Network Working Group Y. Lee (ed.)
Internet Draft Huawei
Intended status: Informational G. Bernstein (ed.)
Expires: March 2010 Grotto Networking
Wataru Imajuku
NTT
September 8, 2009
Framework for GMPLS and PCE Control of Wavelength Switched Optical
Networks (WSON)
draft-ietf-ccamp-rwa-wson-framework-03.txt
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Abstract
This memo provides a framework for applying Generalized Multi-
Protocol Label Switching (GMPLS) and the Path Computation Element
(PCE) architecture to the control of wavelength switched optical
networks (WSON). In particular we provide control plane models for
key wavelength switched optical network subsystems and processes. The
subsystems include wavelength division multiplexed links, tunable
laser transmitters, reconfigurable optical add/drop multiplexers
(ROADM) and wavelength converters.
Lightpath provisioning, in general, requires the routing and
wavelength assignment (RWA) process. This process is reviewed and the
information requirements, both static and dynamic for this process
are presented, along with alternative implementation architectures
that could be realized via various combinations of extended GMPLS and
PCE protocols.
This memo focuses on topological elements and path selection
constraints that are common across different WSON environments as
such it does not address optical impairments in any depth nor does it
address potential incompatibilities between some types of optical
signals and some types of network elements and links.
Table of Contents
1. Introduction...................................................4
1.1. Revision History..........................................4
1.1.1. Changes from 00......................................4
1.1.2. Changes from 01......................................5
1.1.3. Changes from 02......................................5
1.2. Related Documents ........................................5
2. Terminology....................................................6
3. Wavelength Switched Optical Networks...........................7
3.1. WDM and CWDM Links........................................7
3.2. Optical Transmitters......................................9
3.2.1. Lasers...............................................9
3.2.2. WSON Signal Parameters..............................10
3.3. ROADMs, OXCs, Splitters, Combiners and FOADMs............10
3.3.1. Reconfigurable Add/Drop Multiplexers and OXCs.......10
3.3.2. Splitters...........................................12
3.3.3. Combiners...........................................13
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3.3.4. Fixed Optical Add/Drop Multiplexers.................13
3.4. Wavelength Converters....................................14
3.4.1. Wavelength Converter Pool Modeling..................15
4. Routing and Wavelength Assignment and the Control Plane.......19
4.1. Architectural Approaches to RWA..........................20
4.1.1. Combined RWA (R&WA).................................21
4.1.2. Separated R and WA (R+WA)...........................21
4.1.3. Routing and Distributed WA (R+DWA)..................22
4.2. Conveying information needed by RWA......................22
4.3. Lightpath Temporal Characteristics.......................23
5. Modeling Examples and Control Plane Use Cases.................24
5.1. Network Modeling for GMPLS/PCE Control...................24
5.1.1. Describing the WSON nodes...........................25
5.1.2. Describing the links................................27
5.2. RWA Path Computation and Establishment...................28
5.3. Resource Optimization....................................29
5.4. Support for Rerouting....................................30
6. GMPLS & PCE Implications......................................30
6.1. Implications for GMPLS signaling.........................30
6.1.1. Identifying Wavelengths and Signals.................31
6.1.2. Combined RWA/Separate Routing WA support............31
6.1.3. Distributed Wavelength Assignment: Unidirectional, No
Converters.................................................31
6.1.4. Distributed Wavelength Assignment: Unidirectional,
Limited Converters.........................................32
6.1.5. Distributed Wavelength Assignment: Bidirectional, No
Converters.................................................32
6.2. Implications for GMPLS Routing...........................33
6.2.1. Wavelength-Specific Availability Information........33
6.2.2. WSON Routing Information Summary....................34
6.3. Optical Path Computation and Implications for PCE........35
6.3.1. Lightpath Constraints and Characteristics...........35
6.3.2. Discovery of RWA Capable PCEs.......................36
6.4. Summary of Impacts by RWA Architecture...................36
7. Security Considerations.......................................37
8. IANA Considerations...........................................38
9. Acknowledgments...............................................38
10. References...................................................39
10.1. Normative References....................................39
10.2. Informative References..................................40
11. Contributors.................................................43
Author's Addresses...............................................43
Intellectual Property Statement..................................44
Disclaimer of Validity...........................................45
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1. Introduction
This memo provides a framework for applying GMPLS and the Path
Computation Element (PCE) architecture to the control of WSONs. In
particular we provide control plane models for key wavelength
switched optical network subsystems and processes. The subsystems
include wavelength division multiplexed links, tunable laser
transmitters, reconfigurable optical add/drop multiplexers (ROADM)
and wavelength converters.
Lightpath provisioning, in general, requires the routing and
wavelength assignment (RWA) process. This process is reviewed and the
information requirements, both static and dynamic for this process
are presented, along with alternative implementation architectures
that could be realized via various combinations of extended GMPLS and
PCE protocols.
This document will focus on the unique properties of links, switches
and path selection constraints that occur in WSONs. Different WSONs
such as access, metro and long haul may apply different techniques
for dealing with optical impairments hence this document will not
address optical impairments in any depth, but instead focus on
properties that are common across a variety of WSONs. For more on how
the GMPLS control plane can aid in dealing with optical impairments
see [WSON-Imp].
For the purposes of this document we assume that all signals used in
a WSON are compatible with all network elements and links within the
WSON. This can arise in practice for a number of reasons including:
(a) in some WSONs only one class of signal is used throughout the
network, or (b) only "relatively" transparent network elements are
utilized in the WSON. How the GMPLS control plane can deal with
situations where this assumption is not true (i.e., where not all of
the optical signals in the network are compatible with all network
elements and limited to processing only certain types of WSON
signals) is addressed in a separate draft [WSON-Compat].
1.1. Revision History
1.1.1. Changes from 00
o Added new first level section on modeling examples and control
plane use cases.
o Added new third level section on wavelength converter pool
modeling
o Editorial clean up of English and updated references.
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1.1.2. Changes from 01
Fixed error in wavelength converter pool example.
1.1.3. Changes from 02
Updated the abstract to emphasize the focus of this draft and
differentiate it from WSON impairment [WSON-Imp] and WSON
compatibility [WSON-Compat] drafts.
Added references to [WSON-Imp] and [WSON-Compat].
Updated the introduction to explain the relationship between this
document and the [WSON-Imp] and [WSON-Compat] documents.
In section 3.1 removed discussion of optical impairments in fibers.
Merged section 3.2.2 and section 3.2.3. Deferred much of the
discussion of signal types and standards to [WSON-Compat].
In section 3.4 on Wavelength converters removed paragraphs dealing
with signal compatibility discussion as this is addressed in [WSON-
Compat].
In section 6.1 removed discussion of signaling extensions to deal
with different WSON signal types. This is deferred to [WSON-Compat].
In section 6 removed discussion of "Need for Wavelength Specific
Maximum Bandwidth Information".
In section 6 removed discussion of "Relationship to link bundling and
layering".
In section 6 removed discussion of "Computation Architecture
Implications" as this material was redundant with text that occurs
earlier in the document.
In section 6 removed discussion of "Scaling Implications" as this
material was redundant with text that occurs earlier in the document.
1.2. Related Documents
This framework document covers essential concepts and models for the
application and extension of the control plane to WSONs. The
following documents address specific aspects of WSONs and complement
this draft.
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o [WSON-Info] This document provides an information model needed by
the routing and wavelength assignment (RWA) process in WSON.
o [WSON-Encode] This document provides efficient, protocol-agnostic
encodings for the information elements necessary to support the
routing and wavelength assignment (RWA) process in WSONs.
o [WSON-Imp] This document provides a framework for the support of
impairment aware Routing and Wavelength Assignment (RWA) in WSON.
o [WSON-Compat] This document provides an overview of signal
compatibility constraints associated with WSON network elements
including regenerators.
o [PCEP-RWA] This document provides application-specific
requirements for the Path Computation Element communication
Protocol (PCEP) for the support of RWA in WSON.
2. Terminology
CWDM: Coarse Wavelength Division Multiplexing.
DWDM: Dense Wavelength Division Multiplexing.
FOADM: Fixed Optical Add/Drop Multiplexer.
OXC: Optical cross connect. A symmetric optical switching element in
which a signal on any ingress port can reach any egress port.
ROADM: Reconfigurable Optical Add/Drop Multiplexer. An asymmetric
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.
