IPO Working Group D. Papadimitriou
Category: Internet Draft J.-P. Faure
O. Audouin
Alcatel
July 2001
Non-linear Routing Impairments
in Wavelength Switched Optical Networks
draft-papadimitriou-ipo-non-linear-routing-impairm-00.txt
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026 [1].
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts. Internet-Drafts are draft documents valid for a maximum of
six months and may be updated, replaced, or obsoleted by other
documents at any time. It is inappropriate to use Internet- Drafts
as reference material or to cite them other than as "work in
progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
1. Abstract
Today, in transparent optical networks, the increasing bit-rate (10
Gbit/s and up to 40 Gbit/s in the future), combined with the
increasing number of wavelengths (16 and higher up to 320) and a
narrowing of the channels spacing, enhance the impact of non-linear
effects on optical signal quality.
Thus, non-linear effects like Self-Phase Modulation (SPM), Cross-
Phase Modulation (XPM), Four-Wave Mixing (FWM) as well as Stimulated
Raman Scattering (SRS) and Brillouin scattering have to be examined
in order to evaluate their impacts on the transmission quality. If
these effects appear to be significant, they have to be taken into
account in the routing of a wavelength throughout a transparent
optical network.
The aim of this draft is to extend the previous works dedicated to
routing impairments ([IPO-IMP] and [IPO-ORI]) in order to determine
which are the non-linear effects that must be considered and which
D.Papadimitriou et al. û Internet Draft û Expires January 2002 1
draft-papadimitriou-ipo-non-linear-routing-impairments July 2001
kind of engineering rules may be used to take these effects into
account in constraint-based optical routing.
Moreover, we propose to introduce IGP routing protocol extensions to
transport information related to non-linear impairments relevant for
wavelength routing decisions.
2. 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 [2].
3. Introduction
Non-linear effects are due to the fact that the optical properties
of the medium (refractive index, loss, etc.) become dependent of the
signal power of the optical channels present in this medium. As a
consequence, this power dependency tends to modify the propagation
of the optical waves and also lead to interactions between these
waves. Non-linear interactions depend on the transmission length
(distance between the transmitter and the receiver), the type of
fiber, the cross-sectional area of the fiber, the wavelength and the
power level and also on the modulation format. Basically, these
effects become more intensive when the optical power or the
transmission length increase or when the channel spacing becomes
narrower (the different wavelengths tend to interact more each
other). As a consequence, non-linearities can impose significant
limitations on high bit-rates (10 Gbit/s and higher), Long-haul and
Ultra-long haul systems, or high capacity DWDM systems.
Linear impairments are extensively addressed in [IPO-IMP] and
corresponding IGP routing protocol extensions in [IPO-ORI]. In these
previous works, the approach for non-linear impairments was to
consider that: ôOne could assume that non-linear impairments are
bounded and increase the required OSNR level by X dB, where X for
performance reasons would be limited to 1 or 2 dB, consequently
setting a limit on the maximum number of spans. For the approach
described here to be useful, it is desirable for this span limit to
be longer than that imposed by the constraints which can be treated
explicitly.ö
However, this approximation may lead to both an over or an under
estimation of the real impact of non-linear effects. If the actual
impact is less than 2 dB on the OSNR, then, the margin taken will
forbid some feasible path(s) (as it limits the maximum number of
spans). On the contrary, if the real impact is over 2 dB, the
corresponding route(s) may be chosen despite they are not feasible
from the optical transmission point of view.
Therefore, the objective of this document is first to determine
which kind of non-linear effects must be taken into account, and to
give some simple engineering rules to determine their maximum
D.Papadimitriou - Internet Draft û Expires January 2002 2
draft-papadimitriou-ipo-non-linear-routing-impairments July 2001
tolerable value, second, to propose IGP routing protocol extensions
in order to cover non-linear optical routing impairments.
Additional complexity may arise from the fact that for instance when
minimizing degradations through Self Phase Modulation (SPM) after
setting a distinct Lambda LSP (L-LSP) or optical channel in the
network, this L-LSP will suffer a changing degradation by Cross-
Phase Modulation (XPM) through the changing number of concurrent
optical channels on the fiber links. Thus, as we will point out,
cross channels effects should be minimized at the system design in
order to be compatible with SPM and other impairments.
