draft-ietf-ipo-impairments-05.txt   rfc4054.txt 
Internet Draft John Strand (Editor) Network Working Group J. Strand, Ed.
Document: draft-ietf-ipo-impairments-05.txt Angela Chiu (Editor) Request for Comments: 4054 A. Chiu, Ed.
Informational Track AT&T Category: Informational AT&T
Expiration Date: November 2003 May 2005
May 2003 Impairments and Other Constraints on Optical Layer Routing
Impairments And Other Constraints On Optical Layer Routing Status of This Memo
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Abstract Optical networking poses a number challenges for Generalized Multi-
Optical networking poses a number challenges for GMPLS. Optical Protocol Label Switching (GMPLS). Fundamentally, optical technology
technology is fundamentally an analog rather than digital technology; is an analog rather than digital technology whereby the optical layer
and the optical layer is lowest in the transport hierarchy and hence is lowest in the transport hierarchy and hence has an intimate
has an intimate relationship with the physical geography of the relationship with the physical geography of the network. This
network. This contribution surveys some of the aspects of optical contribution surveys some of the aspects of optical networks that
networks which impact routing and identifies possible GMPLS responses impact routing and identifies possible GMPLS responses for each: (1)
for each: (1) Constraints arising from the design of new software Constraints arising from the design of new software controllable
controllable network elements, (2) Constraints in a single all-optical network elements, (2) Constraints in a single all-optical domain
domain without wavelength conversion, (3) Complications arising in more without wavelength conversion, (3) Complications arising in more
complex networks incorporating both all-optical and opaque complex networks incorporating both all-optical and opaque
architectures, and (4) Impacts of diversity constraints. architectures, and (4) Impacts of diversity constraints.
1. Introduction Table of Contents
GMPLS [GMPLS] aims to extend MPLS to encompass a number of transport 1. Introduction ................................................. 2
architectures. Included are optical networks incorporating a number 2. Sub-IP Area Summary and Justification of Work ................ 3
of all-optical and opto-electronic elements such as optical cross- 3. Reconfigurable Network Elements .............................. 3
connects with both optical and electrical fabrics, transponders, and 3.1. Technology Background .................................. 3
optical add-drop multiplexers. Optical networking poses a number 3.2. Implications for Routing ............................... 6
On Optical Layer Routing 4. Wavelength Routed All-Optical Networks ....................... 6
4.1. Problem Formulation .................................... 7
4.2. Polarization Mode Dispersion (PMD) ..................... 8
4.3. Amplifier Spontaneous Emission ......................... 9
4.4. Approximating the Effects of Some Other
Impairments Constraints ................................ 10
4.5. Other Impairment Considerations ........................ 13
4.6. An Alternative Approach - Using Maximum
Distance as the Only Constraint ........................ 13
4.7. Other Considerations ................................... 15
4.8. Implications for Routing and Control Plane Design ...... 15
5. More Complex Networks ........................................ 17
6. Diversity .................................................... 19
6.1. Background on Diversity ................................ 19
6.2. Implications for Routing ............................... 23
7. Security Considerations ...................................... 23
8. Acknowledgements ............................................. 24
9. References ................................................... 25
9.1. Normative References ................................... 25
9.2. Informative References ................................. 26
10. Contributing Authors ......................................... 26
challenges for GMPLS. Optical technology is fundamentally an analog 1. Introduction
rather than digital technology; and the optical layer is lowest in
the transport hierarchy and hence has an intimate relationship with
the physical geography of the network.
GMPLS already has incorporated extensions to deal with some of the Generalized Multi-Protocol Label Switching (GMPLS) [Mannie04] aims to
unique aspects of the optical layer. This contribution surveys some extend MPLS to encompass a number of transport architectures,
of the aspects of optical networks which impact routing and including optical networks that incorporate a number of all-optical
identifies possible GMPLS responses for each. Routing constraints and opto-electronic elements, such as optical cross-connects with
and/or complications arising from the design of network elements, both optical and electrical fabrics, transponders, and optical add-
the accumulation of signal impairments, and from the need to drop multiplexers. Optical networking poses a number of challenges
guarantee the physical diversity of some circuits are discussed. for GMPLS. Fundamentally, optical technology is an analog rather
than digital technology whereby the optical layer is lowest in the
transport hierarchy and hence has an intimate relationship with the
physical geography of the network.
Since the purpose of this draft is to further the specification of GMPLS already has incorporated extensions to deal with some of the
GMPLS, alternative approaches to controlling an optical network are unique aspects of the optical layer. This contribution surveys some
not discussed. For discussions of some broader issues, see of the aspects of optical networks that impact routing and identifies
[Gerstel2000] and [Strand2001]. possible GMPLS responses for each. Routing constraints and/or
complications arising from the design of network elements, the
accumulation of signal impairments, and the need to guarantee the
physical diversity of some circuits are discussed.
The organization of the contribution is as follows: Since the purpose of this document is to further the specification of
GMPLS, alternative approaches to controlling an optical network are
not discussed. For discussions of some broader issues, see
[Gerstel2000] and [Strand02].
- Section 2 is a section requested by the sub-IP Area management The organization of the contribution is as follows:
for all new drafts. It explains how this document fits into the
Area and into the IPO WG, and why it is appropriate for these
groups.
- Section 3 describes constraints arising from the design of new
software controllable network elements.
- Section 4 addresses the constraints in a single all-optical
domain without wavelength conversion.
- Section 5 extends the discussion to more complex networks
incorporating both all-optical and opaque architectures.
- Section 6 discusses the impacts of diversity constraints.
- Section 7 deals with security requirements.
- Section 8 contains acknowledgments.
- Section 9 contains references.
- Section 10 contains contributing authors' addresses.
- Section 11 contains editors' addresses.
2. Sub-IP Area Summary And Justification Of Work - Section 2 is a section requested by the sub-IP Area management for
This draft merges and extends two previous drafts, draft-chiu- all new documents. It explains how this document fits into the
strand-unique-olcp-02.txt and draft-banerjee-routing-impairments- Area and into the IPO WG, and why it is appropriate for these
00.txt. These two drafts were made IPO working group documents to groups.
form a basis for a design team at the Minneapolis meeting, where it
was also requested that they be merged to create a requirements
document for the WG.
In the larger sub-IP Area structure, this merged document describes - Section 3 describes constraints arising from the design of new
specific characteristics of optical technology and the requirements software controllable network elements.
On Optical Layer Routing
they place on routing and path selection. It is appropriate for the - Section 4 addresses the constraints in a single all-optical domain
IPO working group because the material is specific to optical without wavelength conversion.
networks. It identifies and documents the characteristics of the
optical transport network that are important for selecting paths for
optical channels, which is a work area for the IPO WG. It is
appropriate work for this WG because the material covered is
directly aimed at establishing a framework and requirements for
routing in an optical network.
Related documents are: - Section 5 extends the discussion to more complex networks and
draft-banerjee-routing-impairments-00.txt incorporates both all-optical and opaque architectures.
draft-parent-obgp-01.txt
draft-bernstein-optical-bgp-00.txt
draft-hayata-ipo-carrier-needs-00.txt
draft-many-carrier-framework-uni-01.txt
draft-papadimitriou-ipo-non-linear-routing-impairm-01.txt
3. Reconfigurable Network Elements - Section 6 discusses the impacts of diversity constraints.
3.1 Technology Background - Section 7 deals with security requirements.
Control plane architectural discussions (e.g., [Awduche99]) usually - Section 8 contains acknowledgments.
assume that the only software reconfigurable network element is an
optical layer cross-connect (OLXC). There are however other software
reconfigurable elements on the horizon, specifically tunable lasers and
receivers and reconfigurable optical add-drop multiplexers (OADM's).
These elements are illustrated in the following simple example, which
is modeled on announced Optical Transport System (OTS) products:
+ +
---+---+ |\ /| +---+---
---| A |----|D| X Y |D|----| A |---
---+---+ |W| +--------+ +--------+ |W| +---+---
: |D|-----| OADM |-----| OADM |-----|D| :
---+---+ |M| +--------+ +--------+ |M| +---+---
---| A |----| | | | | | | |----| A |---
---+---+ |/ | | | | \| +---+---
+ +---+ +---+ +---+ +---+ +
D | A | | A | | A | | A | E
+---+ +---+ +---+ +---+
| | | | | | | |
Figure 3-1: An OTS With OADM's - Functional Architecture - Section 9 contains references.
In Fig.3-1, the part that is on the inner side of all boxes labeled - Section 10 contains contributing authors' addresses.
"A" defines an all-optical subnetwork. From a routing perspective
two aspects are critical:
On Optical Layer Routing 2. Sub-IP Area Summary and Justification of Work
- Adaptation: These are the functions done at the edges of the This document merges and extends two previous expired Internet-Drafts
subnetwork that transform the incoming optical channel into the that were made IPO working group documents to form a basis for a
physical wavelength to be transported through the subnetwork. design team at the Minneapolis IETF meeting, where it was also
- Connectivity: This defines which pairs of edge Adaptation requested that they be merged to create a requirements document for
functions can be interconnected through the subnetwork. the WG.
In Fig. 3-1, D and E are DWDM's and X and Y are OADM's. The boxes In the larger sub-IP Area structure, this merged document describes
labeled "A" are adaptation functions. They map one or more input specific characteristics of optical technology and the requirements
optical channels assumed to be standard short reach signals into a they place on routing and path selection. It is appropriate for the
long reach (LR) wavelength or wavelength group which will pass IPO working group because the material is specific to optical
transparently to a distant adaptation function. Adaptation networks. It identifies and documents the characteristics of the
functionality which affects routing includes: optical transport network that are important for selecting paths for
- Multiplexing: Either electrical or optical TDM may be used to optical channels, which is a work area for the IPO WG. The material
combine the input channels into a single wavelength. This is covered is directly aimed at establishing a framework and
done to increase effective capacity: A typical DWDM might be requirements for routing in an optical network.
able to handle 100 2.5 Gb/sec signals (250 Gb/sec total) or 50
10 Gb/sec (500 Gb/sec total); combining the 2.5 Gb/sec signals
together thus effectively doubles capacity. After multiplexing
the combined signal must be routed as a group to the distant
adaptation function.
- Adaptation Grouping: In this technique, groups of k (e.g., 4)
wavelengths are managed as a group within the system and must be
added/dropped as a group. We will call such a group an
"adaptation grouping". Examples include so called "wave group"
and "waveband" [Passmore01]. Groupings on the same system may
differ in basics such as wavelength spacing, which constrain the
type of channels that can be accommodated.
- Laser Tunability: The lasers producing the LR wavelengths may
have a fixed frequency, may be tunable over a limited range, or
be tunable over the entire range of wavelengths supported by the
DWDM. Tunability speeds may also vary.
Connectivity between adaptation functions may also be limited: 3. Reconfigurable Network Elements
- As pointed out above, TDM multiplexing and/or adaptation
grouping by the adaptation function forces groups of input
channels to be delivered together to the same distant adaptation
function.
- Only adaptation functions whose lasers/receivers are tunable to
compatible frequencies can be connected.
- The switching capability of the OADM's may also be constrained.
For example:
o There may be some wavelengths that can not be dropped at
all.
o There may be a fixed relationship between the frequency
dropped and the physical port on the OADM to which it is
dropped.
o OADM physical design may put an upper bound on the number
of adaptation groupings dropped at any single OADM.
On Optical Layer Routing 3.1. Technology Background
For a fixed configuration of the OADM's and adaptation functions Control plane architectural discussions (e.g., [Awduche99]) usually
connectivity will be fixed: Each input port will essentially be assume that the only software reconfigurable network element is an
hard-wired to some specific distant port. However this connectivity optical layer cross-connect (OLXC). There are however other software
can be changed by changing the configurations of the OADM's and reconfigurable elements on the horizon, specifically tunable lasers
adaptation functions. For example, an additional adaptation grouping and receivers and reconfigurable optical add-drop multiplexers
might be dropped at an OADM or a tunable laser retuned. In each case (OADM). These elements are illustrated in the following simple
the port-to-port connectivity is changed. example, which is modeled on announced Optical Transport System (OTS)
products:
These capabilities can be expected to be under software control. + +
Today the control would rest in the vendor-supplied Element ---+---+ |\ /| +---+---
Management system (EMS), which in turn would be controlled by the ---| A |----|D| X Y |D|----| A |---
operator's OS's. However in principle the EMS could participate in ---+---+ |W| +--------+ +--------+ |W| +---+---
the GMPLS routing process. : |D|-----| OADM |-----| OADM |-----|D| :
---+---+ |M| +--------+ +--------+ |M| +---+---
---| A |----| | | | | | | |----| A |---
---+---+ |/ | | | | \| +---+---
+ +---+ +---+ +---+ +---+ +
D | A | | A | | A | | A | E
+---+ +---+ +---+ +---+
| | | | | | | |
3.2 Implications For Routing Figure 3-1: An OTS With OADMs - Functional Architecture
An OTS of the sort discussed in Sec. 3.1 is essentially a In Fig. 3-1, the part that is on the inner side of all boxes labeled
geographically distributed but blocking cross-connect system. The "A" defines an all-optical subnetwork. From a routing perspective
specific port connectivity is dependent on the vendor design and two aspects are critical:
also on exactly what line cards have been deployed.