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3. Wavelength Switched Optical Networks
WSONs come in a variety of shapes and sizes from continent spanning
long haul networks, to metropolitan networks, to residential access
networks. In all these cases we are concerned with those properties
that constrain the choice of wavelengths that can be used, i.e.,
restrict the wavelength label set, impact the path selection process,
and limit the topological connectivity. In the following we examine
and model some major subsystems of a WSON with an emphasis on those
aspects that are of relevance to the control plane. In particular we
look at WDM links, Optical Transmitters, ROADMs, and Wavelength
Converters.
3.1. WDM and CWDM Links
WDM and CWDM links run over optical fibers, and optical fibers come
in a wide range of types that tend to be optimized for various
applications from access networks, metro, long haul, and submarine
links to name a few. ITU-T standards exist for various types of
fibers. For the purposes here we are concerned only with single mode
fibers (SMF). The following SMF fiber types are typically encountered
in optical networks:
ITU-T Standard | Common Name
------------------------------------------------------------
G.652 [G.652] | Standard SMF |
G.653 [G.653] | Dispersion shifted SMF |
G.654 [G.654] | Cut-off shifted SMF |
G.655 [G.655] | Non-zero dispersion shifted SMF |
G.656 [G.656] | Wideband non-zero dispersion shifted SMF |
------------------------------------------------------------
Typically WDM links operate in one or more of the approximately
defined optical bands [G.Sup39]:
Band Range (nm) Common Name Raw Bandwidth (THz)
O-band 1260-1360 Original 17.5
E-band 1360-1460 Extended 15.1
S-band 1460-1530 Short 9.4
C-band 1530-1565 Conventional 4.4
L-band 1565-1625 Long 7.1
U-band 1625-1675 Ultra-long 5.5
Not all of a band may be usable, for example in many fibers that
support E-band there is significant attenuation due to a water
absorption peak at 1383nm. Hence we can have a discontinuous
acceptable wavelength range for a particular link. Also some systems
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will utilize more than one band. This is particularly true for coarse
WDM (CWDM) systems.
Current technology breaks up the bandwidth capacity of fibers into
distinct channels based on either wavelength or frequency. There are
two standards covering wavelengths and channel spacing. ITU-T
recommendation [G.694.1] describes a DWDM grid defined in terms of
frequency grids of 12.5GHz, 25GHz, 50GHz, 100GHz, and other multiples
of 100GHz around a 193.1THz center frequency. At the narrowest
channel spacing this provides less than 4800 channels across the O
through U bands. ITU-T recommendation [G.694.2] describes a CWDM grid
defined in terms of wavelength increments of 20nm running from 1271nm
to 1611nm for 18 or so channels. The number of channels is
significantly smaller than the 32 bit GMPLS label space allocated to
lambda switching. A label representation for these ITU-T grids is
given in [Otani] and allows a common vocabulary to be used in
signaling lightpaths. Further, these ITU-T grid based labels can also
be used to describe WDM links, ROADM ports, and wavelength converters
for the purposes path selection.
With a tremendous existing base of fiber many WDM links are designed
to take advantage of particular fiber characteristics or to try to
avoid undesirable properties. For example dispersion shifted SMF
[G.653] was originally designed for good long distance performance in
single channel systems, however putting WDM over this type of fiber
requires much system engineering and a fairly limited range of
wavelengths. Hence for our basic, impairment unaware, modeling of a
WDM link we will need the following information:
o Wavelength range(s): Given a mapping between labels and the ITU-T
grids each range could be expressed in terms of a doublet
(lambda1, lambda2) or (freq1, freq1) where the lambdas or
frequencies can be represented by 32 bit integers.
o Channel spacing: currently there are about five channel spacings
used in DWDM systems 12.5GHz to 200GHz and one defined CWDM
spacing.
For a particular link this information is relatively static, i.e.,
changes to these properties generally require hardware upgrades. Such
information could be used locally during wavelength assignment via
signaling, similar to label restrictions in MPLS or used by a PCE in
solving the combined routing and wavelength assignment problem.
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3.2. Optical Transmitters
3.2.1. Lasers
WDM optical systems make use of laser transmitters utilizing
different wavelengths (frequencies). Some laser transmitters were and
are manufactured for a specific wavelength of operation, that is, the
manufactured frequency cannot be changed. First introduced to reduce
inventory costs, tunable optical laser transmitters are becoming
widely deployed in some systems [Coldren04], [Buus06]. This allows
flexibility in the wavelength used for optical transmission and aids
in path selection.
Fundamental modeling parameters from the control plane perspective
optical transmitters are:
o Tunable: Is this transmitter tunable or fixed.
o Tuning range: This is the frequency or wavelength range over which
the laser can be tuned. With the fixed mapping of labels to
lambdas of [Otani] this can be expressed as a doublet (lambda1,
lambda2) or (freq1, freq2) where lambda1 and lambda2 or freq1 and
freq2 are the labels representing the lower and upper bounds in
wavelength or frequency.
o Tuning time: Tuning times highly depend on the technology used.
Thermal drift based tuning may take seconds to stabilize, whilst
electronic tuning might provide sub-ms tuning times. Depending on
the application this might be critical. For example, thermal drift
might not be applicable for fast protection applications.
o Spectral Characteristics and stability: The spectral shape of the
laser's emissions and its frequency stability put limits on
various properties of the overall WDM system. One relatively easy
to characterize constraint is the finest channel spacing on which
the transmitter can be used.
Note that ITU-T recommendations specify many aspects of a laser
transmitter.. Many of these parameters, such as spectral
characteristics and stability, are used in the design of WDM
subsystems consisting of transmitters, WDM links and receivers
however they do not furnish additional information that will
influence label switched path (LSP) provisioning in a properly
designed system.
Also note that lasers transmitters as a component can degrade and
fail over time. This presents the possibility of the failure of a LSP
(lightpath) without either a node or link failure. Hence, additional
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mechanisms may be necessary to detect and differentiate this failure
from the others, e.g., one doesn't not want to initiate mesh
restoration if the source transmitter has failed, since the laser
transmitter will still be failed on the alternate optical path.
3.2.2. WSON Signal Parameters
As pointed out in the introduction, for the purposes of this document
we assume that all optical signals used in a WSON are compatible with
all links, network elements, and receivers in that WSON. In [WSON-
Compat] we discuss how the GMPLS control plane can be extended to
deal with incompatibilities between signals and network elements.
Key WSON signal parameters include modulation type, bit rate and
forward error correction coding technique. Multiple modulation
formats have been standardized [G.959.1] and many others are used
industry and discussed in the literature [Winzer06]. When signals
with different modulation types are used in a WSON then it can be
important to check these signals for compatibility with network
elements such as regenerators, OEO switches, wavelength converters
and receivers [WSON-Compat].
3.3. ROADMs, OXCs, Splitters, Combiners and FOADMs
Definitions of various optical devices and their parameters can be
found in [G.671], we only look at a subset of these and their non-
impairment related properties.
3.3.1. Reconfigurable Add/Drop Multiplexers and OXCs
Reconfigurable add/drop optical multiplexers (ROADM) have matured and
are available in different forms and technologies [Basch06]. This is
a key technology that allows wavelength based optical switching. A
classic degree-2 ROADM is shown in Figure 1.
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Line side ingress +---------------------+ Line side egress
--->| |--->
| |
| ROADM |
| |
| |
+---------------------+
| | | | o o o o
| | | | | | | |
O O O O | | | |
Tributary Side: Drop (egress) Add (ingress)
Figure 1 Degree-2 ROADM
The key feature across all ROADM types is their highly asymmetric
switching capability. In the ROADM of Figure 1, the "add" ingress
ports can only egress on the line side egress port and not on any of
the "drop" egress ports. The degree of a ROADM or switch is given by
the number of line side ports (ingress and egress) and does not
include the number of "add" or "drop" ports. Sometimes the "add"
"drop" ports are also called tributary ports. As the degree of the
ROADM increases beyond two it can have properties of both a switch
(OXC) and a multiplexer and hence we must know the switched
connectivity offered by such a network element to effectively utilize
it. A straight forward way to do this is via a "switched
connectivity" matrix A where Amn = 0 or 1, depending upon whether a
wavelength on ingress port m can be connected to egress port n
[Imajuku]. For the ROADM of Figure 1 the switched connectivity matrix
can be expressed as
Ingress Egress Port
Port #1 #2 #3 #4 #5
--------------
#1: 1 1 1 1 1
#2 1 0 0 0 0
A = #3 1 0 0 0 0
#4 1 0 0 0 0
#5 1 0 0 0 0
Where ingress ports 2-5 are add ports, egress ports 2-5 are drop
ports and ingress port #1 and egress port #1 are the line side (WDM)
ports.