4. Non-Linear Impairments
Non-linear optical impairments can be classified into two
categories. The first category consists of effects occurring due to
the dependence of the refractive index on the optical signal power
(generally called Kerr effect). This category includes Self-Phase
Modulation (SPM), Cross-Phase Modulation (XPM) and Four-Wave Mixing
(FWM). The second category of effects consists of inelastic
scattering effects in the fiber medium and are due to the
interaction of the light waves with the optical phonons of the fiber
medium leading to Stimulated Raman Scattering (SRS) or with the
acoustic phonons (sound waves) of the medium leading to Stimulated
Brillouin Scattering (SBS).
SPM and XPM essentially affect the phase of the signals and cause
its spectral broadening which lead to temporal distorsions because
of dispersion. FWM lead to energy exchange between signals that
induces in-band crosstalk, whereas SBS and SRS provide gain or loss
to the light waves. Nevertheless, the actual impact of these non-
linear effects on transmission quality depends strongly on
dispersion management.
4.1 Refractive Index
The general equation for the refractive index of the core in an
optical fiber is given by:
n = n(0) + [n(2) x P / A(eff)]
where:
- n(0) = the refractive index of the fiber core at low optical
power level (no unit)
- n(2) = the non-linear refractive index coefficient (for
instance, 2.35 x 10^(-20) m^2/W for silica)
- P = optical signal power in Watts (W)
- A(eff) = the effective area of the core in square meters (m^2)
Clearly, this equation indicates that two strategies for minimizing
non-linearities due to refractive index power dependence are to
minimize the launched optical power P and/or to maximize the
effective area of the fiber A(eff).
D.Papadimitriou - Internet Draft û Expires January 2002 3
draft-papadimitriou-ipo-non-linear-routing-impairments July 2001
Minimizing P is limited by the fact that during network design,
there is a strong trade-off between non-linear effects and optical
signal to noise ratio (decreasing P will decrease non-linear effects
but also the OSNR). On the other hand, augmenting A(eff) is being
targeted by some fiber vendors while keeping other non-linear
effects unchanged.
This optical power dependence of the refractive index introduces the
following non-linear effects: SPM, XPM, FWM, SBS and SRS as
explained here below.
4.1.1 Self-Phase Modulation (SPM)
Self-Phase Modulation (SPM) arises from the power dependency of the
refractive index of the fiber core. Fluctuations in the optical
signal power cause changes in the phase of the signal referred to as
a non-linear phase shift. This induces an additional frequency chirp
on the spectrum of the optical pulse which interacts with the
fiberÆs dispersion to broaden the pulse and lead to intensity
fluctuations. Therefore, this effect leads to higher penalties due
to Inter-Symbol Interference (ISI). This chirping effect affects
each channel independently of the other and is proportional to the
optical channel power. Therefore SPM effects are more pronounced in
systems using higher transmitted signal power. Moreover because the
SPM effect leads to extra ISI, higher bit-rate systems will be more
affected.
It is important to point out that in a DWDM transmission system at
10 Gbit/s with 100 GHz channel spacing, SPM is generally the most
significant non-linear effect (except may be when low dispersion
fibers are used).
4.1.2 Cross-Phase Modulation (XPM)
For a given optical signal, Cross-Phase Modulation (XPM) is a
consequence of a modification of the refractive index of the medium
due to the optical power of the closest neighboring channels present
in the fiber. As for SMP, the induced phase shift lead to intensity
fluctuations after interaction with dispersion. XPM increases when
optical channel spacing becomes narrower as long as the adjacent
channels are closer in the spectral domain, so that they travel
roughly at the same velocity and interact over a longer time period.
As XPM effect depends on channel spacing, it can be a significant
problem for high capacity DWDM system with channel spacing of 50 GHz
or lower.
On the other hand, when the bit rate increases (from 10 Gbit/s to
40 Gbit/s), the impact of XPM decrease as long as the time during
which the interacting channels temporally overlap is considerably
reduced.
D.Papadimitriou - Internet Draft û Expires January 2002 4
draft-papadimitriou-ipo-non-linear-routing-impairments July 2001
For moderate bit rate systems (10 Gbit/s), XPM effect generally
becomes significant compared to SPM when channel spacing is lower
than 100 GHz, and when the local dispersion is low. Here, we want
to point out that solutions demonstrated in laboratory environments
may be implemented to decrease the effect of XPM by introducing a
phase-mismatch between optical channels at the link input by using
interleaved polarization or using a suitable dispersion management.
4.1.3 Four-Wave Mixing (FWM)
In a (high capacity) DWDM system, based on different optical
channels at different wavelengths (i.e. frequencies), the power
dependence of the refractive index of the fiber core also gives rise
to the generation of new frequencies (i.e. new optical signal).
