One document matched: draft-bernstein-ccamp-wson-impairments-02.txt
Differences from draft-bernstein-ccamp-wson-impairments-01.txt
Network Working Group G. Bernstein
Internet Draft Grotto Networking
Y. Lee
D. Li
Huawei
Intended status: Informational February 6, 2009
Expires: August 2009
A Framework for the Control of Wavelength Switched Optical Networks
(WSON) with Impairments
draft-bernstein-ccamp-wson-impairments-02.txt
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Abstract
The operation of optical networks can require a level of detail in
the characterization of network elements, subsystems, devices, and
cabling not typically encountered with other networking technologies.
In addition, these physical characteristics may be important to
consider during typical day-to-day operations such as optical path
establishment, as well as during the network planning, installation,
and turn-up phases. This document discusses how the definition and
characterization of optical fiber, devices, subsystems, and network
elements contained in various ITU-T recommendations can be combined
with common control and measurement plane and path computation
element technologies to support Impairment Aware Routing and
Wavelength Assignment (IA-RWA) in optical networks.
Table of Contents
1. Introduction...................................................3
2. Impairment Aware Optical Path Computation......................4
2.1. Optical Network Requirements and Constraints..............4
2.1.1. Impairment Estimation Level of Detail................5
2.1.2. Impairment Information Sharing Constraints...........6
2.2. IA-RWA Computing and Control Plane Architectures..........7
2.2.1. Combined Routing, WA, and IV.........................8
2.2.2. Separate Routing, WA, or IV..........................8
2.2.3. Distributed WA and/or IV.............................9
2.3. Mapping Network Requirements to Architectures.............9
3. Protocol Implications.........................................12
3.1. Information Model for Impairments........................12
3.2. Routing..................................................13
3.3. Signaling................................................13
3.4. PCE......................................................14
3.4.1. Combined IV & RWA...................................14
3.4.2. IV-Candidates + RWA.................................14
3.4.3. Approximate IA-RWA + Separate Detailed IV...........16
4. Security Considerations.......................................18
5. IANA Considerations...........................................18
6. Acknowledgments...............................................18
APPENDIX A: Overview of Optical Layer ITU-T Recommendations......19
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A.1. Fiber and Cables.........................................19
A.2. Devices..................................................20
A.2.1. Optical Amplifiers..................................20
A.2.2. Dispersion Compensation.............................21
A.2.3. Optical Transmitters................................22
A.2.4. Optical Receivers...................................22
A.3. Components and Subsystems................................23
A.4. Network Elements.........................................24
7. References....................................................26
7.1. Normative References.....................................26
7.2. Informative References...................................27
Author's Addresses...............................................28
Intellectual Property Statement..................................29
Disclaimer of Validity...........................................30
1. Introduction
As an optical signal progresses along its path it may be altered by
the various physical processes in the optical fibers and devices it
encounters. When such alterations result in signal degradation, we
usually refer to these processes as "impairments". An overview of
some critical optical impairments and their routing (path selection)
implications can be found in [RFC4054]. Roughly speaking, optical
impairments accumulate along the path (without 3R regeneration)
traversed by the signal. They are influenced by the type of fiber
used, the types and placement of various optical devices and the
presence of other optical signals that may share a fiber segment
along the signal's path. The degradation of the optical signals due
to impairments can result in unacceptable bit error rates or even a
complete failure to demodulate and/or detect the received signal.
Therefore, path selection in any WSON requires consideration of
optical impairments so that the signal will be propagated from the
network ingress point to the egress point with acceptable amount of
degradation.
Some optical subnetworks are designed such that over any path the
degradation to an optical signal due to impairments never exceeds
prescribed bounds. This may be due to the limited geographic extent
of the network, the network topology, and/or the quality of the
fiber and devices employed. In such networks the path selection
problem reduces to determining a continuous wavelength from source
to destination (the Routing and Wavelength Assignment problem).
These networks are discussed in [WSON-Frame]. In other optical
networks, impairments are important and the path selection process
must be impairment-aware.
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Although [RFC4054] describes a number of key optical impairments, a
more complete description of optical impairments and the processes
that spawn them can be found in textbooks or reference books on
optical communications [Agrawal02], [Agrawal07]. To be useful to
consumers and producers of optical fiber, components, and subsystems,
optical characteristics need to be precisely defined along with
methods for their measurement, estimation and approximation. The ITU-
T and other SDOs have assumed this responsibility and as optical
technology has advanced these documents have been updated. Appendix
A of this document provides an overview of the extensive ITU-T
documentation in this area.
The benefits of operating networks of different technologies using an
intelligent control plane have been described in many places, and the
Generalized Multiprotocol Label Switching (GMPLS) control plane is
described in [RFC3945]. The advantages of using a path computation
element (PCE) to perform complex path computations are discussed in
[RFC4655].
Given the existing standards covering optical characteristics
(impairments) and the knowledge of how the impact of impairments may
be estimated along a path, this document provides a framework for
impairment aware path computation and establishment utilizing GMPLS
protocols and the PCE architecture. As in the impairment free case
covered in [WSON-Frame] a number of different control plane
architectural options are described.
2. Impairment Aware Optical Path Computation
One of the most basic questions in communications is whether one can
successfully transmit information from a transmitter to a receiver
within a prescribed error tolerance, usually specified as a maximum
permissible bit error ratio (BER). This generally depends on the
nature of the signal transmitted between the sender and receiver and
the nature of the communications channel between the sender and
receiver. The optical path utilized (along with the wavelength)
determines the communications channel.
