Requirements for GMPLS Control of Flexible Grids
draft-zhang-ccamp-flexible-grid-requirements-01.txt
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with
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This Internet-Draft will expire on April 27, 2012.
Abstract
A new flexible grid of DWDM is being developed within the ITU-T
Study Group 15 to allow more efficient spectrum allocation. This
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memo describes the requirements of GMPLS control of flexible grid
DWDM 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 [RFC2119].
Table of Contents
1. Introduction ................................................. 3
2. Terminology .................................................. 3
3. Characteristics of Flexible Grid ............................. 4
3.1. Central Frequency ....................................... 4
3.2. Slot Width .............................................. 5
4. Impact on WSON ............................................... 5
4.1. Fiber Links ............................................. 5
4.2. Optical Transmitters and Receivers ...................... 6
5. Routing and Spectrum Assignment .............................. 7
5.1. Architecture Approaches to RSA .......................... 8
5.1.1. Combined RSA (R&SA) ................................ 8
5.1.2. Separated RSA (R+SA) ............................... 9
5.1.3. Routing and Distributed SA (R+DSA) ................. 9
6. Requirements of GMPLS Control ................................ 9
6.1. Routing ................................................. 9
6.1.1. Available Frequency Ranges of DWDM Links .......... 10
6.1.2. Tunable Optical Transmitters and Receivers ........ 10
6.2. Signaling .............................................. 10
6.2.1. Slot Width Requirement ............................ 10
6.2.2. Frequency Slot Representation ..................... 11
6.3. PCE .................................................... 11
6.3.1. RSA Computation Type .............................. 11
6.3.2. RSA path re-optimization request/reply ............ 12
6.3.3. Frequency Constraints ............................. 12
7. Security Considerations ..................................... 12
8. References .................................................. 13
8.1. Normative References ................................... 13
8.2. Informative References ................................. 13
9. Authors' Addresses .......................................... 14
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1. Introduction
[G.694.1v1] defines the DWDM frequency grids for WDM applications. A
frequency grid is a reference set of frequencies used to denote
allowed nominal central frequencies that may be used for defining
applications. The channel spacing, i.e. the frequency spacing
between two allowed nominal central frequencies could be 12.5 GHz,
25 GHz, 50 GHz, 100 GHz and integer multiples of 100 GHz as defined
in [G.694.1v1]. The frequency spacing of the channels on a fiber is
fixed.
The speed of the optical signal becomes higher and higher with the
advancement of the optical technology. In the near future, high-
speed signals (beyond 100 Gbit/s or even 400 Gbit/s) will be
deployed in optical networks. These signals may not be accommodated
in the channel spacing specified in [G.694.1v1]. On the other hand,
''mixed rate'' scenarios will be commonplace, and bandwidth
requirements of the optical signals with different speed will
probably be quite different. As a consequence, when the optical
signals with different speed are mixed to be transmitted on a fiber,
the frequency allocation needs to be more flexible to promote the
efficiency.
An updated version of [G.694.1v1] will be consented in December 2011
in support of flexible grids. The terms ''frequency slot (the
frequency range allocated to a channel and unavailable to other
channels within a flexible grid)'' and ''slot width'' (the full width
of a frequency slot in a flexible grid) are introduced to address
flexible grid. A channel is represented as a LSC (Lambda Switching
Capable) LSP in the control plane and it means a LSC LSP should
occupy a frequency slot on each fiber it traverses. In the case of
flexible grid, different LSC LSPs may have different slot widths on
a fiber, i.e. the slot width is flexible on a fiber.
WSON related documents are being developed currently with the focus
of the GMPLS control of fixed grid optical networks. This document
describes the new characteristics of flexible grids and analyses the
requirements of GMPLS control for the new ''flexible grid'' based
optical transmission.
2. Terminology
Flexible Grid: a new WDM frequency grid defined with the aim of
allowing flexible optical spectrum management, in which the Slot
Width of the frequency ranges allocated to different channels are
flexible (variable sized).
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Frequency Range: a frequency range is defined by a lowest frequency
and a highest frequency.
Frequency Slot: The frequency range allocated to a channel and
unavailable to other channels within a flexible grid. A frequency
slot is defined by its nominal central frequency and its slot width.