Practically, there is an interaction between the different channels,
leading to energy transfer between these channels. This effect is
called Four-Wave Mixing (FWM) because if three optical channels with
frequencies f1, f2 and f3 propagates simultaneously within the same
fiber, a fourth optical channel is generated and frequency f4 which
is related to the other frequencies by the following relation: f4 =
f1 (+ or -) f2 (+ or -) f3. In theory, several frequencies
corresponding to different combinations are possible. However, in
practice only the frequency combinations of the form f4 = f1 + f2 û
f3 are the most troublesome for (high capacity) DWDM systems. These
fourth optical channels can become even nearly phase-matched when
optical channel wavelengths are close to the zero-dispersion point.
As a consequence, significant optical power can be transferred
between neighboring optical channels through the FWM effect. Though,
in contrast to SPM and XPM, which are bit-rate dependent, the FWM
effect is not really dependent of the bit-rate. Nevertheless, like
XPM, it depends strongly on the optical channels spacing and the
fiber dispersion. Clearly, FWM becomes significant only at narrow
channel spacing (50 GHz or lower) or when the local dispersion is
low.
Consequently, we have not considered the impact of FWM in this first
approach since considering it is minimized at system design.
4.2 Scattering Effects
Scattering effects, the second set of mechanisms generating non-
linearities give rise to SRS and SBS.
4.2.1 Stimulated Raman Scattering (SRS)
If two or more signals at different wavelengths are injected into a
fiber, SRS cause optical signal power to be transferred from the
lower wavelength optical channels to the higher wavelength optical
channels. The gain coefficient increases with increasing channel
spacing up to 125 nm. The coupling between channels occurs only if
both optical channels are launched simultaneously so that the impact
D.Papadimitriou - Internet Draft û Expires January 2002 5
draft-papadimitriou-ipo-non-linear-routing-impairments July 2001
of the SRS is reduced by the dispersion introduced by the silica
medium. Basically, SRS induces a gain tilt over the whole bandwidth
of the fiber, which is proportional to the total power of all
channels present in the fiber.
As a matter of fact, SRS should not be considered for impairment
based optical routing, as long as its induced Raman tilt will be
managed link by link during the network design.
4.2.2 Stimulated Brillouin Scattering (SBS)
Scattering effects in the optical fiber occur due to the interaction
of the optical channels with the sound waves (acoustic phonons)
present in the silica medium. In SBS, the scattering process is
stimulated by photons with a wavelength higher than the wavelength
of the incident signal. This interaction takes place over a very
narrow band of 20 MHz at 1550nm. The scattered waves and the
incident optical light waves propagate in opposite directions. Thus,
SBS produces an additional loss in the propagating signal but does
not induce any interaction between different optical channels.
In practical implemented systems, the SBS threshold power is kept
always higher than the optical signal power, so that the losses
induced by the SBS effect are not a real problem when considering
impairment-based optical routing.
5. Fiber and Optical Amplifiers
In the course of moving from pure optical centrally managed
transmission to flexible wavelength switched networks many problems
have to be solved. One of these problems is the fiber diversity
among the different links, and the impact of non-linear effects
inside the different transmission fibers.
5.1 Influence of the fiber medium
An optical transparent network is composed of many nodes (optical
LSR) connected by links (a link is a transmission system between two
nodes). Each link is composed of transmission spans of identical
fiber, whereas in the most general case, different types of fibers
may be deployed among the different links. The main types of fibers
that are generally deployed in today optical networks are mainly
based on G.652 as well as G.655 and G.653 ITU-T Recommendations.
Also and Dispersion Compensating Fibers (DCF) used to compensate the
dispersion is present inside the network element.
The intensity of non-linear effects is also dependent on the
intrinsic optical properties of each fiber, thus, fiber diversity in
optical networks must be taken into account when regarding the
non-linear impairments. In particular, non-linear effects arise not
only in the transmission fiber but also inside the dispersion
compensating fiber present in the network elements.
D.Papadimitriou - Internet Draft û Expires January 2002 6
draft-papadimitriou-ipo-non-linear-routing-impairments July 2001
An additional important point is that in today optical networks, the
dispersion of the transmission fiber is compensated for each link by
a specific dispersion map. The residual dispersion after cascading a
certain number of links must be compatible with the cumulated impact
of non-linear effect such as SPM or XPM.
In most common optical networks, of moderate bit-rate (10 Gbit/s)
and channel spacing (100 GHz), Self-phase modulation (SPM) is the
strongest non-linear degradation effect. Changing the path length in
the network will then increase the SPM contribution when it
increases the total path length, or when the optical path crosses
highly non-linear fiber links.
