draft-ietf-ipo-impairments-02.txt   draft-ietf-ipo-impairments-03.txt 
Internet Draft Internet Draft John Strand (Editor)
Document: draft-ietf-ipo-impairments-02.txt Angela Chiu (Editor) Document: draft-ietf-ipo-impairments-03.txt AT&T
Expiration Date: August 2002 Celion Networks Informational Track
Expiration Date: March 2003 Angela Chiu (Editor)
John Strand (Editor)
AT&T
Robert Tkach
Celion Networks Celion Networks
James Luciani September 2002
Crescent Networks
Ayan Banerjee
John Drake
Dan Blumenthal
Calient Networks
Andre Fredette
Hatteras Networks
Nan Froberg
PhotonEx
Yong Xue
Taha Landolsi
Worldcom
Impairments And Other Constraints On Optical Layer Routing Impairments And Other Constraints On Optical Layer Routing
Status of this Memo Status of this Memo
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Impairments And Other Constraints February 2002
On Optical Layer Routing
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Abstract Abstract
Optical networking poses a number challenges for GMPLS. Optical Optical networking poses a number challenges for GMPLS. Optical
technology is fundamentally an analog rather than digital technology; technology is fundamentally an analog rather than digital technology;
and the optical layer is lowest in the transport hierarchy and hence and the optical layer is lowest in the transport hierarchy and hence
skipping to change at page 2, line 33 skipping to change at page 2, line 4
opaque architectures, and (4) Impacts of diversity constraints. opaque architectures, and (4) Impacts of diversity constraints.
1. Introduction 1. Introduction
GMPLS [GMPLS] aims to extend MPLS to encompass a number of transport GMPLS [GMPLS] aims to extend MPLS to encompass a number of transport
architectures. Included are optical networks incorporating a number architectures. Included are optical networks incorporating a number
of all-optical and opto-electronic elements such as optical cross- of all-optical and opto-electronic elements such as optical cross-
connects with both optical and electrical fabrics, transponders, and connects with both optical and electrical fabrics, transponders, and
optical add-drop multiplexers. Optical networking poses a number optical add-drop multiplexers. Optical networking poses a number
challenges for GMPLS. Optical technology is fundamentally an analog challenges for GMPLS. Optical technology is fundamentally an analog
Impairments And Other Constraints September 2002
On Optical Layer Routing
rather than digital technology; and the optical layer is lowest in rather than digital technology; and the optical layer is lowest in
the transport hierarchy and hence has an intimate relationship with the transport hierarchy and hence has an intimate relationship with
the physical geography of the network. the physical geography of the network.
GMPLS already has incorporated extensions to deal with some of the GMPLS already has incorporated extensions to deal with some of the
unique aspects of the optical layer. This contribution surveys some unique aspects of the optical layer. This contribution surveys some
of the aspects of optical networks which impact routing and of the aspects of optical networks which impact routing and
identifies possible GMPLS responses for each. Routing constraints identifies possible GMPLS responses for each. Routing constraints
and/or complications arising from the design of network elements, and/or complications arising from the design of network elements,
the accumulation of signal impairments, and from the need to the accumulation of signal impairments, and from the need to
guarantee the physical diversity of some circuits are discussed. guarantee the physical diversity of some circuits are discussed.
Since the purpose of this draft is to further the specification of Since the purpose of this draft is to further the specification of
GMPLS, alternative approaches to controlling an optical network are GMPLS, alternative approaches to controlling an optical network are
not discussed. For discussions of some broader issues, see not discussed. For discussions of some broader issues, see
[Gerstel2000] and [Strand2001]. [Gerstel2000] and [Strand2001].
The organization of the contribution is as follows: The organization of the contribution is as follows:
Impairments And Other Constraints February 2002 - Section 2 is a section requested by the sub-IP Area management
On Optical Layer Routing 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
- Section 2 is a section requested by the sub-IP Area groups.
management for all new drafts. It explains how this document - Section 3 describes constraints arising from the design of new
fits into the Area and into the IPO WG, and why it is software controllable network elements.
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 - Section 4 addresses the constraints in a single all-optical
domain without wavelength conversion. domain without wavelength conversion.
- Section 5 extends the discussion to more complex networks. - Section 5 extends the discussion to more complex networks.
incorporating both all-optical and opaque architectures. incorporating both all-optical and opaque architectures.
- Section 6 discusses the impacts of diversity constraints. - Section 6 discusses the impacts of diversity constraints.
- Section 7 deals with security requirements. - Section 7 deals with security requirements.
- Section 8 contains acknowledgments. - 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 2. Sub-IP Area Summary And Justification Of Work
This draft merges and extends two previous drafts, draft-chiu- This draft merges and extends two previous drafts, draft-chiu-
strand-unique-olcp-02.txt and draft-banerjee-routing-impairments- strand-unique-olcp-02.txt and draft-banerjee-routing-impairments-
00.txt. These two drafts were made IPO working group documents to 00.txt. These two drafts were made IPO working group documents to
form a basis for a design team at the Minneapolis meeting, where it form a basis for a design team at the Minneapolis meeting, where it
was also requested that they be merged to create a requirements was also requested that they be merged to create a requirements
document for the WG. document for the WG.
In the larger sub-IP Area structure, this merged document describes In the larger sub-IP Area structure, this merged document describes
specific characteristics of optical technology and the requirements specific characteristics of optical technology and the requirements
they place on routing and path selection. It is appropriate for the they place on routing and path selection. It is appropriate for the
Impairments And Other Constraints September 2002
On Optical Layer Routing
IPO working group because the material is specific to optical IPO working group because the material is specific to optical
networks. It identifies and documents the characteristics of the networks. It identifies and documents the characteristics of the
optical transport network that are important for selecting paths for optical transport network that are important for selecting paths for
optical channels, which is a work area for the IPO WG. It is optical channels, which is a work area for the IPO WG. It is
appropriate work for this WG because the material covered is appropriate work for this WG because the material covered is
directly aimed at establishing a framework and requirements for directly aimed at establishing a framework and requirements for
routing in an optical network. routing in an optical network.
Related documents are: Related documents are:
draft-banerjee-routing-impairments-00.txt draft-banerjee-routing-impairments-00.txt
skipping to change at page 4, line 4 skipping to change at page 3, line 30
draft-many-carrier-framework-uni-01.txt draft-many-carrier-framework-uni-01.txt
draft-papadimitriou-ipo-non-linear-routing-impairm-01.txt draft-papadimitriou-ipo-non-linear-routing-impairm-01.txt
3. Reconfigurable Network Elements 3. Reconfigurable Network Elements
3.1 Technology Background 3.1 Technology Background
Control plane architectural discussions (e.g., [Awduche99]) usually Control plane architectural discussions (e.g., [Awduche99]) usually
assume that the only software reconfigurable network element is an assume that the only software reconfigurable network element is an
optical layer cross-connect (OLXC). There are however other optical layer cross-connect (OLXC). There are however other
Impairments And Other Constraints February 2002
On Optical Layer Routing
software reconfigurable elements on the horizon, specifically software reconfigurable elements on the horizon, specifically
tunable lasers and receivers and reconfigurable optical add-drop tunable lasers and receivers and reconfigurable optical add-drop
multiplexers (OADMÆs). These elements are illustrated in the multiplexers (OADMs). These elements are illustrated in the
following simple example, which is modeled on announced Optical following simple example, which is modeled on announced Optical
Transport System (OTS) products: Transport System (OTS) products:
+ + + +
---+---+ |\ /| +---+--- ---+---+ |\ /| +---+---
---| A |----|D| X Y |D|----| A |--- ---| A |----|D| X Y |D|----| A |---
---+---+ |W| +--------+ +--------+ |W| +---+--- ---+---+ |W| +--------+ +--------+ |W| +---+---
: |D|-----| OADM |-----| OADM |-----|D| : : |D|-----| OADM |-----| OADM |-----|D| :
---+---+ |M| +--------+ +--------+ |M| +---+--- ---+---+ |M| +--------+ +--------+ |M| +---+---
---| A |----| | | | | | | |----| A |--- ---| A |----| | | | | | | |----| A |---
---+---+ |/ | | | | \| +---+--- ---+---+ |/ | | | | \| +---+---
+ +---+ +---+ +---+ +---+ + + +---+ +---+ +---+ +---+ +
D | A | | A | | A | | A | E D | A | | A | | A | | A | E
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+ +---+ +---+ +---+ +---+ + + +---+ +---+ +---+ +---+ +
D | A | | A | | A | | A | E D | A | | A | | A | | A | E
+---+ +---+ +---+ +---+ +---+ +---+ +---+ +---+
| | | | | | | | | | | | | | | |
Figure 3-1: An OTS With OADM's - Functional Architecture Figure 3-1: An OTS With OADM's - Functional Architecture
In Fig.3-1, the part that is on the inner side of all boxes labeled In Fig.3-1, the part that is on the inner side of all boxes labeled
"A" defines an all-optical subnetwork. From a routing perspective "A" defines an all-optical subnetwork. From a routing perspective
two aspects are critical: two aspects are critical:
Impairments And Other Constraints September 2002
On Optical Layer Routing
- Adaptation: These are the functions done at the edges of the - Adaptation: These are the functions done at the edges of the
subnetwork that transform the incoming optical channel into the subnetwork that transform the incoming optical channel into the
physical wavelength to be transported through the subnetwork. physical wavelength to be transported through the subnetwork.
- Connectivity: This defines which pairs of edge Adaptation - Connectivity: This defines which pairs of edge Adaptation
functions can be interconnected through the subnetwork. functions can be interconnected through the subnetwork.
In Fig. 3-1, D and E are DWDMÆs and X and Y are OADMÆs. The boxes In Fig. 3-1, D and E are DWDM’s and X and Y are OADM’s. The boxes
labeled "A" are adaptation functions. They map one or more input labeled "A" are adaptation functions. They map one or more input
optical channels assumed to be standard short reach signals into a optical channels assumed to be standard short reach signals into a
long reach (LR) wavelength or wavelength group which will pass long reach (LR) wavelength or wavelength group which will pass
transparently to a distant adaptation function. Adaptation transparently to a distant adaptation function. Adaptation
functionality which affects routing includes: functionality which affects routing includes:
- Multiplexing: Either electrical or optical TDM may be used to - Multiplexing: Either electrical or optical TDM may be used to
combine the input channels into a single wavelength. This is combine the input channels into a single wavelength. This is
done to increase effective capacity: A typical DWDM might be done to increase effective capacity: A typical DWDM might be
able to handle 100 2.5 Gb/sec signals (250 Gb/sec total) or 50 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 10 Gb/sec (500 Gb/sec total); combining the 2.5 Gb/sec signals
together thus effectively doubles capacity. After multiplexing together thus effectively doubles capacity. After multiplexing
the combined signal must be routed as a group to the distant the combined signal must be routed as a group to the distant
adaptation function. adaptation function.