One way for GMPLS to deal with this architecture would be to view - Adaptation: These are the functions done at the edges of the
the port connectivity as externally determined. In this case the subnetwork that transform the incoming optical channel into the
links known to GMPLS would be groups of identically routed physical wavelength to be transported through the subnetwork.
wavebands. If these were reconfigured by the external EMS the
resulting connectivity changes would need to be detected and
advertised within GMPLS. If the topology shown in Fig. 3-1 became a
tree or a mesh instead of the linear topology shown, the
connectivity changes could result in SRLG changes.
Alternatively, GMPLS could attempt to directly control this port - Connectivity: This defines which pairs of edge Adaptation
connectivity. The state information needed to do this is likely to functions can be interconnected through the subnetwork.
be voluminous and vendor specific.
4. Wavelength Routed All-Optical Networks In Fig. 3-1, D and E are DWDMs and X and Y are OADMs. The boxes
labeled "A" are adaptation functions. They map one or more input
optical channels assumed to be standard short reach signals into a
long reach (LR) wavelength or wavelength group that will pass
transparently to a distant adaptation function. Adaptation
functionality that affects routing includes:
The optical networks presently being deployed may be called "opaque" - Multiplexing: Either electrical or optical TDM may be used to
([Tkach98]): each link is optically isolated by transponders doing combine the input channels into a single wavelength. This is done
O/E/O conversions. They provide regeneration with retiming and to increase effective capacity: A typical DWDM might be able to
reshaping, also called 3R, which eliminates transparency to bit handle 100 2.5 Gb/sec signals (250 Gb/sec total) or 50 10 Gb/sec
rates and frame format. These transponders are quite expensive and (500 Gb/sec total); combining the 2.5 Gb/sec signals together thus
their lack of transparency also constrains the rapid introduction of effectively doubles capacity. After multiplexing the combined
new services. Thus there are strong motivators to introduce signal must be routed as a group to the distant adaptation
"domains of transparency" - all-optical subnetworks - larger than an function.
OTS.
The routing of lightpaths through an all-optical network has - Adaptation Grouping: In this technique, groups of k (e.g., 4)
received extensive attention. (See [Yates99] or [Ramaswami98]). wavelengths are managed as a group within the system and must be
added/dropped as a group. We will call such a group an
"adaptation grouping". Examples include so called "wave group"
and "waveband" [Passmore01]. Groupings on the same system may
differ in basics such as wavelength spacing, which constrain the
type of channels that can be accommodated.
On Optical Layer Routing - Laser Tunability: The lasers producing the LR wavelengths may have
a fixed frequency, may be tunable over a limited range, or may be
tunable over the entire range of wavelengths supported by the
DWDM. Tunability speeds may also vary.
When discussing routing in an all-optical network it is usually Connectivity between adaptation functions may also be limited:
assumed that all routes have adequate signal quality. This may be
ensured by limiting all-optical networks to subnetworks of limited
geographic size which are optically isolated from other parts of the
optical layer by transponders. This approach is very practical and
has been applied to date, e.g. when determining the maximum length
of an Optical Transport System (OTS). Furthermore operational
considerations like fault isolation also make limiting the size of
domains of transparency attractive.
There are however reasons to consider contained domains of - As pointed out above, TDM multiplexing and/or adaptation grouping
transparency in which not all routes have adequate signal quality. by the adaptation function forces groups of input channels to be
From a demand perspective, maximum bit rates have rapidly increased delivered together to the same distant adaptation function.
from DS3 to OC-192 and soon OC-768 (40 Gb/sec). As bit rates
increase it is necessary to increase power. This makes impairments
and nonlinearities more troublesome. From a supply perspective,
optical technology is advancing very rapidly, making ever-larger
domains possible. In this section we assume that these
considerations will lead to the deployment of a domain of
transparency that is too large to ensure that all potential routes
have adequate signal quality for all circuits. Our goal is to
understand the impacts of the various types of impairments in this
environment.
Note that as we describe later in the section there are many types - Only adaptation functions whose lasers/receivers are tunable to
of physical impairments. Which of these needs to be dealt with compatible frequencies can be connected.
explicitly when performing on-line distributed routing will vary
considerably and will depend on many variables, including:
- Equipment vendor design choices,
- Fiber characteristics,
- Service characteristics (e.g., circuit speeds),
- Network size,
- Network operator engineering and deployment strategies.
For example, a metropolitan network which does not intend to support
bit rates above 2.5 Gb/sec may not be constrained by any of these
impairments, while a continental or international network which
wished to minimize O/E/O regeneration investment and support 40
Gb/sec connections might have to explicitly consider many of them.
Also, a network operator may reduce or even eliminate their
constraint set by building a relatively small domain of transparency
to ensure that all the paths are feasible, or by using some
proprietary tools based on rules from the OTS vendor to pre-qualify
paths between node pairs and put them in a table that can be
accessed each time a routing decision has to be made through that
domain.
4.1 Problem Formulation - The switching capability of the OADMs may also be constrained.
On Optical Layer Routing
We consider a single domain of transparency without wavelength For example:
translation. Additionally due to the proprietary nature of DWDM
transmission technology, we assume that the domain is either single
vendor or architected using a single coherent design, particularly
with regard to the management of impairments.
We wish to route a unidirectional circuit from ingress client node X o There may be some wavelengths that can not be dropped at all.
to egress client node Y. At both X and Y, the circuit goes through
an O/E/O conversion which optically isolates the portion within our
domain. We assume that we know the bit rate of the circuit. Also,
we assume that the adaptation function at X may apply some Forward
Error Correction (FEC) method to the circuit. We also assume we know
the launch power of the laser at X.
Impairments can be classified into two categories, linear and o There may be a fixed relationship between the frequency dropped
nonlinear. (See [Tkach98] for more on impairment constraints). and the physical port on the OADM to which it is dropped.
Linear effects are independent of signal power and affect
wavelengths individually. Amplifier spontaneous emission (ASE),
polarization mode dispersion (PMD), and chromatic dispersion are
examples. Nonlinearities are significantly more complex: they
generate not only impairments on each channel, but also crosstalk
between channels.
In the remainder of this section we first outline how two key linear o OADM physical design may put an upper bound on the number of
impairments (PMD and ASE) might be handled by a set of analytical adaptation groupings dropped at any single OADM.
formulae as additional constraints on routing. We next discuss how
the remaining constraints might be approached. Finally we take a
broader perspective and discuss the implications of such constraints
on control plane architecture and also on broader constrained domain
of transparency architecture issues.
4.2 Polarization Mode Dispersion (PMD) For a fixed configuration of the OADMs and adaptation functions
connectivity will be fixed: Each input port will essentially be
hard-wired to some specific distant port. However this connectivity
can be changed by changing the configurations of the OADMs and
adaptation functions. For example, an additional adaptation grouping
might be dropped at an OADM or a tunable laser retuned. In each case
the port-to-port connectivity is changed.
For a transparent fiber segment, the general PMD requirement is that These capabilities can be expected to be under software control.
the time-average differential group delay (DGD) between two Today the control would rest in the vendor-supplied Element
orthogonal state of polarizations should be less than fraction a of Management system (EMS), which in turn would be controlled by the
the bit duration, T=1/B, where B is the bit rate. The value of the operator's OSes. However in principle the EMS could participate in
parameter a depends on three major factors: 1) margin allocated to the GMPLS routing process.
PMD, e.g. 1dB; 2) targeted outage probability, e.g. 4x10-5, and 3)
sensitivity of the receiver to DGD. A typical value for a is 10%
[ITU]. More aggressive designs to compensate for PMD may allow
values higher than 10%. (This would be a system parameter dependent
on the system design. It would need to be known to the routing
process.)
The PMD parameter (Dpmd) is measured in pico-seconds (ps) per 3.2. Implications for Routing
sqrt(km). The square of the PMD in a fiber span, denoted as span-
PMD-square is then given by the product of Dpmd**2 and the span
length. (A fiber span in a transparent network refers to a segment
between two optical amplifiers.) If Dpmd is constant, this results
On Optical Layer Routing
in a upper bound on the maximum length of an M-fiber-span An OTS of the sort discussed in Sec. 3.1 is essentially a
transparent segment, which is inversely proportional to the square geographically distributed but blocking cross-connect system. The
of the product of bit rate and Dpmd (the detailed equation is specific port connectivity is dependent on the vendor design and also
omitted due to the format constraint - see [Strand01] for details). on exactly what line cards have been deployed.
For older fibers with a typical PMD parameter of 0.5 picoseconds per One way for GMPLS to deal with this architecture would be to view the
square root of km, based on the constraint, the maximum length of port connectivity as externally determined. In this case the links
the transparent segment should not exceed 400km and 25km for bit known to GMPLS would be groups of identically routed wavebands. If
rates of 10Gb/s and 40Gb/s, respectively. Due to recent advances in these were reconfigured by the external EMS the resulting
fiber technology, the PMD-limited distance has increased connectivity changes would need to be detected and advertised within
dramatically. For newer fibers with a PMD parameter of 0.1 GMPLS. If the topology shown in Fig. 3-1 became a tree or a mesh
picosecond per square root of km, the maximum length of the instead of the linear topology shown, the connectivity changes could
transparent segment (without PMD compensation) is limited to 10000km result in Shared Risk Link Group (SRLG - see Section 6.2) changes.
and 625km for bit rates of 10Gb/s and 40Gb/, respectively. Still
lower values of PMD are attainable in commercially available fiber
today, and the PMD limit can be further extended if a larger value
of the parameter a (ratio of DGD to the bit period) can be
tolerated. In general, the PMD requirement is not an issue for most
types of fibers at 10Gb/s or lower bit rate. But it will become an
issue at bit rates of 40Gb/s and higher.
If the PMD parameter varies between spans, a slightly more Alternatively, GMPLS could attempt to directly control this port
complicated equation results (see [Strand01]), but in any event the connectivity. The state information needed to do this is likely to
only link dependent information needed by the routing algorithm is be voluminous and vendor specific.
the square of the link PMD, denoted as link-PMD-square. It is the
sum of the span-PMD-square of all spans on the link.
Note that when one has some viable PMD compensation devices and 4. Wavelength Routed All-Optical Networks
deploy them ubiquitously on all routes with potential PMD issues in
the network, then the PMD constraint disappears from the routing
perspective.
4.3 Amplifier Spontaneous Emission The optical networks deployed until recently may be called "opaque"
([Tkach98]): each link is optically isolated by transponders doing
O/E/O conversions. They provide regeneration with retiming and
reshaping, also called 3R, which eliminates transparency to bit rates
and frame format. These transponders are quite expensive and their
lack of transparency also constrains the rapid introduction of new
services. Thus there are strong motivators to introduce "domains of
transparency" - all-optical subnetworks - larger than an OTS.
ASE degrades the optical signal to noise ratio (OSNR). An acceptable The routing of lightpaths through an all-optical network has received
optical SNR level (SNRmin) which depends on the bit rate, extensive attention. (See [Yates99] or [Ramaswami98]). When
transmitter-receiver technology (e.g., FEC), and margins allocated discussing routing in an all-optical network it is usually assumed
for the impairments, needs to be maintained at the receiver. In that all routes have adequate signal quality. This may be ensured by
order to satisfy this requirement, vendors often provide some limiting all-optical networks to subnetworks of limited geographic
general engineering rule in terms of maximum length of the size that are optically isolated from other parts of the optical
transparent segment and number of spans. For example, current layer by transponders. This approach is very practical and has been
transmission systems are often limited to up to 6 spans each 80km applied to date, e.g., when determining the maximum length of an
long. For larger transparent domains, more detailed OSNR Optical Transport System (OTS). Furthermore operational
computations will be needed to determine whether the OSNR level considerations like fault isolation also make limiting the size of
through a domain of transparency is acceptable. This would provide domains of transparency attractive.
flexibility in provisioning or restoring a lightpath through a
transparent subnetwork.