Only for highly dense DWDM systems (channel spacing of 50 GHz or
less), XPM at its turn will take more and more importance, as well
as FWM (even if this latter effect can be reduced with specific
design in the link). In that case, after setting a distinct path in
the network, this path may suffer a changing degradation by XPM and
eventually FWM through the changing number of concurrent channels on
a same fiber link. Thus, XPM and FWM must be minimized at system
design.
5.2 Optical Amplifiers
Potentially, non-linear effects may also occur in the fiber
amplifiers, but they can be considerably reduced provided a specific
design will be done. Then, it is not worth to take them into
account, as long as they are kept under control at system design.
6. Non-linear Phase
The intensity of non-linear effects can be quantified through the
non-linear phase shift NLP (see [AGR-FOCS]) induced in the fiber by
the Kerr effect.
6.1 Definition
The Non-Linear Phase (NLP) for a given transmission span NLP(span)
is given by the following formula (see [AGR-NFO]):
NLP(span) = P x F(w, A(eff), n(2), L(eff))
where:
- P = optical power in Watts (W) at the span input
- F = a function assumed to be constant for a given span.
The F function depends on the following parameters:
- w = wavelength of the optical channel (m)
- A(eff) = effective area of the fiber core in square meters (m^2)
- n(2) = non-linear refractive index coefficient (m^2/W)
- L(eff) = effective interaction length (m)
While the effective interaction length L(eff) is defined as:
D.Papadimitriou - Internet Draft û Expires January 2002 7
draft-papadimitriou-ipo-non-linear-routing-impairments July 2001
L(eff) = [1 û exp(-aL)] / a
where:
- a = linear absorption coefficient of the fiber (m^-1)
- L = fiber length (m)
It is important to point out that the function F is a standard
integral which can be easily calculated and leads to an analytical
formula. Then, for each span, the NLP can be easily computed using
the above coefficients (w, A(eff), n(2) and L(eff)).
For a transmission span, one must take into account the NLP due to
the transmission fiber and Dispersion Compensating Fiber (DCF). The
total NLP for a whole span (NLP(span)) is then given by:
NLP(span) = NLP(fiber) + NLP(DCF) = P x F(span)
where:
- P = optical power at the span input
- F = global function containing the parameters of the span (fiber
+ DCF)
6.1.1 Calculation of the NLP for a link
A link between two optical LSR is constituted by N transmission
spans. Then, the cumulated NLP for a given link (NLP(link)) is equal
to the sum of the different NLP due to each span and is given by:
NLP(link) = Sum(NLP[i]) = Sum(P[i] x F[i])
where:
- NLP[i] = NLP due to span ôiö
- P[i] = power at the input of span ôiö
- F[i] = F function of span ôiö.
This formula allows the calculation of the cumulated NLP for any
link, provided one knows the optical power at each span input, and
the F function for each span.
An interesting case is when the N spans can be considered as roughly
identical, so that we can simplify the above formula to:
NLP(link) = N x NLP(span) = N x P x F(span)
where:
- NLP(span) = NLP of the spans
- P = power at the input of the link
- F(span) = F function of the spans
It is important to note that the NLP(link) is a static information
which characterizes the link, and is calculated locally. Therefore,
it is the only required parameter to be flooded by IGP protocols.
D.Papadimitriou - Internet Draft û Expires January 2002 8
draft-papadimitriou-ipo-non-linear-routing-impairments July 2001
6.1.2 Calculation of the NLP for an optical path
Considering the establishment of an optical path within a network,
this path will be a succession of ôjö different links, each link
being composed of a specific fiber type.
Then, the total cumulated NLP over the whole optical path NLP(path)
is given by:
NLP(path) = Sum(NLP(link)[j])
where NLP(link)[j] is the NLP due to link ôjö.
With this simple formula, it is possible to calculate the total
cumulated NLP for any optical path, using the previously calculated
NLP of the different links.
6.2 SPM Constraint
For a given optical path, the total cumulated NLP due to SPM can be
calculated according to the previous formula.
The routing constraint can be expressed as follow: for a given
residual dispersion after crossing an optical path, the total
cumulated dispersion NLP(path) must be:
NLP(path) < NLPmax(B)
where NLPmax is the maximum tolerable value for the NLP.
It is important to point out that we assume in this approach that
the dispersion is managed at the link level using available
technology being developed, so that for each link, the residual
dispersion is compatible with the NLP of the link.