- Adaptation Grouping: In this technique, groups of k (e.g., 4) - Adaptation Grouping: In this technique, groups of k (e.g., 4)
wavelengths are managed as a group within the system and must be wavelengths are managed as a group within the system and must be
added/dropped as a group. We will call such a group an added/dropped as a group. We will call such a group an
Impairments And Other Constraints February 2002
On Optical Layer Routing
"adaptation grouping". Examples include so called "wave group" "adaptation grouping". Examples include so called "wave group"
and ôwavebandö [Passmore01]. Groupings on the same system may and "waveband" [Passmore01]. Groupings on the same system may
differ in basics such as wavelength spacing, which constrain the differ in basics such as wavelength spacing, which constrain the
type of channels that can be accommodated. type of channels that can be accommodated.
- Laser Tunability: The lasers producing the LR wavelengths may - Laser Tunability: The lasers producing the LR wavelengths may
have a fixed frequency, may be tunable over a limited range, or have a fixed frequency, may be tunable over a limited range, or
be tunable over the entire range of wavelengths supported by the be tunable over the entire range of wavelengths supported by the
DWDM. Tunability speeds may also vary. DWDM. Tunability speeds may also vary.
Connectivity between adaptation functions may also be limited: Connectivity between adaptation functions may also be limited:
- As pointed out above, TDM multiplexing and/or adaptation - As pointed out above, TDM multiplexing and/or adaptation
grouping by the adaptation function forces groups of input grouping by the adaptation function forces groups of input
channels to be delivered together to the same distant adaptation channels to be delivered together to the same distant adaptation
function. function.
- Only adaptation functions whose lasers/receivers are tunable to - Only adaptation functions whose lasers/receivers are tunable to
compatible frequencies can be connected. compatible frequencies can be connected.
- The switching capability of the OADMÆs may also be constrained. - The switching capability of the OADMs may also be constrained.
For example: For example:
o There may be some wavelengths that can not be dropped at o There may be some wavelengths that can not be dropped at
all. all.
o There may be a fixed relationship between the frequency o There may be a fixed relationship between the frequency
dropped and the physical port on the OADM to which it is dropped and the physical port on the OADM to which it is
dropped. dropped.
o OADM physical design may put an upper bound on the number o OADM physical design may put an upper bound on the number
of adaptation groupings dropped at any single OADM. of adaptation groupings dropped at any single OADM.
For a fixed configuration of the OADMÆs and adaptation functions Impairments And Other Constraints September 2002
On Optical Layer Routing
For a fixed configuration of the OADM’s and adaptation functions
connectivity will be fixed: Each input port will essentially be connectivity will be fixed: Each input port will essentially be
hard-wired to some specific distant port. However this connectivity hard-wired to some specific distant port. However this connectivity
can be changed by changing the configurations of the OADMÆs and can be changed by changing the configurations of the OADMs and
adaptation functions. For example, an additional adaptation grouping adaptation functions. For example, an additional adaptation grouping
might be dropped at an OADM or a tunable laser retuned. In each case might be dropped at an OADM or a tunable laser retuned. In each case
the port-to-port connectivity is changed. the port-to-port connectivity is changed.
These capabilities can be expected to be under software control. These capabilities can be expected to be under software control.
Today the control would rest in the vendor-supplied Element Today the control would rest in the vendor-supplied Element
Management system (EMS), which in turn would be controlled by the Management system (EMS), which in turn would be controlled by the
operatorÆs OSÆs. However in principle the EMS could participate in operator’s OS’s. However in principle the EMS could participate in
the GMPLS routing process. the GMPLS routing process.
3.2 Implications For Routing 3.2 Implications For Routing
An OTS of the sort discussed in Sec. 3.1 is essentially a An OTS of the sort discussed in Sec. 3.1 is essentially a
geographically distributed but blocking cross-connect system. The geographically distributed but blocking cross-connect system. The
specific port connectivity is dependent on the vendor design and specific port connectivity is dependent on the vendor design and
also on exactly what line cards have been deployed. also on exactly what line cards have been deployed.
One way for GMPLS to deal with this architecture would be to view One way for GMPLS to deal with this architecture would be to view
the port connectivity as externally determined. In this case the the port connectivity as externally determined. In this case the
links known to GMPLS would be groups of identically routed links known to GMPLS would be groups of identically routed
Impairments And Other Constraints February 2002
On Optical Layer Routing
wavebands. If these were reconfigured by the external EMS the wavebands. If these were reconfigured by the external EMS the
resulting connectivity changes would need to be detected and resulting connectivity changes would need to be detected and
advertised within GMPLS. If the topology shown in Fig. 3-1 became a advertised within GMPLS. If the topology shown in Fig. 3-1 became a
tree or a mesh instead of the linear topology shown, the tree or a mesh instead of the linear topology shown, the
connectivity changes could result in SRLG changes. connectivity changes could result in SRLG changes.
Alternatively, GMPLS could attempt to directly control this port Alternatively, GMPLS could attempt to directly control this port
connectivity. The state information needed to do this is likely to connectivity. The state information needed to do this is likely to
be voluminous and vendor specific. be voluminous and vendor specific.
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O/E/O conversions. They provide regeneration with retiming and O/E/O conversions. They provide regeneration with retiming and
reshaping, also called 3R, which eliminates transparency to bit reshaping, also called 3R, which eliminates transparency to bit
rates and frame format. These transponders are quite expensive and rates and frame format. These transponders are quite expensive and
their lack of transparency also constrains the rapid introduction of their lack of transparency also constrains the rapid introduction of
new services. Thus there are strong motivators to introduce new services. Thus there are strong motivators to introduce
"domains of transparency" - all-optical subnetworks - larger than an "domains of transparency" - all-optical subnetworks - larger than an
OTS. OTS.
The routing of lightpaths through an all-optical network has The routing of lightpaths through an all-optical network has
received extensive attention. (See [Yates99] or [Ramaswami98]). received extensive attention. (See [Yates99] or [Ramaswami98]).
Impairments And Other Constraints September 2002
On Optical Layer Routing
When discussing routing in an all-optical network it is usually When discussing routing in an all-optical network it is usually
assumed that all routes have adequate signal quality. This may be assumed that all routes have adequate signal quality. This may be
ensured by limiting all-optical networks to subnetworks of limited ensured by limiting all-optical networks to subnetworks of limited
geographic size which are optically isolated from other parts of the geographic size which are optically isolated from other parts of the
optical layer by transponders. This approach is very practical and optical layer by transponders. This approach is very practical and
has been applied to date, e.g. when determining the maximum length has been applied to date, e.g. when determining the maximum length
of an Optical Transport System (OTS). Furthermore operational of an Optical Transport System (OTS). Furthermore operational
considerations like fault isolation also make limiting the size of considerations like fault isolation also make limiting the size of
domains of transparency attractive. domains of transparency attractive.
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increase it is necessary to increase power. This makes impairments increase it is necessary to increase power. This makes impairments
and nonlinearities more troublesome. From a supply perspective, and nonlinearities more troublesome. From a supply perspective,
optical technology is advancing very rapidly, making ever-larger optical technology is advancing very rapidly, making ever-larger
domains possible. In this section we assume that these domains possible. In this section we assume that these
considerations will lead to the deployment of a domain of considerations will lead to the deployment of a domain of
transparency that is too large to ensure that all potential routes transparency that is too large to ensure that all potential routes
have adequate signal quality for all circuits. Our goal is to have adequate signal quality for all circuits. Our goal is to
understand the impacts of the various types of impairments in this understand the impacts of the various types of impairments in this
environment. environment.
Impairments And Other Constraints February 2002
On Optical Layer Routing
Note that as we describe later in the section there are many types Note that as we describe later in the section there are many types
of physical impairments. Which of these need to be dealt with of physical impairments. Which of these need to be dealt with
explicitly when performing on-line distributed routing will vary explicitly when performing on-line distributed routing will vary
considerably and will depend on many variables, including: considerably and will depend on many variables, including:
. Equipment vendor design choices, - Equipment vendor design choices,
. Fiber characteristics, - Fiber characteristics,
. Service characteristics (e.g., circuit speeds), - Service characteristics (e.g., circuit speeds),
. Network size, - Network size,
. Network operator engineering and deployment strategies. - Network operator engineering and deployment strategies.
For example, a metropolitan network which does not intend to support 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 bit rates above 2.5 Gb/sec may not be constrained by any of these
impairments, while a continental or international network which impairments, while a continental or international network which
wished to minimize O/E/O regeneration investment and support 40 wished to minimize O/E/O regeneration investment and support 40
Gb/sec connections might have to explicitly consider many of them. Gb/sec connections might have to explicitly consider many of them.
Also, a network operator may reduce or even eliminate their Also, a network operator may reduce or even eliminate their
constraint set by building a relatively small domain of transparency constraint set by building a relatively small domain of transparency
to ensure that all the paths are feasible, or by using some to ensure that all the paths are feasible, or by using some
proprietary tools based on rules from the OTS vendor to pre-qualify 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 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 accessed each time a routing decision has to be made through that
domain. domain.
4.1 Problem Formulation 4.1 Problem Formulation
Impairments And Other Constraints September 2002
On Optical Layer Routing
We consider a single domain of transparency without wavelength We consider a single domain of transparency without wavelength
translation. Additionally due to the proprietary nature of DWDM translation. Additionally due to the proprietary nature of DWDM
transmission technology, we assume that the domain is either single transmission technology, we assume that the domain is either single
vendor or architected using a single coherent design, particularly vendor or architected using a single coherent design, particularly
with regard to the management of impairments. with regard to the management of impairments.
We wish to route a unidirectional circuit from ingress client node X We wish to route a unidirectional circuit from ingress client node X
to egress client node Y. At both X and Y, the circuit goes through 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 an O/E/O conversion which optically isolates the portion within our
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Linear effects are independent of signal power and affect Linear effects are independent of signal power and affect
wavelengths individually. Amplifier spontaneous emission (ASE), wavelengths individually. Amplifier spontaneous emission (ASE),
polarization mode dispersion (PMD), and chromatic dispersion are polarization mode dispersion (PMD), and chromatic dispersion are
examples. Nonlinearities are significantly more complex: they examples. Nonlinearities are significantly more complex: they
generate not only impairments on each channel, but also crosstalk generate not only impairments on each channel, but also crosstalk
between channels. between channels.
In the remainder of this section we first outline how two key linear In the remainder of this section we first outline how two key linear
impairments (PMD and ASE) might be handled by a set of analytical impairments (PMD and ASE) might be handled by a set of analytical
formulae as additional constraints on routing. We next discuss how formulae as additional constraints on routing. We next discuss how
Impairments And Other Constraints February 2002
On Optical Layer Routing
the remaining constraints might be approached. Finally we take a the remaining constraints might be approached. Finally we take a
broader perspective and discuss the implications of such constraints broader perspective and discuss the implications of such constraints
on control plane architecture and also on broader constrained domain on control plane architecture and also on broader constrained domain
of transparency architecture issues. of transparency architecture issues.
