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+Network Working Group J. Strand, Ed.
+Request for Comments: 4054 A. Chiu, Ed.
+Category: Informational AT&T
+ May 2005
+
+
+ Impairments and Other Constraints on Optical Layer Routing
+
+Status of This Memo
+
+ This memo provides information for the Internet community. It does
+ not specify an Internet standard of any kind. Distribution of this
+ memo is unlimited.
+
+Copyright Notice
+
+ Copyright (C) The Internet Society (2005).
+
+Abstract
+
+ Optical networking poses a number challenges for Generalized Multi-
+ Protocol Label Switching (GMPLS). Fundamentally, optical technology
+ is an analog rather than digital technology whereby the optical layer
+ is lowest in the transport hierarchy and hence has an intimate
+ relationship with the physical geography of the network. This
+ contribution surveys some of the aspects of optical networks that
+ impact routing and identifies possible GMPLS responses for each: (1)
+ Constraints arising from the design of new software controllable
+ network elements, (2) Constraints in a single all-optical domain
+ without wavelength conversion, (3) Complications arising in more
+ complex networks incorporating both all-optical and opaque
+ architectures, and (4) Impacts of diversity constraints.
+
+Table of Contents
+
+ 1. Introduction ................................................. 2
+ 2. Sub-IP Area Summary and Justification of Work ................ 3
+ 3. Reconfigurable Network Elements .............................. 3
+ 3.1. Technology Background .................................. 3
+ 3.2. Implications for Routing ............................... 6
+ 4. Wavelength Routed All-Optical Networks ....................... 6
+ 4.1. Problem Formulation .................................... 7
+ 4.2. Polarization Mode Dispersion (PMD) ..................... 8
+ 4.3. Amplifier Spontaneous Emission ......................... 9
+ 4.4. Approximating the Effects of Some Other
+ Impairments Constraints ................................ 10
+ 4.5. Other Impairment Considerations ........................ 13
+
+
+
+
+Strand & Chiu Informational [Page 1]
+
+RFC 4054 Optical Layer Routing May 2005
+
+
+ 4.6. An Alternative Approach - Using Maximum
+ Distance as the Only Constraint ........................ 13
+ 4.7. Other Considerations ................................... 15
+ 4.8. Implications for Routing and Control Plane Design ...... 15
+ 5. More Complex Networks ........................................ 17
+ 6. Diversity .................................................... 19
+ 6.1. Background on Diversity ................................ 19
+ 6.2. Implications for Routing ............................... 23
+ 7. Security Considerations ...................................... 23
+ 8. Acknowledgements ............................................. 24
+ 9. References ................................................... 25
+ 9.1. Normative References ................................... 25
+ 9.2. Informative References ................................. 26
+ 10. Contributing Authors ......................................... 26
+
+1. Introduction
+
+ Generalized Multi-Protocol Label Switching (GMPLS) [Mannie04] aims to
+ extend MPLS to encompass a number of transport architectures,
+ including optical networks that incorporate a number of all-optical
+ and opto-electronic elements, such as optical cross-connects with
+ both optical and electrical fabrics, transponders, and optical add-
+ drop multiplexers. Optical networking poses a number of challenges
+ for GMPLS. Fundamentally, optical technology is an analog rather
+ than digital technology whereby the optical layer is lowest in the
+ transport hierarchy and hence has an intimate relationship with the
+ physical geography of the network.
+
+ GMPLS already has incorporated extensions to deal with some of the
+ unique aspects of the optical layer. This contribution surveys some
+ of the aspects of optical networks that impact routing and identifies
+ possible GMPLS responses for each. Routing constraints and/or
+ complications arising from the design of network elements, the
+ accumulation of signal impairments, and the need to guarantee the
+ physical diversity of some circuits are discussed.
+
+ Since the purpose of this document is to further the specification of
+ GMPLS, alternative approaches to controlling an optical network are
+ not discussed. For discussions of some broader issues, see
+ [Gerstel2000] and [Strand02].