Slot Width: the full width (in Hz) of a frequency slot in a flexible
grid. A slot width can be expressed as a multiple (m) of a basic
slot width (e.g. 12.5 GHz)
SSON: Spectrum-Switched Optical Network. An optical network in which
a data plane connection is switched based on an optical spectrum
frequency slot of a variable (flexible) slot width, rather than
based on a fixed grid. Note than a wavelength switched optical
network (WSON) can be seen as a particular case of SSON in which all
slot widths are equal and depend on the used channel spacing.
LSC SS-LSP or flexi-LSP (Lambda Switch Capable Spectrum-Switched
Label Switched Path): a control plane construct that represents a
data plane connection in which the switching involves a frequency
slot of a variable (flexible) slot width. The mapped client signal
is transported over the frequency slot, using spectrum efficient
modulations such as Coherent Optical Orthogonal Frequency Division
Multiplexing (CO-OFDM) and Forward Error Correction (FEC) techniques.
Although still in the scope of LSC, the term flexi-LSP is used, when
needed, to differentiate from regular WSON LSP in which switching is
based on a nominal wavelength.
3. Characteristics of Flexible Grid
Per [G.FLEXIGRID], a flexible grid is defined for the DWDM system.
Compared with the fixed grids (i.e. traditional DWDM), flexible grid
has a smaller granularity for the central frequency and the slot
width of the LSC LSPs is more flexible on a fiber.
3.1. Central Frequency
According to the definition of flexible DWDM grid in [G.FLEXIGRID],
the step granularity for the central frequency of flexible grid is
6.25 GHz. The allowed nominal central frequencies are calculated as
in the case of flexible grid:
Central Frequency = 193.1 THz + n * 0.00625 THz
Where 193.1 THz is ITU-T ''anchor frequency'' for transmission over
the C band and n is a positive or negative integer including 0.
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3.2. Slot Width
A slot width is defined by:
12.5 GHz * m, where m is a positive integer.
Note that, when flexi-grid is supported on a fiber or DWDM link, the
slot width of different flexi-LSPs may be different.
4. Impact on WSON
Wavelength Switched Optical Networks (WSONs) are constructed from
subsystems that include Wavelength Division Multiplexing (WDM) links,
tunable transmitters and receivers, Reconfigurable Optical Add/Drop
Multiplexers (ROADMs), wavelength converters, and electro-optical
network elements. WSON framework is described in [RFC6163]. The
introduced flexible grid brings some changes on WSON.
The concept of WSON is extended to SSON, to highlight that such
subsystems are extended with flexible and/or elastic capabilities
(i.e. flexi-grid). Note that, when modeling SSONs, a WSON can be
seen as a particular case of SSON where all LSC LSP use a fixed (and
equal) slot width which depends on the used channel spacing.
Transceivers may be able to fully leverage flexible optical channels
with advanced modulation formats, and ROADMs may need to be extended
to allow flexible spectrum switching, based in, for example,
Spectrum Selective Switches (SSS).
4.1. Fiber Links
The nominal (central) frequencies for the flexible grid are defined
with a granularity of 6.25 GHz and the allocated frequency slot
widths are defined as a multiple of 12.5 GHz. The fiber link for
flexible grid can be modeled as shown in figure 1.
-9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 11
...+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--...
^
193.1 THz
<--+-->
6.25 6.25 m=1, n=-4
Figure 1 Fiber link model for flexible grid
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The symbol '+' represents the allowed nominal central frequency. The
symbol ''--" represents a 6.25 GHz frequency unit. The number on the
top of the line represents the 'n' in the frequency calculation
formula. The nominal central frequency is 193.1 THz when n equals
zero.
Because the resource allocated to each flexi-LSP is a frequency
range on a fiber link, the following information is needed as
parameters to perform resource allocation for the LSPs:
o Available frequency ranges: The set or union of frequency ranges
that are not allocated (i.e., available or unused) to flexi-LSPs
crossing the DWDM link. The relative grouping and distribution of
available frequency ranges in a fiber is usually referred to as
''fragmentation'' and it is common design criterion for optical
resource control and management.
4.2. Optical Transmitters and Receivers
In WSON, the optical transmitter is the wavelength source and the
optical receiver is the wavelength sink of the WDM system. In each
direction, the wavelength used by the transmitter and receiver along
a path shall be consistent if there is no wavelength converter in
the path.