4.2 Polarization Mode Dispersion (PMD) 4.2 Polarization Mode Dispersion (PMD)
For a transparent fiber segment, the general PMD requirement is that For a transparent fiber segment, the general PMD requirement is that
the time-average differential group delay (DGD) between two the time-average differential group delay (DGD) between two
orthogonal state of polarizations should be less than fraction a of orthogonal state of polarizations should be less than fraction a of
skipping to change at page 8, line 31 skipping to change at page 8, line 4
[ITU]. More aggressive designs to compensate for PMD may allow [ITU]. More aggressive designs to compensate for PMD may allow
values higher than 10%. (This would be a system parameter dependent values higher than 10%. (This would be a system parameter dependent
on the system design. It would need to be known to the routing on the system design. It would need to be known to the routing
process.) process.)
The PMD parameter (Dpmd) is measured in pico-seconds (ps) per The PMD parameter (Dpmd) is measured in pico-seconds (ps) per
sqrt(km). The square of the PMD in a fiber span, denoted as span- 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 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 length. (A fiber span in a transparent network refers to a segment
between two optical amplifiers.) If Dpmd is constant, this results between two optical amplifiers.) If Dpmd is constant, this results
Impairments And Other Constraints September 2002
On Optical Layer Routing
in a upper bound on the maximum length of an M-fiber-span in a upper bound on the maximum length of an M-fiber-span
transparent segment, which is inversely proportional to the square transparent segment, which is inversely proportional to the square
of the product of bit rate and Dpmd(the detailed equation is omitted of the product of bit rate and Dpmd (the detailed equation is
due to the format constraint - see [Strand01] for details). omitted due to the format constraint - see [Strand01] for details).
For older fibers with a typical PMD parameter of 0.5 picoseconds per For older fibers with a typical PMD parameter of 0.5 picoseconds per
square root of km, based on the constraint, the maximum length of square root of km, based on the constraint, the maximum length of
the transparent segment should not exceed 400km and 25km for bit the transparent segment should not exceed 400km and 25km for bit
rates of 10Gb/s and 40Gb/s, respectively. Due to recent advances in rates of 10Gb/s and 40Gb/s, respectively. Due to recent advances in
fiber technology, the PMD-limited distance has increased fiber technology, the PMD-limited distance has increased
dramatically. For newer fibers with a PMD parameter of 0.1 dramatically. For newer fibers with a PMD parameter of 0.1
picosecond per square root of km, the maximum length of the picosecond per square root of km, the maximum length of the
transparent segment (without PMD compensation) is limited to 10000km transparent segment (without PMD compensation) is limited to 10000km
and 625km for bit rates of 10Gb/s and 40Gb/, respectively. Still and 625km for bit rates of 10Gb/s and 40Gb/, respectively. Still
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tolerated. In general, the PMD requirement is not an issue for most 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 types of fibers at 10Gb/s or lower bit rate. But it will become an
issue at bit rates of 40Gb/s and higher. issue at bit rates of 40Gb/s and higher.
If the PMD parameter varies between spans, a slightly more If the PMD parameter varies between spans, a slightly more
complicated equation results (see [Strand01]), but in any event the complicated equation results (see [Strand01]), but in any event the
only link dependent information needed by the routing algorithm is only link dependent information needed by the routing algorithm is
the square of the link PMD, denoted as link-PMD-square. It is the 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. sum of the span-PMD-square of all spans on the link.
Impairments And Other Constraints February 2002
On Optical Layer Routing
Note that when one has some viable PMD compensation devices and Note that when one has some viable PMD compensation devices and
deploy them ubiquitously on all routes with potential PMD issues in deploy them ubiquitously on all routes with potential PMD issues in
the network, then the PMD constraint disappears from the routing the network, then the PMD constraint disappears from the routing
perspective. perspective.
4.3 Amplifier Spontaneous Emission 4.3 Amplifier Spontaneous Emission
ASE degrades the optical signal to noise ratio (OSNR). An acceptable ASE degrades the optical signal to noise ratio (OSNR). An acceptable
optical SNR level (SNRmin) which depends on the bit rate, optical SNR level (SNRmin) which depends on the bit rate,
transmitter-receiver technology (e.g., FEC), and margins allocated transmitter-receiver technology (e.g., FEC), and margins allocated
skipping to change at page 9, line 31 skipping to change at page 9, line 4
transparent segment and number of spans. For example, current transparent segment and number of spans. For example, current
transmission systems are often limited to up to 6 spans each 80km transmission systems are often limited to up to 6 spans each 80km
long. For larger transparent domains, more detailed OSNR long. For larger transparent domains, more detailed OSNR
computations will be needed to determine whether the OSNR level computations will be needed to determine whether the OSNR level
through a domain of transparency is acceptable. This would provide through a domain of transparency is acceptable. This would provide
flexibility in provisioning or restoring a lightpath through a flexibility in provisioning or restoring a lightpath through a
transparent subnetwork. transparent subnetwork.
Assume that the average optical power launched at the transmitter is Assume that the average optical power launched at the transmitter is
P. The lightpath from the transmitter to the receiver goes through M P. The lightpath from the transmitter to the receiver goes through M
Impairments And Other Constraints September 2002
On Optical Layer Routing
optical amplifiers, with each introducing some noise power. Unity optical amplifiers, with each introducing some noise power. Unity
gain can be used at all amplifier sites to maintain constant signal gain can be used at all amplifier sites to maintain constant signal
power at the input of each span to minimize noise power and power at the input of each span to minimize noise power and
nonlinearity. A constraint on the maximum number of spans can be nonlinearity. A constraint on the maximum number of spans can be
obtained [Kaminow97] which is proportional to P and inversely obtained [Kaminow97] which is proportional to P and inversely
proportional to SNRmin, optical bandwidth B, amplifier gain G-1 and proportional to SNRmin, optical bandwidth B, amplifier gain G-1 and
spontaneous emission factor n of the optical amplifier, assuming all spontaneous emission factor n of the optical amplifier, assuming all
spans have identical gain and noise figure. (Again, the detailed spans have identical gain and noise figure. (Again, the detailed
equation is omitted due to the format constraint - see [Strand01] equation is omitted due to the format constraint - see [Strand01]
for details.) LetÆs take a typical example. Assuming P=4dBm, for details.) Lets take a typical example. Assuming P=4dBm,
SNRmin=20dB with FEC, B=12.5GHz, n=2.5, G=25dB, based on the 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 constraint, the maximum number of spans is at most 10. However, if
FEC is not used and the requirement on SNRmin becomes 25dB, the FEC is not used and the requirement on SNRmin becomes 25dB, the
maximum number of spans drops down to 3. maximum number of spans drops down to 3.
For ASE the only link-dependent information needed by the routing For ASE the only link-dependent information needed by the routing
algorithm is the noise of the link, denoted as link-noise, which is 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 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 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 which is the sum of the link-noise of all links can not exceed
P/SNRmin. P/SNRmin.
4.4 Approximating The Effects Of Some Other Impairment Constraints 4.4 Approximating The Effects Of Some Other Impairment Constraints
There are a number of other impairment constraints that we believe There are a number of other impairment constraints that we believe
could be approximated with a domain-wide margin on the OSNR, plus in could be approximated with a domain-wide margin on the OSNR, plus in
Impairments And Other Constraints February 2002
On Optical Layer Routing
some cases a constraint on the total number of networking elements some cases a constraint on the total number of networking elements
(OXC or OADM) along the path. Most impairments generated at OXCs or (OXC or OADM) along the path. Most impairments generated at OXCs or
OADMs, including polarization dependent loss, coherent crosstalk, OADMs, including polarization dependent loss, coherent crosstalk,
and effective passband width, could be dealt with using this and effective passband width, could be dealt with using this
approach. In principle, impairments generated at the nodes can be approach. In principle, impairments generated at the nodes can be
bounded by system engineering rules because the node elements can be bounded by system engineering rules because the node elements can be
designed and specified in a uniform manner. This approach is not designed and specified in a uniform manner. This approach is not
feasible with PMD and noise because neither can be uniformly feasible with PMD and noise because neither can be uniformly
specified. Instead, they depend on node spacing and the specified. Instead, they depend on node spacing and the
characteristics of the installed fiber plant, neither of which are characteristics of the installed fiber plant, neither of which are
likely to be under the system designerÆs control. likely to be under the system designers control.
Examples of the constraints we propose to approximate with a domain- Examples of the constraints we propose to approximate with a domain-
wide margin are given in the remaining paragraphs in this section. wide margin are given in the remaining paragraphs in this section.
It should be kept in mind that as optical transport technology It should be kept in mind that as optical transport technology
evolves it may become necessary to include some of these impairments evolves it may become necessary to include some of these impairments
explicitly in the routing process. Other impairments not mentioned explicitly in the routing process. Other impairments not mentioned
here at all may also become sufficiently important to require here at all may also become sufficiently important to require
incorporation either explicitly or via a domain-wide margin. incorporation either explicitly or via a domain-wide margin.
Other Polarization Dependent Impairments Other polarization- Other Polarization Dependent Impairments Other polarization-
dependent effects besides PMD influence system performance. For dependent effects besides PMD influence system performance. For
example, many components have polarization-dependent loss (PDL) example, many components have polarization-dependent loss (PDL)
[Ramaswami98], which accumulates in a system with many components on [Ramaswami98], which accumulates in a system with many components on
Impairments And Other Constraints September 2002
On Optical Layer Routing
the transmission path. The state of polarization fluctuates with the transmission path. The state of polarization fluctuates with
time and its distribution is very important also. It is generally time and its distribution is very important also. It is generally
required to maintain the total PDL on the path to be within some required to maintain the total PDL on the path to be within some
acceptable limit, potentially by using some compensation technology acceptable limit, potentially by using some compensation technology
for relatively long transmission systems, plus a small built-in for relatively long transmission systems, plus a small built-in
margin in OSNR. Since the total PDL increases with the number of margin in OSNR. Since the total PDL increases with the number of
components in the data path, it must be taken into account by the components in the data path, it must be taken into account by the
system vendor when determining the maximum allowable number of system vendor when determining the maximum allowable number of
spans. spans.