The optical impairments incurred by the signal along the fiber and at
each optical network element along the path determine whether the BER
performance or any other measure of signal quality can be met for
this particular signal on this particular path.
2.1. Optical Network Requirements and Constraints
This section examines the various optical network requirements and
constraints that an impairment aware optical control plane may have
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to operate under. These requirements and constraints motivate the IA-
RWA architectural alternatives to be presented in the following
section. We can break the different optical networks contexts up
along two main criteria: (a) the level impairment detail and
validation required, and (b) the constraints on sharing of impairment
information.
2.1.1. Impairment Estimation Level of Detail
A. Ignored Impairments
This situation is applicable to networks designed such that every
possible path is valid for the signal types permitted on the network.
In this case impairments are only taken into account during network
design and after that, for example during optical path computation,
they can be ignored. This is the case discussed in [WSON-Frame] where
impairments could be ignored by the control plane.
B. Approximated Impairment Estimation
This situation is applicable to networks in which impairment effects
can be estimated via approximation techniques such as link budgets
and dispersion (rise time) budgets [Agrawal02],[G.680],[G.sup39]. As
networks grow larger listing all useable paths for each signal type
can encounter scaling issues. Instead the viability of most optical
paths for a particular class of signals is performed using well
defined approximation techniques [G.680], [G.sup39].
Much work at ITU-T has gone into developing impairment models at this
and more detailed levels. Impairment characterization of network
elements could then made available via the control plane and then
used to calculate which paths are conformant with a specified BER for
a particular signal type. In such a case we have the potential, but
not the requirement, to combine the IV process with RWA process to
permit more optimal IA-RWA computations.
C. Detailed Impairment Estimation
This situation is applicable to networks in which impairment effects
must be more accurately estimated. This includes evaluation of the
impact to any existing paths prior to the addition of a new path.
This is currently performed via methods that solve the partial
differential equations describing signal propagation in fiber along
with more detailed models for the other network elements
[Agrawal02],[Agrawal07]. The estimation/simulation time required can
be very situation dependent.
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The implication is that a significant amount of time could be
required to "qualify" a path. To increase the probability of passing
this validation it is strongly recommended to use approximate
impairment information prior to the detailed path validation via
simulations. Since the detailed path validation utilizes calculations
and techniques that are much different from that of RWA it could be
useful to allow the IV process to run on a separate computation
entity. Hence it can be useful to consider any necessary PCEP
extensions required to allow this detailed IV computation to take
place on an additional PCE.
2.1.2. Impairment Information Sharing Constraints
In GMPLS information used in for path computation is shared without
constraints amongst the elements participating in the control plane.
For optical impairments while this would typically be the case,
however it may be the case that some of the information concerning
impairments in an optical network is considered private and not to be
shared with other vendors control systems. This might occur if
proprietary impairment models are used to evaluate paths or a vendor
chooses not to publish impairment information for a set of network
elements. An example of this is document in "situation 1" of [G.680].
In "situation 1" in [G.680] there is a single WDM line system vendor
and multiple vendors of OADMs and PXCs and the impairment information
for the line system can be considered "private", while the impairment
information for any OADM or PXCs can be considered shared.
To maintain the privacy of impairment information and provide for IA-
RWA we have the following options:
A. The vendor who wishes to keep impairment information private can
furnish a list of all viable paths between all viable node pairs
to a computational entity. This information would be particularly
useful as an input to RWA optimization to be performed by another
computation entity. The difficulty here is for larger networks
such a list of paths along with any wavelength constraints could
get unmanageably large.
B. The vendor who wishes to keep impairment information private could
furnish a PCE like entity that would furnish a list of viable
paths/wavelengths between two requested nodes. This is useful as
an input to RWA optimizations and can reduce the scaling issue
previously mentioned. Such a PCE like entity would not need to
perform a full RWA computation, i.e., it would not need to take
into account current wavelength availability on links. Such an
approach may require PCEP extensions for both the request and
response information.
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C. The vendor who wishes to keep impairment information private can
furnish a PCE performs full IA-RWA services. The difficulty is
this requires the one vendor to also become the sole source of all
RWA optimization algorithms and such.
In all the above cases it would be the responsibility of the vendor
wishing to keep its information private to import the shared
impairment information from the other NEs via the control plane or
other means.
2.2. IA-RWA Computing and Control Plane Architectures
From a control plane point of view optical impairments are additional
constraints to the impairment-free RWA process described in [WSON-
Frame]. In impairment aware routing and wavelength assignment (IA-
RWA), there are conceptually three general classes of processes to be
considered: Routing (R), Wavelength Assignment (WA), and Impairment
Validation (IV).
Impairment validation may come in many forms, and maybe invoked at
different levels of detail in the IA-RWA process. From a process
point of view we will consider the following three forms of
impairment validation:
o IV-Candidates
In this case an Impairment Validation (IV) process furnishes a set of
paths between two nodes along with any wavelength restrictions such
that the paths are valid with respect to optical impairments. These
paths and wavelengths may not be actually available in the network
due to its current usage state. This set of paths would be returned
in response to a request for a set of at most K valid paths between
two specified nodes. Note that such a process never directly
discloses optical impairment information.
o IV-Detailed Verification
In this case an IV process is given a particular path and wavelength
through an optical network and is asked to verify whether the overall
quality objectives for the signal over this path can be met. Note
that such a process never directly discloses optical impairment
information.
o IV-Distributed
In this distributed IV process impairment approximate degradation
measures such as OSNR, dispersion, DGD, etc. are accumulated along
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the path via a signaling like protocol. When the accumulated measures
reach the destination node a decision on the impairment validity of
the path can be made. Note that such a process would entail revealing
an individual network element's impairment information.