Assume that the average optical power launched at the transmitter is There are however reasons to consider contained domains of
P. The lightpath from the transmitter to the receiver goes through M transparency in which not all routes have adequate signal quality.
On Optical Layer Routing From a demand perspective, maximum bit rates have rapidly increased
from DS3 to OC-192 and soon OC-768 (40 Gb/sec). As bit rates
increase it is necessary to increase power. This makes impairments
and nonlinearities more troublesome. From a supply perspective,
optical technology is advancing very rapidly, making ever-larger
domains possible. In this section, we assume that these
considerations will lead to the deployment of a domain of
transparency that is too large to ensure that all potential routes
have adequate signal quality for all circuits. Our goal is to
understand the impacts of the various types of impairments in this
environment.
optical amplifiers, with each introducing some noise power. Unity Note that, as we describe later in the section, there are many types
gain can be used at all amplifier sites to maintain constant signal of physical impairments. Which of these needs to be dealt with
power at the input of each span to minimize noise power and explicitly when performing on-line distributed routing will vary
nonlinearity. A constraint on the maximum number of spans can be considerably and will depend on many variables, including:
obtained [Kaminow97] which is proportional to P and inversely
proportional to SNRmin, optical bandwidth B, amplifier gain G-1 and
spontaneous emission factor n of the optical amplifier, assuming all
spans have identical gain and noise figure. (Again, the detailed
equation is omitted due to the format constraint - see [Strand01]
for details.) Let's take a typical example. Assuming P=4dBm,
SNRmin=20dB with FEC, B=12.5GHz, n=2.5, G=25dB, based on the
constraint, the maximum number of spans is at most 10. However, if
FEC is not used and the requirement on SNRmin becomes 25dB, the
maximum number of spans drops down to 3.
For ASE the only link-dependent information needed by the routing - Equipment vendor design choices,
algorithm is the noise of the link, denoted as link-noise, which is - Fiber characteristics,
the sum of the noise of all spans on the link. Hence the constraint - Service characteristics (e.g., circuit speeds),
on ASE becomes that the aggregate noise of the transparent segment - Network size,
which is the sum of the link-noise of all links can not exceed - Network operator engineering and deployment strategies.
P/SNRmin.
4.4 Approximating The Effects Of Some Other Impairment Constraints For example, a metropolitan network that does not intend to support
bit rates above 2.5 Gb/sec may not be constrained by any of these
impairments, while a continental or international network that wished
to minimize O/E/O regeneration investment and support 40 Gb/sec
connections might have to explicitly consider many of them. Also, a
network operator may reduce or even eliminate their constraint set by
building a relatively small domain of transparency to ensure that all
the paths are feasible, or by using some proprietary tools based on
rules from the OTS vendor to pre-qualify paths between node pairs and
put them in a table that can be accessed each time a routing decision
has to be made through that domain.
There are a number of other impairment constraints that we believe 4.1. Problem Formulation
could be approximated with a domain-wide margin on the OSNR, plus in
some cases a constraint on the total number of networking elements
(OXC or OADM) along the path. Most impairments generated at OXCs or
OADMs, including polarization dependent loss, coherent crosstalk,
and effective passband width, could be dealt with using this
approach. In principle, impairments generated at the nodes can be
bounded by system engineering rules because the node elements can be
designed and specified in a uniform manner. This approach is not
feasible with PMD and noise because neither can be uniformly
specified. Instead, they depend on node spacing and the
characteristics of the installed fiber plant, neither of which are
likely to be under the system designer's control.
Examples of the constraints we propose to approximate with a domain- We consider a single domain of transparency without wavelength
wide margin are given in the remaining paragraphs in this section. translation. Additionally, due to the proprietary nature of DWDM
It should be kept in mind that as optical transport technology transmission technology, we assume that the domain is either single
evolves it may become necessary to include some of these impairments vendor or architected using a single coherent design, particularly
explicitly in the routing process. Other impairments not mentioned with regard to the management of impairments.
here at all may also become sufficiently important to require
incorporation either explicitly or via a domain-wide margin.
Other Polarization Dependent Impairments Other polarization- We wish to route a unidirectional circuit from ingress client node X
dependent effects besides PMD influence system performance. For to egress client node Y. At both X and Y, the circuit goes through
example, many components have polarization-dependent loss (PDL) an O/E/O conversion that optically isolates the portion within our
[Ramaswami98], which accumulates in a system with many components on domain. We assume that we know the bit rate of the circuit. Also,
On Optical Layer Routing we assume that the adaptation function at X may apply some Forward
Error Correction (FEC) method to the circuit. We also assume we know
the launch power of the laser at X.
the transmission path. The state of polarization fluctuates with Impairments can be classified into two categories, linear and
time and its distribution is very important also. It is generally nonlinear. (See [Tkach98] or [Kaminow02] for more on impairment
required to maintain the total PDL on the path to be within some constraints.) Linear effects are independent of signal power and
acceptable limit, potentially by using some compensation technology affect wavelengths individually. Amplifier spontaneous emission
for relatively long transmission systems, plus a small built-in (ASE), polarization mode dispersion (PMD), and chromatic dispersion
margin in OSNR. Since the total PDL increases with the number of are examples. Nonlinearities are significantly more complex: they
components in the data path, it must be taken into account by the generate not only impairments on each channel, but also crosstalk
system vendor when determining the maximum allowable number of between channels.
spans.
Chromatic Dispersion In general this impairment can be adequately In the remainder of this section we first outline how two key linear
(but not optimally) compensated for on a per-link basis, and/or at impairments (PMD and ASE) might be handled by a set of analytical
system initial setup time. Today most deployed compensation devices formulae as additional constraints on routing. We next discuss how
are based on DCF (Dispersion Compensation Fiber). DCF provides per the remaining constraints might be approached. Finally we take a
fiber compensation by means of a spool of fiber with a CD coefficient broader perspective and discuss the implications of such constraints
opposite to the fiber. Due to the imperfect matching between the CD on control plane architecture and also on broader constrained domain
slope of the fiber and the DCF some lambdas can be over compensated of transparency architecture issues.
while others can be under compensated. Moreover DCF modules may only
be available in fixed lengths of compensating fiber; this means that
sometimes it is impossible to find a DCF module that exactly
compensates the CD introduced by the fiber. These effects introduce
what is known as residual CD. Residual CD varies with the frequency
of the wavelength. Knowing the characteristics of both of the fiber
and the DCF modules along the path, this can be calculated with a
sufficient degree of precision. However this is a very challenging
task. In fact the per-wavelength residual dispersion needs to be
combined with other information in the system (e.g. types fibers to
figure out the amount of nonlinearities) to obtain the net effect of
CD either by simulation or by some analytical approximation. It
appears that the routing/control plane should not be burdened by such
a large set of information while it can be handled at the system
design level. Therefore it will be assumed until proven otherwise
that residual dispersion should not be reported. For high bit rates,
dynamic dispersion compensation may be required at the receiver to
clean up any residual dispersion.
Crosstalk Optical crosstalk refers to the effect of other signals on 4.2. Polarization Mode Dispersion (PMD)
the desired signal. It includes both coherent (i.e. intrachannel)
crosstalk and incoherent (i.e. interchannel) crosstalk. Main
contributors of crosstalk are the OADM and OXC sites that use a DWDM
multiplexer/demultiplexer (MUX/DEMUX) pair. For a relatively sparse
network where the number of OADM/OXC nodes on a path is low,
crosstalk can be treated with a low margin in OSNR without being a
binding constraint. But for some relatively dense networks where
crosstalk might become a binding constraint, one needs to propagate
the per-link crosstalk information to make sure that the end-to-end
On Optical Layer Routing
path crosstalk which is the sum of the crosstalks on all the For a transparent fiber segment, the general PMD requirement is that
corresponding links to be within some limit, e.g. -25dB threshold the time-average differential group delay (DGD) between two
with 1dB penalty ([Goldstein94]). Another way to treat it without orthogonal state of polarizations should be less than some fraction a
having to propagate per-link crosstalk information is to have the of the bit duration, T=1/B, where B is the bit rate. The value of
system evaluate what the maximum number of OADM/OXC nodes that has a the parameter a depends on three major factors: 1) margin allocated
MUX/DEMUX pair for the worst route in the transparent domain for a to PMD, e.g., 1dB; 2) targeted outage probability, e.g., 4x10-5, and
low built-in margin. The latter one should work well where all the 3) sensitivity of the receiver to DGD. A typical value for a is 10%
OXC/OADM nodes have similar level of crosstalk. [ITU]. More aggressive designs to compensate for PMD may allow
values higher than 10%. (This would be a system parameter dependent
on the system design. It would need to be known to the routing
process.)
Effective Passband As more and more DWDM components are cascaded, The PMD parameter (Dpmd) is measured in pico-seconds (ps) per
the effective passband narrows. The number of filters along the sqrt(km). The square of the PMD in a fiber span, denoted as span-
link, their passband width and their shape will determine the end- PMD-square is then given by the product of Dpmd**2 and the span
to-end effective passband. In general, this is a system design length. (A fiber span in a transparent network refers to a segment
issue, i.e., the system is designed with certain maximum bit rate between two optical amplifiers.) If Dpmd is constant, this results
using the proper modulation format and filter spacing. For linear in a upper bound on the maximum length of an M-fiber-span transparent
systems, the filter effect can be turned into a constraint on the segment, which is inversely proportional to the square of the product
maximum number of narrow filters with the condition that filters in of bit rate and Dpmd (the detailed equation is omitted due to the
the systems are at least as wide as the one in the receiver. format constraint - see [Strand01] for details).
Because traffic at lower bit rates can tolerate a narrower passband,
the maximum allowable number of narrow filters will increase as the
bit rate decreases.
Nonlinear Impairments It seems unlikely that these can be dealt with For older fibers with a typical PMD parameter of 0.5 picoseconds per
explicitly in a routing algorithm because they lead to constraints square root of km, based on the constraint, the maximum length of the
that can couple routes together and lead to complex dependencies, transparent segment should not exceed 400km and 25km for bit rates of
e.g. on the order in which specific fiber types are traversed 10Gb/s and 40Gb/s, respectively. Due to recent advances in fiber
[Kaminow97]. Note that different fiber types (standard single mode technology, the PMD-limited distance has increased dramatically. For
fiber, dispersion shifted fiber, dispersion compensated fiber, etc.) newer fibers with a PMD parameter of 0.1 picosecond per square root
have very different effects from nonlinear impairments. A full of km, the maximum length of the transparent segment (without PMD
treatment of the nonlinear constraints would likely require very compensation) is limited to 10000km and 625km for bit rates of 10Gb/s
detailed knowledge of the physical infrastructure, including and 40Gb/, respectively. Still lower values of PMD are attainable in
measured dispersion values for each span, fiber core area and commercially available fiber today, and the PMD limit can be further
composition, as well as knowledge of subsystem details such as extended if a larger value of the parameter a (ratio of DGD to the
dispersion compensation technology. This information would need to bit period) can be tolerated. In general, the PMD requirement is not
be combined with knowledge of the current loading of optical signals an issue for most types of fibers at 10Gb/s or lower bit rate. But
on the links of interest to determine the level of nonlinear it will become an issue at bit rates of 40Gb/s and higher.
impairment. Alternatively, one could assume that nonlinear
impairments are bounded and result in X dB margin in the required
OSNR level for a given bit rate, 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 length limit to be longer than
that imposed by the constraints which can be treated explicitly.
When designing a DWDM transport system, there are tradeoffs between
signal power launched at the transmitter, span length, and nonlinear
effects on BER that need to be considered jointly. Here, we assume
that an X dB margin is obtained after the transport system has been
designed with a fixed signal power and maximum span length for a
given bit rate. Note that OTSs can be designed in very different
ways, in linear, pseudo-linear, or nonlinear environments. The X-dB
On Optical Layer Routing
margin approach may be valid for some but not for others. However, If the PMD parameter varies between spans, a slightly more
it is likely that there is an advantage in designing systems that complicated equation results (see [Strand01]), but in any event the
are less aggressive with respect to nonlinearities, and therefore only link dependent information needed by the routing algorithm is
somewhat sub-optimal, in exchange for improved scalability, the square of the link PMD, denoted as link-PMD-square. It is the
simplicity and flexibility in routing and control plane design. sum of the span-PMD-square of all spans on the link.
