Network Working Group Y. Lee
Internet Draft Huawei
Intended status: Informational G. Bernstein
Grotto Networking
February 24, 2009
Expires: August 2009
Information Model for Impaired Optical Path Validation
draft-bernstein-wson-impairment-info-01.txt
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Abstract
This document provides an information model for the optical
impairment characteristics of optical network elements for use in
path computation and optical path validation. This model is based on
ITU-T defined optical network element characteristics as given in
ITU-T recommendation G.680 and related specifications. This model is
intentionally compatible with a previous impairment free optical
information model used in optical path computations and wavelength
assignment.
Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC-2119 [RFC2119].
Table of Contents
1. Introduction...................................................3
2. Optical Impairment Information Model...........................3
2.1. Network Element Wide Parameters...........................3
2.2. Per Port Parameters.......................................4
2.3. Port to Port Parameters...................................4
2.4. Frequency Dependent Parameters............................5
3. Encoding Considerations........................................5
4. Usage of Parameters in Optical Path Validation.................6
5. Security Considerations........................................7
6. IANA Considerations............................................7
7. Conclusions....................................................7
8. Acknowledgments................................................7
APPENDIX A: Distributed Impairment Accumulation Model.............8
A.1. Distributed Computation of OSNR...........................9
A.2. Distributed Computation of Residual Dispersion...........10
A.3. Distributed Computation of PMD...........................10
A.4. Distributed Computation of PDL...........................11
APPENDIX B: Optical Parameters...................................12
B.1. Parameters for NEs without optical amplifiers............12
B.2. Additional parameters for NEs with optical amplifiers....14
References.......................................................16
8.1. Normative References.....................................16
8.2. Informative References...................................16
Author's Addresses...............................................17
Intellectual Property Statement..................................17
Disclaimer of Validity...........................................17
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1. Introduction
Impairments in optical networks can be accounted for in a number of
ways as discussed in reference [Imp-Frame]. This document provides an
information model for path validation in optical networks utilizing
approximate computations. The definitions, characteristics and usage
of the optical parameters that form this model are based on ITU-T
recommendation G.680 [G.680]. This impairment related model is
intentionally compatible with the impairment free model of reference
[RWA-Info]. Although this document focuses on the optical impairment
parameters from a control plane point of view, Appendix B provides a
list of optical parameter definitions from ITU-T G.680 and related
documents.
2. Optical Impairment Information Model
The definitions of optical impairment parameters of network elements
and examples of their use can be found in [G.680] and related
documents (also see Appendix B). From an information modeling and
control plane perspective, one basic aspect of a given parameter is
the scope of its applicability within a network element. In
particular we need to know which parameters will (a) apply to the
network element as a whole, (b) can vary on a per port basis for a
network element, and (c) can vary based on ingress to egress port
pairs. A second orthogonal aspect of impairment parameters is whether
a parameter exhibits a strong frequency variation over the optical
frequencies supported by the subnetwork.
2.1. Network Element Wide Parameters
Based on the definitions in [G.680] and related documents the
following parameters apply to the network element as a whole. At most
one of these parameters is required per network element.
1. Channel frequency range (GHz, Max, Min)
2. Channel insertion loss deviation (dB, Max)
3. Ripple (dB, Max)
4. Channel chromatic dispersion (ps/nm, Max, Min)
5. Differential group delay (ps, Max)
6. Polarization dependent loss (dB, Max)
7. Reflectance (passive component) (dB, Max)
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8. Reconfigure time/Switching time (ms, Max, Min)
9. Channel uniformity (dB, Max)
10.Channel addition/removal (steady-state) gain response (dB, Max,
Min)
11.Transient duration (ms, Max)
12.Transient gain increase (dB, Max)
13.Transient gain reduction (dB, Max)
14.Multichannel gain-change difference (inter-channel gain-change
difference) (dB, Max)
15.Multichannel gain tilt (inter-channel gain-change ratio)(dB, Max)
2.2. Per Port Parameters
The following optical parameters may exhibit per port dependence,
hence may be specified at most once for each port of the network
element.
1. Total input power range (dBm, Max, Min)
2. Channel input power range (dBm, Max, Min)
3. Channel output power range (dBm, Max, Min)
4. Input reflectance (dB, Max) (with amplifiers)
5. Output reflectance (dB, Max) (with amplifiers)
6. Maximum reflectance tolerable at input (dB, Min)
7. Maximum reflectance tolerable at output (dB, Min)
8. Maximum total output power (dBm, Max)
2.3. Port to Port Parameters
The following optical parameters may exhibit a port-to-port
dependence and hence may be specified at most once for each
ingress/egress port pair of the network element.