The following subsections present three major classes of IA-RWA path
computation architectures and their respective advantages and
disadvantages.
2.2.1. Combined Routing, WA, and IV
From the point of view of optimality, the "best" IA-RWA solutions can
be achieved if the path computation entity (PCE) can
conceptually/algorithmically combine the processes of routing,
wavelength assignment and impairment validation.
Such a combination can take place if the PCE is given: (a) the
impairment-free WSON network information as discussed in [WSON-Frame]
and (b) impairment information to validate potential paths.
2.2.2. Separate Routing, WA, or IV
Separating the processes of routing, WA and/or IV can reduce the need
for sharing of different types of information used in path
computation. This was discussed for routing separate from WA in
[WSON-Frame]. In addition, as will be discussed in the section on
network contexts some impairment information may not be shared and
this may lead to the need to separate IV from RWA. In addition, as
also discussed in the section on network contexts, if IV needs to be
done at a high level of precision it may be advantageous to offload
this computation to a specialized server.
The following conceptual architectures belong in this general
category:
o R+WA+IV -- separate routing, wavelength assignment, and impairment
validation.
o R + (WA & IV) -- routing separate from a combined wavelength
assignment and impairment validation process. Note that impairment
validation is typically wavelength dependent hence combining WA
with IV can lead to efficiencies.
o (RWA)+IV - combined routing and wavelength assignment with a
separate impairment validation process.
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Note that the IV process may come before or after the RWA processes.
If RWA comes first then IV is just rendering a yes/no decision on the
selected path and wavelength. If IV comes first it would need to
furnish a list of possible (valid with respect to impairments) routes
and wavelengths to the RWA processes.
2.2.3. Distributed WA and/or IV
In the non-impairment RWA situation [WSON-Frame] it was shown that a
distributed wavelength assignment (WA) process carried out via
signaling can eliminate the need to distribute wavelength
availability information via an IGP. A similar approach can allow for
the distributed computation of impairment effects and avoid the need
to distribute impairment characteristics of network elements and
links via route protocols or by other means. An example of such an
approach is given in [Martinelli] and utilizes enhancements to RSVP
signaling to carry accumulated impairment related information.
A distributed impairment validation for a prescribed network path
requires that the effects of impairments can be calculated by
approximate models with cumulative quality measures such as those in
[G.680].
For such a system to be interoperable the various impairment measures
to be accumulated would need to be agreed upon. Section 9 of [G.680]
can be useful in deriving such cumulative measures but doesn't
explicitly state how a distributed computation would take place. For
example in the computation of the optical signal to noise ratio along
a path (see equation 9-3 of [G.680]) one could accumulate the linear
sum terms and convert to the optical signal to noise ratio (OSNR) in
(dBs) at the destination or one could convert in and out of the OSNR
in (dBs) at each intermediate point along the path.
If distributed WA is being done at the same time as distributed IV
then we may need to accumulate impairment related information for all
wavelengths that could be used. This is somewhat winnowed down as
potential wavelengths are discovered to be in use, but could be a
significant burden for lightly loaded high channel count networks.
2.3. Mapping Network Requirements to Architectures
In Figure 1 we show process flows for three main architectural
alternatives to IA-RWA when approximate impairment validation
suffices. In Figure 2 we show process flows for two main
architectural alternatives when detailed impairment verification is
required.
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+-----------------------------------+
| +--+ +-------+ +--+ |
| |IV| |Routing| |WA| |
| +--+ +-------+ +--+ |
| |
| Combined Processes |
+-----------------------------------+
(a)
+--------------+ +----------------------+
| +----------+ | | +-------+ +--+ |
| | IV | | | |Routing| |WA| |
| |candidates| |----->| +-------+ +--+ |
| +----------+ | | Combined Processes |
+--------------+ +----------------------+
(b)
+-----------+ +----------------------+
| +-------+ | | +--+ +--+ |
| |Routing| |------->| |WA| |IV| |
| +-------+ | | +--+ +--+ |
+-----------+ | Distributed Processes|
+----------------------+
(c)
Figure 1 Process flows for the three main approximate impairment
architectural alternatives.
The advantages, requirements and suitability of these options are as
follows:
o Combined IV & RWA process
This alternative combines RWA and IV within a single computation
entity enabling highest potential optimality and efficiency in IA-
RWA. This alternative requires that the computational entity knows
impairment information as well as non-impairment RWA information.
This alternative can be used when impairment information is required
to be kept private, but would then need to be provided by the non-
sharing vendor.
o IV-Candidates + RWA process
This alternative allows separation of impairment information into two
computational entities while still maintaining a high degree of
potential optimality and efficiency in IA-RWA. The candidates IV
process needs to know impairment information from all optical network
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elements, while the RWA process needs to know non-impairment RWA
information from the network elements. This alternative can be used
when impairment information is required to be kept private, but the
non-sharing vendor would need to provide the functionality of the IV-
candidates process. Note that this is still very useful since the
algorithmic areas of IV and RWA are very different and prone to
specialization.
o Routing + Distributed WA and IV
In this alternative a signaling protocol is extended and leveraged in
the wavelength assignment and impairment validation processes.