skipping to change at page 11, line 4 skipping to change at page 10, line 31
fiber compensation by means of a spool of fiber with a CD coefficient fiber compensation by means of a spool of fiber with a CD coefficient
opposite to the fiber. Due to the imperfect matching between the CD opposite to the fiber. Due to the imperfect matching between the CD
slope of the fiber and the DCF some lambdas can be over compensated slope of the fiber and the DCF some lambdas can be over compensated
while others can be under compensated. Moreover DCF modules may only while others can be under compensated. Moreover DCF modules may only
be available in fixed lengths of compensating fiber; this means that be available in fixed lengths of compensating fiber; this means that
sometimes it is impossible to find a DCF module that exactly sometimes it is impossible to find a DCF module that exactly
compensates the CD introduced by the fiber. These effects introduce compensates the CD introduced by the fiber. These effects introduce
what is known as residual CD. Residual CD varies with the frequency what is known as residual CD. Residual CD varies with the frequency
of the wavelength. Knowing the characteristics of both of the fiber of the wavelength. Knowing the characteristics of both of the fiber
and the DCF modules along the path, this can be calculated with a and the DCF modules along the path, this can be calculated with a
Impairments And Other Constraints February 2002
On Optical Layer Routing
sufficient degree of precision. However this is a very challenging sufficient degree of precision. However this is a very challenging
task. In fact the per-wavelength residual dispersion needs to be task. In fact the per-wavelength residual dispersion needs to be
combined with other information in the system (e.g. types fibers to combined with other information in the system (e.g. types fibers to
figure out the amount of nonlinearities) to obtain the net effect of figure out the amount of nonlinearities) to obtain the net effect of
CD either by simulation or by some analytical approximation. It CD either by simulation or by some analytical approximation. It
appears that the routing/control plane should not be burdened by such 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 a large set of information while it can be handled at the system
design level. Therefore it will be assumed until proven otherwise design level. Therefore it will be assumed until proven otherwise
that residual dispersion should not be reported. For high bit rates, that residual dispersion should not be reported. For high bit rates,
dynamic dispersion compensation may be required at the receiver to dynamic dispersion compensation may be required at the receiver to
skipping to change at page 11, line 29 skipping to change at page 11, line 4
Crosstalk Optical crosstalk refers to the effect of other signals on Crosstalk Optical crosstalk refers to the effect of other signals on
the desired signal. It includes both coherent (i.e. intrachannel) the desired signal. It includes both coherent (i.e. intrachannel)
crosstalk and incoherent (i.e. interchannel) crosstalk. Main crosstalk and incoherent (i.e. interchannel) crosstalk. Main
contributors of crosstalk are the OADM and OXC sites that use a DWDM contributors of crosstalk are the OADM and OXC sites that use a DWDM
multiplexer/demultiplexer (MUX/DEMUX) pair. For a relatively sparse multiplexer/demultiplexer (MUX/DEMUX) pair. For a relatively sparse
network where the number of OADM/OXC nodes on a path is low, 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 crosstalk can be treated with a low margin in OSNR without being a
binding constraint. But for some relatively dense networks where binding constraint. But for some relatively dense networks where
crosstalk might become a binding constraint, one needs to propagate crosstalk might become a binding constraint, one needs to propagate
the per-link crosstalk information to make sure that the end-to-end the per-link crosstalk information to make sure that the end-to-end
Impairments And Other Constraints September 2002
On Optical Layer Routing
path crosstalk which is the sum of the crosstalks on all the path crosstalk which is the sum of the crosstalks on all the
corresponding links to be within some limit, e.g. û25dB threshold corresponding links to be within some limit, e.g. -25dB threshold
with 1dB penalty ([Goldstein94]). Another way to treat it without with 1dB penalty ([Goldstein94]). Another way to treat it without
having to propagate per-link crosstalk information is to have the having to propagate per-link crosstalk information is to have the
system evaluate what the maximum number of OADM/OXC nodes that has a 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 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 low built-in margin. The latter one should work well where all the
OXC/OADM nodes have similar level of crosstalk. OXC/OADM nodes have similar level of crosstalk.
Effective Passband As more and more DWDM components are cascaded, Effective Passband As more and more DWDM components are cascaded,
the effective passband narrows. The number of filters along the the effective passband narrows. The number of filters along the
link, their passband width and their shape will determine the end- link, their passband width and their shape will determine the end-
skipping to change at page 12, line 4 skipping to change at page 11, line 33
maximum number of narrow filters with the condition that filters in maximum number of narrow filters with the condition that filters in
the systems are at least as wide as the one in the receiver. the systems are at least as wide as the one in the receiver.
Because traffic at lower bit rates can tolerate a narrower passband, Because traffic at lower bit rates can tolerate a narrower passband,
the maximum allowable number of narrow filters will increase as the the maximum allowable number of narrow filters will increase as the
bit rate decreases. bit rate decreases.
Nonlinear Impairments It seems unlikely that these can be dealt with Nonlinear Impairments It seems unlikely that these can be dealt with
explicitly in a routing algorithm because they lead to constraints explicitly in a routing algorithm because they lead to constraints
that can couple routes together and lead to complex dependencies, that can couple routes together and lead to complex dependencies,
e.g. on the order in which specific fiber types are traversed e.g. on the order in which specific fiber types are traversed
Impairments And Other Constraints February 2002
On Optical Layer Routing
[Kaminow97]. Note that different fiber types (standard single mode [Kaminow97]. Note that different fiber types (standard single mode
fiber, dispersion shifted fiber, dispersion compensated fiber, etc.) fiber, dispersion shifted fiber, dispersion compensated fiber, etc.)
have very different effects from nonlinear impairments. A full have very different effects from nonlinear impairments. A full
treatment of the nonlinear constraints would likely require very treatment of the nonlinear constraints would likely require very
detailed knowledge of the physical infrastructure, including detailed knowledge of the physical infrastructure, including
measured dispersion values for each span, fiber core area and measured dispersion values for each span, fiber core area and
composition, as well as knowledge of subsystem details such as composition, as well as knowledge of subsystem details such as
dispersion compensation technology. This information would need to dispersion compensation technology. This information would need to
be combined with knowledge of the current loading of optical signals be combined with knowledge of the current loading of optical signals
on the links of interest to determine the level of nonlinear on the links of interest to determine the level of nonlinear
skipping to change at page 12, line 31 skipping to change at page 12, line 4
maximum number of spans. For the approach described here to be maximum number of spans. For the approach described here to be
useful, it is desirable for this span length limit to be longer than useful, it is desirable for this span length limit to be longer than
that imposed by the constraints which can be treated explicitly. that imposed by the constraints which can be treated explicitly.
When designing a DWDM transport system, there are tradeoffs between When designing a DWDM transport system, there are tradeoffs between
signal power launched at the transmitter, span length, and nonlinear signal power launched at the transmitter, span length, and nonlinear
effects on BER that need to be considered jointly. Here, we assume 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 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 designed with a fixed signal power and maximum span length for a
given bit rate. Note that OTSs can be designed in very different given bit rate. Note that OTSs can be designed in very different
ways, in linear, pseudo-linear, or nonlinear environments. The X-dB ways, in linear, pseudo-linear, or nonlinear environments. The X-dB
margin approach maybe valid for some but not for others. However, it Impairments And Other Constraints September 2002
is likely that there is an advantage in designing systems which are On Optical Layer Routing
less aggressive with respect to nonlinearities, and therefore
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, somewhat sub-optimal, in exchange for improved scalability,
simplicity and flexibility in routing and control plane design. simplicity and flexibility in routing and control plane design.
4.5 Other Impairment Considerations 4.5 Other Impairment Considerations
There are many other types of impairments that can degrade There are many other types of impairments that can degrade
performance. In this section we briefly mention one other type of performance. In this section we briefly mention one other type of
impairment, which we propose be dealt with by either by the system impairment, which we propose be dealt with by either by the system
designer or by the transmission engineers at the time the system is designer or by the transmission engineers at the time the system is
installed. If dealt with successfully in this manner they should not installed. If dealt with successfully in this manner they should not
need to be considered in the dynamic routing process. need to be considered in the dynamic routing process.
Gain Nonuniformity and Gain Transients For simple noise estimates to Gain Nonuniformity and Gain Transients For simple noise estimates to
be of use, the amplifiers must be gain-flattened and must have be of use, the amplifiers must be gain-flattened and must have
automatic gain control (AGC). Furthermore, each link should have automatic gain control (AGC). Furthermore, each link should have
dynamic gain equalization (DGE) to optimize power levels each time dynamic gain equalization (DGE) to optimize power levels each time
wavelengths are added or dropped. Variable optical attenuators on wavelengths are added or dropped. Variable optical attenuators on
the output ports of an OXC or OADM can be used for this purpose, and the output ports of an OXC or OADM can be used for this purpose, and
in-line devices are starting to become commercially available. in-line devices are starting to become commercially available.
Optical channel monitors are also required to provide feedback to Optical channel monitors are also required to provide feedback to
Impairments And Other Constraints February 2002
On Optical Layer Routing
the DGEs. AGC must be done rapidly if signal degradation after a the DGEs. AGC must be done rapidly if signal degradation after a
protection switch or link failure is to be avoided. protection switch or link failure is to be avoided.
Note that the impairments considered here are treated more or less Note that the impairments considered here are treated more or less
independently. By considering them jointly and varying the tradeoffs independently. By considering them jointly and varying the tradeoffs
between the effects from different components may allow more routes between the effects from different components may allow more routes
to be feasible. If that is desirable or the system is designed such to be feasible. If that is desirable or the system is designed such
that certain impairments (e.g. nonlinearities) need to be considered that certain impairments (e.g. nonlinearities) need to be considered
by a centralized process, then distributed routing is not the one to by a centralized process, then distributed routing is not the one to
use. use.
4.6 An Alternative Approach û Using Maximum Distance As The only 4.6 An Alternative Approach - Using Maximum Distance As The only
Constraint Constraint
Today, carriers often use maximum distance to engineer point-to- Today, carriers often use maximum distance to engineer point-to-
point OTS systems given a fixed per-span length based on the OSNR 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 constraint for a given bit rate. They may desire to keep the same
engineering rule when they move to all-optical networks. Here, we engineering rule when they move to all-optical networks. Here, we
discuss the assumptions that need to be satisfied to keep this discuss the assumptions that need to be satisfied to keep this
approach viable and how to treat the network elements between two approach viable and how to treat the network elements between two
adjacent links. adjacent links.
Impairments And Other Constraints September 2002
On Optical Layer Routing
In order to use the maximum distance for a given bit rate to meet an In order to use the maximum distance for a given bit rate to meet an
OSNR constraint as the only binding constraint, the operators need OSNR constraint as the only binding constraint, the operators need
to satisfy the following constraints in their all-optical networks: to satisfy the following constraints in their all-optical networks:
- All the other non-OSNR constraints described in the previous - All the other non-OSNR constraints described in the previous
subsections are not binding factors as long as the maximum subsections are not binding factors as long as the maximum
distance constraint is met. distance constraint is met.
- Specifically for PMD, this means that the whole all-optical - Specifically for PMD, this means that the whole all-optical
network is built on top of sufficiently low-PMD fiber such that network is built on top of sufficiently low-PMD fiber such that
the upper bound on the mean aggregate path DGD is always the upper bound on the mean aggregate path DGD is always
skipping to change at page 14, line 4 skipping to change at page 13, line 30
with high-PMD fibers. with high-PMD fibers.