1. Insertion loss (dB, Max, Min)
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2. Isolation, adjacent channel (dB, Min)
3. Isolation, non-adjacent channel (dB, Min)
4. Channel extinction (dB, Min)
5. Channel signal-spontaneous noise figure (dB, Max)
6. Channel gain (dB, Max, Min)
2.4. Frequency Dependent Parameters
Many of the previously mentioned parameters can exhibit a significant
frequency dependence over the range of wavelength supported by a
subnetwork. In reference [G.680] parameters denoted as related to
"channel" could exhibit significant frequency variation that would
need to be encoded efficiently. These parameters may include:
1. Channel insertion loss deviation (dB, Max)
2. Channel chromatic dispersion (ps/nm, Max, Min)
3. Channel uniformity (dB, Max)
4. Insertion loss (dB, Max, Min)
5. Channel extinction (dB, Min)
6. Channel signal-spontaneous noise figure (dB, Max)
7. Channel gain (dB, Max, Min)
Finalization of this list is TBD and will need liaison with ITU-T.
3. Encoding Considerations
The units for the various parameters include GHz, dB, dBm, ms, ps,
and ps/nm. These are typically expressed as floating point numbers.
Due to the measurement limitations inherent in these parameters
single precision floating point, e.g., 32 bit IEEE floating point,
numbers should be sufficient.
For realistic optical network elements per port and port-to-port
parameters typically only assume a few values. For example, the
channel gain of a ROADM is usually specified in terms of input to
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drop, add to output, and input to output. This implies that many port
and port-to-port parameters could be efficiently specified, stored
and transported by making use of the Link Set Sub-TLV and
Connectivity Matrix Sub-TLV of reference [Encode].
For parameters that vary with frequency we have the following
options:
1. Explicit parameter list with associated frequencies: Here we would
give the parameter and frequencies it applies to. We would need
as many of these parameter/frequency pairs as necessary to cover
all the frequencies and parameters. This could get large for a
high channel count system with strong frequency dependencies in
some parameters.
2. Provide "standardized" general interpolation formulas and
parameters for use over an entire frequency range or sub-range.
3. Use parameter specific interpolation formulas based on ITU-T and
other standards. For example in reference [G.650.1]Annex A
equations and fitting coefficients are given for chromatic
dispersion interpolation. Such formulas may be valid over an
entire frequency range or a sub-range.
4. Usage of Parameters in Optical Path Validation
Given an optical path and the optical characteristics of each network
element along the path we can then use these characteristics to
validate the path. We envision that these parameters will be made
available via some mechanism to the entity which validates optical
paths. Refer to [Imp-Frame] for architectural options in which
impairment validation for an optical path is defined.
Sections 9 and 10 of G.680 give techniques and formulas for use in
calculating the impact of a cascade of network elements such as
occurs along an optical signal path. These range from relatively
simple bounds on the sum of uncompensated chromatic dispersion
(residual dispersion) to more elaborate formulas for overall optical
signal to noise ration (OSNR) computations based on multiple
parameters including noise factor.
To further aid understanding and use of these optical parameters
Appendix I of [G.680] provides example parameter values for different
network element types and appendix II provides examples of
computations involving the cascades of network elements along a path.
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5. Security Considerations
This document defines an information model for impairments in optical
networks. If such a model 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.
6. IANA Considerations
This draft does not currently require any consideration from IANA.
7. Conclusions
The state of standardization of optical device characteristics has
matured from when initial IETF work concerning optical impairments
was investigated in [RFC4054]. Relatively recent ITU-T
recommendations provide a standardized based of optical
characteristic definitions and parameters that control plane
technologies such as GMPLS and PCE can make use of in performing
optical path validation. The enclosed information model shows how
readily such ITU-T optical work can be utilized within the control
plane.
8. Acknowledgments
This document was prepared using 2-Word-v2.0.template.dot.
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APPENDIX A: Distributed Impairment Accumulation Model
In reference [Imp-Frame] an alternative impairment aware RWA control
plane based on distributed impairment validation was discussed. In
such a scheme the preceding impairment information model would not be
distributed via a link state IGP, instead a set of impairment
parameters would be computed along the proposed path and a final
decision on whether the path is viable would be made based on these
accumulated impairment parameters. It should be noted that these
accumulated impairment parameters are estimated at each node along
the path and not measured.