+
+ The organization of the contribution is as follows:
+
+ - Section 2 is a section requested by the sub-IP Area management for
+ all new documents. It explains how this document fits into the
+ Area and into the IPO WG, and why it is appropriate for these
+ groups.
+
+
+
+
+Strand & Chiu Informational [Page 2]
+
+RFC 4054 Optical Layer Routing May 2005
+
+
+ - Section 3 describes constraints arising from the design of new
+ software controllable network elements.
+
+ - Section 4 addresses the constraints in a single all-optical domain
+ without wavelength conversion.
+
+ - Section 5 extends the discussion to more complex networks and
+ incorporates both all-optical and opaque architectures.
+
+ - Section 6 discusses the impacts of diversity constraints.
+
+ - Section 7 deals with security requirements.
+
+ - Section 8 contains acknowledgments.
+
+ - Section 9 contains references.
+
+ - Section 10 contains contributing authors' addresses.
+
+2. Sub-IP Area Summary and Justification of Work
+
+ This document merges and extends two previous expired Internet-Drafts
+ that were made IPO working group documents to form a basis for a
+ design team at the Minneapolis IETF meeting, where it was also
+ requested that they be merged to create a requirements document for
+ the WG.
+
+ In the larger sub-IP Area structure, this merged document describes
+ specific characteristics of optical technology and the requirements
+ they place on routing and path selection. It is appropriate for the
+ IPO working group because the material is specific to optical
+ networks. It identifies and documents the characteristics of the
+ optical transport network that are important for selecting paths for
+ optical channels, which is a work area for the IPO WG. The material
+ covered is directly aimed at establishing a framework and
+ requirements for routing in an optical network.
+
+3. Reconfigurable Network Elements
+
+3.1. Technology Background
+
+ Control plane architectural discussions (e.g., [Awduche99]) usually
+ assume that the only software reconfigurable network element is an
+ optical layer cross-connect (OLXC). There are however other software
+ reconfigurable elements on the horizon, specifically tunable lasers
+ and receivers and reconfigurable optical add-drop multiplexers
+
+
+
+
+
+Strand & Chiu Informational [Page 3]
+
+RFC 4054 Optical Layer Routing May 2005
+
+
+ (OADM). These elements are illustrated in the following simple
+ example, which is modeled on announced Optical Transport System (OTS)
+ products:
+
+ + +
+ ---+---+ |\ /| +---+---
+ ---| A |----|D| X Y |D|----| A |---
+ ---+---+ |W| +--------+ +--------+ |W| +---+---
+ : |D|-----| OADM |-----| OADM |-----|D| :
+ ---+---+ |M| +--------+ +--------+ |M| +---+---
+ ---| A |----| | | | | | | |----| A |---
+ ---+---+ |/ | | | | \| +---+---
+ + +---+ +---+ +---+ +---+ +
+ D | A | | A | | A | | A | E
+ +---+ +---+ +---+ +---+
+ | | | | | | | |
+
+ Figure 3-1: An OTS With OADMs - Functional Architecture
+
+ 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
+ two aspects are critical:
+
+ - Adaptation: These are the functions done at the edges of the
+ subnetwork that transform the incoming optical channel into the
+ physical wavelength to be transported through the subnetwork.
+
+ - Connectivity: This defines which pairs of edge Adaptation
+ functions can be interconnected through the subnetwork.
+
+ In Fig. 3-1, D and E are DWDMs and X and Y are OADMs. The boxes
+ labeled "A" are adaptation functions. They map one or more input
+ optical channels assumed to be standard short reach signals into a
+ long reach (LR) wavelength or wavelength group that will pass
+ transparently to a distant adaptation function. Adaptation
+ functionality that affects routing includes:
+
+ - Multiplexing: Either electrical or optical TDM may be used to
+ combine the input channels into a single wavelength. This is 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 10 Gb/sec
+ (500 Gb/sec total); combining the 2.5 Gb/sec signals together thus
+ effectively doubles capacity. After multiplexing the combined
+ signal must be routed as a group to the distant adaptation
+ function.