4.5 Other Impairment Considerations Note that when one has some viable PMD compensation devices and
deploy them ubiquitously on all routes with potential PMD issues in
the network, then the PMD constraint disappears from the routing
perspective.
There are many other types of impairments that can degrade 4.3. Amplifier Spontaneous Emission
performance. In this section we briefly mention one other type of
impairment, which we propose be dealt with by either by the system
designer or by the transmission engineers at the time the system is
installed. If dealt with successfully in this manner they should not
need to be considered in the dynamic routing process.
Gain Nonuniformity and Gain Transients For simple noise estimates to ASE degrades the optical signal to noise ratio (OSNR). An acceptable
be of use, the amplifiers must be gain-flattened and must have optical SNR level (SNRmin), which depends on the bit rate,
automatic gain control (AGC). Furthermore, each link should have transmitter-receiver technology (e.g., FEC), and margins allocated
dynamic gain equalization (DGE) to optimize power levels each time for the impairments, needs to be maintained at the receiver. In
wavelengths are added or dropped. Variable optical attenuators on order to satisfy this requirement, vendors often provide some general
the output ports of an OXC or OADM can be used for this purpose, and engineering rule in terms of maximum length of the transparent
in-line devices are starting to become commercially available. segment and number of spans. For example, current transmission
Optical channel monitors are also required to provide feedback to systems are often limited to up to 6 spans each 80km long. For
the DGEs. AGC must be done rapidly if signal degradation after a larger transparent domains, more detailed OSNR computations will be
protection switch or link failure is to be avoided. needed to determine whether the OSNR level through a domain of
transparency is acceptable. This would provide flexibility in
provisioning or restoring a lightpath through a transparent
subnetwork.
Note that the impairments considered here are treated more or less Assume that the average optical power launched at the transmitter is
independently. By considering them jointly and varying the tradeoffs P. The lightpath from the transmitter to the receiver goes through M
between the effects from different components may allow more routes optical amplifiers, with each introducing some noise power. Unity
to be feasible. If that is desirable or the system is designed such gain can be used at all amplifier sites to maintain constant signal
that certain impairments (e.g. nonlinearities) need to be considered power at the input of each span to minimize noise power and
by a centralized process, then distributed routing is not the one to nonlinearity. A constraint on the maximum number of spans can be
use. obtained [Kaminow97] which is proportional to P and inversely
proportional to SNRmin, optical bandwidth B, amplifier gain G-1 and
spontaneous emission factor n of the optical amplifier, assuming all
spans have identical gain and noise figure. (Again, the detailed
equation is omitted due to the format constraint - see [Strand01] for
details.) Let's take a typical example. Assuming P=4dBm,
SNRmin=20dB with FEC, B=12.5GHz, n=2.5, G=25dB, based on the
constraint, the maximum number of spans is at most 10. However, if
FEC is not used and the requirement on SNRmin becomes 25dB, the
maximum number of spans drops down to 3.
4.6 An Alternative Approach - Using Maximum Distance As The only For ASE the only link-dependent information needed by the routing
Constraint algorithm is the noise of the link, denoted as link-noise, which is
the sum of the noise of all spans on the link. Hence the constraint
on ASE becomes that the aggregate noise of the transparent segment
which is the sum of the link-noise of all links can not exceed
P/SNRmin.
Today, carriers often use maximum distance to engineer point-to- 4.4. Approximating the Effects of Some Other Impairment Constraints
point OTS systems given a fixed per-span length based on the OSNR
constraint for a given bit rate. They may desire to keep the same
engineering rule when they move to all-optical networks. Here, we
discuss the assumptions that need to be satisfied to keep this
approach viable and how to treat the network elements between two
adjacent links.
On Optical Layer Routing There are a number of other impairment constraints that we believe
could be approximated with a domain-wide margin on the OSNR, plus in
some cases a constraint on the total number of networking elements
(OXC or OADM) along the path. Most impairments generated at OXCs or
OADMs, including polarization dependent loss, coherent crosstalk, and
effective passband width, could be dealt with using this approach.
In principle, impairments generated at the nodes can be bounded by
system engineering rules because the node elements can be designed
and specified in a uniform manner. This approach is not feasible
with PMD and noise because neither can be uniformly specified.
Instead, they depend on node spacing and the characteristics of the
installed fiber plant, neither of which are likely to be under the
system designer's control.
In order to use the maximum distance for a given bit rate to meet an Examples of the constraints we propose to approximate with a domain-
OSNR constraint as the only binding constraint, the operators need wide margin are given in the remaining paragraphs in this section.
to satisfy the following constraints in their all-optical networks: It should be kept in mind that as optical transport technology
evolves it may become necessary to include some of these impairments
explicitly in the routing process. Other impairments not mentioned
here at all may also become sufficiently important to require
incorporation either explicitly or via a domain-wide margin.
- All the other non-OSNR constraints described in the previous Other Polarization Dependent Impairments
subsections are not binding factors as long as the maximum Other polarization-dependent effects besides PMD influence system
distance constraint is met. performance. For example, many components have polarization-
- Specifically for PMD, this means that the whole all-optical dependent loss (PDL) [Ramaswami98], which accumulates in a system
network is built on top of sufficiently low-PMD fiber such that with many components on the transmission path. The state of
the upper bound on the mean aggregate path DGD is always polarization fluctuates with time and its distribution is very
satisfied for any path that does not exceed the maximum important also. It is generally required that the total PDL on
distance, or PMD compensation devices might be used for routes the path be maintained within some acceptable limit, potentially
with high-PMD fibers. by using some compensation technology for relatively long
- In terms of the ASE/OSNR constraint, in order to convert the ASE transmission systems, plus a small built-in margin in OSNR. Since
constraint into a distance constraint directly, the network the total PDL increases with the number of components in the data
needs to have a fixed fiber distance D for each span (so that path, it must be taken into account by the system vendor when
ASE can be directly mapped by the gain of the amplifier which determining the maximum allowable number of spans.
equals to the loss of the previous fiber span), e.g., 80km
spacing which is commonly chosen by carriers. However, when
spans have variable lengths, certain adjustment and compromise
need to be made in order to avoid treating ASE explicitly as in
section 4.3. These include: 1) If a span is shorter than a
typical span length D, unless certain mechanism is built in the
OTS to take advantages of shorter spans, it needs to be treated
as a span of length D instead of with its real length. 2) When
there are spans that are longer than D, it means that paths with
these longer spans would have higher average span loss. In
general, the maximum system reach decreases when the average
span loss increases. Thus, in order to accommodate longer spans
in the network, the maximum distance upper bound has to be set
with respect to the average span loss of the worst path in the
network. This sub-optimality may be acceptable for some networks
if the variance is not too large, but may be too conservative
for others.
If these assumptions are satisfied, the second issue we need to Chromatic Dispersion
address is how to treat a transparent network element (e.g., MEMS- In general this impairment can be adequately (but not optimally)
based switch) between two adjacent links in terms of a distance compensated for on a per-link basis, and/or at system initial
constraint since it also introduces an insertion loss. If the setup time. Today most deployed compensation devices are based on
network element cannot somehow compensate for this OSNR degradation, Dispersion Compensation Fiber (DCF). DCF provides per fiber
one approach is to convert each network element into an equivalent compensation by means of a spool of fiber with a CD coefficient
length of fiber based on its loss/ASE contribution. Hence, in opposite to the fiber. Due to the imperfect matching between the
general, introducing a set of transparent network elements would CD slope of the fiber and the DCF some lambdas can be over
On Optical Layer Routing compensated while others can be under compensated. Moreover DCF
modules may only be available in fixed lengths of compensating
fiber; this means that sometimes it is impossible to find a DCF
module that exactly compensates the CD introduced by the fiber.
These effects introduce what is known as residual CD. Residual CD
varies with the frequency of the wavelength. Knowing the
characteristics of both of the fiber and the DCF modules along the
path, this can be calculated with a sufficient degree of
precision. However this is a very challenging task. In fact the
per-wavelength residual dispersion needs to be combined with other
information in the system (e.g., types fibers to figure out the
amount of nonlinearities) to obtain the net effect of CD either by
simulation or by some analytical approximation. It appears that
the routing/control plane should not be burdened by such a large
set of information while it can be handled at the system design
level. Therefore it will be assumed until proven otherwise that
residual dispersion should not be reported. For high bit rates,
dynamic dispersion compensation may be required at the receiver to
clean up any residual dispersion.
effectively result in reducing the overall actual transmission Crosstalk
distance between the OEO edges. Optical crosstalk refers to the effect of other signals on the
desired signal. It includes both coherent (i.e., intrachannel)
crosstalk and incoherent (i.e., interchannel) crosstalk. Main
contributors of crosstalk are the OADM and OXC sites that use a
DWDM multiplexer/demultiplexer (MUX/DEMUX) pair. For a relatively
sparse network where the number of OADM/OXC nodes on a path is
low, crosstalk can be treated with a low margin in OSNR without
being a binding constraint. But for some relatively dense
networks where crosstalk might become a binding constraint, one
needs to propagate the per-link crosstalk information to make sure
that the end-to-end path crosstalk which is the sum of the
crosstalks on all the corresponding links to be within some limit,
e.g., -25dB threshold with 1dB penalty ([Goldstein94]). Another
way to treat it without having to propagate per-link crosstalk
information is to have the system evaluate what the maximum number
of OADM/OXC nodes that has a MUX/DEMUX pair for the worst route in
the transparent domain for a low built-in margin. The latter one
should work well where all the OXC/OADM nodes have similar level
of crosstalk.
With this approach, the link-specific state information is link- Effective Passband
distance, the length of a link. It equals to the distance sum of all As more and more DWDM components are cascaded, the effective
fiber spans on the link and the equivalent length of fiber for the passband narrows. The number of filters along the link, their
network element(s) on the link. The constraint is that the sum of passband width and their shape will determine the end-to-end
all the link-distance over all links of a path should be less than effective passband. In general, this is a system design issue,
the maximum-path-distance, the upper bound of all paths. i.e., the system is designed with certain maximum bit rate using
the proper modulation format and filter spacing. For linear
systems, the filter effect can be turned into a constraint on the
maximum number of narrow filters with the condition that filters
in the systems are at least as wide as the one in the receiver.
Because traffic at lower bit rates can tolerate a narrower
passband, the maximum allowable number of narrow filters will
increase as the bit rate decreases.
4.7 Other Considerations Nonlinear Impairments
It seems unlikely that these can be dealt with explicitly in a
routing algorithm because they lead to constraints that can couple
routes together and lead to complex dependencies, e.g., on the
order in which specific fiber types are traversed [Kaminow97].
Note that different fiber types (standard single mode fiber,
dispersion shifted fiber, dispersion compensated fiber, etc.) have
very different effects from nonlinear impairments. A full
treatment of the nonlinear constraints would likely require very
detailed knowledge of the physical infrastructure, including
measured dispersion values for each span, fiber core area and
composition, as well as knowledge of subsystem details such as
dispersion compensation technology. This information would need
to be combined with knowledge of the current loading of optical
signals on the links of interest to determine the level of
nonlinear impairment. Alternatively, one could assume that
nonlinear impairments are bounded and result in X dB margin in the
required OSNR level for a given bit rate, 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 length limit to
be longer than that imposed by the constraints which can be
treated explicitly. When designing a DWDM transport system, there
are tradeoffs between signal power launched at the transmitter,
span length, and nonlinear effects on BER that need to be
considered jointly. Here, we assume that an X dB margin is
obtained after the transport system has been designed with a fixed
signal power and maximum span length for a given bit rate. Note
that OTSs can be designed in very different ways, in linear,
pseudo-linear, or nonlinear environments. The X-dB margin
approach may be valid for some but not for others. However, it is
likely that there is an advantage in designing systems that are
less aggressive with respect to nonlinearities, and therefore
somewhat sub-optimal, in exchange for improved scalability,
simplicity and flexibility in routing and control plane design.