When signaling a path we think of the "nodes" as being the switching
nodes along the path. In the case of optical impairments the
properties of the links (WDM line systems) are just as important as
the properties of the nodes. In the following we will assume that the
switching nodes (GMPLS nodes) will act on behalf of all the line
systems corresponding to their egress ports. In particular this
implies that some how these nodes will obtain the line system
impairment information.
Mux PXC PXC Demux
|\ +--+ ROADM ROADM +--+ /|
-|| BA LA LA | | LA LA +---+ LA LA +---+ LA LA | | LA LA | |-
-||_|\__|\__|\_| |_|\__|\_| |_|\__|\_| |_|\__|\_| |_|\__|\_| |-
-|| |/ |/ |/ | | |/ |/ | | |/ |/ | | |/ |/ | | |/ |/ | |-
-|| -| |- +---+ +---+ | |- | |-
|/ +--+ | | | | +--+ \|
<---- NE1 ----><--- NE2 --><--- NE3 ---><--- NE4 ---><--- NE5 -->
Figure 1 A path through an optical network with line systems, PXCs,
ROADMs, and multiplexers.
In Figure 1 we show an example system from appendix II of [G.680].
This diagram shows the DWDM line systems including amplifiers, BA =
booster amplifier, LA = line amplifier. For distributed impairment
validation we would group the line systems with their preceding nodes
as shown for computational purposes.
Section 9 of ITU-T G.680 [G.680] shows how various impairment
parameters accumulate and this suggests that the following parameters
or subset thereof could be used in distributed impairment estimation:
o Optical Signal to Noise Ratio (OSNR)
o Residual Dispersion (chromatic)
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o Polarization Mode Dispersion (PMD)
o Polarization Dependent Loss (PDL)
o Ripple
o Channel Uniformity
For each of the above the units and accumulation procedure needs to
be defined. In the following we suggest units and procedures for the
above for which computation of cascaded elements are suitably defined
in [G.680]. Note: ONE = Optical Network Element.
A.1. Distributed Computation of OSNR
Section 9.1 of ITU-T G.680 gives several equivalent formulas for the
estimation of OSNR. For distributed impairment validation the
following formula from [G.680] is convenient:
OSNR_out = -10*log(Term1 + Term2)
Where
Term1 = 10^-(ONSR_in/10), and
Term2 = 10^-((P_in-NF-10*log(h*v*vr))/10)
and we have the following additional definitions:
OSNR_out is the output optical signal to noise ratio in dB of the ONE
OSNR_in is the input optical signal to noise ratio in dB of the ONE
P_in is the channel power (dBm) at the input port of the ONE
NF is the noise figure (dB) of the relevant path through the ONE
h is Planck's constant (in mJ*s to be consistent with P_in in dBm)
v is the optical frequency in Hz
vr is the reference bandwidth in Hz (usually the frequency equivalent
of 0.1nm)
From the previous formula, a distributed computation of OSNR requires
knowing the OSNR_in and the P_in based on computations from the
previous node along the path. The noise figure, F, is something that
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the current node performing the computation would know along with the
frequency, v, and the reference bandwidth vr (TBD: confirm with ITU-
T).
The control plane will need to distribute the following information
from node to node along the path:
o OSNR_in (this is the accumulated OSNR along the path)(dB)
o P_in (this is the estimated power into the next node)(dBm)
The input power would be calculated by the previous node by taking
into account gain and attenuation on the link between the nodes.
A.2. Distributed Computation of Residual Dispersion
The residual dispersion for a path is required to be bounded, in
particular from [G.680] equation 9-4:
Min RD < Residual Dispersion < Max RD
Where Min RD and Max RD are the minimum and maximum tolerable
residual dispersion for a particular transmitter/receiver
combination.
The residual dispersion for a cascade of network elements can be
computed by [G.680] equation 9-5:
Residual dispersion = sum(fiber dispersion) + sum(DCM dispersion)
+ sum(ONE dispersion)
Where DCM dispersion is from Dispersion Compensation Modules (DCM),
and ONE dispersion is due to optical network elements.
Although the residual dispersion formula is a relatively simple
linear formula [G.680] indicates two possible methods for its
evaluation (a) Worst-case upper and lower bounds, or (b)Statistical
approach. In case (a) two parameters would need to be accumulated
along the path a worst case upper and lower bound. In case (b) some
type of statistical information would be needed in [G.680] mean and
standard deviation are used under a Gaussian assumption.