- In terms of the ASE/OSNR constraint, in order to convert the ASE - In terms of the ASE/OSNR constraint, in order to convert the ASE
constraint into a distance constraint directly, the network constraint into a distance constraint directly, the network
needs to have a fixed fiber distance D for each span (so that needs to have a fixed fiber distance D for each span (so that
ASE can be directly mapped by the gain of the amplifier which ASE can be directly mapped by the gain of the amplifier which
equals to the loss of the previous fiber span), e.g., 80km equals to the loss of the previous fiber span), e.g., 80km
spacing which is commonly chosen by carriers. However, when spacing which is commonly chosen by carriers. However, when
spans have variable lengths, certain adjustment and compromise spans have variable lengths, certain adjustment and compromise
need to be made in order to avoid treating ASE explicitly as in 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 section 4.3. These include: 1) If a span is shorter than a
Impairments And Other Constraints February 2002
On Optical Layer Routing
typical span length D, unless certain mechanism is built in the typical span length D, unless certain mechanism is built in the
OTS to take advantages of shorter spans, it needs to be treated 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 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 there are spans that are longer than D, it means that paths with
these longer spans would have higher average span loss. In these longer spans would have higher average span loss. In
general, the maximum system reach decreases when the average general, the maximum system reach decreases when the average
span loss increases. Thus, in order to accommodate longer spans span loss increases. Thus, in order to accommodate longer spans
in the network, the maximum distance upper bound has to be set 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 with respect to the average span loss of the worst path in the
network. This sub-optimality may be acceptable for some networks network. This sub-optimality may be acceptable for some networks
skipping to change at page 14, line 28 skipping to change at page 14, line 4
for others. for others.
If these assumptions are satisfied, the second issue we need to If these assumptions are satisfied, the second issue we need to
address is how to treat a transparent network element (e.g., MEMS- address is how to treat a transparent network element (e.g., MEMS-
based switch) between two adjacent links in terms of a distance based switch) between two adjacent links in terms of a distance
constraint since it also introduces an insertion loss. If the constraint since it also introduces an insertion loss. If the
network element cannot somehow compensate for this OSNR degradation, network element cannot somehow compensate for this OSNR degradation,
one approach is to convert each network element into an equivalent one approach is to convert each network element into an equivalent
length of fiber based on its loss/ASE contribution. Hence, in length of fiber based on its loss/ASE contribution. Hence, in
general, introducing a set of transparent network elements would general, introducing a set of transparent network elements would
Impairments And Other Constraints September 2002
On Optical Layer Routing
effectively result in reducing the overall actual transmission effectively result in reducing the overall actual transmission
distance between the OEO edges. distance between the OEO edges.
With this approach, the link-specific state information is link- With this approach, the link-specific state information is link-
distance, the length of a link. It equals to the distance sum of all distance, the length of a link. It equals to the distance sum of all
fiber spans on the link and the equivalent length of fiber for the fiber spans on the link and the equivalent length of fiber for the
network element(s) on the link. The constraint is that the sum of network element(s) on the link. The constraint is that the sum of
all the link-distance over all links of a path should be less than all the link-distance over all links of a path should be less than
the maximum-path-distance, the upper bound of all paths. the maximum-path-distance, the upper bound of all paths.
4.7 Other Considerations 4.7 Other Considerations
Routing in an all-optical network without wavelength conversion Routing in an all-optical network without wavelength conversion
raises several additional issues: raises several additional issues:
- Since the route selected must have the chosen wavelength - Since the route selected must have the chosen wavelength
available on all links, this information needs to be considered available on all links, this information needs to be considered
in the routing process. This is discussed in [Chaudhuri00], where in the routing process. This is discussed in [Chaudhuri00],
it is concluded that advertising detailed wavelength where it is concluded that advertising detailed wavelength
availabilities on each link is not likely to scale. Instead they availabilities on each link is not likely to scale. Instead
propose an alternative method which probes along a chosen path to they propose an alternative method which probes along a chosen
determine which wavelengths (if any) are available. This would path to determine which wavelengths (if any) are available.
require a significant addition to the routing logic normally used This would require a significant addition to the routing logic
in OSPF. Others have proposed simultaneously probing along normally used in OSPF. Others have proposed simultaneously
multiple paths. probing along multiple paths.
Impairments And Other Constraints February 2002
On Optical Layer Routing
- Choosing a path first and then a wavelength along the path is - Choosing a path first and then a wavelength along the path is
known to give adequate results in simple topologies such as rings known to give adequate results in simple topologies such as
and trees ([Yates99]). This does not appear to be true in large rings and trees ([Yates99]). This does not appear to be true in
mesh networks under realistic provisioning scenarios, however. large mesh networks under realistic provisioning scenarios,
Instead significantly better results are achieved if wavelength however. Instead significantly better results are achieved if
and route are chosen simultaneously ([Strand01b]). This approach wavelength and route are chosen simultaneously ([Strand01b]).
would however also have a significant effect on OSPF. This approach would however also have a significant effect on
OSPF.
4.8 Implications For Routing and Control Plane Design 4.8 Implications For Routing and Control Plane Design
If distributed routing is desired, additional state information will If distributed routing is desired, additional state information will
be required by the routing to deal with the impairments described in be required by the routing to deal with the impairments described in
Sections 4.2 - 4.4: Sections 4.2 - 4.4:
- As mentioned earlier, an operator who wants to avoid having to - As mentioned earlier, an operator who wants to avoid having to
provide impairment-related parameters to the control plane may provide impairment-related parameters to the control plane may
elect not to deal with them at the routing level, instead treating elect not to deal with them at the routing level, instead
them at the system design and planning level if that is a viable treating them at the system design and planning level if that is
approach for their network. In this approach the operator can a viable approach for their network. In this approach the
pre-qualifies all or a set of feasible end-to-end optical paths operator can pre-qualify all or a set of feasible end-to-end
through the domain of transparency for each bit rate. This Impairments And Other Constraints September 2002
approach may work well with relatively small and sparse networks, On Optical Layer Routing
but it may not be scalable for large and dense networks where the
number of feasible paths can be very large. 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- - If the optical paths are not pre-qualified, additional link-
specific state information will be required by the routing specific state information will be required by the routing
algorithm for each type of impairment that has the potential of algorithm for each type of impairment that has the potential of
being limiting for some routes. Note that for one operator, PMD being limiting for some routes. Note that for one operator, PMD
might be the only limiting constraint while for another, ASE might might be the only limiting constraint while for another, ASE
be the only one, or it could be both plus some other constraints might be the only one, or it could be both plus some other
considered in this document. Some networks might not be limited by constraints considered in this document. Some networks might not
any of these constraints. be limited by any of these constraints.
- For an operator needing to deal explicitly with these constraints, - For an operator needing to deal explicitly with these
the link-dependent information identified above for PMD is link- constraints, the link-dependent information identified above for
PMD-square which is the square of the total PMD on a link. For ASE PMD is link-PMD-square which is the square of the total PMD on a
the link-dependent information identified is link-noise which is link. For ASE the link-dependent information identified is link-
the total noise on a link. Other link-dependent information noise which is the total noise on a link. Other link-dependent
includes link-span-length which is the total number of spans on a information includes link-span-length which is the total number
link, link-crosstalk or OADM-OXC-number which is the total of spans on a link, link-crosstalk or OADM-OXC-number which is
crosstalk or the number of OADM/OXC nodes on a link, respectively, the total crosstalk or the number of OADM/OXC nodes on a link,
and filter-number which is the number of narrow filters on a link. respectively, and filter-number which is the number of narrow
When the alternative distance-only approach is chosen, the link- filters on a link. When the alternative distance-only approach
specific information is link-distance. is chosen, the link-specific information is link-distance.
. In addition to the link-specific information, bounds on each of - In addition to the link-specific information, bounds on each of
the impairments need to be quantified. Since these bounds are the impairments need to be quantified. Since these bounds are
Impairments And Other Constraints February 2002 determined by the system designer's impairment allocations,
On Optical Layer Routing these will be system dependent. For PMD, the constraint is that
the sum of the link-PMD-square of all links on the transparent
segment is less than the square of (a/B) where B is the bit
rate. Hence, the required information is the parameter "a". For
ASE, the constraint is that the sum of the link-noise of all
links is no larger than P/SNRmin. Thus, the information needed
include the launch power P and OSNR requirement SNRmin. The
minimum acceptable OSNR, in turn, depends on the strength of the
FEC being 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
determined by the system designer's impairment allocations, these - It is likely that the physical layer parameters do not change
will be system dependent. For PMD, the constraint is that the sum value rapidly and could be stored in some database; however
of the link-PMD-square of all links on the transparent segment is these are physical layer parameters that today are frequently
less than the square of (a/B) where B is the bit rate. Hence, the Impairments And Other Constraints September 2002
required information is the parameter "a". For ASE, the constraint On Optical Layer Routing
is that the sum of the link-noise of all links is no larger than
P/SNRmin. Thus, the information needed include the launch power P
and OSNR requirement SNRmin. The minimum acceptable OSNR, in
turn, depends on the strength of the FEC being 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 ôislandsö
some of these parameters may be available in a local or EMS
database and would not need to be advertised
. It is likely that the physical layer parameters do not change not known at the granularity required. If the ingress node of a
value rapidly and could be stored in some database; however these lightpath does path selection these parameters would need to be
are physical layer parameters that today are frequently not known available at this node.
at the granularity required. If the ingress node of a lightpath
does path selection these parameters would need to be available at
this node.
. The specific constraints required in a given situation will depend - The specific constraints required in a given situation will
on the design and engineering of the domain of transparency; for depend on the design and engineering of the domain of
example it will be essential to know whether chromatic dispersion transparency; for example it will be essential to know whether
has been dealt with on a per-link basis, and whether the domain is chromatic dispersion has been dealt with on a per-link basis,
operating in a linear or nonlinear regime. and whether the domain is operating in a linear or nonlinear
regime.
. As optical transport technology evolves, the set of constraints - As optical transport technology evolves, the set of constraints
that will need to be considered either explicitly or via a domain- that will need to be considered either explicitly or via a
wide margin may change. The routing and control plane design domain-wide margin may change. The routing and control plane
should therefore be as open as possible, allowing parameters to be design should therefore be as open as possible, allowing
included as necessary. parameters to be included as necessary.
. In the absence of wavelength conversion, the necessity of finding - In the absence of wavelength conversion, the necessity of
a single wavelength that is available on all links introduces the finding a single wavelength that is available on all links
need to either advertise detailed information on wavelength introduces the need to either advertise detailed information on
availability, which probably doesn't scale, or have some mechanism wavelength availability, which probably doesn't scale, or have
for probing potential routes with or without crankback to some mechanism for probing potential routes with or without
determine wavelength availability. Choosing the route first, and crankback to determine wavelength availability. Choosing the
then the wavelength, may not yield acceptable utilization levels route first, and then the wavelength, may not yield acceptable
in mesh-type networks. utilization levels in mesh-type networks.
5. More Complex Networks 5. More Complex Networks
Impairments And Other Constraints February 2002
On Optical Layer Routing
Mixing optical equipment in a single domain of transparency that has Mixing optical equipment in a single domain of transparency that has
not been explicitly designed to interwork is beyond the scope of not been explicitly designed to interwork is beyond the scope of
this document. This includes most multi-vendor all-optical networks. this document. This includes most multi-vendor all-optical networks.