Although this doesn't enable as high a potential degree of optimality
of optimality as (a) or (b), it does not require distribution of
either link wavelength usage or link/node impairment information.
Note that this is most likely not acceptable for keeping impairment
information "private".
+-----------------------------------+ +------------+
| +-----------+ +-------+ +--+ | | +--------+ |
| | IV | |Routing| |WA| | | | IV | |
| |approximate| +-------+ +--+ |---->| |Detailed| |
| +-----------+ | | +--------+ |
| Combined Processes | | |
+-----------------------------------+ +------------+
(a)
+--------------+ +----------------------+ +------------+
| +----------+ | | +-------+ +--+ | | +--------+ |
| | IV | | | |Routing| |WA| |---->| | IV | |
| |candidates| |----->| +-------+ +--+ | | |Detailed| |
| +----------+ | | Combined Processes | | +--------+ |
+--------------+ +----------------------+ | |
(b) +------------+
Figure 2 Process flows for the two main detailed impairment
validation architectural options.
The advantages, requirements and suitability of these detailed
validation options are as follows:
o Combined approximate IV & RWA + Detailed-IV
This alternative combines RWA and approximate IV within a single
computation entity enabling highest potential optimality and
efficiency in IA-RWA; then has a separate entity performing detailed
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impairment validation. In the case of non-shared impairment
information the non-sharing vendor would need to provide all
functionality.
o Candidates-IV + RWA + Detailed-IV
This alternative allows separation of approximate impairment
information into a computational entity while still maintaining a
high degree of potential optimality and efficiency in IA-RWA; then a
separate computation entity performs detailed impairment validation.
In the case of non-shared impairment information the non-sharing
vendor would need to provide both the candidates-IV and Detailed-IV
processes, but not necessarily the RWA process.
3. Protocol Implications
The previous IA-RWA architectural alternatives and process flows make
differing demands on a GMPLS/PCE based control plane. In this section
we discuss the use of (a) an impairment information model, (b) PCE as
computational entity assuming the various process roles and
consequences for PCEP, (c)any needed extensions to signaling, and (d)
extensions to routing. The impacts to the control plane for IA-RWA
are summarized in Figure 3.
+-------------------+----+----+----------+--------+
| IA-RWA Option |PCE |Sig |Info Model| Routing|
+-------------------+----+----+----------+--------+
| Combined |Yes | No | Yes | Yes |
| IV & RWA | | | | |
+-------------------+----+----+----------+--------+-
| IV-Candidates |Yes | No | Yes | Yes |
| + RWA | | | | |
+-------------------+----+----+----------+--------+
| Routing + |No | Yes| Yes | No |
|Distributed IV, RWA| | | | |
+-------------------+----+----+----------+--------+
| Detailed IV |Yes | No | Yes | Yes |
+-------------------+----+----+----------+--------+
Figure 3 IA-RWA architectural options and control plane impacts.
3.1. Information Model for Impairments
As previously discussed all IA-RWA scenarios to a greater or lesser
extent rely on a common impairment information model. A number of
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ITU-T recommendations cover detailed as well as approximate
impairment characteristics of fibers and a variety of devices and
subsystems. A well integrated impairment model for optical network
elements is given in [G.680] and is used to form the basis for an
optical impairment model in a companion document [Imp-Info].
It should be noted that the current version of [G.680] is limited to
the networks composed of a single WDM line system vendor combined
with OADMs and/or PXCs from potentially multiple other vendors, this
is known as situation 1 and is shown in Figure 1-1 of [G.680]. It is
planed in the future that [G.680] will include networks incorporating
line systems from multiple vendors as well as OADMs and/or PXCs from
potentially multiple other vendors, this is known as situation 2 and
is shown in Figure 1-2 of [G.680].
The case of distributed impairment validation actually requires a bit
more than an impairment information model. In particular, it needs a
common impairment "computation" model. In the distributed IV case one
needs to standardize the accumulated impairment measures that will be
conveyed and updated at each node. Section 9 of [G.680] provides
guidance in this area with specific formulas given for OSNR, residual
dispersion, polarization mode dispersion/polarization dependent loss,
effects of channel uniformity, etc... However, specifics of what
intermediate results are kept and in what form would need to be
standardized.
3.2. Routing
Different approaches to path/wavelength impairment validation gives
rise to different demands placed on GMPLS routing protocols. In the
case where approximate impairment information is used to validate
paths GMPLS routing may be used to distribute the impairment
characteristics of the network elements and links based on the
impairment information model previously discussed. In the case of
distributed-IV no new demands would be placed on the routing
protocol.
3.3. Signaling
The largest impacts on signaling occur in the cases where distributed
impairment validation is performed. In this we need to accumulate
impairment information as previously discussed. In addition, since
the characteristics of the signal itself, such as modulation type,
can play a major role in the tolerance of impairments, this type of
information will need to be implicitly or explicitly signaled so that
an impairment validation decision can be made at the destination
node.
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It remains for further study if it may be beneficial to include
additional information to a connection request such as desired egress
signal quality (defined in some appropriate sense) in non-distributed
IV scenarios.