For ROADMs this matrix will be very sparse, and for OXCs the
complement of the matrix will be very sparse, compact encodings and
examples, including high degree ROADMs/OXCs, are given in [WSON-
Encode].
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Additional constraints may also apply to the various ports in a
ROADM/OXC. In the literature of optical switches and ROADMs the
following restrictions/terms are used:
Colored port: An ingress or more typically an egress (drop) port
restricted to a single channel of fixed wavelength.
Colorless port: An ingress or more typically an egress (drop) port
restricted to a single channel of arbitrary wavelength.
In general a port on a ROADM could have any of the following
wavelength restrictions:
o Multiple wavelengths, full range port
o Single wavelength, full range port
o Single wavelength, fixed lambda port
o Multiple wavelengths, reduced range port (for example wave band
switching)
To model these restrictions we need two pieces of information for
each port: (a) number of wavelengths, (b) wavelength range and
spacing. Note that this information is relatively static. More
complicated wavelength constraints are modeled in [WSON-Info].
3.3.2. Splitters
An optical splitter consists of a single ingress port and two or more
egress ports. The ingress optical signaled is essentially copied
(with loss) to all egress ports.
Using the modeling notions of section 3.3.1. the ingress and egress
ports of a splitter would have the same wavelength restrictions. In
addition we can describe a splitter by a connectivity matrix Amn as
follows:
Ingress Egress Port
Port #1 #2 #3 ... #N
-----------------
A = #1 1 1 1 ... 1
The difference from a simple ROADM is that this is not a switched
(potential) connectivity matrix but the fixed connectivity matrix of
the device.
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3.3.3. Combiners
A optical combiner is somewhat the dual of a splitter in that it has
a single multi-wavelength egress port and multiple ingress ports.
The contents of all the ingress ports are copied and combined to the
single egress port. The various ports may have different wavelength
restrictions. It is generally the responsibility of those using the
combiner to assure that wavelength collision does not occur on the
egress port. The fixed connectivity matrix Amn for a combiner would
look like:
Ingress Egress Port
Port #1
---
#1: 1
#2 1
A = #3 1
... 1
#N 1
3.3.4. Fixed Optical Add/Drop Multiplexers
A fixed optical add/drop multiplexer can alter the course of an
ingress wavelength in a preset way. In particular a given wavelength
(or waveband) from a line side ingress port would be dropped to a
fixed "tributary" egress port. Depending on the device's construction
that same wavelength may or may not be "continued" to the line side
egress port ("drop and continue" operation). Further there may exist
tributary ingress ports ("add" ports) whose signals are combined with
each other and "continued" line side signals.
In general to represent the routing properties of an FOADM we need a
fixed connectivity matrix Amn as previously discussed and we need the
precise wavelength restrictions for all ingress and egress ports.
From the wavelength restrictions on the tributary egress ports (drop
ports) we can see what wavelengths have been dropped. From the
wavelength restrictions on the tributary ingress (add) ports we can
see which wavelengths have been added to the line side egress port.
Finally from the added wavelength information and the line side
egress wavelength restrictions we can infer which wavelengths have
been continued.
To summarize, the modeling methodology introduced in section 3.3.1.
consisting of a connectivity matrix and port wavelength restrictions
can be used to describe a large set of fixed optical devices such as
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combiners, splitters and FOADMs. Hybrid devices consisting of both
switched and fixed parts are modeled in [WSON-Info].
3.4. Wavelength Converters
Wavelength converters take an ingress optical signal at one
wavelength and emit an equivalent content optical signal at another
wavelength on egress. There are currently two approaches to building
wavelength converters. One approach is based on optical to electrical
to optical (OEO) conversion with tunable lasers on egress. This
approach can be dependent upon the signal rate and format, i.e., this
is basically an electrical regenerator combined with a tunable laser.
The other approach performs the wavelength conversion, optically via
non-linear optical effects, similar in spirit to the familiar
frequency mixing used in radio frequency systems, but significantly
harder to implement. Such processes/effects may place limits on the
range of achievable conversion. These may depend on the wavelength of
the input signal and the properties of the converter as opposed to
only the properties of the converter in the OEO case. Different WSON
system designs may choose to utilize this component to varying
degrees or not at all.
Current or envisioned contexts for wavelength converters are:
1. Wavelength conversion associated with OEO switches and tunable
laser transmitters. In this case there are plenty of converters to
go around since we can think of each tunable output laser
transmitter on an OEO switch as a potential wavelength converter.
2. Wavelength conversion associated with ROADMs/OXCs. In this case we
may have a limited pool of wavelength converters available.
Conversion could be either all optical or via an OEO method.
3. Wavelength conversion associated with fixed devices such as FOADMs.
In this case we may have a limited amount of conversion. Also in
this case the conversion may be used as part of light path routing.
Based on the above contexts a modeling approach for wavelength
converters could be as follows:
1. Wavelength converters can always be modeled as associated with
network elements. This includes fixed wavelength routing elements.
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2. A network element may have full wavelength conversion capability,
i.e., any ingress port and wavelength, or a limited number of
wavelengths and ports. On a box with a limited number of
converters there also may exist restrictions on which ports can
reach the converters. Hence regardless of where the converters
actually are we can associate them with ingress ports.
3. Wavelength converters have range restrictions that are either
independent or dependent upon the ingress wavelength.
In WSONs where wavelength converters are sparse we may actually see a
light path appear to loop or "backtrack" upon itself in order to
reach a wavelength converter prior to continuing on to its
destination. The lambda used on the "detour" out to the wavelength
converter would be different from that coming back from the "detour"
to the wavelength converter.
A model for an individual O-E-O wavelength converter would consist
of:
o Input lambda or frequency range
o Output lambda or frequency range
3.4.1. Wavelength Converter Pool Modeling
A WSON node may include multiple wavelength converters. These are
usually arranged into some type of pool to promote resource sharing.
There are a number of different approaches used in the design of
switches with converter pools. However, from the point of view of
path computation we need to know the following:
1. The nodes that support wavelength conversion.
2. The accessibility and availability of a wavelength converter to
convert from a given ingress wavelength on a particular ingress
port to a desired egress wavelength on a particular egress port.
3. Limitations on the types of signals that can be converted and the
conversions that can be performed.
To model point 2 above we can use a similar technique as used to
model ROADMs and optical switches, i.e., matrices to indicate
possible connectivity along with wavelength constraints for
links/ports. Since wavelength converters are considered a scarce
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resource we will also want our model to include as a minimum the
usage state of individual wavelength converters in the pool.
We utilize a three stage model as shown schematically in Figure 2. In
this model we assume N ingress ports (fibers), P wavelength
converters, and M egress ports (fibers). Since not all ingress ports
can necessarily reach the converter pool, the model starts with a
wavelength pool ingress matrix WI(i,p) = {0,1} whether ingress port i
can reach potentially reach wavelength converter p.
Since not all wavelength can necessarily reach all the converters or
the converters may have limited input wavelength range we have a set
of ingress port constraints for each wavelength converter. Currently
we assume that a wavelength converter can only take a single
wavelength on input. We can model each wavelength converter ingress
port constraint via a wavelength set mechanism.
Next we have a state vector WC(j) = {0,1} dependent upon whether
wavelength converter j in the pool is in use. This is the only state
kept in the converter pool model. This state is not necessary for
modeling "fixed" transponder system, i.e., systems where there is no
sharing. In addition, this state information may be encoded in a
much more compact form depending on the overall connectivity
structure [WC-Pool].
After that, we have a set of wavelength converter egress wavelength
constraints. These constraints indicate what wavelengths a particular
wavelength converter can generate or are restricted to generating due
to internal switch structure.
Finally, we have a wavelength pool egress matrix WE(p,k) = {0,1}
depending on whether the output from wavelength converter p can reach
egress port k. Examples of this method being used to model wavelength
converter pools for several switch architectures from the literature
are given in reference [WC-Pool].