An optical network composed of multiple domains of transparency An optical network composed of multiple domains of transparency
optically isolated from each other by O/E/O devices (transponders) optically isolated from each other by O/E/O devices (transponders)
is more plausible. A network composed of both "opaque" (optically is more plausible. A network composed of both "opaque" (optically
isolated) OLXC's and one or more all-optical "islands" isolated by isolated) OLXC's and one or more all-optical "islands" isolated by
transponders is of particular interest because this is most likely transponders is of particular interest because this is most likely
how all-optical technologies (such as that described in Sec. 2) are how all-optical technologies (such as that described in Sec. 2) are
going to be introduced. (We use the term "island" in this discussion going to be introduced. (We use the term "island" in this discussion
rather than a term like "domain" or "area" because these terms are rather than a term like "domain" or "area" because these terms are
associated with specific approaches like BGP or OSPF.) associated with specific approaches like BGP or OSPF.)
We consider the complexities raised by these alternatives now. We consider the complexities raised by these alternatives now.
The first requirement for routing in a multi-island network is that The first requirement for routing in a multi-island network is that
the routing process needs to know the extent of each island. There the routing process needs to know the extent of each island. There
are several reasons for this: are several reasons for this:
. When entering or leaving an all-optical island, the regeneration
Impairments And Other Constraints September 2002
On Optical Layer Routing
- When entering or leaving an all-optical island, the regeneration
process cleans up the optical impairments discussed in Sec. 3. process cleans up the optical impairments discussed in Sec. 3.
. Each all-optical island may have its own bounds on each - Each all-optical island may have its own bounds on each
impairment. impairment.
. The routing process needs to be sensitive to the costs associated - The routing process needs to be sensitive to the costs
with "island-hopping". associated with "island-hopping".
This last point needs elaboration. It is extremely important to This last point needs elaboration. It is extremely important to
realize that, at least in the short to intermediate term, the realize that, at least in the short to intermediate term, the
resources committed by a single routing decision can be very resources committed by a single routing decision can be very
significant: The equipment tied up by a single coast-to-coast OC-192 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 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. circuit once established is likely to be measured in months.
Carriers will expect the routing algorithms used to be sensitive to Carriers will expect the routing algorithms used to be sensitive to
these costs. Simplistic measures of cost such as the number of these costs. Simplistic measures of cost such as the number of
"hops" are not likely to be acceptable. "hops" are not likely to be acceptable.
skipping to change at page 18, line 4 skipping to change at page 17, line 39
economical for longer distances. It is therefore likely to be economical for longer distances. It is therefore likely to be
deployed as a sort of "express backbone". In this scenario a carrier deployed as a sort of "express backbone". In this scenario a carrier
is likely to expect the routing algorithm to balance OEO costs is likely to expect the routing algorithm to balance OEO costs
against the additional costs associated with ULH technology and against the additional costs associated with ULH technology and
route circuitously to make maximum use of the backbone where route circuitously to make maximum use of the backbone where
appropriate. Note that the metrics used to do this must be appropriate. Note that the metrics used to do this must be
consistent throughout the routing domain if this expectation is to consistent throughout the routing domain if this expectation is to
be met. be met.
The first-order implications for GMPLS seem to be: The first-order implications for GMPLS seem to be:
- Information about island boundaries needs to be advertised.
Impairments And Other Constraints February 2002 - The routing algorithm needs to be sensitive to island
On Optical Layer Routing transitions and to the connectivity limitations and impairment
constraints particular to each island.
. Information about island boundaries needs to be advertised. - The cost function used in routing must allow the balancing of
. The routing algorithm needs to be sensitive to island transitions transponder costs, OXC and OADM costs, and line haul costs
and to the connectivity limitations and impairment constraints across the entire routing domain.
particular to each island.
. 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.
Several distributed approaches to multi-island routing seem worth Several distributed approaches to multi-island routing seem worth
investigating: investigating:
. Advertise the internal topology and constraints of each island - Advertise the internal topology and constraints of each island
globally; let the ingress node compute an end-to-end strict globally; let the ingress node compute an end-to-end strict
explicit route sensitive to all constraints and wavelength explicit route sensitive to all constraints and wavelength
availabilities. In this approach the routing algorithm used by the availabilities. In this approach the routing algorithm used by
ingress node must be able to deal with the details of routing the ingress node must be able to deal with the details of
within each island. routing within each island.
. Have the EMS or control plane of each island determine and - Have the EMS or control plane of each island determine and
advertise the connectivity between its boundary nodes together advertise the connectivity between its boundary nodes together
with additional information such as costs and the bit rates and with additional information such as costs and the bit rates and
Impairments And Other Constraints September 2002
On Optical Layer Routing
formats supported. As the spare capacity situation changes, formats supported. As the spare capacity situation changes,
updates would be advertised. In this approach impairment updates would be advertised. In this approach impairment
constraints are handled within each island and impairment-related constraints are handled within each island and impairment-
parameters need not be advertised outside of the island. The related parameters need not be advertised outside of the island.
ingress node would then do a loose explicit route and leave the The ingress node would then do a loose explicit route and leave
routing and wavelength selection within each island to the island. the routing and wavelength selection within each island to the
. Have the ingress node send out probes or queries to nearby gateway island.
nodes or to an NMS to get routing guidance. - Have the ingress node send out probes or queries to nearby
gateway nodes or to an NMS to get routing guidance.
6. Diversity 6. Diversity
6.1 Background On Diversity 6.1 Background On Diversity
"Diversity" is a relationship between lightpaths. Two lightpaths are "Diversity" is a relationship between lightpaths. Two lightpaths are
said to be diverse if they have no single point of failure. In said to be diverse if they have no single point of failure. In
traditional telephony the dominant transport failure mode is a traditional telephony the dominant transport failure mode is a
failure in the interoffice plant, such as a fiber cut inflicted by a failure in the interoffice plant, such as a fiber cut inflicted by a
backhoe. backhoe.
Why is diversity a unique problem that needs to be considered for Why is diversity a unique problem that needs to be considered for
optical networks? So far, data network operators have relied on optical networks? So far, data network operators have relied on
their private line providers to ensure diversity and so have not had their private line providers to ensure diversity and so have not had
to deal directly with the problem. GMPLS makes the complexities to deal directly with the problem. GMPLS makes the complexities
handled by the private line provisioning process, including handled by the private line provisioning process, including
diversity, part of the common control plane and so visible to all. diversity, part of the common control plane and so visible to all.
To determine whether two lightpath routings are diverse it is To determine whether two lightpath routings are diverse it is
necessary to identify single points of failure in the interoffice necessary to identify single points of failure in the interoffice
Impairments And Other Constraints February 2002
On Optical Layer Routing
plant. To do so we will use the following terms: A fiber cable is a 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 uniform group of fibers contained in a sheath. An Optical Transport
System will occupy fibers in a sequence of fiber cables. Each fiber System will occupy fibers in a sequence of fiber cables. Each fiber
cable will be placed in a sequence of conduits - buried honeycomb cable will be placed in a sequence of conduits - buried honeycomb
structures through which fiber cables may be pulled - or buried in a 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 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 the right to install his conduit or fiber cable. It is worth noting
that for economic reasons, ROWÆs are frequently obtained from that for economic reasons, ROWs are frequently obtained from
railroads, pipeline companies, or thruways. It is frequently the railroads, pipeline companies, or thruways. It is frequently the
case that several carriers may lease ROW from the same source; this case that several carriers may lease ROW from the same source; this
makes it common to have a number of carriersÆ fiber cables in close makes it common to have a number of carriers fiber cables in close
proximity to each other. Similarly, in a metropolitan network, proximity to each other. Similarly, in a metropolitan network,
several carriers might be leasing duct space in the same RBOC several carriers might be leasing duct space in the same RBOC
conduit. There are also "carrier's carriers" - optical networks conduit. There are also "carrier's carriers" - optical networks
which provide fibers to multiple carriers, all of whom could be which provide fibers to multiple carriers, all of whom could be
affected by a single failure in the "carrier's carrier" network. affected by a single failure in the "carrier's carrier" network.
Impairments And Other Constraints September 2002
On Optical Layer Routing
In a typical intercity facility network there might be on the order In a typical intercity facility network there might be on the order
of 100 offices that are candidates for OLXCÆs. To represent the of 100 offices that are candidates for OLXCs. To represent the
inter-office fiber network accurately a network with an order of inter-office fiber network accurately a network with an order of
magnitude more nodes is required. In addition to Optical Amplifier magnitude more nodes is required. In addition to Optical Amplifier
(OA) sites, these additional nodes include: (OA) sites, these additional nodes include:
- Places where fiber cables enter/leave a conduit or right of way; - Places where fiber cables enter/leave a conduit or right of way;
- Locations where fiber cables cross; - Locations where fiber cables cross;
- Locations where fiber splices are used to interchange fibers Locations where fiber splices are used to interchange fibers
between fiber cables. between fiber cables.
An example of the first might be: An example of the first might be:
A B A B
A-------------B \ / A-------------B \ /
\ / \ /
X-----Y X-----Y
/ \ / \
C-------------D / \ C-------------D / \
C D C D
(a) Fiber Cable Topology (b) Right-Of-Way/Conduit Topology (a) Fiber Cable Topology (b) Right-Of-Way/Conduit Topology
skipping to change at page 20, line 4 skipping to change at page 19, line 32
/ \ / \
C-------------D / \ C-------------D / \
C D C D
(a) Fiber Cable Topology (b) Right-Of-Way/Conduit Topology (a) Fiber Cable Topology (b) Right-Of-Way/Conduit Topology
Figure 6-1: Fiber Cable vs. ROW Topologies Figure 6-1: Fiber Cable vs. ROW Topologies
Here the A-B fiber cable would be physically routed A-X-Y-B and the 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 C-D cable would be physically routed C-X-Y-D. This topology might
Impairments And Other Constraints February 2002
On Optical Layer Routing
arise because of some physical bottleneck: X-Y might be the Lincoln arise because of some physical bottleneck: X-Y might be the Lincoln
Tunnel, for example, or the Bay Bridge. Tunnel, for example, or the Bay Bridge.
Fiber route crossing (the second case) is really a special case of 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 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 not even be a manhole; the fiber routes might just be buried at
different depths. different depths.
Fiber splicing (the third case) often occurs when a major fiber Fiber splicing (the third case) often occurs when a major fiber
route passes near to a small office. To avoid the expense and route passes near to a small office. To avoid the expense and
additional transmission loss only a small number of fibers are additional transmission loss only a small number of fibers are
spliced out of the major route into a smaller route going to the 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 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 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 the manhole, and C the smaller office. The actual fiber topology
would then look like Fig. 6-2(b), where there would typically be would then look like Fig. 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 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 might have different numbers of fibers. (One of the latter might
even be missing.) even be missing.)