In the case of flexible grids, the central frequency utilized by a
transmitter or receiver may be fixed or tunable. The slot width
needed by different transmitters or receivers may be different.
Hence, the changes introduced by flexible grid on fundamental
modeling parameters for optical transmitters and receivers from the
control plane perspective are:
o Available central frequencies: The set of central frequencies
which can be used by an optical transmitter or receiver.
o Slot width: The slot width needed by a transmitter or receiver.
Similarly, information on transmitters and receivers capabilities,
in regard to signal processing is needed to perform efficient RSA,
much like in WSON [WSON-ENCODE]. Additional modeling parameters are:
o Supported Input/Output Modulation formats and spectral efficiency
and reach, as well as Input/Output client signals.
o Supported FEC techniques.
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5. Routing and Spectrum Assignment
A LSC flexi-LSP occupies a frequency slot, i.e. a range of frequency,
on each link the LSP traverses. The route computation and frequency
slot assignment could be called RSA (Routing and Spectrum
Assignment).
Similar to fixed grids network, if there is no (available)
wavelength converter in an optical network, a flexible grid LSC LSP
(flexi-LSP) resource allocation will be subject to the ''wavelength
continuity constraint'', which is described as section 4 of [RFC6163].
Because of the high cost of the wavelength converters, an optical
network is generally deployed with limited or without wavelength
converters (sparse translucent optical network). Hence, the
wavelength/spectrum continuity constraint should always be
considered, and the possibility of wavelength conversion will not be
taking into account during the RSA process. When available,
information regarding spectrum conversion capabilities at the
optical nodes MAY be used by RSA mechanisms
The RSA should determine a route and frequency slot for a flexi-LSP.
Note that the mapping between client signals data rates (10, 40,
100... Gbps) and optical slot widths (which are dependent on
modulation formats and other physical layer parameters) is out of
the scope of the document. The frequency slot can be deduced from
the central frequency and slot width parameters as follows:
Lowest frequency = (central frequency) - (slot width)/2;
Highest frequency = (central frequency) + (slot width)/2.
Hence, when a route is computed (by the routing assignment process
or subprocess, RA) the spectrum assignment process (SA) should
determine the central frequency for a flexi-LSP based on the slot
width and available central frequencies information of the
transmitter and receiver, and the available frequency ranges
information of the links that the route traverses.
Figure 2 shows two LSC LSPs that traverse a link.
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Frequency Slot 1 Frequency Slot 2
------------- -------------------
| | | |
-9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 11
...+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--...
------------- -------------------
^ ^
Central F = 193.1THz Central F = 193.14375 THz
Slot width = 25 GHz Slot width = 37.5 GHz
Figure 2 Two LSC LSPs traverse a Link
The two wavelengths shown in figure 2 have the following meaning:
Flexi-LSP 1: central frequency = 193.1 THz, slot width = 25 GHz. It
means the frequency slot [193.0875 THz, 193.1125 THz] is assigned to
this LSC LSP.
Flexi-LSP 2: central frequency = 193.14375 THz, slot width = 37.5
GHz. It means the frequency slot [193.125 THz, 193.1625 THz] is
assigned to this LSC LSP.
Note that the frequency slots of two LSC flexi-LSPs on a fiber MUST
NOT overlap with each other.
5.1. Architecture Approaches to RSA
Similar to RWA for fixed grids, different ways of performing RSA in
conjunction with the control plane can be considered. The approaches
included in this document are provided for reference purposes only,
other possible options could also be deployed.
5.1.1. Combined RSA (R&SA)
In this case, a computation entity performs both routing and
frequency slot assignment. The computation entity should have the
detailed network information, e.g. connectivity topology constructed
by nodes/links information, available frequency ranges on each link,
node capability, etc.
The computation entity could reside on the following elements, which
depends on the implementation:
o PCE: PCE get the detailed network information and implement the
RSA algorithm for RSA requests from the PCCs.
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o Ingress node: Ingress node gets the detailed network information
through routing protocol and implements the RSA algorithm when a
LSC LSP request is received.
5.1.2. Separated RSA (R+SA)
In this case, routing computation and frequency slot assignment are
performed by different entities. The first entity computes the
routes and provides them to the second entity; the second entity
assigns the frequency slot.