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I1 +-------------+ +-------------+ E1
----->| | +--------+ | |----->
I2 | +------+ WC #1 +-------+ | E2
----->| | +--------+ | |----->
| Wavelength | | Wavelength |
| Converter | +--------+ | Converter |
| Pool +------+ WC #2 +-------+ Pool |
| | +--------+ | |
| Ingress | | Egress |
| Connection | . | Connection |
| Matrix | . | Matrix |
| | . | |
| | | |
IN | | +--------+ | | EM
----->| +------+ WC #P +-------+ |----->
| | +--------+ | |
+-------------+ ^ ^ +-------------+
| |
| |
| |
| |
Ingress wavelength Egress wavelength
constraints for constraints for
each converter each converter
Figure 2 Schematic diagram of wavelength converter pool model.
Example: Shared Per Node
In Figure 3 below we show a simple optical switch in a four
wavelength DWDM system sharing wavelength converters in a general
"per node" fashion.
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___________ +------+
| |--------------------------->| |
| |--------------------------->| C |
/| | |--------------------------->| o | E1
I1 /D+--->| |--------------------------->| m |
+ e+--->| | | b |====>
====>| M| | Optical | +-----------+ +----+ | i |
+ u+--->| Switch | | WC Pool | |O S|-->| n |
\x+--->| | | +-----+ | |p w|-->| e |
\| | +----+->|WC #1|--+->|t i| | r |
| | | +-----+ | |i t| +------+
| | | | |c c| +------+
/| | | | +-----+ | |a h|-->| |
I2 /D+--->| +----+->|WC #2|--+->|l |-->| C | E2
+ e+--->| | | +-----+ | | | | o |
====>| M| | | +-----------+ +----+ | m |====>
+ u+--->| | | b |
\x+--->| |--------------------------->| i |
\| | |--------------------------->| n |
| |--------------------------->| e |
|___________|--------------------------->| r |
+------+
Figure 3 An optical switch featuring a shared per node wavelength
converter pool architecture.
In this case the ingress and egress pool matrices are simply:
+-----+ +-----+
| 1 1 | | 1 1 |
WI =| |, WE =| |
| 1 1 | | 1 1 |
+-----+ +-----+
Example: Shared Per Link
In Figure 4 we show a different wavelength pool architecture know as
"shared per fiber". In this case the ingress and egress pool matrices
are simply:
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+-----+ +-----+
| 1 1 | | 1 0 |
WI =| |, WE =| |
| 1 1 | | 0 1 |
+-----+ +-----+
___________ +------+
| |--------------------------->| |
| |--------------------------->| C |
/| | |--------------------------->| o | E1
I1 /D+--->| |--------------------------->| m |
+ e+--->| | | b |====>
====>| M| | Optical | +-----------+ | i |
+ u+--->| Switch | | WC Pool | | n |
\x+--->| | | +-----+ | | e |
\| | +----+->|WC #1|--+---------->| r |
| | | +-----+ | +------+
| | | | +------+
/| | | | +-----+ | | |
I2 /D+--->| +----+->|WC #2|--+---------->| C | E2
+ e+--->| | | +-----+ | | o |
====>| M| | | +-----------+ | m |====>
+ u+--->| | | b |
\x+--->| |--------------------------->| i |
\| | |--------------------------->| n |
| |--------------------------->| e |
|___________|--------------------------->| r |
+------+
Figure 4 An optical switch featuring a shared per fiber wavelength
converter pool architecture.
4. Routing and Wavelength Assignment and the Control Plane
In wavelength switched optical networks consisting of tunable lasers
and wavelength selective switches with wavelength converters on every
interface, path selection is similar to the MPLS and TDM circuit
switched cases in that the labels, in this case wavelengths
(lambdas), have only local significance. That is, a wavelength-
convertible network with full wavelength-conversion capability at
each node is equivalent to a circuit-switched TDM network with full
time slot interchange capability; thus, the routing problem needs to
be addressed only at the level of the traffic engineered (TE) link
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choice, and wavelength assignment can be resolved locally by the
switches on a hop-by-hop basis.
However, in the limiting case of an optical network with no
wavelength converters, a light path (optical signal) needs a route
from source to destination and must pick a single wavelength that can
be used along that path without "colliding" with the wavelength used
by any other light path that may share an optical fiber. This is
sometimes referred to as a "wavelength continuity constraint". To
ease up on this constraint while keeping network costs in check a
limited number of wavelength converters may be introduced at key
points in the network [Chu03].
In the general case of limited or no wavelength converters this
computation is known as the Routing and Wavelength Assignment (RWA)
problem [HZang00]. The "hardness" of this problem is well documented.
There, however, exist a number of reasonable approximate methods for
its solution [HZang00].
The inputs to the basic RWA problem are the requested light paths
source and destination, the network's topology, the locations and
capabilities of any wavelength converters, and the wavelengths
available on each optical link. The output from an algorithm solving
the RWA problem is an explicit route through ROADMs, a wavelength for
the optical transmitter, and a set of locations (generally associated
with ROADMs or switches) where wavelength conversion is to occur and
the new wavelength to be used on each component link after that point
in the route.
It is to be noted that choice of specific RWA algorithm is out of the
scope for this document. However there are a number of different
approaches to dealing with the RWA algorithm that can affect the
division of effort between signaling, routing and PCE.
4.1. Architectural Approaches to RWA
Two general computational approaches are taken to solving the RWA
problem. Some algorithms utilize a two step procedure of path
selection followed by wavelength assignment, and others solve the
problem in a combined fashion.
In the following, three different ways of performing RWA in
conjunction with the control plane are considered. The choice of one
of these architectural approaches over another generally impacts the
demands placed on the various control plane protocols.
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4.1.1. Combined RWA (R&WA)
In this case, a unique entity is in charge of performing routing and
wavelength assignment. This approach relies on a sufficient knowledge
of network topology, of available network resources and of network
nodes capabilities. This solution is compatible with most known RWA
algorithms, and in particular those concerned with network
optimization. On the other hand, this solution requires up-to-date
and detailed network information.
Such a computational entity could reside in two different logical
places:
o In a separate Path Computation Element (PCE) which hence owns the
complete and updated knowledge of network state and provides path
computation services to node.
o In the Ingress node, in that case all nodes have the R&WA
functionality; the knowledge of the network state is obtained by a
periodic flooding of information provided by the other nodes.
4.1.2. Separated R and WA (R+WA)
In this case a first entity performs routing, while a second performs
wavelength assignment. The first entity furnishes one or more paths
to the second entity that will perform wavelength assignment and
possibly final path selection.
As the entities computing the path and the wavelength assignment are
separated, this constrains the class of RWA algorithms that may be
implemented. Although it may seem that algorithms optimizing a joint
usage of the physical and spectral paths are excluded from this
solution, many practical optimization algorithms only consider a
limited set of possible paths, e.g., as computed via a k-shortest
path algorithm [Ozdaglar03]. Hence although there is no guarantee
that the selected final route and wavelength offers the optimal
solution, by allowing multiple routes to pass to the wavelength
selection process reasonable optimization can be performed.
The entity performing the routing assignment needs the topology
information of the network, whereas the entity performing the
wavelength assignment needs information on the network's available
resources and on network node capabilities.
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4.1.3. Routing and Distributed WA (R+DWA)
In this case a first entity performs routing, while wavelength
assignment is performed on a hop-by-hop manner along the previously
computed route. This mechanism relies on updating of a list of
potential wavelengths used to ensure conformance with the wavelength
continuity constraint.
As currently specified, the GMPLS protocol suite signaling protocol
can accommodate such an approach. Per [RFC3471], the Label Set
selection works according to an AND scheme. Each hop restricts the
Label Set sent to the next hop from the one received from the
previous hop by performing an AND operation between the wavelength
referred by the labels the message includes with the one available on
the ongoing interface. The constraint to perform this AND operation
is up to the node local policy (even if one expects a consistent
policy configuration throughout a given transparency domain). When
wavelength conversion is performed at an intermediate node, a new
Label Set is generated. The egress nodes selects one label in the
Label Set received at the node, which is also up to the node local
policy.
Depending on these policies a spectral assignment may not be found or
one consuming too many conversion resources relative to what a
dedicated wavelength assignment policy would have achieved. Hence,
this approach may generate higher blocking probabilities in a heavily
loaded network.
On the one hand, this solution may be empowered with some signaling
extensions to ease its functioning and possibly enhance its
performances relatively to blocking. Note that this approach requires
less information dissemination than the others.
The first entity may be a PCE or the ingress node of the LSP. This
solution is applicable inside networks where resource optimization is
not as critical.