Impairments And Other Constraints September 2002
On Optical Layer Routing
C C C C
| / \ | / \
| / \ | / \
| / \ | / \
A------X------B A---------------B A------X------B A---------------B
(a) Fiber Cable Topology (b) Fiber Topology (a) Fiber Cable Topology (b) Fiber Topology
Figure 6-2. Fiber Cable vs Fiber Topologies Figure 6-2. Fiber Cable vs Fiber Topologies
skipping to change at page 21, line 5 skipping to change at page 20, line 30
Atlanta OTS and a Philadelphia - Orlando OTS might ride on the same Atlanta OTS and a Philadelphia - Orlando OTS might ride on the same
right of way for x miles in Maryland and then again for y miles in 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 Georgia. They might also cross at Raleigh or some other intermediate
node without sharing right of way. node without sharing right of way.
Diversity is often equated to routing two lightpaths between a Diversity is often equated to routing two lightpaths between a
single pair of points, or different pairs of points so that no single pair of points, or different pairs of points so that no
single route failure will disrupt them both. This is too simplistic, single route failure will disrupt them both. This is too simplistic,
for a number of reasons: for a number of reasons:
Impairments And Other Constraints February 2002
On Optical Layer Routing
- A sophisticated client of an optical network will want to derive - A sophisticated client of an optical network will want to derive
diversity needs from his/her end customers' availability diversity needs from his/her end customers' availability
requirements. These often lead to more complex diversity requirements. These often lead to more complex diversity
requirements than simply providing diversity between two requirements than simply providing diversity between two
lightpaths. For example, a common requirement is that no single lightpaths. For example, a common requirement is that no single
failure should isolate a node or nodes. If a node A has 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 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 diverse. In real applications, a large data network with N
lightpaths between its routers might describe their needs in an lightpaths between its routers might describe their needs in an
NxN matrix, where (i,j) defines whether lightpaths i and j must NxN matrix, where (i,j) defines whether lightpaths i and j must
be diverse. be diverse.
- Two circuits that might be considered diverse for one - Two circuits that might be considered diverse for one
application might not be considered diverse for in another application might not be considered diverse for in another
situation. Diversity is usually thought of as a reaction to situation. Diversity is usually thought of as a reaction to
interoffice route failures. High reliability applications may interoffice route failures. High reliability applications may
require other types of failures to be taken into account. Some require other types of failures to be taken into account. Some
examples: examples:
o Office Outages: Although less frequent than route failures, o Office Outages: Although less frequent than route failures,
fires, power outages, and floods do occur. Many network fires, power outages, and floods do occur. Many network
Impairments And Other Constraints September 2002
On Optical Layer Routing
managers require that diverse routes have no (intermediate) managers require that diverse routes have no (intermediate)
nodes in common. In other cases an intermediate node might nodes in common. In other cases an intermediate node might
be acceptable as long as there is power diversity within be acceptable as long as there is power diversity within
the office. the office.
o Shared Rings: Many applications are willing to allow o Shared Rings: Many applications are willing to allow
"diverse" circuits to share a SONET ring-protected link; "diverse" circuits to share a SONET ring-protected link;
presumably they would allow the same for optical layer presumably they would allow the same for optical layer
rings. rings.
o Disasters: Earthquakes and floods can cause failures over o Disasters: Earthquakes and floods can cause failures over
an extended area. Defense Department circuits might need an extended area. Defense Department circuits might need
to be routed with nuclear damage radii taken into account. to be routed with nuclear damage radii taken into account.
- Conversely, some networks may be willing to take somewhat larger - Conversely, some networks may be willing to take somewhat larger
risks. Taking route failures as an example: Such a network risks. Taking route failures as an example: Such a network
might be willing to consider two fiber cables in heavy duty might be willing to consider two fiber cables in heavy duty
concrete conduit as having a low enough chance of simultaneous concrete conduit as having a low enough chance of simultaneous
failure to be considered "diverse". They might also be willing failure to be considered "diverse". They might also be willing
to view two fiber cables buried on opposite sides of a railroad to view two fiber cables buried on opposite sides of a railroad
track as being diverse because there is minimal danger of a track as being diverse because there is minimal danger of a
single backhoe disrupting them both even though a bad train single backhoe disrupting them both even though a bad train
wreck might jeopardize them both. A network seeking N mutually wreck might jeopardize them both. A network seeking N mutually
diverse paths from an office with less than N diverse ROWÆs will diverse paths from an office with less than N diverse ROWs will
need to live with some level of compromise in the immediate need to live with some level of compromise in the immediate
vicinity of the office. vicinity of the office.
Impairments And Other Constraints February 2002
On Optical Layer Routing
These considerations strongly suggest that the routing algorithm These considerations strongly suggest that the routing algorithm
should be sensitive to the types of threat considered unacceptable should be sensitive to the types of threat considered unacceptable
by the requester. Note that the impairment constraints described in by the requester. Note that the impairment constraints described in
the previous section may eliminate some of the long circuitous the previous section may eliminate some of the long circuitous
routes sometimes needed to provide diversity. This would make it routes sometimes needed to provide diversity. This would make it
harder to find many diverse paths through an all-optical network harder to find many diverse paths through an all-optical network
than an opaque one. than an opaque one.
[Chaudhuri00] introduced the term "Shared Risk Link Group" (SRLG) to [Chaudhuri00] introduced the term "Shared Risk Link Group" (SRLG) to
describe the relationship between two non-diverse links. The above describe the relationship between two non-diverse links. The above
discussion suggests that an SRLG should be characterized by 2 discussion suggests that an SRLG should be characterized by 2
parameters: parameters:
- Type of Compromise: Examples would be shared fiber cable, shared - Type of Compromise: Examples would be shared fiber cable, shared
conduit, shared ROW, shared optical ring, shared office without conduit, shared ROW, shared optical ring, shared office without
power sharing, etc.) power sharing, etc.)
- Extent of Compromise: For compromised outside plant, this would - Extent of Compromise: For compromised outside plant, this would
be the length of the sharing. be the length of the sharing.
A CSPF algorithm could then penalize a diversity compromise by an A CSPF algorithm could then penalize a diversity compromise by an
amount dependent on these two parameters. amount dependent on these two parameters.
Impairments And Other Constraints September 2002
On Optical Layer Routing
Two links could be related by many SRLG's (AT&T's experience Two links could be related by many SRLG's (AT&T's experience
indicates that a link may belong to over 100 SRLG's, each indicates that a link may belong to over 100 SRLG's, each
corresponding to a separate fiber group. Each SRLG might relate a corresponding to a separate fiber group. Each SRLG might relate a
single link to many other links. For the optical layer, similar single link to many other links. For the optical layer, similar
situations can be expected where a link is an ultra-long OTS). situations can be expected where a link is an ultra-long OTS).
The mapping between links and different types of SRLGÆs is in The mapping between links and different types of SRLGs is in
general defined by network operators based on the definition of each general defined by network operators based on the definition of each
SRLG type. Since SRLG information is not yet ready to be SRLG type. Since SRLG information is not yet ready to be
discoverable by a network element and does not change dynamically, discoverable by a network element and does not change dynamically,
it need not be advertised with other resource availability it need not be advertised with other resource availability
information by network elements. It could be configured in some information by network elements. It could be configured in some
central database and be distributed to or retrieved by the nodes, or central database and be distributed to or retrieved by the nodes, or
advertised by network elements at the topology discovery stage. advertised by network elements at the topology discovery stage.
6.2 Implications For Routing 6.2 Implications For Routing
Dealing with diversity is an unavoidable requirement for routing in Dealing with diversity is an unavoidable requirement for routing in
the optical layer. It requires dealing with constraints in the the optical layer. It requires dealing with constraints in the
routing process but most importantly requires additional state routing process but most importantly requires additional state
information û the SRLG relationships and also the routings of any information the SRLG relationships and also the routings of any
existing circuits from the new circuit is to be diverse û to be existing circuits from the new circuit is to be diverse to be
available to the routing process. available to the routing process.
Impairments And Other Constraints February 2002
On Optical Layer Routing
At present SRLG information cannot be self-discovered. Indeed, in a At present SRLG information cannot be self-discovered. Indeed, in a
large network it is very difficult to maintain accurate SRLG large network it is very difficult to maintain accurate SRLG
information. The problem becomes particularly daunting whenever information. The problem becomes particularly daunting whenever
multiple administrative domains are involved, for instance after the multiple administrative domains are involved, for instance after the
acquisition of one network by another, because there normally is a acquisition of one network by another, because there normally is a
likelihood that there are diversity violations between the domains. likelihood that there are diversity violations between the domains.
It is very unlikely that diversity relationships between carriers It is very unlikely that diversity relationships between carriers
will be known any time in the near future. will be known any time in the near future.
Considerable variation in what different customers will mean by Considerable variation in what different customers will mean by
acceptable diversity should be anticipated. Consequently we suggest acceptable diversity should be anticipated. Consequently we suggest
that an SRLG should be defined as follows: (i) It is a relationship 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 between two or more links, and (ii) it is characterized by two
parameters, the type of compromise (shared conduit, shared ROW, parameters, the type of compromise (shared conduit, shared ROW,
shared optical ring, etc.) and the extent of the compromise (e.g., shared optical ring, etc.) and the extent of the compromise (e.g.,
the number of miles over which the compromise persisted). This will the number of miles over which the compromise persisted). This will
allow the SRLGÆs appropriate to a particular routing request to be allow the SRLGs appropriate to a particular routing request to be
easily identified. easily identified.
7. Security Considerations 7. Security Considerations
Impairments And Other Constraints September 2002
On Optical Layer Routing
The solution developed to address the requirements defined in this We are assuming OEO interfaces to the domain(s) covered by our
document must address security aspects. discussion (see, e.g., Sec. 4.1 above). If this assumption were to
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.
A key layer one security issue is resilience in the face of physical
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
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-
plane security is out of scope.
8. Acknowledgments 8. Acknowledgments
This document has benefited from discussions with Michael Eiselt, This document has benefited from discussions with Michael Eiselt,
Jonathan Lang, Mark Shtaif, Jennifer Yates, Dongmei Wang, Guangzhi Jonathan Lang, Mark Shtaif, Jennifer Yates, Dongmei Wang, Guangzhi
Li, Robert Doverspike, Albert Greenberg, Jim Maloney, John Jacob, Li, Robert Doverspike, Albert Greenberg, Jim Maloney, John Jacob,
Katie Hall, Diego Caviglia, D. Papadimitriou, O. Audouin, J. P. Katie Hall, Diego Caviglia, D. Papadimitriou, O. Audouin, J. P.
Faure, L. Noirie, and with our OIF colleagues. Faure, L. Noirie, and with our OIF colleagues.
References: 9. References
9.1 Normative References
Impairments And Other Constraints September 2002
On Optical Layer Routing
[Chaudhuri00] Chaudhuri, S., Hjalmtysson, G., and Yates, J.,
"Control of Lightpaths in an Optical Network", Work in Progress,
draft-chaudhuri-ip-olxc-control-00.txt.