A.3. Distributed Computation of PMD
The accumulated impact of line system and ONE polarization mode
dispersion can be estimated via the formula [G.680] equation (9-6):
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DGDmax_link = {DGDmaxf^2 + S^2*sum_i(PMDc_i^2)}^(1/2)
where
DGDmax_link is the max link DGD (ps)
DGDmxf is the max concatenated optical fiber cable DGD (ps)
S is the Maxwell adjustment factor(Table 9-2 of [G.680])
PMDc_i is the PMD value for the ith component (ps)
Under a distributed computation approach the above could be computed
by keeping track of DGDmaxf and the running sum of PMDc_i^2. The
Maxwell adjustment factor and final square root can be applied at the
final node in the path. [Question for Q6: does DGDMaxf^2 need to be
accumulated over the different link segments?]
A.4. Distributed Computation of PDL
See section 9.3.2 of [G.680]
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APPENDIX B: Optical Parameters
The following provides an annotated list of optical characteristics
from ITU-T recommendation G.680 [G.680] for use in optical path
impairment computations. For each parameter we specify the units to
be used, whether minimum or maximum values are used, and whether the
parameters applies to the optical network element as a whole, on a
per port basis or on a port-to-port pair basis.
Not all these parameters will apply to all devices. The main
differentiation in G.680 comes from those network elements that
include or do not include optical amplifiers.
B.1. Parameters for NEs without optical amplifiers
Channel frequency range (GHz, Max, Min): [G.671] The frequency range
within which a DWDM device is required to operate with a specified
performance. For a particular nominal channel central frequency,
fnomi, this frequency range is from fimin = (fnomi - dfmax) to fimax
= (fnomi + dfmax), where dfmax is the maximum channel central
frequency deviation. Nominal channel central frequency and maximum
channel central frequency deviation are defined in ITU-T Rec. G.692.
Insertion loss (dB, Port-Port, Max, Min):[G.671] It is the reduction
in optical power between an input and output port of a WDM device in
decibels (dB).
Channel insertion loss deviation (dB, Max):[G.671] This is the
maximum variation of insertion loss at any frequency within the
channel frequency range (DWDM devices) or channel wavelength range
(CWDM and WWDM devices).
Ripple (dB, Max): [G.671] For WDM devices and tuneable filters, the
peak-to-peak difference in insertion loss within a channel frequency
(or wavelength) range.
Channel chromatic dispersion (ps/nm, Max, Min): [G.650.1] Change of
the group delay of a light pulse for a unit fibre length caused by a
unit wavelength change.
Differential group delay (ps, Max): [G.671] Polarization Mode
Dispersion (PMD) is usually described in terms of a Differential
Group Delay (DGD), which is the time difference between the principal
States of Polarization (SOPs) of an optical signal at a particular
wavelength and time.
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Polarization dependent loss (dB, Max): [G.671] Maximum variation of
insertion loss due to a variation of the state of polarization (SOP)
over all SOPs.
Reflectance (dB, Max): [G.671] The ratio of reflected power Pr to
incident power, Pi at a given port of a passive component, for given
conditions of spectral composition, polarization and geometrical
distribution.
Isolation, adjacent channel (dB, Min, Port-Port): [G.671] The
adjacent channel isolation (of a WDM device) is defined to be equal
to the unidirectional (far-end) isolation of that device with the
restriction that x, the isolation wavelength number, is restricted to
the channels immediately adjacent to the (channel) wavelength number
associated with port o.
Isolation, non-adjacent channel (dB, Min, Port-Port): [G.671] The
non-adjacent channel isolation (of a WDM device) is defined to be
equal to the unidirectional (far-end) isolation of that device with
the restriction that x, the isolation wavelength number, is
restricted to each of the channels not immediately adjacent to the
(channel) wavelength number associated with port o.
Note: [G.671] In a WDM device able to separate k wavelengths (w1, w2,
... , wk) radiation coming from one input port into k output ports,
each one nominally passing radiation at one specific wavelength only.
The unidirectional (far-end) isolation is a measure of the part of
the optical power at each wavelength exiting from the port at
wavelengths different from the nominal wavelength relative to the
power at the nominal wavelength.
Channel extinction (dB, Min, Port-Port): [G.671] Within the operating
wavelength range, the difference (in dB) between the maximum
insertion loss for the non-extinguished (non-blocked) channels and
the minimum insertion loss for the extinguished (blocked) channels.
Reconfigure time (ms, Max, Min): [G.680] The reconfigure time (of an
ROADM) is the elapsed time measured from the earliest point that the
actuation energy is applied to reconfigure the ONE to the time when
the channel insertion loss for all wanted channels has settled to
within 0.5 dB of its final steady state value and all other
parameters of the device (e.g., isolation and channel extinction)are
within the allowed limits.