4.2. Conveying information needed by RWA
The previous sections have characterized WSONs and lightpath
requests. In particular, high level models of the information used by
the RWA process were presented. We can view this information as
either static, changing with hardware changes (including possibly
failures), or dynamic, those that can change with subsequent
lightpath provisioning. The timeliness in which an entity involved in
the RWA process is notified of such changes is fairly situational.
For example, for network restoration purposes, learning of a hardware
failure or of new hardware coming online to provide restoration
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capability can be critical.
Currently there are various methods for communicating RWA relevant
information, these include, but are not limited to:
o Existing control plane protocols such as GMPLS routing and
signaling. Note that routing protocols can be used to convey both
static and dynamic information. Static information currently
conveyed includes items like router options and such.
o Management protocols such as NetConf, SNMPv3, CLI, CORBA, or
others.
o Directory services and accompanying protocols. These are good for
the dissemination of relatively static information. Not intended
for dynamic information.
o Other techniques for dynamic information: messaging straight from
NEs to PCE to avoid flooding. This would be useful if the number
of PCEs is significantly less than number of WSON NEs. Or other
ways to limit flooding to "interested" NEs.
Mechanisms to improve scaling of dynamic information:
o Tailor message content to WSON. For example the use of wavelength
ranges, or wavelength occupation bit maps.
Utilize incremental updates if feasible.
4.3. Lightpath Temporal Characteristics
The temporal characteristics of a light path connection can affect
the choice of solution to the RWA process. For our purposes here we
look at the timeliness of connection establishment/teardown, and the
duration of the connection.
Connection Establishment/Teardown Timeliness can be thought of in
approximately three time frames:
1. Time Critical: For example those lightpath establishments used for
restoration of service or other high priority real time service
requests.
2. Soft time bounds: This is a more typical new connection request.
While expected to be responsive, there should be more time to take
into account network optimization.
3. Scheduled or Advanced reservations. Here lightpath connections are
requested significantly ahead of their intended "in service" time.
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There is the potential for significant network optimization if
multiple lightpaths can be computed concurrently to achieve network
optimization objectives.
Lightpath connection duration has typically been thought of as
approximately three time frames:
1. Dynamic: those lightpaths with relatively short duration (holding
times).
2. Pseudo-static: lightpaths with moderately long durations.
3. Static: lightpaths with long durations.
Different types of RWA algorithms have been developed for dealing
with dynamic versus pseudo-static conditions. These can address
service provider's needs for: (a) network optimization, (b)
restoration, and (c) highly dynamic lightpath provisioning.
Hence we can model timescale related lightpath requirements via the
following notions:
o Batch or Sequential light path connection requests
o Timeliness of Connection establishment
o Duration of lightpath connection
5. Modeling Examples and Control Plane Use Cases
This section provides examples of the fixed and switch optical node
and wavelength constraint models of section 3. and WSON control plane
use cases related to path computation, establishment, rerouting, and
optimization.
5.1. Network Modeling for GMPLS/PCE Control
Consider a network containing three routers (R1 through R3), eight
WSON nodes (N1 through N8) and 18 links (L1 through L18) and one OEO
converter (O1) in a topology shown below.
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+--+ +--+ +--+ +--------+
+-L3-+N2+-L5-+ +--------L12--+N6+--L15--+ N8 +--
| +--+ |N4+-L8---+ +--+ ++--+---++
| | +-L9--+| | | |
+--+ +-+-+ ++-+ || | L17 L18
| ++-L1--+ | | ++++ +----L16---+ | |
|R1| | N1| L7 |R2| | | |
| ++-L2--+ | | ++-+ | ++---++
+--+ +-+-+ | | | + R3 |
| +--+ ++-+ | | +-----+
+-L4-+N3+-L6-+N5+-L10-+ ++----+
+--+ | +--------L11--+ N7 +----
+--+ ++---++
| |
L13 L14
| |
++-+ |
|O1+-+
+--+
5.1.1. Describing the WSON nodes
The eight WSON nodes in this example have the following properties:
o Nodes N1, N2, N3 have fixed OADMs (FOADMs) installed and can
therefore only access a static and pre-defined set of wavelengths
o All other nodes contain ROADMs and can therefore access all
wavelengths.
o Nodes N4, N5, N7 and N8 are multi-degree nodes, allowing any
wavelength to be optically switched between any of the links. Note
however, that this does not automatically apply to wavelengths
that are being added or dropped at the particular node.
o Node N4 is an exception to that: This node can switch any
wavelength from its add/drop ports to any of its outgoing links
(L5, L7 and L12 in this case)
o The links from the routers are always only able to carry one
wavelength with the exception of links L8 and L9 which are capable
to add/drop any wavelength.
o Node N7 contains an OEO transponder (O1) connected to the node via
links L13 and L14. That transponder operates in 3R mode and does
not change the wavelength of the signal. Assume that it can
regenerate any of the client signals, however only for a specific
wavelength.
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Given the above restrictions, the node information for the eight
nodes can be expressed as follows: (where ID == identifier, SCM ==
switched connectivity matrix, and FCM == fixed connectivity matrix).
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+ID+SCM +FCM +
| | |L1 |L2 |L3 |L4 | | |L1 |L2 |L3 |L4 | |
| |L1 |0 |0 |0 |0 | |L1 |0 |0 |1 |0 | |
|N1|L2 |0 |0 |0 |0 | |L2 |0 |0 |0 |1 | |
| |L3 |0 |0 |0 |0 | |L3 |1 |0 |0 |1 | |
| |L4 |0 |0 |0 |0 | |L4 |0 |1 |1 |0 | |
+--+---+---+---+---+---+---+---+---+---+---+---+---+
| | |L3 |L5 | | | | |L3 |L5 | | | |
|N2|L3 |0 |0 | | | |L3 |0 |1 | | | |
| |L5 |0 |0 | | | |L5 |1 |0 | | | |
+--+---+---+---+---+---+---+---+---+---+---+---+---+
| | |L4 |L6 | | | | |L4 |L6 | | | |
|N3|L4 |0 |0 | | | |L4 |0 |1 | | | |
| |L6 |0 |0 | | | |L6 |1 |0 | | | |
+--+---+---+---+---+---+---+---+---+---+---+---+---+
| | |L5 |L7 |L8 |L9 |L12| |L5 |L7 |L8 |L9 |L12|
| |L5 |0 |1 |1 |1 |1 |L5 |0 |0 |0 |0 |0 |
|N4|L7 |1 |0 |1 |1 |1 |L7 |0 |0 |0 |0 |0 |
| |L8 |1 |1 |0 |1 |1 |L8 |0 |0 |0 |0 |0 |
| |L9 |1 |1 |1 |0 |1 |L9 |0 |0 |0 |0 |0 |
| |L12|1 |1 |1 |1 |0 |L12|0 |0 |0 |0 |0 |
+--+---+---+---+---+---+---+---+---+---+---+---+---+
| | |L6 |L7 |L10|L11| | |L6 |L7 |L10|L11| |
| |L6 |0 |1 |0 |1 | |L6 |0 |0 |1 |0 | |
|N5|L7 |1 |0 |0 |1 | |L7 |0 |0 |0 |0 | |
| |L10|0 |0 |0 |0 | |L10|1 |0 |0 |0 | |
| |L11|1 |1 |0 |0 | |L11|0 |0 |0 |0 | |
+--+---+---+---+---+---+---+---+---+---+---+---+---+
| | |L12|L15| | | | |L12|L15| | | |
|N6|L12|0 |1 | | | |L12|0 |0 | | | |
| |L15|1 |0 | | | |L15|0 |0 | | | |
+--+---+---+---+---+---+---+---+---+---+---+---+---+
| | |L11|L13|L14|L16| | |L11|L13|L14|L16| |
| |L11|0 |1 |0 |1 | |L11|0 |0 |0 |0 | |
|N7|L13|1 |0 |0 |0 | |L13|0 |0 |1 |0 | |
| |L14|0 |0 |0 |1 | |L14|0 |1 |0 |0 | |
| |L16|1 |0 |1 |0 | |L16|0 |0 |1 |0 | |
+--+---+---+---+---+---+---+---+---+---+---+---+---+
| | |L15|L16|L17|L18| | |L15|L16|L17|L18| |
| |L15|0 |1 |0 |0 | |L15|0 |0 |0 |1 | |
|N8|L16|1 |0 |0 |0 | |L16|0 |0 |1 |0 | |
| |L17|0 |0 |0 |0 | |L17|0 |1 |0 |0 | |
| |L18|0 |0 |0 |0 | |L18|1 |0 |1 |0 | |
+--+---+---+---+---+---+---+---+---+---+---+---+---+
5.1.2. Describing the links
For the following discussion some simplifying assumptions are made:
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o It is assumed that the WSON node support a total of four
wavelengths designated WL1 through WL4.
o It is assumed that the impairment feasibility of a path or path
segment is independent from the wavelength chosen.