The first entity should get the connectivity topology to compute the
proper routes; the second entity should get the available frequency
ranges of the links and nodes' capabilities information to assign
the spectrum.
5.1.3. Routing and Distributed SA (R+DSA)
In this case, one entity computes the route but the frequency slot
assignment is performed hop-by-hop in a distributed way along the
route. The available central frequencies which meet the wavelength
continuity constraint should be collected hop by hop along the route.
This procedure can be implemented by the GMPLS signaling protocol.
The GMPLS signaling procedure is similar to the one described in
section 4.1.3 of [RFC6163] except that the label set should specify
the available central frequencies that meet the slot width
requirement of the LSC LSP, i.e. the frequency slot which is
determined by the central frequency and slot width MUST NOT overlap
with the existing LSC LSPs.
6. Requirements of GMPLS Control
According to the different architecture approaches to RSA some
additional requirements have to be considered for the GMPLS control.
6.1. Routing
In the case of combined RSA architecture, the computation entity
needs to get the detailed network information, i.e. connectivity
topology, node capabilities and available frequency ranges of the
links. Route computation is performed based on the connectivity
topology and node capabilities; spectrum assignment is performed
based on the available frequency ranges of the links. The
computation entity may get the detailed network information by the
GMPLS routing protocol.
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Compared with [RFC6163], except wavelength-specific availability
information, the connectivity topology and node capabilities are the
same as WSON, which can be advertised by GMPLS routing protocol
(refer to section 6.2 of [RFC6163]. This section analyses the
necessary changes on link information brought by flexible grids.
6.1.1. Available Frequency Ranges of DWDM Links
In the case of flexible grids, channel central frequencies span from
193.1 THz towards both ends of the spectrum with 6.25 GHz
granularity. Different LSC LSPs could make use of different slot
widths on the same link. Hence, the available frequency ranges
should be advertised.
6.1.2. Tunable Optical Transmitters and Receivers
The slot width of a LSC LSP is determined by the transmitter and
receiver. The transmitters and receivers could be mapped to ADD/DROP
interfaces in WSON. Hence, the slot width of an ADD/DROP interface
should be advertised.
The central frequency of a transmitter or receiver could be fixed or
tunable. Hence, the available central frequencies should be
advertised.
6.2. Signaling
Compared with [RFC6163], except identifying the resource (i.e.,
fixed wavelength for WSON and frequency resource for flexible grids),
the other signaling requirements (e.g., unidirectional or
bidirectional, with or without converters) are the same as WSON
described in the section 6.1 of [RFC6163].
In the case of routing and distributed SA, GMPLS signaling can be
used to allocate the frequency slot to a LSC LSP. This brings the
following changes to the GMPLS signaling.
6.2.1. Slot Width Requirement
In order to allocate a proper frequency slot for a LSC LSP, the
signaling should specify the slot width requirement of a LSC LSP.
Then the intermediate nodes can collect the acceptable central
frequencies that meet the slot width requirement hop by hop.
The tail node also needs to know the slot width of a LSC LSP to
assign the proper frequency resource. Hence, the slot width
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requirement should be specified in the signaling message when a LSC
LSP is being set up.
6.2.2. Frequency Slot Representation
The frequency slot can be determined by the two parameters, which
are central frequency and slot width as described in section 5.
Hence, the signaling messages should be able to specify the central
frequency and slot width of a LSC LSP.
6.3. PCE
[WSON-PCE] describes the architecture and requirements of PCE for
WSON. In the case of flexible grid, RSA instead of RWA is used for
routing and frequency slot assignment. Hence PCE should implement
RSA for flexible grids. The architecture and requirements of PCE for
flexible grids are similar to what is described in [WSON-PCE]. This
section describes the changes brought by flexible grids.
6.3.1. RSA Computation Type
A PCEP request within a PCReq message MUST be able to specify the
computation type of the request:
o Combined RSA: Both of the route and frequency slot should be
provided by PCE.
o Routing Only: Only the route is requested to be provided by PCE.
The PCEP response within a PCRep Message MUST be able to specify the
route and the frequency slot assigned to the route.