Routing in an all-optical network without wavelength conversion 4.5. Other Impairment Considerations
raises several additional issues:
- Since the route selected must have the chosen wavelength There are many other types of impairments that can degrade
available on all links, this information needs to be considered performance. In this section, we briefly mention one other type of
in the routing process. One approach is to propagate impairment, which we propose be dealt with by either the system
information throughout the network about the state of every designer or by the transmission engineers at the time the system is
wavelength on every link in the network. However, the state installed. If dealt with successfully in this manner they should not
required and the overhead involved in processing and need to be considered in the dynamic routing process.
maintaining this information is proportional to the total
number of links (thus, number of nodes squared), maximum number
of wavelengths which keeps doubling every couple of years), and
the frequency of wavelength availability changes, which can be
very high. Instead [Hjlmtsson00] proposes an alternative
method which probes along a chosen path to determine which
wavelengths (if any) are available. This would require a
significant addition to the routing logic normally used in
OSPF. Others have proposed simultaneously probing along
multiple paths.
- Choosing a path first and then a wavelength along the path is Gain Nonuniformity and Gain Transients For simple noise estimates to
known to give adequate results in simple topologies such as be of use, the amplifiers must be gain-flattened and must have
rings and trees ([Yates99]). This does not appear to be true in automatic gain control (AGC). Furthermore, each link should have
large mesh networks under realistic provisioning scenarios, dynamic gain equalization (DGE) to optimize power levels each time
however. Instead significantly better results are achieved if wavelengths are added or dropped. Variable optical attenuators on
wavelength and route are chosen simultaneously ([Strand01b]). the output ports of an OXC or OADM can be used for this purpose, and
This approach would however also have a significant effect on in-line devices are starting to become commercially available.
OSPF. Optical channel monitors are also required to provide feedback to the
DGEs. AGC must be done rapidly if signal degradation after a
protection switch or link failure is to be avoided.
4.8 Implications For Routing and Control Plane Design Note that the impairments considered here are treated more or less
independently. By considering them jointly and varying the tradeoffs
between the effects from different components may allow more routes
to be feasible. If that is desirable or the system is designed such
that certain impairments (e.g., nonlinearities) need to be considered
by a centralized process, then distributed routing is not the one to
use.
If distributed routing is desired, additional state information will 4.6. An Alternative Approach - Using Maximum Distance as the Only
be required by the routing to deal with the impairments described in Constraint
Sections 4.2 - 4.4:
On Optical Layer Routing Today, carriers often use maximum distance to engineer point-to-point
OTS systems given a fixed per-span length based on the OSNR
constraint for a given bit rate. They may desire to keep the same
engineering rule when they move to all-optical networks. Here, we
discuss the assumptions that need to be satisfied to keep this
approach viable and how to treat the network elements between two
adjacent links.
- As mentioned earlier, an operator who wants to avoid having to In order to use the maximum distance for a given bit rate to meet an
provide impairment-related parameters to the control plane may OSNR constraint as the only binding constraint, the operators need to
elect not to deal with them at the routing level, instead satisfy the following constraints in their all-optical networks:
treating them at the system design and planning level if that is
a viable approach for their network. In this approach the
operator can pre-qualify all or a set of feasible end-to-end
optical paths through the domain of transparency for each bit
rate. This approach may work well with relatively small and
sparse networks, but it may not be scalable for large and dense
networks where the number of feasible paths can be very large.
- If the optical paths are not pre-qualified, additional link- - All the other non-OSNR constraints described in the previous
specific state information will be required by the routing subsections are not binding factors as long as the maximum
algorithm for each type of impairment that has the potential of distance constraint is met.
being limiting for some routes. Note that for one operator, PMD
might be the only limiting constraint while for another, ASE
might be the only one, or it could be both plus some other
constraints considered in this document. Some networks might not
be limited by any of these constraints.
- For an operator needing to deal explicitly with these - Specifically for PMD, this means that the whole all-optical
constraints, the link-dependent information identified above for network is built on top of sufficiently low-PMD fiber such that
PMD is link-PMD-square which is the square of the total PMD on a the upper bound on the mean aggregate path DGD is always satisfied
link. For ASE the link-dependent information identified is link- for any path that does not exceed the maximum distance, or PMD
noise which is the total noise on a link. Other link-dependent compensation devices might be used for routes with high-PMD
information includes link-span-length which is the total number fibers.
of spans on a link, link-crosstalk or OADM-OXC-number which is
the total crosstalk or the number of OADM/OXC nodes on a link,
respectively, and filter-number which is the number of narrow
filters on a link. When the alternative distance-only approach
is chosen, the link-specific information is link-distance.
- In addition to the link-specific information, bounds on each of - In terms of the ASE/OSNR constraint, in order to convert the ASE
the impairments need to be quantified. Since these bounds are constraint into a distance constraint directly, the network needs
determined by the system designer's impairment allocations, to have a fixed fiber distance D for each span (so that ASE can be
these will be system dependent. For PMD, the constraint is that directly mapped by the gain of the amplifier which equals to the
the sum of the link-PMD-square of all links on the transparent loss of the previous fiber span), e.g., 80km spacing which is
segment is less than the square of (a/B) where B is the bit commonly chosen by carriers. However, when spans have variable
rate. Hence, the required information is the parameter "a". For lengths, certain adjustment and compromise need to be made in
ASE, the constraint is that the sum of the link-noise of all order to avoid treating ASE explicitly as in section 4.3. These
links is no larger than P/SNRmin. Thus, the information needed include: 1) Unless a certain mechanism is built in the OTS to take
include the launch power P and OSNR requirement SNRmin. The advantage of shorter spans, spans shorter than a typical span
minimum acceptable OSNR, in turn, depends on the strength of the length D need to be treated as a span of length D instead of with
FEC being used and the margins reserved for other types of its real length. 2) Spans that are longer than D would have a
impairments. Other bounds include the maximum span length of the higher average span loss. In general, the maximum system reach
transmission system, the maximum path crosstalk or the maximum decreases when the average span loss increases. Thus, in order to
number of OADM/OXC nodes, and the maximum number of narrow accommodate longer spans in the network, the maximum distance
filters, all are bit rate dependent. With the alternative upper bound has to be set with respect to the average span loss of
distance-only approach, the upper bound is the maximum-path- the worst path in the network. This sub-optimality may be
distance. In single-vendor "islands" some of these parameters acceptable for some networks if the variance is not too large, but
On Optical Layer Routing may be too conservative for others.
may be available in a local or EMS database and would not need If these assumptions are satisfied, the second issue we need to
to be advertised address is how to treat a transparent network element (e.g., MEMS-
based switch) between two adjacent links in terms of a distance
constraint since it also introduces an insertion loss. If the
network element cannot somehow compensate for this OSNR degradation,
one approach is to convert each network element into an equivalent
length of fiber based on its loss/ASE contribution. Hence, in
general, introducing a set of transparent network elements would
effectively result in reducing the overall actual transmission
distance between the OEO edges.
- It is likely that the physical layer parameters do not change With this approach, the link-specific state information is link-
value rapidly and could be stored in some database; however distance, the length of a link. It equals the distance sum of all
these are physical layer parameters that today are frequently fiber spans on the link and the equivalent length of fiber for the
not known at the granularity required. If the ingress node of a network element(s) on the link. The constraint is that the sum of
lightpath does path selection these parameters would need to be all the link-distance over all links of a path should be less than
available at this node. the maximum-path-distance, the upper bound of all paths.
- The specific constraints required in a given situation will 4.7. Other Considerations
depend on the design and engineering of the domain of
transparency; for example it will be essential to know whether
chromatic dispersion has been dealt with on a per-link basis,
and whether the domain is operating in a linear or nonlinear
regime.
- As optical transport technology evolves, the set of constraints Routing in an all-optical network without wavelength conversion
that will need to be considered either explicitly or via a raises several additional issues:
domain-wide margin may change. The routing and control plane
design should therefore be as open as possible, allowing
parameters to be included as necessary.
- In the absence of wavelength conversion, the necessity of - Since the route selected must have the chosen wavelength available
finding a single wavelength that is available on all links on all links, this information needs to be considered in the
introduces the need to either advertise detailed information on routing process. One approach is to propagate information
wavelength availability, which probably doesn't scale, or have throughout the network about the state of every wavelength on
some mechanism for probing potential routes with or without every link in the network. However, the state required and the
crankback to determine wavelength availability. Choosing the overhead involved in processing and maintaining this information
route first, and then the wavelength, may not yield acceptable is proportional to the total number of links (thus, number of
utilization levels in mesh-type networks. nodes squared), maximum number of wavelengths (which keeps
doubling every couple of years), and the frequency of wavelength
availability changes, which can be very high. Instead
[Hjalmtysson00], proposes an alternative method which probes along
a chosen path to determine which wavelengths (if any) are
available. This would require a significant addition to the
routing logic normally used in OSPF. Others have proposed
simultaneously probing along multiple paths.
5. More Complex Networks - Choosing a path first and then a wavelength along the path is
known to give adequate results in simple topologies such as rings
and trees ([Yates99]). This does not appear to be true in large
mesh networks under realistic provisioning scenarios, however.
Instead significantly better results are achieved if wavelength
and route are chosen simultaneously ([Strand01b]). This approach
would however also have a significant effect on OSPF.
Mixing optical equipment in a single domain of transparency that has 4.8. Implications For Routing and Control Plane Design
not been explicitly designed to interwork is beyond the scope of
this document. This includes most multi-vendor all-optical networks.
An optical network composed of multiple domains of transparency If distributed routing is desired, additional state information will
optically isolated from each other by O/E/O devices (transponders) be required by the routing to deal with the impairments described in
is more plausible. A network composed of both "opaque" (optically Sections 4.2 - 4.4:
isolated) OLXC's and one or more all-optical "islands" isolated by
transponders is of particular interest because this is most likely
how all-optical technologies (such as that described in Sec. 2) are
going to be introduced. (We use the term "island" in this discussion
rather than a term like "domain" or "area" because these terms are
associated with specific approaches like BGP or OSPF.)
On Optical Layer Routing
We consider the complexities raised by these alternatives now. - As mentioned earlier, an operator who wants to avoid having to
provide impairment-related parameters to the control plane may
elect not to deal with them at the routing level, instead treating
them at the system design and planning level if that is a viable
approach for their network. In this approach the operator can
pre-qualify all or a set of feasible end-to-end optical paths
through the domain of transparency for each bit rate. This
approach may work well with relatively small and sparse networks,
but it may not be scalable for large and dense networks where the
number of feasible paths can be very large.
The first requirement for routing in a multi-island network is that - If the optical paths are not pre-qualified, additional link-
the routing process needs to know the extent of each island. There specific state information will be required by the routing
are several reasons for this: algorithm for each type of impairment that has the potential of
- When entering or leaving an all-optical island, the regeneration being limiting for some routes. Note that for one operator, PMD
process cleans up the optical impairments discussed in Sec. 3. might be the only limiting constraint while for another, ASE might
- Each all-optical island may have its own bounds on each be the only one, or it could be both plus some other constraints
impairment. considered in this document. Some networks might not be limited
- The routing process needs to be sensitive to the costs by any of these constraints.
associated with "island-hopping".
This last point needs elaboration. It is extremely important to - For an operator needing to deal explicitly with these constraints,
realize that, at least in the short to intermediate term, the the link-dependent information identified above for PMD is link-
resources committed by a single routing decision can be very PMD-square which is the square of the total PMD on a link. For
significant: The equipment tied up by a single coast-to-coast OC-192 ASE the link-dependent information identified is link-noise which
can easily have a first cost of $10**6, and the holding times on a is the total noise on a link. Other link-dependent information
circuit once established is likely to be measured in months. includes link-span-length which is the total number of spans on a
Carriers will expect the routing algorithms used to be sensitive to link, link-crosstalk or OADM-OXC-number which is the total
these costs. Simplistic measures of cost such as the number of crosstalk or the number of OADM/OXC nodes on a link, respectively,
"hops" are not likely to be acceptable. and filter-number which is the number of narrow filters on a link.
When the alternative distance-only approach is chosen, the link-
specific information is link-distance.