+
+
+
+
+
+
+Strand & Chiu Informational [Page 4]
+
+RFC 4054 Optical Layer Routing May 2005
+
+
+ - Adaptation Grouping: In this technique, groups of k (e.g., 4)
+ wavelengths are managed as a group within the system and must be
+ added/dropped as a group. We will call such a group an
+ "adaptation grouping". Examples include so called "wave group"
+ and "waveband" [Passmore01]. Groupings on the same system may
+ differ in basics such as wavelength spacing, which constrain the
+ type of channels that can be accommodated.
+
+ - Laser Tunability: The lasers producing the LR wavelengths may have
+ a fixed frequency, may be tunable over a limited range, or may be
+ tunable over the entire range of wavelengths supported by the
+ DWDM. Tunability speeds may also vary.
+
+ Connectivity between adaptation functions may also be limited:
+
+ - As pointed out above, TDM multiplexing and/or adaptation grouping
+ by the adaptation function forces groups of input channels to be
+ delivered together to the same distant adaptation function.
+
+ - Only adaptation functions whose lasers/receivers are tunable to
+ compatible frequencies can be connected.
+
+ - The switching capability of the OADMs may also be constrained.
+
+ For example:
+
+ o There may be some wavelengths that can not be dropped at all.
+
+ o There may be a fixed relationship between the frequency dropped
+ and the physical port on the OADM to which it is dropped.
+
+ o OADM physical design may put an upper bound on the number of
+ adaptation groupings dropped at any single OADM.
+
+ For a fixed configuration of the OADMs and adaptation functions
+ connectivity will be fixed: Each input port will essentially be
+ hard-wired to some specific distant port. However this connectivity
+ can be changed by changing the configurations of the OADMs and
+ adaptation functions. For example, an additional adaptation grouping
+ might be dropped at an OADM or a tunable laser retuned. In each case
+ the port-to-port connectivity is changed.
+
+ These capabilities can be expected to be under software control.
+ Today the control would rest in the vendor-supplied Element
+ Management system (EMS), which in turn would be controlled by the
+ operator's OSes. However in principle the EMS could participate in
+ the GMPLS routing process.
+
+
+
+
+Strand & Chiu Informational [Page 5]
+
+RFC 4054 Optical Layer Routing May 2005
+
+
+3.2. Implications for Routing
+
+ An OTS of the sort discussed in Sec. 3.1 is essentially a
+ geographically distributed but blocking cross-connect system. The
+ specific port connectivity is dependent on the vendor design and also
+ on exactly what line cards have been deployed.
+
+ One way for GMPLS to deal with this architecture would be to view the
+ port connectivity as externally determined. In this case the links
+ known to GMPLS would be groups of identically routed wavebands. If
+ these were reconfigured by the external EMS the resulting
+ connectivity changes would need to be detected and advertised within
+ GMPLS. If the topology shown in Fig. 3-1 became a tree or a mesh
+ instead of the linear topology shown, the connectivity changes could
+ result in Shared Risk Link Group (SRLG - see Section 6.2) changes.
+
+ Alternatively, GMPLS could attempt to directly control this port
+ connectivity. The state information needed to do this is likely to
+ be voluminous and vendor specific.
+
+4. Wavelength Routed All-Optical Networks
+
+ The optical networks deployed until recently may be called "opaque"
+ ([Tkach98]): each link is optically isolated by transponders doing
+ O/E/O conversions. They provide regeneration with retiming and
+ reshaping, also called 3R, which eliminates transparency to bit rates
+ and frame format. These transponders are quite expensive and their
+ lack of transparency also constrains the rapid introduction of new
+ services. Thus there are strong motivators to introduce "domains of
+ transparency" - all-optical subnetworks - larger than an OTS.