For the discussion of the RWA operation to build LSPs between two
routers, the wavelength constraints on the links between the routers
and the WSON nodes as well as the connectivity matrix of these links
needs to be specified:
+Link+WLs supported +Possible egress links+
| L1 | WL1 | L3 |
+----+-----------------+---------------------+
| L2 | WL2 | L4 |
+----+-----------------+---------------------+
| L8 | WL1 WL2 WL3 WL4 | L5 L7 L12 |
+----+-----------------+---------------------+
| L9 | WL1 WL2 WL3 WL4 | L5 L7 L12 |
+----+-----------------+---------------------+
| L10| WL2 | L6 |
+----+-----------------+---------------------+
| L13| WL1 WL2 WL3 WL4 | L11 L14 |
+----+-----------------+---------------------+
| L14| WL1 WL2 WL3 WL4 | L13 L16 |
+----+-----------------+---------------------+
| L17| WL2 | L16 |
+----+-----------------+---------------------+
| L18| WL1 | L15 |
+----+-----------------+---------------------+
Note that the possible egress links for the links connecting to the
routers is inferred from the Switched Connectivity Matrix and the
Fixed Connectivity Matrix of the Nodes N1 through N8 and is show here
for convenience, i.e., this information does not need to be repeated.
5.2. RWA Path Computation and Establishment
The calculation of optical impairment feasible routes is outside the
scope of this framework document. In general impairment feasible
routes serve as an input to the RWA algorithm.
For the example use case shown here, assume the following feasible
routes:
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+Endpoint 1+Endpoint 2+Feasible Route +
| R1 | R2 | L1 L3 L5 L8 |
| R1 | R2 | L1 L3 L5 L9 |
| R1 | R2 | L2 L4 L6 L7 L8 |
| R1 | R2 | L2 L4 L6 L7 L9 |
| R1 | R2 | L2 L4 L6 L10 |
| R1 | R3 | L1 L3 L5 L12 L15 L18 |
| R1 | N7 | L2 L4 L6 L11 |
| N7 | R3 | L16 L17 |
| N7 | R2 | L16 L15 L12 L9 |
| R2 | R3 | L8 L12 L15 L18 |
| R2 | R3 | L8 L7 L11 L16 L17 |
| R2 | R3 | L9 L12 L15 L18 |
| R2 | R3 | L9 L7 L11 L16 L17 |
Given a request to establish a LSP between R1 and R2 the RWA
algorithm finds the following possible solutions:
+WL + Path +
| WL1| L1 L3 L5 L8 |
| WL1| L1 L3 L5 L9 |
| WL2| L2 L4 L6 L7 L8|
| WL2| L2 L4 L6 L7 L9|
| WL2| L2 L4 L6 L10 |
Assume now that the RWA chooses WL1 and the Path L1 L3 L5 L8 for the
requested LSP.
Next, another LSP is signaled from R1 to R2. Given the established
LSP using WL1, the following table shows the available paths:
+WL + Path +
| WL2| L2 L4 L6 L7 L9|
| WL2| L2 L4 L6 L10 |
Assume now that the RWA chooses WL2 and the path L2 L4 L6 L7 L9 for
the establishment of the new LSP.
Faced with another LSP request -this time from R2 to R3 - can not be
fulfilled since the only four possible paths (starting at L8 and L9)
are already in use.
5.3. Resource Optimization
The preceding example gives rise to another use case: The
optimization of network resources. Optimization can be achieved on a
number of layers (e.g. through electrical or optical multiplexing of
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client signals) or by re-optimizing the solutions found by the RWA
algorithm.
Given the above example again, assume that the RWA algorithm should
find a path between R2 and R3. The only possible path to reach R3
from R2 needs to use L9. L9 however is blocked by one of the LSPs
from R1.
5.4. Support for Rerouting
It is also envisioned that the extensions to GMPLS and PCE support
rerouting of wavelengths in case of failures.
Assume for this discussion that the only two LSPs in use in the
system are:
LSP1: WL1 L1 L3 L5 L8
LSP2: WL2 L2 L4 L6 L7 L9
Assume furthermore that the link L5 fails. The RWA can now find the
following alternate path and and establish that path:
R1 -> N7 -> R2
Level 3 regeneration will take place at N7, so that the complete path
looks like this:
R1 -> L2 L4 L6 L11 L13 -> O1 -> L14 L16 L15 L12 L9 -> R2
6. GMPLS & PCE Implications
The presence and amount of wavelength conversion available at a
wavelength switching interface has an impact on the information that
needs to be transferred by the control plane (GMPLS) and the PCE
architecture. Current GMPLS and PCE standards can address the full
wavelength conversion case so the following will only address the
limited and no wavelength conversion cases.
6.1. Implications for GMPLS signaling
Basic support for WSON signaling already exists in GMPLS with the
lambda (value 9) LSP encoding type [RFC3471], or for G.709 compatible
optical channels, the LSP encoding type (value = 13) "G.709 Optical
Channel" from [RFC4328]. However a number of practical issues arise
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in the identification of wavelengths and signals, and distributed
wavelength assignment processes which are discussed below.
6.1.1. Identifying Wavelengths and Signals
As previously stated a global fixed mapping between wavelengths and
labels simplifies the characterization of WDM links and WSON devices.
Furthermore such a mapping as described in [Otani] eases
communication between PCE and WSON PCCs.
6.1.2. Combined RWA/Separate Routing WA support
In either the combined RWA or separate routing WA cases, the node
initiating the signaling will have a route from the source to
destination along with the wavelengths (generalized labels) to be
used along portions of the path. Current GMPLS signaling supports an
explicit route object (ERO) and within an ERO an ERO Label subobject
can be use to indicate the wavelength to be used at a particular
node. In case the local label map approach is used the label sub-
object entry in the ERO has to be translated appropriately.
6.1.3. Distributed Wavelength Assignment: Unidirectional, No
Converters
GMPLS signaling for a uni-directional lightpath LSP allows for the
use of a label set object in the RSVP-TE path message. The processing
of the label set object to take the intersection of available lambdas
along a path can be performed resulting in the set of available
lambda being known to the destination that can then use a wavelength
selection algorithm to choose a lambda. For example, the following is
a non-exhaustive subset of wavelength assignment (WA) approaches
discussed in [HZang00]:
1. Random: Looks at all available wavelengths for the light path then
chooses from those available at random.
2. First Fit: Wavelengths are ordered, first available (on all links)
is chosen.
3. Most Used: Out of the wavelengths available on the path attempts
to select most use wavelength in network.
4. Least Loaded: For multi-fiber networks. Chooses the wavelength j
that maximizes minimum of the difference between the number of
fibers on link l and the number of fibers on link l with
wavelength j occupied.
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As can be seen from the above short list, wavelength assignment
methods have differing information or processing requirements. The
information requirements of these methods are as follows:
1. Random: nothing more than the available wavelength set.
2. First Fit: nothing more than the available wavelength set.
3. Most Used: the available wavelength set and information on global
wavelength use in the network.
4. Least Loaded: the available wavelength set and information
concerning the wavelength dependent loading for each link (this
applies to multi-fiber links). This could be obtained via global
information or via supplemental information passed via the
signaling protocol.
In case (3) above the global information needed by the wavelength
assignment could be derived from suitably enhanced GMPLS routing.
Note however this information need not be accurate enough for
combined RWA computation. GMPLS signaling does not provide a way to
indicate that a particular wavelength assignment algorithm should be
used.
6.1.4. Distributed Wavelength Assignment: Unidirectional, Limited
Converters
The previous outlined the case with no wavelength converters. In the
case of wavelength converters, nodes with wavelength converters would
need to make the decision as to whether to perform conversion. One
indicator for this would be that the set of available wavelengths
which is obtained via the intersection of the incoming label set and
the egress links available wavelengths is either null or deemed too
small to permit successful completion.
At this point the node would need to remember that it will apply
wavelength conversion and will be responsible for assigning the
wavelength on the previous lambda-contiguous segment when the RSVP-TE
RESV message passes by. The node will pass on an enlarged label set
reflecting only the limitations of the wavelength converter and the
egress link. The record route option in RVSP-TE signaling can be used
to show where wavelength conversion has taken place.