Taking the case of an all-optical island consisting of an "ultra - In addition to the link-specific information, bounds on each of
long-haul" system like that in Fig. 3-1 embedded in an OEO network the impairments need to be quantified. Since these bounds are
of electrical fabric OLXC's as an example: It is likely that the ULH determined by the system designer's impairment allocations, these
system will be relatively expensive for short hops but relatively will be system dependent. For PMD, the constraint is that the sum
economical for longer distances. It is therefore likely to be of the link-PMD-square of all links on the transparent segment is
deployed as a sort of "express backbone". In this scenario a carrier less than the square of (a/B) where B is the bit rate. Hence, the
is likely to expect the routing algorithm to balance OEO costs required information is the parameter "a". For ASE, the
against the additional costs associated with ULH technology and constraint is that the sum of the link-noise of all links is no
route circuitously to make maximum use of the backbone where larger than P/SNRmin. Thus, the information needed include the
appropriate. Note that the metrics used to do this must be launch power P and OSNR requirement SNRmin. The minimum
consistent throughout the routing domain if this expectation is to acceptable OSNR, in turn, depends on the strength of the FEC being
be met. used and the margins reserved for other types of impairments.
Other bounds include the maximum span length of the transmission
system, the maximum path crosstalk or the maximum number of
OADM/OXC nodes, and the maximum number of narrow filters, all are
bit rate dependent. With the alternative distance-only approach,
the upper bound is the maximum-path-distance. In single-vendor
"islands" some of these parameters may be available in a local or
EMS database and would not need to be advertised
The first-order implications for GMPLS seem to be: - It is likely that the physical layer parameters do not change
- Information about island boundaries needs to be advertised. value rapidly and could be stored in some database; however these
- The routing algorithm needs to be sensitive to island are physical layer parameters that today are frequently not known
transitions and to the connectivity limitations and impairment at the granularity required. If the ingress node of a lightpath
constraints particular to each island. does path selection these parameters would need to be available at
- The cost function used in routing must allow the balancing of this node.
transponder costs, OXC and OADM costs, and line haul costs
across the entire routing domain.
Several distributed approaches to multi-island routing seem worth - The specific constraints required in a given situation will depend
investigating: on the design and engineering of the domain of transparency; for
- Advertise the internal topology and constraints of each island example it will be essential to know whether chromatic dispersion
globally; let the ingress node compute an end-to-end strict has been dealt with on a per-link basis, and whether the domain is
explicit route sensitive to all constraints and wavelength operating in a linear or nonlinear regime.
availabilities. In this approach the routing algorithm used by
On Optical Layer Routing
the ingress node must be able to deal with the details of - As optical transport technology evolves, the set of constraints
routing within each island. that will need to be considered either explicitly or via a
- Have the EMS or control plane of each island determine and domain-wide margin may change. The routing and control plane
advertise the connectivity between its boundary nodes together design should therefore be as open as possible, allowing
with additional information such as costs and the bit rates and parameters to be included as necessary.
formats supported. As the spare capacity situation changes,
updates would be advertised. In this approach impairment
constraints are handled within each island and impairment-
related parameters need not be advertised outside of the island.
The ingress node would then do a loose explicit route and leave
the routing and wavelength selection within each island to the
island.
- Have the ingress node send out probes or queries to nearby
gateway nodes or to an NMS to get routing guidance.
6. Diversity - In the absence of wavelength conversion, the necessity of finding
a single wavelength that is available on all links introduces the
need to either advertise detailed information on wavelength
availability, which probably doesn't scale, or have some mechanism
for probing potential routes with or without crankback to
determine wavelength availability. Choosing the route first, and
then the wavelength, may not yield acceptable utilization levels
in mesh-type networks.
6.1 Background On Diversity 5. More Complex Networks
"Diversity" is a relationship between lightpaths. Two lightpaths are Mixing optical equipment in a single domain of transparency that has
said to be diverse if they have no single point of failure. In not been explicitly designed to interwork is beyond the scope of this
traditional telephony the dominant transport failure mode is a document. This includes most multi-vendor all-optical networks.
failure in the interoffice plant, such as a fiber cut inflicted by a
backhoe.
Why is diversity a unique problem that needs to be considered for An optical network composed of multiple domains of transparency
optical networks? So far, data network operators have relied on optically isolated from each other by O/E/O devices (transponders) is
their private line providers to ensure diversity and so have not had more plausible. A network composed of both "opaque" (optically
to deal directly with the problem. GMPLS makes the complexities isolated) OLXCs and one or more all-optical "islands" isolated by
handled by the private line provisioning process, including transponders is of particular interest because this is most likely
diversity, part of the common control plane and so visible to all. how all-optical technologies (such as that described in Sec. 2) are
going to be introduced. (We use the term "island" in this discussion
rather than a term like "domain" or "area" because these terms are
associated with specific approaches like BGP or OSPF.)
To determine whether two lightpath routings are diverse it is We consider the complexities raised by these alternatives now.
necessary to identify single points of failure in the interoffice
plant. To do so we will use the following terms: A fiber cable is a
uniform group of fibers contained in a sheath. An Optical Transport
System will occupy fibers in a sequence of fiber cables. Each fiber
cable will be placed in a sequence of conduits - buried honeycomb
structures through which fiber cables may be pulled - or buried in a
right of way (ROW). A ROW is land in which the network operator has
the right to install his conduit or fiber cable. It is worth noting
that for economic reasons, ROW's are frequently obtained from
railroads, pipeline companies, or thruways. It is frequently the
case that several carriers may lease ROW from the same source; this
makes it common to have a number of carriers' fiber cables in close
proximity to each other. Similarly, in a metropolitan network,
On Optical Layer Routing
several carriers might be leasing duct space in the same RBOC The first requirement for routing in a multi-island network is that
conduit. There are also "carrier's carriers" - optical networks the routing process needs to know the extent of each island. There
which provide fibers to multiple carriers, all of whom could be are several reasons for this:
affected by a single failure in the "carrier's carrier" network.
In a typical intercity facility network there might be on the order - When entering or leaving an all-optical island, the regeneration
of 100 offices that are candidates for OLXC's. To represent the process cleans up the optical impairments discussed in Sec. 3.
inter-office fiber network accurately a network with an order of
magnitude more nodes is required. In addition to Optical Amplifier
(OA) sites, these additional nodes include:
- Places where fiber cables enter/leave a conduit or right of way;
- Locations where fiber cables cross;
Locations where fiber splices are used to interchange fibers between
fiber cables.
An example of the first might be:
A B
A-------------B \ /
\ /
X-----Y
/ \
C-------------D / \
C D
(a) Fiber Cable Topology (b) Right-Of-Way/Conduit Topology - Each all-optical island may have its own bounds on each
impairment.
Figure 6-1: Fiber Cable vs. ROW Topologies - The routing process needs to be sensitive to the costs associated
with "island-hopping".
Here the A-B fiber cable would be physically routed A-X-Y-B and the This last point needs elaboration. It is extremely important to
C-D cable would be physically routed C-X-Y-D. This topology might realize that, at least in the short to intermediate term, the
arise because of some physical bottleneck: X-Y might be the Lincoln resources committed by a single routing decision can be very
Tunnel, for example, or the Bay Bridge. significant: The equipment tied up by a single coast-to-coast OC-192
can easily have a first cost of $10**6, and the holding times on a
circuit once established is likely to be measured in months.
Carriers will expect the routing algorithms used to be sensitive to
these costs. Simplistic measures of cost such as the number of
"hops" are not likely to be acceptable.
Fiber route crossing (the second case) is really a special case of Taking the case of an all-optical island consisting of an "ultra
this, where X and Y coincide. In this case the crossing point may long-haul" system like that in Fig. 3-1 embedded in an OEO network of
not even be a manhole; the fiber routes might just be buried at electrical fabric OLXCs as an example: It is likely that the ULH
different depths. system will be relatively expensive for short hops but relatively
economical for longer distances. It is therefore likely to be
deployed as a sort of "express backbone". In this scenario a carrier
is likely to expect the routing algorithm to balance OEO costs
against the additional costs associated with ULH technology and route
circuitously to make maximum use of the backbone where appropriate.
Note that the metrics used to do this must be consistent throughout
the routing domain if this expectation is to be met.
Fiber splicing (the third case) often occurs when a major fiber The first-order implications for GMPLS seem to be:
route passes near to a small office. To avoid the expense and
additional transmission loss only a small number of fibers are
spliced out of the major route into a smaller route going to the
small office. This might well occur in a manhole or hut. An
example is shown in Fig. 6-2(a), where A-X-B is the major route, X
the manhole, and C the smaller office. The actual fiber topology
On Optical Layer Routing
would then look like Fig. 6-2(b), where there would typically be - Information about island boundaries needs to be advertised.
many more A-B fibers than A-C or C-B fibers, and where A-C and C-B
might have different numbers of fibers. (One of the latter might
even be missing.)
C C - The routing algorithm needs to be sensitive to island transitions
| / \ and to the connectivity limitations and impairment constraints
| / \ particular to each island.
| / \
A------X------B A---------------B
(a) Fiber Cable Topology (b) Fiber Topology - The cost function used in routing must allow the balancing of
transponder costs, OXC and OADM costs, and line haul costs across
the entire routing domain.
Figure 6-2. Fiber Cable vs Fiber Topologies Several distributed approaches to multi-island routing seem worth
investigating:
The imminent deployment of ultra-long (>1000 km) Optical Transport - Advertise the internal topology and constraints of each island
Systems introduces a further complexity: Two OTS's could interact a globally; let the ingress node compute an end-to-end strict
number of times. To make up a hypothetical example: A New York - explicit route sensitive to all constraints and wavelength
Atlanta OTS and a Philadelphia - Orlando OTS might ride on the same availabilities. In this approach the routing algorithm used by
right of way for x miles in Maryland and then again for y miles in the ingress node must be able to deal with the details of routing
Georgia. They might also cross at Raleigh or some other intermediate within each island.
node without sharing right of way.
Diversity is often equated to routing two lightpaths between a - Have the EMS or control plane of each island determine and
single pair of points, or different pairs of points so that no advertise the connectivity between its boundary nodes together
single route failure will disrupt them both. This is too simplistic, with additional information such as costs and the bit rates and
for a number of reasons: formats supported. As the spare capacity situation changes,
updates would be advertised. In this approach impairment
constraints are handled within each island and impairment-related
parameters need not be advertised outside of the island. The
ingress node would then do a loose explicit route and leave the
routing and wavelength selection within each island to the island.
- A sophisticated client of an optical network will want to derive - Have the ingress node send out probes or queries to nearby gateway
diversity needs from his/her end customers' availability nodes or to an NMS to get routing guidance.
requirements. These often lead to more complex diversity
requirements than simply providing diversity between two
lightpaths. For example, a common requirement is that no single
failure should isolate a node or nodes. If a node A has single
lightpaths to nodes B and C, this requires A-B and A-C to be
diverse. In real applications, a large data network with N
lightpaths between its routers might describe their needs in an
NxN matrix, where (i,j) defines whether lightpaths i and j must
be diverse.
- Two circuits that might be considered diverse for one 6. Diversity
application might not be considered diverse for in another
situation. Diversity is usually thought of as a reaction to
interoffice route failures. High reliability applications may
On Optical Layer Routing
require other types of failures to be taken into account. Some 6.1. Background on Diversity
examples:
o Office Outages: Although less frequent than route failures,
fires, power outages, and floods do occur. Many network
managers require that diverse routes have no (intermediate)
nodes in common. In other cases an intermediate node might
be acceptable as long as there is power diversity within
the office.
o Shared Rings: Many applications are willing to allow
"diverse" circuits to share a SONET ring-protected link;
presumably they would allow the same for optical layer
rings.
o Disasters: Earthquakes and floods can cause failures over
an extended area. Defense Department circuits might need
to be routed with nuclear damage radii taken into account.
- Conversely, some networks may be willing to take somewhat larger
risks. Taking route failures as an example: Such a network
might be willing to consider two fiber cables in heavy duty
concrete conduit as having a low enough chance of simultaneous
failure to be considered "diverse". They might also be willing
to view two fiber cables buried on opposite sides of a railroad
track as being diverse because there is minimal danger of a
single backhoe disrupting them both even though a bad train
wreck might jeopardize them both. A network seeking N mutually
diverse paths from an office with less than N diverse ROW's will
need to live with some level of compromise in the immediate
vicinity of the office.
These considerations strongly suggest that the routing algorithm "Diversity" is a relationship between lightpaths. Two lightpaths are
should be sensitive to the types of threat considered unacceptable said to be diverse if they have no single point of failure. In
by the requester. Note that the impairment constraints described in traditional telephony the dominant transport failure mode is a
the previous section may eliminate some of the long circuitous failure in the interoffice plant, such as a fiber cut inflicted by a
routes sometimes needed to provide diversity. This would make it backhoe.
harder to find many diverse paths through an all-optical network
than an opaque one.