+
+ The routing of lightpaths through an all-optical network has received
+ extensive attention. (See [Yates99] or [Ramaswami98]). When
+ discussing routing in an all-optical network it is usually assumed
+ that all routes have adequate signal quality. This may be ensured by
+ limiting all-optical networks to subnetworks of limited geographic
+ size that are optically isolated from other parts of the optical
+ layer by transponders. This approach is very practical and has been
+ applied to date, e.g., when determining the maximum length of an
+ Optical Transport System (OTS). Furthermore operational
+ considerations like fault isolation also make limiting the size of
+ domains of transparency attractive.
+
+ There are however reasons to consider contained domains of
+ transparency in which not all routes have adequate signal quality.
+ From a demand perspective, maximum bit rates have rapidly increased
+ from DS3 to OC-192 and soon OC-768 (40 Gb/sec). As bit rates
+ increase it is necessary to increase power. This makes impairments
+
+
+
+Strand & Chiu Informational [Page 6]
+
+RFC 4054 Optical Layer Routing May 2005
+
+
+ and nonlinearities more troublesome. From a supply perspective,
+ optical technology is advancing very rapidly, making ever-larger
+ domains possible. In this section, we assume that these
+ considerations will lead to the deployment of a domain of
+ transparency that is too large to ensure that all potential routes
+ have adequate signal quality for all circuits. Our goal is to
+ understand the impacts of the various types of impairments in this
+ environment.
+
+ Note that, as we describe later in the section, there are many types
+ of physical impairments. Which of these needs to be dealt with
+ explicitly when performing on-line distributed routing will vary
+ considerably and will depend on many variables, including:
+
+ - Equipment vendor design choices,
+ - Fiber characteristics,
+ - Service characteristics (e.g., circuit speeds),
+ - Network size,
+ - Network operator engineering and deployment strategies.
+
+ For example, a metropolitan network that does not intend to support
+ bit rates above 2.5 Gb/sec may not be constrained by any of these
+ impairments, while a continental or international network that wished
+ to minimize O/E/O regeneration investment and support 40 Gb/sec
+ connections might have to explicitly consider many of them. Also, a
+ network operator may reduce or even eliminate their constraint set by
+ building a relatively small domain of transparency to ensure that all
+ the paths are feasible, or by using some proprietary tools based on
+ rules from the OTS vendor to pre-qualify paths between node pairs and
+ put them in a table that can be accessed each time a routing decision
+ has to be made through that domain.
+
+4.1. Problem Formulation
+
+ We consider a single domain of transparency without wavelength
+ translation. Additionally, due to the proprietary nature of DWDM
+ transmission technology, we assume that the domain is either single
+ vendor or architected using a single coherent design, particularly
+ with regard to the management of impairments.
+
+ We wish to route a unidirectional circuit from ingress client node X
+ to egress client node Y. At both X and Y, the circuit goes through
+ an O/E/O conversion that optically isolates the portion within our
+ domain. We assume that we know the bit rate of the circuit. Also,
+ we assume that the adaptation function at X may apply some Forward
+ Error Correction (FEC) method to the circuit. We also assume we know
+ the launch power of the laser at X.
+
+
+
+
+Strand & Chiu Informational [Page 7]
+
+RFC 4054 Optical Layer Routing May 2005
+
+
+ Impairments can be classified into two categories, linear and
+ nonlinear. (See [Tkach98] or [Kaminow02] for more on impairment
+ constraints.) Linear effects are independent of signal power and
+ affect wavelengths individually. Amplifier spontaneous emission
+ (ASE), polarization mode dispersion (PMD), and chromatic dispersion
+ are examples. Nonlinearities are significantly more complex: they
+ generate not only impairments on each channel, but also crosstalk
+ between channels.