[Goldstein94] Goldstein, E. L., Eskildsen, L., and Elrefaie, A. F.,
Performance Implications of Component Crosstalk in Transparent
Lightwave Networks", IEEE Photonics Technology Letters, Vol.6, No.5,
May 1994.
[ITU] ITU-T Doc. G.663, Optical Fibers and Amplifiers, Section
II.4.1.2.
[Kaminow97] Kaminow, I. P. and Koch, T. L., editors, Optical Fiber
Telecommunications IIIA, Academic Press, 1997.
[Strand01] J. Strand, A. Chiu, and R. Tkach, "Issues for Routing in
the Optical Layer", IEEE Communications Magazine, Feb. 2001, vol. 39
No. 2, pp. 81-88; also see "Unique Features and Requirements for The
Optical Layer Control Plane", Internet Draft, draft-chiu-strand-
unique-olcp-01.txt, work in progress, November 2000.
[Strand01b] J. Strand, R. Doverspike, and G. Li, "Importance of
Wavelength Conversion In An Optical Network", Optical Networks
Magazine, May/June 2001, pp. 33-44.
[Yates99] Yates, J. M., Rumsewicz, M. P. and Lacey, J. P. R.,
"Wavelength Converters in Dynamically-Reconfigurable WDM Networks",
IEEE Communications Surveys, 2Q1999 (online at
www.comsoc.org/pubs/surveys/2q99issue/yates.html).
9.2 Informative References
[Awduche99] Awduche, D. O., Rekhter, Y., Drake, J., and Coltun, R., [Awduche99] Awduche, D. O., Rekhter, Y., Drake, J., and Coltun, R.,
"Multi-Protocol Lambda Switching: Combining MPLS Traffic Engineering "Multi-Protocol Lambda Switching: Combining MPLS Traffic Engineering
Control With Optical Crossconnects", Work in Progress, draft- Control With Optical Crossconnects", Work in Progress, draft-
awduche-mpls-te-optical-01.txt. awduche-mpls-te-optical-01.txt.
[Bra96] Bradner, S., "The Internet Standards Process -- Revision 3," [Bra96] Bradner, S., "The Internet Standards Process -- Revision 3,"
BCP 9, RFC 2026, October 1996. BCP 9, RFC 2026, October 1996.
[CBD00] Ceuppens, L., Blumenthal, D., Drake, J., Chrostowski, J., [CBD00] Ceuppens, L., Blumenthal, D., Drake, J., Chrostowski, J.,
Edwards, W., "Performance Monitoring in Photonic Networks in Support Edwards, W., "Performance Monitoring in Photonic Networks in Support
of MPL(ambda)S", Internet draft, work in progress, March 2000. of MPL(ambda)S", Internet draft, work in progress, March 2000.
Impairments And Other Constraints February 2002 Impairments And Other Constraints September 2002
On Optical Layer Routing On Optical Layer Routing
[Chaudhuri00] Chaudhuri, S., Hjalmtysson, G., and Yates, J.,
"Control of Lightpaths in an Optical Network", Work in Progress,
draft-chaudhuri-ip-olxc-control-00.txt.
[Doverspike00] Doverspike, R. and Yates, J., "Challenges For MPLS in [Doverspike00] Doverspike, R. and Yates, J., "Challenges For MPLS in
Optical Network Restoration", IEEE Communication Magazine, February, Optical Network Restoration", IEEE Communication Magazine, February,
2001. 2001.
[Gerstel 2000] O. Gorstel, "Optical Layer Signaling: How Much Is [Gerstel 2000] O. Gorstel, "Optical Layer Signaling: How Much Is
Really Needed?" IEEE Communications Magazine, vol. 38 no. 10, Oct. Really Needed?" IEEE Communications Magazine, vol. 38 no. 10, Oct.
2000, pp. 154-160 2000, pp. 154-160
[GMPLS] E. Mannie (ed), ôGeneralized Multi-Protocol Label Switching
(GMPLS) Architectureö, Work in Progress, draft-ietf-ccamp-gmpls-
architecture-01.txt, Nov. 2001.
[Goldstein94] Goldstein, E. L., Eskildsen, L., and Elrefaie, A. F.,
Performance Implications of Component Crosstalk in Transparent
Lightwave Networks", IEEE Photonics Technology Letters, Vol.6, No.5,
May 1994.
[ITU] ITU-T Doc. G.663, Optical Fibers and Amplifiers, Section
II.4.1.2.
[Kaminow97] Kaminow, I. P. and Koch, T. L., editors, Optical Fiber
Telecommunications IIIA, Academic Press, 1997.
[KRB01a] Kompella, K., et.al., "IS-IS extensions in support of [KRB01a] Kompella, K., et.al., "IS-IS extensions in support of
Generalized MPLS," Internet Draft, draft-ietf-gmpls- extensions- Generalized MPLS," Internet Draft, draft-ietf-gmpls- extensions-
01.txt, work in progress, 2001. 01.txt, work in progress, 2001.
[KRB01b] Kompella, K., et. al., "OSPF extensions in support of [KRB01b] Kompella, K., et. al., "OSPF extensions in support of
Generalized MPLS," Internet draft, draft-ospf-generalized- mpls- Generalized MPLS," Internet draft, draft-ospf-generalized- mpls-
00.txt, work in progress, March 2001. 00.txt, work in progress, March 2001.
[Moy98] Moy, John T., OSPF: Anatomy of an Internet Routing Protocol, [Moy98] Moy, John T., OSPF: Anatomy of an Internet Routing Protocol,
Addison-Wesley, 1998. Addison-Wesley, 1998.
[Passmore01] Passmore, D. ôManaging Fatter Pipes,ö Business [Passmore01] Passmore, D., "Managing Fatter Pipes," Business
Communications Review, August, 2001, pp. 20-21. Communications Review, August 2001, pp. 20-21.
[Ramaswami98] Ramaswami, R. and Sivarajan, K. N., Optical Networks: [Ramaswami98] Ramaswami, R. and Sivarajan, K. N., Optical Networks:
A Practical Perspective, Morgan Kaufmann Publishers, 1998. A Practical Perspective, Morgan Kaufmann Publishers, 1998.
Impairments And Other Constraints February 2002
On Optical Layer Routing
[Strand01] J. Strand, A. Chiu, and R. Tkach, ôIssues for Routing in
the Optical Layerö, IEEE Communications Magazine, Feb. 2001, vol. 39
No. 2, pp. 81-88; also see "Unique Features and Requirements for The
Optical Layer Control Plane", Internet Draft, draft-chiu-strand-
unique-olcp-01.txt, work in progress, November 2000.
[Strand01b] J. Strand, R. Doverspike, and G. Li, ôImportance of
Wavelength Conversion In An Optical Networkö, Optical Networks
Magazine, accepted for publication. Available from the author.
[Tkach98] Tkach, R., Goldstein, E., Nagel, J., and Strand, J., [Tkach98] Tkach, R., Goldstein, E., Nagel, J., and Strand, J.,
"Fundamental Limits of Optical Transparency", Optical Fiber "Fundamental Limits of Optical Transparency", Optical Fiber
Communication Conf., Feb. 1998, pp. 161-162. Communication Conf., Feb. 1998, pp. 161-162.
[Yates99] Yates, J. M., Rumsewicz, M. P. and Lacey, J. P. R., 10. Contributing Authors
"Wavelength Converters in Dynamically-Reconfigurable WDM Networks",
IEEE Communications Surveys, 2Q1999 (online at
www.comsoc.org/pubs/surveys/2q99issue/yates.html).
Authors' Addresses: 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 Ayan Banerjee
Calient Networks Calient Networks
5853 Rue Ferrari 5853 Rue Ferrari
San Jose, CA 95138 San Jose, CA 95138
Email: abanerjee@calient.net Email: abanerjee@calient.net
Dan Blumenthal Dan Blumenthal
Calient Networks Calient Networks
Impairments And Other Constraints September 2002
On Optical Layer Routing
5853 Rue Ferrari 5853 Rue Ferrari
San Jose, CA 95138 San Jose, CA 95138
Email: dblumenthal@calient.net Email: dblumenthal@calient.net
Angela Chiu
Celion Networks
1 Sheila Dr., Suite 2
Tinton Falls, NJ 07724
Phone:(732) 747-9987
Email: angela.chiu@celion.com
John Drake John Drake
Calient Networks Calient Networks
5853 Rue Ferrari 5853 Rue Ferrari
San Jose, CA 95138 San Jose, CA 95138
Email: jdrake@calient.net Email: jdrake@calient.net
Andre Fredette Andre Fredette
Impairments And Other Constraints February 2002
On Optical Layer Routing
Hatteras Networks Hatteras Networks
PO Box 110025 PO Box 110025
Research Triangle Park, NC 27709 Research Triangle Park, NC 27709
Email: afredette@charter.net Email: afredette@hatterasnetworks.com
Nan Froberg Nan Froberg
PhotonEx Corporation PhotonEx Corporation
200 Metrowest Technology Dr. 200 Metrowest Technology Dr.
Maynard, MA 01754 Maynard, MA 01754
Email: nfroberg@photonex.com Email: nfroberg@photonex.com
Taha Landolsi Taha Landolsi
WorldCom, Inc.
2400 North Glenville Drive 2400 North Glenville Drive
Richardson, TX 75082 Richardson, TX 75082
Telephone: 972-729-5201
Email: taha.landolsi@wcom.com Email: taha.landolsi@wcom.com
James V. Luciani James V. Luciani
900 Chelmsford St. 900 Chelmsford St.
Lowell, MA 01851 Lowell, MA 01851
+1 978 275 3182 Email: james_luciani@mindspring.com
james_luciani@mindspring.com
John Strand
AT&T Labs
200 Laurel Ave., Rm A5-1D06
Middletown, NJ 07748
Phone:(732) 420-9036
Email: jls@research.att.com
Robert Tkach Robert Tkach
Celion Networks Celion Networks
1 Sheila Dr., Suite 2 1 Sheila Dr., Suite 2
Tinton Falls, NJ 07724 Tinton Falls, NJ 07724
Phone:(732) 747-9909
Email: bob.tkach@celion.com Email: bob.tkach@celion.com
Yong Xue Yong Xue
WorldCom, Inc. WorldCom, Inc.
22001 Loudoun County Parkway 22001 Loudoun County Parkway
Ashburn, VA 20147 Ashburn, VA 20147
Telephone: (703) 886-5358 Email: yxue@cox.com
Email: yong.xue@wcom.com
11. Editors’ Addresses
Angela Chiu
Impairments And Other Constraints September 2002
On Optical Layer Routing
Celion Networks
1 Sheila Dr., Suite 2
Tinton Falls, NJ 07724
Phone:(732) 747-9987
Email: alchiu@ieee.org
John Strand
AT&T Labs
200 Laurel Ave., Rm A5-1D33
Middletown, NJ 07748
Phone:(732) 420-9036
Email: jls@research.att.com
 End of changes. 

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