[Hjlmtsson00] introduced the term "Shared Risk Link Group" (SRLG) Why is diversity a unique problem that needs to be considered for
to describe the relationship between two non-diverse links. The optical networks? Traditionally, data network operators have relied
above examples and discussion given at the start of this section on their private line providers to ensure diversity and so have not
suggests that an SRLG should be characterized by 2 parameters: had to deal directly with the problem. GMPLS makes the complexities
- Type of Compromise: Examples would be shared fiber cable, shared handled by the private line provisioning process, including
conduit, shared ROW, shared optical ring, shared office without diversity, part of the common control plane and so visible to all.
power sharing, etc.)
On Optical Layer Routing
- Extent of Compromise: For compromised outside plant, this would To determine whether two lightpath routings are diverse it is
be the length of the sharing. necessary to identify single points of failure in the interoffice
A CSPF algorithm could then penalize a diversity compromise by an plant. To do so we will use the following terms: A fiber cable is a
amount dependent on these two parameters. uniform group of fibers contained in a sheath. An Optical Transport
System will occupy fibers in a sequence of fiber cables. Each fiber
cable will be placed in a sequence of conduits - buried honeycomb
structures through which fiber cables may be pulled - or buried in a
right of way (ROW). A ROW is land in which the network operator has
the right to install his conduit or fiber cable. It is worth noting
that for economic reasons, ROWs are frequently obtained from
railroads, pipeline companies, or thruways. It is frequently the
case that several carriers may lease ROW from the same source; this
makes it common to have a number of carriers' fiber cables in close
proximity to each other. Similarly, in a metropolitan network,
several carriers might be leasing duct space in the same RBOC
conduit. There are also "carrier's carriers" - optical networks
which provide fibers to multiple carriers, all of whom could be
affected by a single failure in the "carrier's carrier" network. In
a typical intercity facility network there might be on the order of
100 offices that are candidates for OLXCs. To represent the inter-
office fiber network accurately a network with an order of magnitude
more nodes is required. In addition to Optical Amplifier (OA) sites,
these additional nodes include:
Two links could be related by many SRLG's (AT&T's experience - Places where fiber cables enter/leave a conduit or right of way;
indicates that a link may belong to over 100 SRLG's, each
corresponding to a separate fiber group. Each SRLG might relate a
single link to many other links. For the optical layer, similar
situations can be expected where a link is an ultra-long OTS).
The mapping between links and different types of SRLG's is in - Locations where fiber cables cross; Locations where fiber splices
general defined by network operators based on the definition of each are used to interchange fibers between fiber cables.
SRLG type. Since SRLG information is not yet ready to be
discoverable by a network element and does not change dynamically,
it need not be advertised with other resource availability
information by network elements. It could be configured in some
central database and be distributed to or retrieved by the nodes, or
advertised by network elements at the topology discovery stage.
6.2 Implications For Routing An example of the first might be:
Dealing with diversity is an unavoidable requirement for routing in A B
the optical layer. It requires dealing with constraints in the A-------------B \ /
routing process but most importantly requires additional state \ /
information the SRLG relationships and also the routings of any X-----Y
existing circuits from the new circuit is to be diverse to be / \
available to the routing process. C-------------D / \
C D
At present SRLG information cannot be self-discovered. Indeed, in a (a) Fiber Cable Topology (b) Right-Of-Way/Conduit Topology
large network it is very difficult to maintain accurate SRLG
information. The problem becomes particularly daunting whenever
multiple administrative domains are involved, for instance after the
acquisition of one network by another, because there normally is a
likelihood that there are diversity violations between the domains.
It is very unlikely that diversity relationships between carriers
will be known any time in the near future.
Considerable variation in what different customers will mean by Figure 6-1: Fiber Cable vs. ROW Topologies
acceptable diversity should be anticipated. Consequently we suggest
that an SRLG should be defined as follows: (i) It is a relationship
between two or more links, and (ii) it is characterized by two
parameters, the type of compromise (shared conduit, shared ROW,
shared optical ring, etc.) and the extent of the compromise (e.g.,
the number of miles over which the compromise persisted). This will
allow the SRLG's appropriate to a particular routing request to be
easily identified.
On Optical Layer Routing Here the A-B fiber cable would be physically routed A-X-Y-B and the
C-D cable would be physically routed C-X-Y-D. This topology might
arise because of some physical bottleneck: X-Y might be the Lincoln
Tunnel, for example, or the Bay Bridge.
7. Security Considerations Fiber route crossing (the second case) is really a special case of
this, where X and Y coincide. In this case the crossing point may
not even be a manhole; the fiber routes might just be buried at
different depths.
We are assuming OEO interfaces to the domain(s) covered by our Fiber splicing (the third case) often occurs when a major fiber route
discussion (see, e.g., Sec. 4.1 above). If this assumption were to passes near to a small office. To avoid the expense and additional
be relaxed and externally generated optical signals allowed into the transmission loss only a small number of fibers are spliced out of
domain, network security issues would arise. Specifically, the major route into a smaller route going to the small office. This
unauthorized usage in the form of signals at improper wavelengths or might well occur in a manhole or hut. An example is shown in Fig.
with power levels or impairments inconsistent with those assumed by 6-2(a), where A-X-B is the major route, X the manhole, and C the
the domain would be possible. With OEO interfaces, these types of smaller office. The actual fiber topology would then look like Fig.
layer one threats should be controllable. 6-2(b), where there would typically be many more A-B fibers than A-C
or C-B fibers, and where A-C and C-B might have different numbers of
fibers. (One of the latter might even be missing.)
C C
| / \
| / \
| / \
A------X------B A---------------B
A key layer one security issue is resilience in the face of physical (a) Fiber Cable Topology (b) Fiber Topology
attack. Diversity, as describe in Sec. 6, is a part of the
solution. However, it is ineffective if there is not sufficient
spare capacity available to make the network whole after an attack.
Several major related issues are:
- Defining the threat: If, for example, an electro-magnetic
interference (EMI) burst is an in-scope threat, then (in the
terminology of Sec. 6) all of the links sufficiently close
together to be disrupted by such a burst must be included in a
single SRLG. Similarly for other threats: For each in-scope
threat, SRLG's must be defined so that all links vulnerable to a
single incident of the threat must be grouped together in a
single SRLG.
- Allocating responsibility for responding to a layer one failure
between the various layers (especially the optical and IP
layers): This must be clearly specified to avoid churning and
unnecessary service interruptions.
The whole proposed process depends on the integrity of the Figure 6-2. Fiber Cable vs Fiber Topologies
impairment characterization information (PMD parameters, etc.) and
also the SRLG definitions. Security of this information, both when
stored and when distributed, is essential.
This document does not address control plane issues, and so control- The imminent deployment of ultra-long (>1000 km) Optical Transport
plane security is out of scope. IPO control plane security Systems introduces a further complexity: Two OTSes could interact a
considerations are discussed in [Rajagopalam02]. Security number of times. To make up a hypothetical example: A New York -
considerations for GMPLS, a likely control plane candidate, are Atlanta OTS and a Philadelphia - Orlando OTS might ride on the same
discussed in [Mannie02]. right of way for x miles in Maryland and then again for y miles in
Georgia. They might also cross at Raleigh or some other intermediate
node without sharing right of way.
8. Acknowledgments Diversity is often equated to routing two lightpaths between a single
pair of points, or different pairs of points so that no single route
failure will disrupt them both. This is too simplistic, for a number
of reasons:
This document has benefited from discussions with Michael Eiselt, - A sophisticated client of an optical network will want to derive
Jonathan Lang, Mark Shtaif, Jennifer Yates, Dongmei Wang, Guangzhi diversity needs from his/her end customers' availability
Li, Robert Doverspike, Albert Greenberg, Jim Maloney, John Jacob, requirements. These often lead to more complex diversity
Katie Hall, Diego Caviglia, D. Papadimitriou, O. Audouin, J. P. requirements than simply providing diversity between two
Faure, L. Noirie, and with our OIF colleagues. lightpaths. For example, a common requirement is that no single
failure should isolate a node or nodes. If a node A has single
lightpaths to nodes B and C, this requires A-B and A-C to be
diverse. In real applications, a large data network with N
lightpaths between its routers might describe their needs in an
NxN matrix, where (i,j) defines whether lightpaths i and j must be
diverse.
On Optical Layer Routing - Two circuits that might be considered diverse for one application
might not be considered diverse for in another situation.
Diversity is usually thought of as a reaction to interoffice route
failures. High reliability applications may require other types
of failures to be taken into account. Some examples:
9. References o Office Outages: Although less frequent than route failures,
fires, power outages, and floods do occur. Many network
managers require that diverse routes have no (intermediate)
nodes in common. In other cases an intermediate node might be
acceptable as long as there is power diversity within the
office.
9.1 Normative References o Shared Rings: Many applications are willing to allow "diverse"
circuits to share a SONET ring-protected link; presumably they
would allow the same for optical layer rings.
[Goldstein94] Goldstein, E. L., Eskildsen, L., and Elrefaie, A. F., o Disasters: Earthquakes and floods can cause failures over an
Performance Implications of Component Crosstalk in Transparent extended area. Defense Department circuits might need to be
Lightwave Networks", IEEE Photonics Technology Letters, Vol.6, No.5, routed with nuclear damage radii taken into account.
May 1994.
[Hjlmtsson00] Gsli Hjlmtsson, Jennifer Yates, Sid Chaudhuri and - Conversely, some networks may be willing to take somewhat larger
Albert risks. Taking route failures as an example: Such a network might
Greenberg, "Smart Routers - Simple Optics: An Architecture for the be willing to consider two fiber cables in heavy duty concrete
Optical Internet, IEEE/OSA Journal of Lightwave Technology, December conduit as having a low enough chance of simultaneous failure to
2000,, Vo 18, Issue 12 , Dec. 2000 , pp. 1880 -1891. be considered "diverse". They might also be willing to view two
fiber cables buried on opposite sides of a railroad track as being
diverse because there is minimal danger of a single backhoe
disrupting them both even though a bad train wreck might
jeopardize them both. A network seeking N mutually diverse paths
from an office with less than N diverse ROWs will need to live
with some level of compromise in the immediate vicinity of the
office.
[ITU] ITU-T Doc. G.663, Optical Fibers and Amplifiers, Section These considerations strongly suggest that the routing algorithm
II.4.1.2. should be sensitive to the types of threat considered unacceptable by
the requester. Note that the impairment constraints described in the
previous section may eliminate some of the long circuitous routes
sometimes needed to provide diversity. This would make it harder to
find many diverse paths through an all-optical network than an opaque
one.
[Kaminow97] Kaminow, I. P. and Koch, T. L., editors, Optical Fiber [Hjalmtysson00] introduced the term "Shared Risk Link Group" (SRLG)
Telecommunications IIIA, Academic Press, 1997. to describe the relationship between two non-diverse links. The
above examples and discussion given at the start of this section
suggests that an SRLG should be characterized by 2 parameters:
[Mannie02] Mannie, E. (ed.), "Generalized Multi-Protocol Label - Type of Compromise: Examples would be shared fiber cable, shared
Switching (GMPLS) Architecture", Interned Draft, draft-ietf-ccamp- conduit, shared ROW, shared optical ring, shared office without
gmpls-architecture-03.txt, August, 2002. power sharing, etc.)
[Rajagopalam02] Rajagopalam, B., et. al., "IP over Optical Networks: - Extent of Compromise: For compromised outside plant, this would
A Framework", Internet Draft, draft-ietf-ipo-framework-02.txt June, be the length of the sharing.
2002.
[Strand01] J. Strand, A. Chiu, and R. Tkach, "Issues for Routing in A CSPF algorithm could then penalize a diversity compromise by an
the Optical Layer", IEEE Communications Magazine, Feb. 2001, vol. 39 amount dependent on these two parameters.
No. 2, pp. 81-88.
[Strand01b] J. Strand, R. Doverspike, and G. Li, "Importance of Two links could be related by many SRLGs. (AT&T's experience
Wavelength Conversion In An Optical Network", Optical Networks indicates that a link may belong to over 100 SRLGs, each
Magazine, May/June 2001, pp. 33-44. corresponding to a separate fiber group.) Each SRLG might relate a
single link to many other links. For the optical layer, similar
situations can be expected where a link is an ultra-long OTS.