+
+ In the remainder of this section we first outline how two key linear
+ impairments (PMD and ASE) might be handled by a set of analytical
+ formulae as additional constraints on routing. We next discuss how
+ the remaining constraints might be approached. Finally we take a
+ broader perspective and discuss the implications of such constraints
+ on control plane architecture and also on broader constrained domain
+ of transparency architecture issues.
+
+4.2. Polarization Mode Dispersion (PMD)
+
+ For a transparent fiber segment, the general PMD requirement is that
+ the time-average differential group delay (DGD) between two
+ orthogonal state of polarizations should be less than some fraction a
+ of the bit duration, T=1/B, where B is the bit rate. The value of
+ the parameter a depends on three major factors: 1) margin allocated
+ to PMD, e.g., 1dB; 2) targeted outage probability, e.g., 4x10-5, and
+ 3) sensitivity of the receiver to DGD. A typical value for a is 10%
+ [ITU]. More aggressive designs to compensate for PMD may allow
+ values higher than 10%. (This would be a system parameter dependent
+ on the system design. It would need to be known to the routing
+ process.)
+
+ The PMD parameter (Dpmd) is measured in pico-seconds (ps) per
+ sqrt(km). The square of the PMD in a fiber span, denoted as span-
+ PMD-square is then given by the product of Dpmd**2 and the span
+ length. (A fiber span in a transparent network refers to a segment
+ between two optical amplifiers.) If Dpmd is constant, this results
+ in a upper bound on the maximum length of an M-fiber-span transparent
+ segment, which is inversely proportional to the square of the product
+ of bit rate and Dpmd (the detailed equation is omitted due to the
+ format constraint - see [Strand01] for details).
+
+ 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 the
+ transparent segment should not exceed 400km and 25km for bit rates of
+ 10Gb/s and 40Gb/s, respectively. Due to recent advances in fiber
+ technology, the PMD-limited distance has increased dramatically. For
+ newer fibers with a PMD parameter of 0.1 picosecond per square root
+ of km, the maximum length of the transparent segment (without PMD
+
+
+
+Strand & Chiu Informational [Page 8]
+
+RFC 4054 Optical Layer Routing May 2005
+
+
+ compensation) is limited to 10000km and 625km for bit rates of 10Gb/s
+ and 40Gb/, respectively. Still lower values of PMD are attainable in
+ commercially available fiber today, and the PMD limit can be further
+ extended if a larger value of the parameter a (ratio of DGD to the
+ bit period) can be tolerated. In general, the PMD requirement is not
+ an issue for most types of fibers at 10Gb/s or lower bit rate. But
+ it will become an issue at bit rates of 40Gb/s and higher.
+
+ If the PMD parameter varies between spans, a slightly more
+ complicated equation results (see [Strand01]), but in any event the
+ only link dependent information needed by the routing algorithm is
+ the square of the link PMD, denoted as link-PMD-square. It is the
+ sum of the span-PMD-square of all spans on the link.
+
+ Note that when one has some viable PMD compensation devices and
+ deploy them ubiquitously on all routes with potential PMD issues in
+ the network, then the PMD constraint disappears from the routing
+ perspective.
+
+4.3. Amplifier Spontaneous Emission
+
+ ASE degrades the optical signal to noise ratio (OSNR). An acceptable
+ optical SNR level (SNRmin), which depends on the bit rate,
+ transmitter-receiver technology (e.g., FEC), and margins allocated
+ for the impairments, needs to be maintained at the receiver. In
+ order to satisfy this requirement, vendors often provide some general
+ engineering rule in terms of maximum length of the transparent
+ segment and number of spans. For example, current transmission
+ systems are often limited to up to 6 spans each 80km long. For
+ larger transparent domains, more detailed OSNR computations will be
+ needed to determine whether the OSNR level through a domain of
+ transparency is acceptable. This would provide flexibility in
+ provisioning or restoring a lightpath through a transparent
+ subnetwork.