Switching time (for PXC) (ms, Max, Min): [G.671] The elapsed time it
takes the switch to turn path io on or off from a particular initial
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state, measured from the time the actuation energy is applied or
removed.
Channel uniformity (dB, Max): [G.671] The difference (in dB) between
the powers of the channel with the most power (in dBm) and the
channel with the least power (in dBm). This applies to a multichannel
signal across the operating wavelength range.
B.2. Additional parameters for NEs with optical amplifiers
Total input power range (dBm, Max, Min, Port): [G.661] The range of
optical power levels at the input for which the corresponding output
signal optical power lies in the specified output power range, where
the OA performance is ensured.
Channel input power range (dBm, Max, Min, Port): see above.
Channel output power range (dBm, Max, Min, Port): [G.661] The range
of optical power levels at the output of the OA for which the
corresponding input signal power lies in the specified input power
range, where the OA performance is ensured.
Channel signal-spontaneous noise figure (dB, Max, Port-Port) [G.661]
The signal-spontaneous beat noise contribution to the noise figure,
expressed in dB.
Input reflectance (dB, Max, Port): [G.661] The maximum fraction of
incident optical power, at the operating wavelength and over all
states of input light polarization, reflected by the OA from the
input port, under nominal specified operating conditions, expressed
in dB.
Output reflectance (dB, Max, Port): [G.661] The fraction of incident
optical power at the operating wavelength reflected by the OA from
the output port, under nominal operating conditions, expressed in dB.
Maximum reflectance tolerable at input (dB, Min, Port): [G.661] The
maximum fraction of power, expressed in dB, exiting the optical input
port of the OA which, when reflected back into the OA, allows the
device to still meet its specifications.
Maximum reflectance tolerable at output (dB, Min, Port): [G.661] The
maximum fraction of power, expressed in dB, exiting the optical
output port of the OA which, when reflected back into the OA, allows
the device to still meet its specifications.
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Maximum total output power (dBm, Max, Port): [G.661] The highest
signal optical power at the output that can be obtained from the OA
under nominal operating conditions.
Channel addition/removal (steady-state) gain response (dB, Max, Min):
[G.661] For a specified multichannel configuration, the steady-state
change in channel gain of any one of the channels due to the
addition/removal of one or more other channels, expressed in dB.
Transient duration (ms, Max): [G.661] The time period from the
addition/removal of a channel to the time when the output power level
of that or another channel reaches and remains within +- N dB from
its steady-state value.
Transient gain increase (dB, Max): [G.661] For a specified
multichannel configuration, the maximum change in channel gain of any
one of the channels due to the addition/removal of one or more other
channels during the transient period after channel addition/removal,
expressed in dB.
Transient gain reduction (dB, Max): see above.
Channel gain (dB, Max, Min, Port-Port): [G.661] Gain for each channel
(at wavelength wj) in a specified multichannel configuration,
expressed in dB.
Multichannel gain-change difference (inter-channel gain-change
difference) (dB, Max): [G.661] For a specified channel allocation,
the difference of change in gain in one channel with respect to the
change in gain of another channel for two specified sets of channel
input powers, expressed in dB.
Multichannel gain tilt (inter-channel gain-change ratio)(dB, Max):
[G.661] The ratio of the changes in gain in each channel to the
change in gain at a reference channel as the input conditions are
varied from one set of input channel powers to a second set of input
channel powers, expressed in dB per dB.
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References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[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.
[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.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.
[Imp-Frame] G. Bernstein, Y. Lee, D. Li, A Framework for the Control
and Measurement of Wavelength Switched Optical Networks
(WSON) with Impairments, Work in Progress, October 2008.
[RWA-Info] G. Bernstein, Y. Lee, D. Li, W. Imajuku, Routing and
Wavelength Assignment Information Model for Wavelength
Switched Optical Networks, Work in Progress: draft-ietf-
ccamp-rwa-info-01.txt, November 2008.
8.2. Informative References
[RFC4054] Strand, J., Ed., and A. Chiu, Ed., "Impairments and Other
Constraints on Optical Layer Routing", RFC 4054, May 2005.
[Encode] G. Bernstein, Y. Lee, D. Li, W. Imajuku, "Routing and
Wavelength Assignment Information Encoding for Wavelength
Switched Optical Networks" Work in progress: draft-
bernstein-ccamp-wson-encode-01.txt, November 2008.
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Author's Addresses
Greg Bernstein
Grotto Networking
Fremont CA, 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
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Acknowledgment
Funding for the RFC Editor function is currently provided by the
Internet Society.
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