[Yates99] Yates, J. M., Rumsewicz, M. P. and Lacey, J. P. R., The mapping between links and different types of SRLGs is in general
"Wavelength Converters in Dynamically-Reconfigurable WDM Networks", defined by network operators based on the definition of each SRLG
IEEE Communications Surveys, 2Q1999 (online at type. Since SRLG information is not yet ready to be discoverable by
www.comsoc.org/pubs/surveys/2q99issue/yates.html). a network element and does not change dynamically, it need not be
advertised with other resource availability information by network
elements. It could be configured in some central database and be
distributed to or retrieved by the nodes, or advertised by network
elements at the topology discovery stage.
On Optical Layer Routing 6.2. Implications For Routing
9.2 Informative References Dealing with diversity is an unavoidable requirement for routing in
the optical layer. It requires dealing with constraints in the
routing process, but most importantly requires additional state
information (e.g., the SRLG relationships). The routings of any
existing circuits from which the new circuit must be diverse must
also be available to the routing process.
[Awduche99] Awduche, D. O., Rekhter, Y., Drake, J., and Coltun, R., At present SRLG information cannot be self-discovered. Indeed, in a
"Multi-Protocol Lambda Switching: Combining MPLS Traffic Engineering large network it is very difficult to maintain accurate SRLG
Control With Optical Crossconnects", Work in Progress, draft- information. The problem becomes particularly daunting whenever
awduche-mpls-te-optical-01.txt. multiple administrative domains are involved, for instance after the
acquisition of one network by another, because there normally is a
likelihood that there are diversity violations between the domains.
It is very unlikely that diversity relationships between carriers
will be known any time in the near future.
[Bra96] Bradner, S., "The Internet Standards Process -- Revision 3," Considerable variation in what different customers will mean by
BCP 9, RFC 2026, October 1996. acceptable diversity should be anticipated. Consequently we suggest
that an SRLG should be defined as follows: (i) It is a relationship
between two or more links, and (ii) it is characterized by two
parameters, the type of compromise (shared conduit, shared ROW,
shared optical ring, etc.) and the extent of the compromise (e.g.,
the number of miles over which the compromise persisted). This will
allow the SRLGs appropriate to a particular routing request to be
easily identified.
[CBD00] Ceuppens, L., Blumenthal, D., Drake, J., Chrostowski, J., 7. Security Considerations
Edwards, W., "Performance Monitoring in Photonic Networks in Support
of MPL(ambda)S", Internet draft, work in progress, March 2000.
[Doverspike00] Doverspike, R. and Yates, J., "Challenges For MPLS in We are assuming OEO interfaces to the domain(s) covered by our
Optical Network Restoration", IEEE Communication Magazine, February, discussion (see, e.g., Sec. 4.1 above). If this assumption were to
2001. be relaxed and externally generated optical signals allowed into the
domain, network security issues would arise. Specifically,
unauthorized usage in the form of signals at improper wavelengths or
with power levels or impairments inconsistent with those assumed by
the domain would be possible. With OEO interfaces, these types of
layer one threats should be controllable.
[Gerstel 2000] O. Gorstel, "Optical Layer Signaling: How Much Is A key layer one security issue is resilience in the face of physical
Really Needed?" IEEE Communications Magazine, vol. 38 no. 10, Oct. attack. Diversity, as describe in Sec. 6, is a part of the solution.
2000, pp. 154-160 However, it is ineffective if there is not sufficient spare capacity
available to make the network whole after an attack. Several major
related issues are:
[KRB01a] Kompella, K., et.al., "IS-IS extensions in support of - Defining the threat: If, for example, an electro-magnetic
Generalized MPLS," Internet Draft, draft-ietf-gmpls- extensions- interference (EMI) burst is an in-scope threat, then (in the
01.txt, work in progress, 2001. terminology of Sec. 6) all of the links sufficiently close
together to be disrupted by such a burst must be included in a
single SRLG. Similarly for other threats: For each in-scope
threat, SRLGs must be defined so that all links vulnerable to a
single incident of the threat must be grouped together in a single
SRLG.
[KRB01b] Kompella, K., et. al., "OSPF extensions in support of - Allocating responsibility for responding to a layer one failure
Generalized MPLS," Internet draft, draft-ospf-generalized- mpls- between the various layers (especially the optical and IP layers):
00.txt, work in progress, March 2001. This must be clearly specified to avoid churning and unnecessary
service interruptions.
[Moy98] Moy, John T., OSPF: Anatomy of an Internet Routing Protocol, The whole proposed process depends on the integrity of the impairment
Addison-Wesley, 1998. characterization information (PMD parameters, etc.) and also the SRLG
definitions. Security of this information, both when stored and when
distributed, is essential.
[Passmore01] Passmore, D., "Managing Fatter Pipes," Business This document does not address control plane issues, and so control-
Communications Review, August 2001, pp. 20-21. plane security is out of scope. IPO control plane security
considerations are discussed in [Rajagopalam04]. Security
considerations for GMPLS, a likely control plane candidate, are
discussed in [Mannie04].
[Ramaswami98] Ramaswami, R. and Sivarajan, K. N., Optical Networks: 8. Acknowledgments
A Practical Perspective, Morgan Kaufmann Publishers, 1998.
[Tkach98] Tkach, R., Goldstein, E., Nagel, J., and Strand, J., This document has benefited from discussions with Michael Eiselt,
"Fundamental Limits of Optical Transparency", Optical Fiber Jonathan Lang, Mark Shtaif, Jennifer Yates, Dongmei Wang, Guangzhi
Communication Conf., Feb. 1998, pp. 161-162. Li, Robert Doverspike, Albert Greenberg, Jim Maloney, John Jacob,
Katie Hall, Diego Caviglia, D. Papadimitriou, O. Audouin, J. P.
Faure, L. Noirie, and with our OIF colleagues.
On Optical Layer Routing 9. References
10. Contributing Authors 9.1. Normative References
This document was a collective work of a number of people. The text [Goldstein94] Goldstein, E. L., Eskildsen, L., and Elrefaie, A. F.,
and content of this document was contributed by the editors and the Performance Implications of Component Crosstalk in
co-authors listed below. Transparent Lightwave Networks", IEEE Photonics
Technology Letters, Vol.6, No.5, May 1994.
Ayan Banerjee [Hjalmtysson00] Gsli Hjalmtysson, Jennifer Yates, Sid Chaudhuri and
Calient Networks Albert Greenberg, "Smart Routers - Simple Optics: An
5853 Rue Ferrari Architecture for the Optical Internet, IEEE/OSA
San Jose, CA 95138 Journal of Lightwave Technology, December 2000, Vo
Email: abanerjee@calient.net 18, Issue 12, Dec. 2000, pp. 1880-1891.
Dan Blumenthal [ITU] ITU-T Doc. G.663, Optical Fibers and Amplifiers,
Calient Networks Section II.4.1.2.
5853 Rue Ferrari
San Jose, CA 95138
Email: dblumenthal@calient.net
John Drake [Kaminow97] Kaminow, I. P. and Koch, T. L., editors, Optical
Calient Networks Fiber Telecommunications IIIA, Academic Press, 1997.
5853 Rue Ferrari
San Jose, CA 95138
Email: jdrake@calient.net
Andre Fredette [Mannie04] Mannie, E., Ed., "Generalized Multi-Protocol Label
Hatteras Networks Switching (GMPLS) Architecture", RFC 3945, October
PO Box 110025 2004.
Research Triangle Park, NC 27709
Email: afredette@hatterasnetworks.com
Nan Froberg [Rajagopalam04] Rajagopalan, B., Luciani, J., and D. Awduche, "IP
PhotonEx Corporation over Optical Networks: A Framework", RFC 3717, March
200 Metrowest Technology Dr. 2004.
Maynard, MA 01754
Email: nfroberg@photonex.com
Taha Landolsi [Strand01] Strand, J., Chiu, A., and R. Tkach, "Issues for
WorldCom, Inc. Routing in the Optical Layer", IEEE Communications
2400 North Glenville Drive Magazine, Feb. 2001, vol. 39 No. 2, pp. 81-88.
Richardson, TX 75082
Email: taha.landolsi@wcom.com
James V. Luciani [Strand01b] Strand, J., Doverspike, R., and G. Li, "Importance of
900 Chelmsford St. Wavelength Conversion In An Optical Network", Optical
Lowell, MA 01851 Networks Magazine, May/June 2001, pp. 33-44.
Email: james_luciani@mindspring.com
Robert Tkach [Yates99] Yates, J. M., Rumsewicz, M. P., and J. P. R. Lacey,
On Optical Layer Routing "Wavelength Converters in Dynamically-Reconfigurable
WDM Networks", IEEE Communications Surveys, 2Q1999
(online at
www.comsoc.org/pubs/surveys/2q99issue/yates.html).
Celion Networks 9.2. Informative References
1 Sheila Dr., Suite 2
Tinton Falls, NJ 07724
Email: bob.tkach@celion.com
Yong Xue [Awduche99] Awduche, D. O., Rekhter, Y., Drake, J., R. and
WorldCom, Inc. Coltun, "Multi-Protocol Lambda Switching: Combining
22001 Loudoun County Parkway MPLS Traffic Engineering Control With Optical
Ashburn, VA 20147 Crossconnects", Work in Progress.
Email: yxue@cox.com
11. Editors' Addresses [Gerstel2000] Gorstel, O., "Optical Layer Signaling: How Much Is
Really Needed?" IEEE Communications Magazine, vol. 38
no. 10, Oct. 2000, pp. 154-160
Angela Chiu [Kaminow02] Ivan P. Kaminow and Tingye Li (editors), "Optical
AT&T Labs Fiber Communications IV: Systems and Impairments",
200 Laurel Ave., Rm A5-1F13 Elsevier Press, 2002.
Middletown, NJ 07748
Phone:(732) 420-9061
Email: chiu@research.att.com
John Strand [Passmore01] Passmore, D., "Managing Fatter Pipes," Business
AT&T Labs Communications Review, August 2001, pp. 20-21.
200 Laurel Ave., Rm A5-1D33
Middletown, NJ 07748 [Ramaswami98] Ramaswami, R. and K. N. Sivarajan, Optical Networks:
Phone:(732) 420-9036 A Practical Perspective, Morgan Kaufmann Publishers,
Email: jls@research.att.com 1998.
[Strand02] John Strand, "Optical Network Architecture
Evolution", in [Kaminow02].
[Tkach98] Tkach, R., Goldstein, E., Nagel, J., and J. Strand,
"Fundamental Limits of Optical Transparency", Optical
Fiber Communication Conf., Feb. 1998, pp. 161-162.
10. Contributing Authors
This document was a collective work of a number of people. The text
and content of this document was contributed by the editors and the
co-authors listed below.
Ayan Banerjee
Calient Networks
6620 Via Del Oro
San Jose, CA 95119
EMail: abanerjee@calient.net
Prof. Dan Blumenthal
Eng. Science Bldg., Room 2221F
Department of Electrical and Computer Engineering
University of California
Santa Barbara, CA 93106-9560
EMail: danb@ece.ucsb.edu
Dr. John Drake
Boeing
2260 E Imperial Highway
El Segundo, Ca 90245
EMail: John.E.Drake2@boeing.com
Andre Fredette
Hatteras Networks
PO Box 110025
Research Triangle Park, NC 27709
EMail: afredette@hatterasnetworks.com
Change Nan Froberg's reach info to:
Dr. Nan Froberg
Photonic Systems, Inc.
900 Middlesex Turnpike, Bldg #5
Billerica, MA 01821
EMail: nfroberg@photonicsinc.com
Dr. Taha Landolsi
King Fahd University
KFUPM Mail Box 1026
Dhahran 31261, Saudi Arabia
EMail: landolsi@kfupm.edu.sa
James V. Luciani
900 Chelmsford St.
Lowell, MA 01851
EMail: james_luciani@mindspring.com
Dr. Robert Tkach
32 Carriage House Lane
Little Silver, NJ 07739
908 246 5048
EMail: tkach@ieee.org
Yong Xue
Dr. Yong Xue
DoD/DISA
5600 Columbia Pike
Falls Church VA 22041
EMail: yong.xue@disa.mil
Editors' Addresses
Angela Chiu
AT&T Labs
200 Laurel Ave., Rm A5-1F13
Middletown, NJ 07748
Phone: (732) 420-9061
EMail: chiu@research.att.com
John Strand
AT&T Labs
200 Laurel Ave., Rm A5-1D33
Middletown, NJ 07748
Phone: (732) 420-9036
EMail: jls@research.att.com
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