3.4. PCE
In section 2.3. we gave a number of computation architectural
alternatives that could be used to meet the various requirements and
constraints of section 2.1. Here we look at how these alternatives
could be implemented via either a single PCE or a set of two or more
cooperating PCEs, and the impacts on the PCEP protocol.
3.4.1. Combined IV & RWA
In this situation, shown in Figure 1(a), a single PCE performs all
the computations needed for IA-RWA.
o TE Database Requirements
WSON Topology and switching capabilities, WSON WDM link wavelength
utilization, and WSON impairment information
o PCC to PCE Request Information
Signal characteristics/type, required quality, source node,
destination node
o PCE to PCC Reply Information
If the computations completed successfully then the PCE returns
the path and its assigned wavelength. If the computations could
not complete successfully it would be potentially useful to know
the reason why. At a very crude level we'd like to know if this
was due to lack of wavelength availability or impairment
considerations or a bit of both. The information to be conveyed is
for further study.
3.4.2. IV-Candidates + RWA
In this situation, shown in Figure 1(b), we have two separate
processes involved in the IA-RWA computation. This requires at least
two cooperating PCEs: one for the Candidates-IV process and another
for the RWA process. In addition, the overall process needs to be
coordinated. This could be done with yet another PCE or we can add
this functionality to one of previously defined PCEs. We choose this
later option and require the RWA PCE to also act as the overall
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process coordinator. The roles, responsibilities and information
requirements for these two PCEs are given below.
RWA and Coordinator PCE (RWA-Coord-PCE):
Responsible for interacting with PCC and for utilizing Candidates-PCE
as needed during RWA computations. In particular it needs to know to
use the Candidates-PCE to obtain potential set of routes and
wavelengths.
o TE Database Requirements
WSON Topology and switching capabilities and WSON WDM link
wavelength utilization (no impairment information).
o PCC to RWA-PCE request: same as in the combined case.
o RWA-PCE to PCC reply: same as in the combined case.
o RWA-PCE to IV-Candidates-PCE request
The RWA-PCE asks for a set of at most K routes along with
acceptable wavelengths between nodes specified in the original PCC
request.
o IV-Candidates-PCE reply to RWA-PCE
The Candidates-PCE returns a set of at most K routes along with
acceptable wavelengths between nodes specified in the RWA-PCE
request.
IV-Candidates-PCE:
The IV-Candidates-PCE is responsible for impairment aware path
computation. It needs not take into account current link
wavelength utilization, but this is not prohibited. The
Candidates-PCE is only required to interact with the RWA-PCE as
indicated above and not the PCC.
o TE Database Requirements
WSON Topology and switching capabilities and WSON impairment
information (no information link wavelength utilization required).
In Figure 4 we show a sequence diagram for the interactions between
the PCC, RWA-PCE and IV-Candidates-PCE.
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+---+ +-------------+ +-----------------+
|PCC| |RWA-Coord-PCE| |IV-Candidates-PCE|
+-+-+ +------+------+ +---------+-------+
...___ (a) | |
| ````---...____ | |
| ```-->| |
| | |
| |--..___ (b) |
| | ```---...___ |
| | ```---->|
| | |
| | |
| | (c) ___...|
| | ___....---'''' |
| |<--'''' |
| | |
| | |
| (d) ___...| |
| ___....---''' | |
|<--''' | |
| | |
| | |
Figure 4 Sequence diagram for the interactions between PCC, RWA-
Coordinating-PCE and the IV-Candidates-PCE.
In step (a) the PCC requests a path meeting specified quality
constraints between two nodes (A and Z) for a given signal
represented either by a specific type or a general class with
associated parameters. In step (b) the RWA-Coordinating-PCE requests
up to K candidate paths between nodes A and Z and associated
acceptable wavelengths. In step (c) The IV-Candidates-PCE returns
this list to the RWA-Coordinating PCE which then uses this set of
paths and wavelengths as input (e.g. a constraint) to its RWA
computation. In step (d) the RWA-Coordinating-PCE returns the overall
IA-RWA computation results to the PCC.
3.4.3. Approximate IA-RWA + Separate Detailed IV
In Figure 2 we showed two cases where a separate detailed impairment
validation process could be utilized. We can place the detailed
validation process into a separate PCE. Assuming that a different PCE
assumes a coordinating role and interacts with the PCC we can keep
the interactions with this separate IV-Detailed-PCE very simple.
IV-Detailed-PCE:
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o TE Database Requirements
The IV-Detailed-PCE will need optical impairment information, WSON
topology, and possibly WDM link wavelength usage information. This
document puts no restrictions on the type of information that may
be used in these computations.
o Coordinating-PCE to IV-Detailed-PCE request
The coordinating-PCE will furnish signal characteristics, quality
requirements, path and wavelength to the IV-Detailed-PCE.
o IV-Detailed-PCE to Coordinating-PCE reply
The reply is essential an yes/no decision as to whether the
requirements could actually be met. In the case where the
impairment validation fails it would be helpful to convey
information related to cause or quantify the failure, e.g., so a
judgment can be made whether to try a different signal or adjust
signal parameters.
In Figure 5 we show a sequence diagram for the interactions for the
process shown in Figure 2(b). This involves interactions between the
PCC, RWA-PCE (acting as coordinator), IV-Candidates-PCE and the IV-
Detailed-PCE.