For example, simulations have shown that the non-linear constraint
can be expressed as follows (assuming an accurate dispersion
management):
- NLP < 0.45 pi at 10Gbit/s
- NLP < 0.3 pi at 40Gbit/s
6.3 XPM Constraint
In a high density DWDM system, the NLP shift per span for a given
optical channel does not only depend on the optical power of that
channel but also on the power of the closest neighboring optical
channels typically (8 in practical applications), as well as on the
channel spacing.
As a consequence, taking into account the effect of XPM in routing
decision would suppose to flood additional information (at least the
channel spacing). So before implementing such a complex solution,
D.Papadimitriou - Internet Draft û Expires January 2002 9
draft-papadimitriou-ipo-non-linear-routing-impairments July 2001
design of link should take care of minimizing the XPM in the worst
case.
However, one approach currently under study for the quantification
of the NLP(XPM) consists to investigate the use of NLP due to SPM
affected by a specific coefficient (called G) to represent the XPM
penalty.
Hence, the total NLP due to the cumulated effects of SMP and XPM
would be given by:
NLPcum(SPM + XPM) = (1 + G) x NLP(SPM)
Where G is a tailored parameter, which must be tabulated (typically,
its value equals 10%).
7. Traffic-Engineering Routing Protocol Extension
In OSPF, the NPL parameter is represented as a sub-TLV of the Link
TLV in the Traffic Engineering LSA whose type is to be attributed
(TBA). The length of the sub-TLV is four-octets and specifies the
NLP (in IEEE floating point format). The format of the NPL sub-TLV
is as shown:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = TBA | Length = 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NLP |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
In IS-IS, we enhance the sub-TLVs for the extended IS-IS
reachability TLV. The length of the NPL sub-TLV is four-octets and
specifies the square of the polarization mode dispersion (in IEEE
floating point format). Specifically, we add the following sub-TLV:
- Sub-TLV type: TBA
- Length(in bytes): 4
- Name: NLP
8. Security Considerations
There are no additional security considerations than the ones
already covered in OSPF and IS-IS.
9. Reference
1 Bradner, S., "The Internet Standards Process -- Revision 3", BCP
9, RFC 2026, October 1996.
2 Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997
D.Papadimitriou - Internet Draft û Expires January 2002 10
draft-papadimitriou-ipo-non-linear-routing-impairments July 2001
3 [AGR-NFO] Govind P. Agrawal, ôNonlinear Fiber Opticsö, (section
2.6.2: ôNonlinear refractionö), Academic Press, 1995.
4. [AGR-FOCS] Govind P. Agrawal, ôFiber-Optic Communication
Systemsö, Second Edition, Wiley Series in Microwave and Optical
Engineering, March 1997.
5. [GYS-XT] T. Gyselings, ôInvestigation and Reduction of CrossTalk
in Wavelength Division Multiplexed All-Optical Cross-Connectsö,
PhD Thesis, INTEC, Universiteit Gent.
6. [IPO-IMP] A. Chiu et al., ôImpairments And Other Constraints On
Optical Layer Routingö, Internet Draft, Work in progress, draft-
ietf-ipo-impairments-00.txt, May 2001.
7. [IPO-ORI] A. Banerjee et al., ôImpairment Constraints for Routing
in All-Optical Networksö, Internet Draft, Work in progress,
draft-banerjee-routing-impairments-00.txt, May 2001.
10. Acknowledgments
The authors would like to thank B. Sales, E. Desmet, J.C. Antona, S.
Bigo and A. Jourdan for their constructive comments and inputs.
11. Author's Addresses
Dimitri Papadimitriou
Senior R&D Engineer û Optical Networking
Alcatel IPO NSG-NA
Francis Wellesplein 1,
B-2018 Antwerpen, Belgium
Phone: +32 3 240-8491
Email: dimitri.papadimitriou@alcatel.be
Jean-Paul Faure
Research Engineer
Alcatel CIT R&I
Route de Nozay
91461 Marcoussis Cedex, France
Phone: +33 1 6963-1307
Email: jean-paul.faure@ms.alcatel.fr
Olivier Audouin
Research Engineer
Alcatel CIT R&I
Route de Nozay
91461 Marcoussis Cedex, France
Phone: +33 1 6963-2365
Email: olivier.audouin@ms.alcatel.fr
D.Papadimitriou - Internet Draft û Expires January 2002 11
draft-papadimitriou-ipo-non-linear-routing-impairments July 2001
Full Copyright Statement
"Copyright (C) The Internet Society (date). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph
are included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE."
D.Papadimitriou - Internet Draft û Expires January 2002 12