+
+ Assume that the average optical power launched at the transmitter is
+ P. The lightpath from the transmitter to the receiver goes through M
+ optical amplifiers, with each introducing some noise power. Unity
+ gain can be used at all amplifier sites to maintain constant signal
+ power at the input of each span to minimize noise power and
+ nonlinearity. A constraint on the maximum number of spans can be
+ obtained [Kaminow97] which is proportional to P and inversely
+ proportional to SNRmin, optical bandwidth B, amplifier gain G-1 and
+ spontaneous emission factor n of the optical amplifier, assuming all
+ spans have identical gain and noise figure. (Again, the detailed
+ equation is omitted due to the format constraint - see [Strand01] for
+ details.) Let's take a typical example. Assuming P=4dBm,
+ SNRmin=20dB with FEC, B=12.5GHz, n=2.5, G=25dB, based on the
+
+
+
+Strand & Chiu Informational [Page 9]
+
+RFC 4054 Optical Layer Routing May 2005
+
+
+ constraint, the maximum number of spans is at most 10. However, if
+ FEC is not used and the requirement on SNRmin becomes 25dB, the
+ maximum number of spans drops down to 3.
+
+ For ASE the only link-dependent information needed by the routing
+ algorithm is the noise of the link, denoted as link-noise, which is
+ the sum of the noise of all spans on the link. Hence the constraint
+ on ASE becomes that the aggregate noise of the transparent segment
+ which is the sum of the link-noise of all links can not exceed
+ P/SNRmin.
+
+4.4. Approximating the Effects of Some Other Impairment Constraints
+
+ There are a number of other impairment constraints that we believe
+ could be approximated with a domain-wide margin on the OSNR, plus in
+ some cases a constraint on the total number of networking elements
+ (OXC or OADM) along the path. Most impairments generated at OXCs or
+ OADMs, including polarization dependent loss, coherent crosstalk, and
+ effective passband width, could be dealt with using this approach.
+ In principle, impairments generated at the nodes can be bounded by
+ system engineering rules because the node elements can be designed
+ and specified in a uniform manner. This approach is not feasible
+ with PMD and noise because neither can be uniformly specified.
+ Instead, they depend on node spacing and the characteristics of the
+ installed fiber plant, neither of which are likely to be under the
+ system designer's control.
+
+ Examples of the constraints we propose to approximate with a domain-
+ wide margin are given in the remaining paragraphs in this section.
+ It should be kept in mind that as optical transport technology
+ evolves it may become necessary to include some of these impairments
+ explicitly in the routing process. Other impairments not mentioned
+ here at all may also become sufficiently important to require
+ incorporation either explicitly or via a domain-wide margin.
+
+ Other Polarization Dependent Impairments
+ Other polarization-dependent effects besides PMD influence system
+ performance. For example, many components have polarization-
+ dependent loss (PDL) [Ramaswami98], which accumulates in a system
+ with many components on the transmission path. The state of
+ polarization fluctuates with time and its distribution is very
+ important also. It is generally required that the total PDL on
+ the path be maintained within some acceptable limit, potentially
+ by using some compensation technology for relatively long
+ transmission systems, plus a small built-in 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 system vendor when
+ determining the maximum allowable number of spans.
+
+
+
+Strand & Chiu Informational [Page 10]
+
+RFC 4054 Optical Layer Routing May 2005
+
+
+ Chromatic Dispersion
+ In general this impairment can be adequately (but not optimally)
+ compensated for on a per-link basis, and/or at system initial
+ setup time. Today most deployed compensation devices are based on
+ Dispersion Compensation Fiber (DCF). DCF provides per 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 slope of the fiber and the DCF some lambdas can be over
+ compensated while others can be under compensated. Moreover DCF
+ modules may only be available in fixed lengths of compensating
+ fiber; this means that sometimes it is impossible to find a DCF
+ module that exactly compensates the CD introduced by the fiber.