In step (a) the PCC requests a path meeting specified quality
constraints between two nodes (A and Z) for a given signal
represented either by a specific type or a general class with
associated parameters. In step (b) the RWA-Coordinating-PCE requests
up to K candidate paths between nodes A and Z and associated
acceptable wavelengths. In step (c) The IV-Candidates-PCE returns
this list to the RWA-Coordinating PCE which then uses this set of
paths and wavelengths as input (e.g. a constraint) to its RWA
computation. In step (d) the RWA-Coordinating-PCE request a detailed
verification of the path and wavelength that it has computed. In step
(e) the IV-Detailed-PCE returns the results of the validation to the
RWA-Coordinating-PCE. Finally in step (f)IA-RWA-Coordinating PCE
returns the final results (either a path and wavelength or cause for
the failure to compute a path and wavelength) to the PCC.
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+----------+ +--------------+ +------------+
+---+ |RWA-Coord | |IV-Candidates | |IV-Detailed |
|PCC| | PCE | | PCE | | PCE |
+-+-+ +----+-----+ +------+-------+ +-----+------+
|.._ (a) | | |
| ``--.__ | | |
| `-->| | |
| | (b) | |
| |--....____ | |
| | ````---.>| |
| | | |
| | (c) __..-| |
| | __..---'' | |
| |<--'' | |
| | |
| |...._____ (d) |
| | `````-----....._____ |
| | `````----->|
| | |
| | (e) _____.....+
| | _____.....-----''''' |
| |<----''''' |
| (f) __.| |
| __.--'' |
|<-'' |
| |
Figure 5 Sequence diagram for the interactions between PCC, RWA-
Coordinating-PCE, IV-Candidates-PCE and IV-Detailed-PCE.
4. Security Considerations
This document discusses a number of control plane architectures that
incorporate knowledge of impairments in optical networks. If such
architecture is put into use within a network it will by its nature
contain details of the physical characteristics of an optical
network. Such information would need to be protected from intentional
or unintentional disclosure.
5. IANA Considerations
This draft does not currently require any consideration from IANA.
6. Acknowledgments
This document was prepared using 2-Word-v2.0.template.dot.
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APPENDIX A: Overview of Optical Layer ITU-T Recommendations
For optical fiber, devices, subsystems and network elements the ITU-T
has a variety of recommendations that include definitions,
characterization parameters and test methods. In the following we
take a bottom up survey to emphasize the breadth and depth of the
existing recommendations. We focus on digital communications over
single mode optical fiber.
A.1. Fiber and Cables
Fibers and cables form a key component of what from the control plane
perspective could be termed an optical link. Due to the wide range of
uses of optical networks a fairly wide range of fiber types are used
in practice. The ITU-T has three main recommendations covering the
definition of attributes and test methods for single mode fiber:
o Definitions and test methods for linear, deterministic attributes
of single-mode fibre and cable [G.650.1]
o Definitions and test methods for statistical and non-linear
related attributes of single-mode fibre and cable [G.650.2]
o Test methods for installed single-mode fibre cable sections
[G.650.3]
General Definitions[G.650.1]: Mechanical Characteristics (numerous),
Mode field characteristics(mode field, mode field diameter, mode
field centre, mode field concentricity error, mode field non-
circularity), Glass geometry characteristics, Chromatic dispersion
definitions (chromatic dispersion, group delay, chromatic dispersion
coefficient, chromatic dispersion slope, zero-dispersion wavelength,
zero-dispersion slope), cut-off wavelength, attenuation. Definition
of equations and fitting coefficients for chromatic dispersion (Annex
A). [G.650.2] polarization mode dispersion (PMD) - phenomenon of PMD,
principal states of polarization (PSP), differential group delay
(DGD), PMD value, PMD coefficient, random mode coupling, negligible
mode coupling, mathematical definitions in terms of Stokes or Jones
vectors. Nonlinear attributes: Effective area, correction factor k,
non-linear coefficient (refractive index dependent on intensity),
Stimulated Billouin scattering.
Tests defined [G.650.1]: Mode field diameter, cladding diameter, core
concentricity error, cut-off wavelength, attenuation, chromatic
dispersion. [G.650.2]: test methods for polarization mode dispersion.
[G.650.3] Test methods for characteristics of fibre cable sections
following installation: attenuation, splice loss, splice location,
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fibre uniformity and length of cable sections (these are OTDR based),
PMD, Chromatic dispersion.
With these definitions a variety of single mode fiber types are
defined as shown in the table below:
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 |
------------------------------------------------------------
A.2. Devices
A.2.1. Optical Amplifiers
Optical amplifiers greatly extend the transmission distance of
optical signals in both single channel and multi channel (WDM)
subsystems. The ITU-T has the following recommendations:
o Definition and test methods for the relevant generic parameters of
optical amplifier devices and subsystems [G.661]
o Generic characteristics of optical amplifier devices and
subsystems [G.662]
o Application related aspects of optical amplifier devices and
subsystems [G.663]
o Generic characteristics of Raman amplifiers and Raman amplified
subsystems [G.665]
Reference [G.661] starts with general classifications of optical
amplifiers based on technology and usage, and include a near
exhaustive list of over 60 definitions for optical amplifier device
attributes and parameters. In references [G.662] and [G.665] we have
characterization of specific devices, e.g., semiconductor optical
amplifier, used in a particular setting, e.g., line amplifier. For
example reference[G.662] gives the following minimum list of relevant
parameters for the specification of an optical amplifier device used
as line amplifier in a multichannel application:
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a) Channel allocation.
b) Total input power range.
c) Channel input power range.
d) Channel output power range.
e) Channel signal-spontaneous noise figure.
f) Input reflectance.
g) Output reflectance.
h) Maximum reflectance tolerable at input.
i) Maximum reflectance tolerable at output.
j) Maximum total output power.
k) Channel addition/removal (steady-state) gain response.
l) Channel addition/removal (transient) gain response.
m) Channel gain.
n) Multichannel gain variation (inter-channel gain difference).
o) Multichannel gain-change difference (inter-channel gain-change
difference).
p) Multichannel gain tilt (inter-channel gain-change ratio).
q) Polarization Mode Dispersion (PMD).