RSA in SSON MAY include the check of signal processing capabilities,
which MAY be provided by the IGP. A PCC should be able to indicate
additional restrictions for such signal compatibility, either on the
endpoint or any given link (such as regeneration points).
A PCC MUST be able to specify whether the PCE MUST also assign a
Modulation list and / or a FEC list, as defined in [WSON-ENCODE] and
[WSON-PCE].
A PCC MUST be able to specify whether the PCE MUST or SHOULD include
or exclude specific modulation formats and FEC mechanisms.
In the case where a valid path is not found, the response MUST be
able to specify the reason (e.g., no route, spectrum not found, etc.)
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6.3.2. RSA path re-optimization request/reply
For a re-optimization request, the PCEP request MUST provide the
path to be re-optimized and include the following options:
o Re-optimize the path keeping the same frequency slot.
o Re-optimize spectrum keeping the same path.
o Re-optimize allowing both frequency slot and the path to change.
The corresponding PCEP response for the re-optimized request MUST
provide the Re-optimized path and frequency slot.
In case the path is not found, the response MUST include the reason
(e.g., no route, frequency slot not found, both of route and
frequency slot not found, etc.)
6.3.3. Frequency Constraints
PCE for flexible grids should consider the following constraints
brought by the transmitters and receivers:
o Available central frequencies: The set of central frequencies that
can be used by an optical transmitter or receiver.
o Slot width: The slot width needed by a transmitter or receiver.
This constraints may be provided by the requester (PCC) in PCReq or
reside within the PCE's TEDB which stores the transponder's
capabilities.
PCC may also specify the frequency constraints for policy reasons.
In this case, the constraints should be specified in the PCReq
message sent to the PCE. In any case, PCE will compute the route and
assign the frequency slot to meet the constraints specified in the
PCReq message. Then return the result to the PCC.
7. Security Considerations
This document does not introduce any further security issues other
than those described in [RFC6163] and [RFC5920].
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8. References
8.1. Normative References
[RFC2119] S. Bradner, "Key words for use in RFCs to indicate
requirements levels", RFC 2119, March 1997.
[WSON-PCE] Y. Lee, G. Bernstein, Jonas Martensson, T. Takeda and T.
Tsuritani, "PCEP Requirements for WSON Routing and
Wavelength Assignment", draft-ietf-pce-wson-routing-
wavelength-05, July 2011.
[WSON-ENCODE] G. Bernstein, Y. Lee, Dan Li and W. Imajuku, "Routing
and Wavelength Assignment Information Encoding for
Wavelength Switched Optical Networks", draft-ietf-ccamp-
rwa-wson-encode, August 2011.
[RFC6163] Y. Lee, G. Bernstein and W. Imajuku, "Framework for GMPLS
and Path Computation Element (PCE) Control of Wavelength
Switched Optical Networks (WSONs)", RFC 6163, April 2011.
[G.FLEXIGRID] Draft revised G.694.1 version 1.3, Unpublished ITU-T
Study Group 15, Question 6.
8.2. Informative References
[G.694.1v1] ITU-T Recommendation G.694.1, Spectral grids for WDM
applications: DWDM frequency grid, June 2002.
[RFC5920] Fang, L., Ed., "Security Framework for MPLS and GMPLS
Networks", RFC 5920, July 2010.
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9. Authors' Addresses
Fatai Zhang
Huawei Technologies
F3-5-B R&D Center, Huawei Base
Bantian, Longgang District
Shenzhen 518129 P.R.China
Phone: +86-755-28972912
Email: zhangfatai@huawei.com
Oscar Gonzalez de Dios
Telefonica Investigacion y Desarrollo
Emilio Vargas 6
Madrid, 28045
Spain
Phone: +34 913374013
Email: ogondio@tid.es
Ramon Casellas
CTTC
Av. Carl Friedrich Gauss, 7
Castelldefels, 08860, Spain
Phone: +34 936452900
Email: ramon.casellas@cttc.es
Xiaobing Zi
Huawei Technologies
F3-5-B R&D Center, Huawei Base
Bantian, Longgang District
Shenzhen 518129 P.R.China
Phone: +86-755-28973229
Email: zixiaobing@huawei.com
Felipe Jimenez Arribas
Telefonica Investigacion y Desarrollo
Emilio Vargas 6
Madrid, 28045
Spain
Email: felipej@tid.es
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