6.1.5. Distributed Wavelength Assignment: Bidirectional, No
Converters
There are potential issues in the case of a bi-directional lightpath
which requires the use of the same lambda in both directions. We can
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try to use the above procedure to determine the available
bidirectional lambda set if we use the interpretation that the
available label set is available in both directions. However, a
problem, arises in that bidirectional LSPs setup, according to
[RFC3471] section 4.1, is indicated by the presence of an upstream
label in the path message.
However, until the intersection of the available label sets is
obtained, e.g., at the destination node and the wavelength assignment
algorithm has been run the upstream label information will not be
available. Hence currently distributed wavelength assignment with
bidirectional lightpaths is not supported.
6.2. Implications for GMPLS Routing
GMPLS routing [RFC4202] currently defines an interface capability
descriptor for "lambda switch capable" (LSC) which we can use to
describe the interfaces on a ROADM or other type of wavelength
selective switch. In addition to the topology information typically
conveyed via an IGP, we would need to convey the following subsystem
properties to minimally characterize a WSON:
1. WDM Link properties (allowed wavelengths).
2. Laser Transmitters (wavelength range).
3. ROADM/FOADM properties (connectivity matrix, port wavelength
restrictions).
4. Wavelength Converter properties (per network element, may change if
a common limited shared pool is used).
This information is modeled in detail in [WSON-Info] and a compact
encoding is given in [WSON-Encode].
6.2.1. Wavelength-Specific Availability Information
For wavelength assignment we need to know which specific wavelengths
are available and which are occupied if we are going to run a
combined RWA process or separate WA process as discussed in sections
4.1.1. 4.1.2. This is currently not possible with GMPLS routing
extensions.
In the routing extensions for GMPLS [RFC4202], requirements for
layer-specific TE attributes are discussed. The RWA problem for
optical networks without wavelength converters imposes an additional
requirement for the lambda (or optical channel) layer: that of
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knowing which specific wavelengths are in use. Note that current
dense WDM (DWDM) systems range from 16 channels to 128 channels with
advanced laboratory systems with as many as 300 channels. Given these
channel limitations and if we take the approach of a global
wavelength to label mapping or furnishing the local mappings to the
PCEs then representing the use of wavelengths via a simple bit-map is
feasible [WSON-Encode].
6.2.2. WSON Routing Information Summary
The following table summarizes the WSON information that could be
conveyed via GMPLS routing and attempts to classify that information
as to its static or dynamic nature and whether that information would
tend to be associated with either a link or a node.
Information Static/Dynamic Node/Link
------------------------------------------------------------------
Connectivity matrix Static Node
Per port wavelength restrictions Static Node(1)
WDM link (fiber) lambda ranges Static Link
WDM link channel spacing Static Link
Laser Transmitter range Static Link(2)
Wavelength conversion capabilities Static(3) Node
Maximum bandwidth per Wavelength Static Link
Wavelength Availability Dynamic(4) Link
Notes:
1. These are the per port wavelength restrictions of an optical
device such as a ROADM and are independent of any optical
constraints imposed by a fiber link.
2. This could also be viewed as a node capability.
3. This could be dynamic in the case of a limited pool of converters
where the number available can change with connection
establishment. Note we may want to include regeneration
capabilities here since OEO converters are also regenerators.
4. Not necessarily needed in the case of distributed wavelength
assignment via signaling.
While the full complement of the information from the previous table
is needed in the Combined RWA and the separate Routing and WA
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architectures, in the case of Routing + distribute WA via signaling
we only need the following information:
Information Static/Dynamic Node/Link
------------------------------------------------------------------
Connectivity matrix Static Node
Wavelength conversion capabilities Static(3) Node
Information models and compact encodings for this information is
provided in [WSON-Info] and [WSON-Encode].
6.3. Optical Path Computation and Implications for PCE
As previously noted the RWA problem can be computationally intensive
[HZang00]. Such computationally intensive path computations and
optimizations were part of the impetus for the PCE (path computation
element) architecture.
As the PCEP defines the procedures necessary to support both
sequential [RFC5440] and global concurrent path computations
[RFC5557], PCE is well positioned to support WSON-enabled RWA
computation with some protocol enhancement.
Implications for PCE generally fall into two main categories: (a)
lightpath constraints and characteristics, (b) computation
architectures.
6.3.1. Lightpath Constraints and Characteristics
For the varying degrees of optimization that may be encountered in a
network the following models of bulk and sequential lightpath
requests are encountered:
o Batch optimization, multiple lightpaths requested at one time.
o Lightpath(s) and backup lightpath(s) requested at one time.
o Single lightpath requested at a time.
PCEP and PCE-GCO can be readily enhanced to support all of the
potential models of RWA computation.
Lightpath constraints include:
o Bidirectional Assignment of wavelengths
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o Possible simultaneous assignment of wavelength to primary and
backup paths.
o Tuning range constraint on optical transmitter.
Lightpath characteristics can include:
o Duration information (how long this connection may last)
o Timeliness/Urgency information (how quickly is this connection
needed)
6.3.2. Discovery of RWA Capable PCEs
The algorithms and network information needed for solving the RWA are
somewhat specialized and computationally intensive hence not all PCEs
within a domain would necessarily need or want this capability.
Hence, it would be useful via the mechanisms being established for
PCE discovery [RFC5088] to indicate that a PCE has the ability to
deal with the RWA problem. Reference [RFC5088] indicates that a sub-
TLV could be allocated for this purpose.
Recent progress on objective functions in PCE [RFC5541] would allow
the operators to flexibly request differing objective functions per
their need and applications. For instance, this would allow the
operator to choose an objective function that minimizes the total
network cost associated with setting up a set of paths concurrently.
This would also allow operators to choose an objective function that
results in a most evenly distributed link utilization.
This implies that PCEP would easily accommodate wavelength selection
algorithm in its objective function to be able to optimize the path
computation from the perspective of wavelength assignment if chosen
by the operators.
6.4. Summary of Impacts by RWA Architecture
The following table summarizes for each RWA strategy the list of
mandatory ("M") and optional ("O") control plane features according
to GMPLS architectural blocks:
o Information required by the path computation entity,
o LSP request parameters used in either PCC to PCE situations or in
signaling,
o RSVP-TE LSP signaling parameters used in LSP establishment.
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The table shows which enhancements are common to all architectures
(R&WA, R+WA, R+DWA), which apply only to R&WA and R+WA (R+&WA), and
which apply only to R+DWA. Note that this summary serves for the
purpose of a generic reference.
+-------------------------------------+-----+-------+-------+-------+
| | |Common | R+&WA | R+DWA |
| Feature | ref +---+---+---+---+---+---+
| | | M | O | M | O | M | O |
+-------------------------------------+-----+---+---+---+---+---+---+
| Generalized Label for Wavelength |5.1.1| x | | | | | |
+-------------------------------------+-----+---+---+---+---+---+---+
| Flooding of information for the | | | | | | | |
| routing phase | | | | | | | |
| Node features | 3.3 | | | | | | |
| Node type | | | x | | | | |
| spectral X-connect constraint | | | | x | | | |
| port X-connect constraint | | | | x | | | |
| Transponders availability | | | x | | | | |
| Transponders features | 3.2 | | x | | | | |
| Converter availability | | | | x | | | |
| Converter features | 3.4 | | | x | | | x |
| TE-parameters of WDM links | 3.1 | x | | | | | |
| Total Number of wavelength | | x | | | | | |
| Number of wavelengths available | | x | | | | | |
| Grid spacing | | x | | | | | |
| Wavelength availability on links | 5.2 | | | x | | | |
+-------------------------------------+-----+---+---+---+---+---+---+
| LSP request parameters | | | | | | | |
| Signal features | 5.1 | | x | | | x | |
| Modulation format | | | x | | | x | |
| Modulation parameters | | | x | | | x | |
| Specification of RWA method | 5.1 | | x | | | x | |
| LSP time features | 4.3 | | x | | | | |
+-------------------------------------+-----+---+---+---+---+---+---+
| Enriching signaling messages | | | | | | | |
| Signal features | 5.1 | | | | | x | |
+-------------------------------------+-----+---+---+---+---+---+---+
7. 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
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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.
8. IANA Considerations
This document makes no request for IANA actions.