A.2.2. Dispersion Compensation
In optical systems two forms of dispersion are commonly encountered
[RFC4054] chromatic dispersion and polarization mode dispersion
(PMD). There are a number of techniques and devices used for
compensating for these effects. The following ITU-T recommendations
characterize such devices:
o Characteristics of PMD compensators and PMD compensating receivers
[G.666]
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o Characteristics of Adaptive Chromatic Dispersion Compensators
[G.667]
The above furnish definitions as well as parameters and
characteristics. For example in [G.667] adaptive chromatic dispersion
compensators are classified as being receiver, transmitter or line
based, while in [G.666] PMD compensators are only defined for line
and receiver configurations. Parameters that are common to both PMD
and chromatic dispersion compensators include: line fiber type,
maximum and minimum input power, maximum and minimum bit rate, and
modulation type. In addition there are a great many parameters that
apply to each type of device and configuration.
A.2.3. Optical Transmitters
The definitions of the characteristics of optical transmitters can be
found in references [G.957], [G.691], [G.692] and [G.959.1]. In
addition references [G.957], [G.691], and [G.959.1] define specific
parameter values or parameter ranges for these characteristics for
interfaces for use in particular situations.
We generally have the following types of parameters
Wavelength related: Central frequency, Channel spacing, Central
frequency deviation[G.692].
Spectral characteristics of the transmitter: Nominal source type
(LED, MLM lasers, SLM lasers) [G.957], Maximum spectral width, Chirp
parameter, Side mode suppression ratio, Maximum spectral power
density [G.691].
Power related: Mean launched power, Extinction ration, Eye pattern
mask [G.691], Maximum and minimum mean channel output power
[G.959.1].
A.2.4. Optical Receivers
References [G.959.1], [G.691], [G.692] and [G.957], define optical
receiver characteristics and [G.959.1], [G.691] and [G.957]give
specific values of these parameters for particular interface types
and network contexts.
The receiver parameters include:
Receiver sensitivity: minimum value of average received power to
achieve a 1x10-10 BER [G.957] or 1x10-12 BER [G.691]. See [G.957] and
[G.691] for assumptions on signal condition.
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Receiver overload: Receiver overload is the maximum acceptable value
of the received average power for a 1x10.10 BER [G.957] or a 1x10-12
BER [G.691].
Receiver reflectance: "Reflections from the receiver back to the
cable plant are specified by the maximum permissible reflectance of
the receiver measured at reference point R."
Optical path power penalty: "The receiver is required to tolerate an
optical path penalty not exceeding X dB to account for total
degradations due to reflections, intersymbol interference, mode
partition noise, and laser chirp."
When dealing with multi-channel systems or systems with optical
amplifiers we may also need:
Optical signal-to-noise ratio: "The minimum value of optical SNR
required to obtain a 1x10-12 BER."[G.692]
Receiver wavelength range: "The receiver wavelength range is defined
as the acceptable range of wavelengths at point Rn. This range must
be wide enough to cover the entire range of central frequencies over
the OA passband." [G.692]
Minimum equivalent sensitivity: "This is the minimum sensitivity that
would be required of a receiver placed at MPI-RM in multichannel
applications to achieve the specified maximum BER of the application
code if all except one of the channels were to be removed (with an
ideal loss-less filter) at point MPI-RM." [G.959.1]
A.3. Components and Subsystems
Reference [G.671] "Transmission characteristics of optical components
and subsystems" covers the following components:
o optical add drop multiplexer (OADM) subsystem;
o asymmetric branching component;
o optical attenuator;
o optical branching component (wavelength non-selective);
o optical connector;
o dynamic channel equalizer (DCE);
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o optical filter;
o optical isolator;
o passive dispersion compensator;
o optical splice;
o optical switch;
o optical termination;
o tuneable filter;
o optical wavelength multiplexer (MUX)/demultiplexer (DMUX);
- coarse WDM device;
- dense WDM device;
- wide WDM device.
Reference [G.671] then specifies applicable parameters for these
components. For example an OADM subsystem will have parameters such
as: insertion loss (input to output, input to drop, add to output),
number of add, drop and through channels, polarization dependent
loss, adjacent channel isolation, allowable input power, polarization
mode dispersion, etc...
A.4. Network Elements
The previously cited ITU-T recommendations provide a plethora of
definitions and characterizations of optical fiber, devices,
components and subsystems. Reference [G.Sup39] "Optical system design
and engineering considerations" provides useful guidance on the use
of such parameters.
In many situations the previous models while good don't encompass the
higher level network structures that one typically deals with in the
control plane, i.e, "links" and "nodes". In addition such models
include the full range of network applications from planning,
installation, and possibly day to day network operations, while with
the control plane we are generally concerned with a subset of the
later. In particular for many control plane applications we are
interested in formulating the total degradation to an optical signal
as it travels through multiple optical subsystems, devices and fiber
segments.