+ These effects introduce what is known as residual CD. Residual CD
+ varies with the frequency of the wavelength. Knowing the
+ characteristics of both of the fiber and the DCF modules along the
+ path, this can be calculated with a sufficient degree of
+ precision. However this is a very challenging task. In fact the
+ per-wavelength residual dispersion needs to be combined with other
+ information in the system (e.g., types fibers to figure out the
+ amount of nonlinearities) to obtain the net effect of CD either by
+ simulation or by some analytical approximation. It appears that
+ the routing/control plane should not be burdened by such a large
+ set of information while it can be handled at the system design
+ level. Therefore it will be assumed until proven otherwise that
+ residual dispersion should not be reported. For high bit rates,
+ dynamic dispersion compensation may be required at the receiver to
+ clean up any residual dispersion.
+
+ Crosstalk
+ Optical crosstalk refers to the effect of other signals on the
+ desired signal. It includes both coherent (i.e., intrachannel)
+ crosstalk and incoherent (i.e., interchannel) crosstalk. Main
+ contributors of crosstalk are the OADM and OXC sites that use a
+ DWDM multiplexer/demultiplexer (MUX/DEMUX) pair. For a relatively
+ sparse network where the number of OADM/OXC nodes on a path is
+ low, crosstalk can be treated with a low margin in OSNR without
+ being a binding constraint. But for some relatively dense
+ networks where crosstalk might become a binding constraint, one
+ needs to propagate the per-link crosstalk information to make sure
+ that the end-to-end path crosstalk which is the sum of the
+ crosstalks on all the corresponding links to be within some limit,
+ e.g., -25dB threshold with 1dB penalty ([Goldstein94]). Another
+ way to treat it without having to propagate per-link crosstalk
+ information is to have the system evaluate what the maximum number
+ of OADM/OXC nodes that has a MUX/DEMUX pair for the worst route in
+ the transparent domain for a low built-in margin. The latter one
+ should work well where all the OXC/OADM nodes have similar level
+ of crosstalk.
+
+
+
+Strand & Chiu Informational [Page 11]
+
+RFC 4054 Optical Layer Routing May 2005
+
+
+ Effective Passband
+ As more and more DWDM components are cascaded, the effective
+ passband narrows. The number of filters along the link, their
+ passband width and their shape will determine the end-to-end
+ effective passband. In general, this is a system design issue,
+ i.e., the system is designed with certain maximum bit rate using
+ the proper modulation format and filter spacing. For linear
+ systems, the filter effect can be turned into a constraint on the
+ maximum number of narrow filters with the condition that filters
+ in the systems are at least as wide as the one in the receiver.
+ Because traffic at lower bit rates can tolerate a narrower
+ passband, the maximum allowable number of narrow filters will
+ increase as the bit rate decreases.
+
+ Nonlinear Impairments
+ It seems unlikely that these can be dealt with explicitly in a
+ routing algorithm because they lead to constraints that can couple
+ routes together and lead to complex dependencies, e.g., on the
+ order in which specific fiber types are traversed [Kaminow97].
+ Note that different fiber types (standard single mode fiber,
+ dispersion shifted fiber, dispersion compensated fiber, etc.) have
+ very different effects from nonlinear impairments. A full
+ treatment of the nonlinear constraints would likely require very
+ detailed knowledge of the physical infrastructure, including
+ measured dispersion values for each span, fiber core area and
+ composition, as well as knowledge of subsystem details such as
+ dispersion compensation technology. This information would need
+ to be combined with knowledge of the current loading of optical
+ signals on the links of interest to determine the level of
+ nonlinear impairment. Alternatively, one could assume that
+ nonlinear impairments are bounded and result in X dB margin in the
+ required OSNR level for a given bit rate, where X for performance
+ reasons would be limited to 1 or 2 dB, consequently setting a
+ limit on the maximum number of spans. For the approach described
+ here to be useful, it is desirable for this span length limit to
+ be longer than that imposed by the constraints which can be
+ treated explicitly. When designing a DWDM transport system, there
+ are tradeoffs between signal power launched at the transmitter,
+ span length, and nonlinear effects on BER that need to be
+ considered jointly. Here, we assume that an X dB margin is
+ obtained after the transport system has been designed with a fixed
+ signal power and maximum span length for a given bit rate. Note
+ that OTSs can be designed in very different ways, in linear,
+ pseudo-linear, or nonlinear environments. The X-dB margin
+ approach may be valid for some but not for others. However, it is
+ likely that there is an advantage in designing systems that are
+
+
+
+
+
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+
+ less aggressive with respect to nonlinearities, and therefore
+ somewhat sub-optimal, in exchange for improved scalability,
+ simplicity and flexibility in routing and control plane design.