9. Acknowledgments
The authors would like to thank Adrian Farrel for many helpful
comments that greatly improved the contents of this draft.
This document was prepared using 2-Word-v2.0.template.dot.
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10. References
10.1. Normative References
[RFC3471] Berger, L., "Generalized Multi-Protocol Label Switching
(GMPLS) Signaling Functional Description", RFC 3471,
January 2003.
[RFC3630] Katz, D., Kompella, K., and D. Yeung, "Traffic Engineering
(TE) Extensions to OSPF Version 2", RFC 3630, September
2003.
[RFC3945] Mannie, E., "Generalized Multi-Protocol Label Switching
(GMPLS) Architecture", RFC 3945, October 2004.
[RFC4201] Kompella, K., Rekhter, Y., and L. Berger, "Link Bundling in
MPLS Traffic Engineering (TE)", RFC 4201, October 2005.
[RFC4202] Kompella, K. and Y. Rekhter, "Routing Extensions in Support
of Generalized Multi-Protocol Label Switching (GMPLS)", RFC
4202, October 2005.
[RFC4328] Papadimitriou, D., "Generalized Multi-Protocol Label
Switching (GMPLS) Signaling Extensions for G.709 Optical
Transport Networks Control", RFC 4328, January 2006.
[G.694.1] ITU-T Recommendation G.694.1, "Spectral grids for WDM
applications: DWDM frequency grid", June, 2002.
[RFC5088] J.L. Le Roux, J.P. Vasseur, Yuichi Ikejiri, and Raymond
Zhang, "OSPF protocol extensions for Path Computation
Element (PCE) Discovery", January 2008.
[RFC5557] Y. Lee, J.L. Le Roux, D. King, and E. Oki, "Path
Computation Element Communication Protocol (PCECP)
Requirements and Protocol Extensions In Support of Global
Concurrent Optimization", RFC 5557, July 2009.
[RFC5440] J.P. Vasseur and J.L. Le Roux (Editors), "Path Computation
Element (PCE) Communication Protocol (PCEP)", RFC 5440, May
2009.
[RFC5541] J.L. Le Roux, J.P. Vasseur, and Y. Lee, "Encoding of
Objective Functions in Path Computation Element (PCE)
communication and discovery protocols", RFC 5541, July
2009.
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[WSON-Compat] G. Bernstein, Y. Lee, B. Mack-Crane, "WSON Signal
Characteristics and Network Element Compatibility
Constraints for GMPLS", draft-bernstein-ccamp-wson-
compatibility, work in progress.
[WSON-Encode] G. Bernstein, Y. Lee, D. Li, and W. Imajuku, "Routing
and Wavelength Assignment Information Encoding for
Wavelength Switched Optical Networks", draft-bernstein-
ccamp-wson-encode, work in progress.
[WSON-Imp] Y. Lee, G. Bernstein, D. Li, G. Martinelli, "A Framework
for the Control of Wavelength Switched Optical Networks
(WSON) with Impairments", draft-ietf-ccamp-wson-
impairments, work in progress.
[WSON-Info] Y. Lee, G. Bernstein, D. Li, W. Imajuku, "Routing and
Wavelength Assignment Information for Wavelength Switched
Optical Networks", draft-bernstein-ccamp-wson-info, work in
progress
[PCEP-RWA] Y. Lee, G. Bernstein, J. Martensson, T. Takeda, T. Otani,
"PCEP Requirements for WSON Routing and Wavelength
Assignment", draft-lee-pce-wson-routing-wavelength, work in
progress.
10.2. Informative References
[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.
[Coldren04] Larry A. Coldren, G. A. Fish, Y. Akulova, J. S.
Barton, L. Johansson and C. W. Coldren, "Tunable
Seiconductor Lasers: A Tutorial", Journal of Lightwave
Technology, vol. 22, no. 1, pp. 193-202, January 2004.
[Chu03] Xiaowen Chu, Bo Li and Chlamtac I, "Wavelength converter
placement under different RWA algorithms in wavelength-
routed all-optical networks", IEEE Transactions on
Communications, vol. 51, no. 4, pp. 607-617, April 2003.
[Buus06] Jens Buus EJM, "Tunable Lasers in Optical Networks",
Journal of Lightware Technology, vol. 24, no. 1, pp. 5-11,
January 2006.
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[Basch06] E. Bert Bash, Roman Egorov, Steven Gringeri and Stuart
Elby, "Architectural Tradeoffs for Reconfigurable Dense
Wavelength-Division Multiplexing Systems", IEEE Journal of
Selected Topics in Quantum Electronics, vol. 12, no. 4, pp.
615-626, July/August 2006.
[Otani] T. Otani, H. Guo, K. Miyazaki, D. Caviglia, "Generalized
Labels of Lambda-Switching Capable Label Switching Routers
(LSR)", work in progress: draft-otani-ccamp-gmpls-g-694-
lambda-labels, work in progress.
[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.652] ITU-T Recommendation G.652, Characteristics of a single-mode
optical fibre and cable, June 2005.
[G.653] ITU-T Recommendation G.653, Characteristics of a dispersion-
shifted single-mode optical fibre and cable, December 2006.
[G.654] ITU-T Recommendation G.654, Characteristics of a cut-off
shifted single-mode optical fibre and cable, December 2006.
[G.655] ITU-T Recommendation G.655, Characteristics of a non-zero
dispersion-shifted single-mode optical fibre and cable,
March 2006.
[G.656] ITU-T Recommendation G.656, Characteristics of a fibre and
cable with non-zero dispersion for wideband optical
transport, December 2006.
[G.671] ITU-T Recommendation G.671, Transmission characteristics of
optical components and subsystems, January 2005.
[G.872] ITU-T Recommendation G.872, Architecture of optical
transport networks, November 2001.
[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.
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[G.Sup39] ITU-T Series G Supplement 39, Optical system design and
engineering considerations, February 2006.
[G.Sup43] ITU-T Series G Supplement 43, Transport of IEEE 10G base-R
in optical transport networks (OTN), November 2006.
[Imajuku] W. Imajuku, Y. Sone, I. Nishioka, S. Seno, "Routing
Extensions to Support Network Elements with Switching
Constraint", work in progress: draft-imajuku-ccamp-rtg-
switching-constraint-02.txt, July 2007.
[Ozdaglar03] Asuman E. Ozdaglar and Dimitri P. Bertsekas, "Routing
and wavelength assignment in optical networks," IEEE/ACM
Transactions on Networking, vol. 11, 2003, pp. 259 -272.
[RFC4054] Strand, J. and A. Chiu, "Impairments and Other Constraints
on Optical Layer Routing", RFC 4054, May 2005.
[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.
[WC-Pool] G. Bernstein, Y. Lee, "Modeling WDM Switching Systems
including Wavelength Converters" to appear www.grotto-
networking.com, 2008.
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11. Contributors
Snigdho Bardalai
Fujitsu
Email: Snigdho.Bardalai@us.fujitsu.com
Diego Caviglia
Ericsson
Via A. Negrone 1/A 16153
Genoa Italy
Phone: +39 010 600 3736
Email: diego.caviglia@(marconi.com, ericsson.com)
Daniel King
Aria Networks
Email: daniel.king@aria-networks.com
Itaru Nishioka
NEC Corp.
1753 Simonumabe, Nakahara-ku, Kawasaki, Kanagawa 211-8666
Japan
Phone: +81 44 396 3287
Email: i-nishioka@cb.jp.nec.com
Lyndon Ong
Ciena
Email: Lyong@Ciena.com
Pierre Peloso
Alcatel-Lucent
Route de Villejust - 91620 Nozay - France
Email: pierre.peloso@alcatel-lucent.fr
Jonathan Sadler
Tellabs
Email: Jonathan.Sadler@tellabs.com
Dirk Schroetter
Cisco
Email: dschroet@cisco.com
Author's Addresses
Greg M. Bernstein (ed.)
Grotto Networking
Fremont California, USA
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Phone: (510) 573-2237
Email: gregb@grotto-networking.com
Young Lee (ed.)
Huawei Technologies
1700 Alma Drive, Suite 100
Plano, TX 75075
USA
Phone: (972) 509-5599 (x2240)
Email: ylee@huawei.com
Wataru Imajuku
NTT Network Innovation Labs
1-1 Hikari-no-oka, Yokosuka, Kanagawa
Japan
Phone: +81-(46) 859-4315
Email: imajuku.wataru@lab.ntt.co.jp
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