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In reference [G.680] "Physical transfer functions of optical networks
elements", a degradation function is currently defined for the
following optical network elements: (a) DWDM Line segment, (b)
Optical Add/Drop Multiplexers (OADM), and (c) Photonic cross-connect
(PXC). The scope of [G.680] is currently for optical networks
consisting of one vendors DWDM line systems along with another
vendors OADMs or PXCs.
The DWDM line system of [G.680] consists of the optical fiber, line
amplifiers and any embedded dispersion compensators. Similarly the
OADM/PXC network element may consist of the basic OADM component and
optionally included optical amplifiers. The parameters for these
optical network elements (ONE) are given under the following
circumstances:
o General ONE without optical amplifiers
o General ONE with optical amplifiers
o OADM without optical amplifiers
o OADM with optical amplifiers
o Reconfigurable OADM (ROADM) without optical amplifiers
o ROADM with optical amplifiers
o PXC without optical amplifiers
o PXC with optical amplifiers
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7. References
7.1. Normative References
[G.650.1] ITU-T Recommendation G.650.1, Definitions and test methods
for linear, deterministic attributes of single-mode fibre
and cable, June 2004.
[650.2] ITU-T Recommendation G.650.2, Definitions and test methods
for statistical and non-linear related attributes of
single-mode fibre and cable, July 2007.
[650.3] ITU-T Recommendation G.650.3
[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.661] ITU-T Recommendation G.661, Definition and test methods for
the relevant generic parameters of optical amplifier
devices and subsystems, March 2006.
[G.662] ITU-T Recommendation G.662, Generic characteristics of
optical amplifier devices and subsystems, July 2005.
[G.671] ITU-T Recommendation G.671, Transmission characteristics of
optical components and subsystems, January 2005.
[G.680] ITU-T Recommendation G.680, Physical transfer functions of
optical network elements, July 2007.
[G.691] ITU-T Recommendation G.691, Optical interfaces for
multichannel systems with optical amplifiers, November
1998.
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[G.692] ITU-T Recommendation G.692, Optical interfaces for single
channel STM-64 and other SDH systems with optical
amplifiers, March 2006.
[G.872] ITU-T Recommendation G.872, Architecture of optical
transport networks, November 2001.
[G.957] ITU-T Recommendation G.957, Optical interfaces for
equipments and systems relating to the synchronous digital
hierarchy, March 2006.
[G.959.1] ITU-T Recommendation G.959.1, Optical Transport Network
Physical Layer Interfaces, March 2006.
[G.694.1] ITU-T Recommendation G.694.1, Spectral grids for WDM
applications: DWDM frequency grid, June 2002.
[G.694.2] ITU-T Recommendation G.694.2, Spectral grids for WDM
applications: CWDM wavelength grid, December 2003.
[G.Sup39] ITU-T Series G Supplement 39, Optical system design and
engineering considerations, February 2006.
[RFC3945] Mannie, E., Ed., "Generalized Multi-Protocol Label
Switching (GMPLS) Architecture", RFC 3945, October 2004.
[RFC4054] Strand, J., Ed., and A. Chiu, Ed., "Impairments and Other
Constraints on Optical Layer Routing", RFC 4054, May 2005.
[RFC4655] Farrel, A., Vasseur, J.-P., and J. Ash, "A Path Computation
Element (PCE)-Based Architecture", RFC 4655, August 2006.
[WSON-Frame] G. Bernstein, Y. Lee, W. Imajuku, "Framework for GMPLS
and PCE Control of Wavelength Switched Optical Networks",
work in progress: draft-ietf-ccamp-wavelength-switched-
framework-01.txt, November 2008.
7.2. Informative References
[Agrawal02] Govind P. Agrawal, Fiber-Optic Communications Systems -
Third Edition, Wiley-Interscience, 2002.
[Agrawal07] Govind P. Agrawal, Nonlinear Fiber Optics - Fourth
Edition, Academic Press, 2007.
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[Imp-Info] G. Bernstein, Y. Lee, D. Li, "A Framework for the Control
and Measurement of Wavelength Switched Optical Networks
(WSON) with Impairments", work in progress: draft-
bernstein-wson-impairment-info-00.txt, October 2008.
[Martinelli] G. Martinelli (ed.) and A. Zanardi (ed.), "GMPLS
Signaling Extensions for Optical Impairment Aware Lightpath
Setup", Work in Progress: draft-martinelli-ccamp-optical-
imp-signaling-01.txt, February 2008.
Author's Addresses
Greg M. Bernstein (ed.)
Grotto Networking
Fremont California, USA
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
Dan Li
Huawei Technologies Co., Ltd.
F3-5-B R&D Center, Huawei Base,
Bantian, Longgang District
Shenzhen 518129 P.R.China
Phone: +86-755-28973237
Email: danli@huawei.com
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Contributor's Addresses
Ming Chen
Huawei Technologies Co., Ltd.
F3-5-B R&D Center, Huawei Base,
Bantian, Longgang District
Shenzhen 518129 P.R.China
Phone: +86-755-28973237
Email: mchen@huawei.com
Rebecca Han
Huawei Technologies Co., Ltd.
F3-5-B R&D Center, Huawei Base,
Bantian, Longgang District
Shenzhen 518129 P.R.China
Phone: +86-755-28973237
Email: hanjianrui@huawei.com
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