+
+4.5. Other Impairment Considerations
+
+ There are many other types of impairments that can degrade
+ performance. In this section, we briefly mention one other type of
+ impairment, which we propose be dealt with by either the system
+ designer or by the transmission engineers at the time the system is
+ installed. If dealt with successfully in this manner they should not
+ need to be considered in the dynamic routing process.
+
+ Gain Nonuniformity and Gain Transients For simple noise estimates to
+ be of use, the amplifiers must be gain-flattened and must have
+ automatic gain control (AGC). Furthermore, each link should have
+ dynamic gain equalization (DGE) to optimize power levels each time
+ wavelengths are added or dropped. Variable optical attenuators on
+ the output ports of an OXC or OADM can be used for this purpose, and
+ in-line devices are starting to become commercially available.
+ Optical channel monitors are also required to provide feedback to the
+ DGEs. AGC must be done rapidly if signal degradation after a
+ protection switch or link failure is to be avoided.
+
+ Note that the impairments considered here are treated more or less
+ independently. By considering them jointly and varying the tradeoffs
+ between the effects from different components may allow more routes
+ to be feasible. If that is desirable or the system is designed such
+ that certain impairments (e.g., nonlinearities) need to be considered
+ by a centralized process, then distributed routing is not the one to
+ use.
+
+4.6. An Alternative Approach - Using Maximum Distance as the Only
+ Constraint
+
+ Today, carriers often use maximum distance to engineer point-to-point
+ OTS systems given a fixed per-span length based on the OSNR
+ constraint for a given bit rate. They may desire to keep the same
+ engineering rule when they move to all-optical networks. Here, we
+ discuss the assumptions that need to be satisfied to keep this
+ approach viable and how to treat the network elements between two
+ adjacent links.
+
+ 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 to
+ satisfy the following constraints in their all-optical networks:
+
+
+
+
+
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+
+
+ - All the other non-OSNR constraints described in the previous
+ subsections are not binding factors as long as the maximum
+ distance constraint is met.
+
+ - Specifically for PMD, this means that the whole all-optical
+ network is built on top of sufficiently low-PMD fiber such that
+ the upper bound on the mean aggregate path DGD is always satisfied
+ for any path that does not exceed the maximum distance, or PMD
+ compensation devices might be used for routes with high-PMD
+ fibers.
+
+ - In terms of the ASE/OSNR constraint, in order to convert the ASE
+ constraint into a distance constraint directly, the network 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 equals to the
+ loss of the previous fiber span), e.g., 80km spacing which is
+ commonly chosen by carriers. However, when spans have variable
+ lengths, certain adjustment and compromise need to be made in
+ order to avoid treating ASE explicitly as in section 4.3. These
+ include: 1) Unless a certain mechanism is built in the OTS to take
+ advantage of shorter spans, spans shorter than a typical span
+ length D need to be treated as a span of length D instead of with
+ its real length. 2) Spans that are longer than D would have a
+ higher average span loss. In general, the maximum system reach
+ decreases when the average span loss increases. Thus, in order to
+ accommodate longer spans in the network, the maximum distance
+ upper bound has to be set with respect to the average span loss of
+ the worst path in the network. This sub-optimality may be
+ acceptable for some networks if the variance is not too large, but
+ may be too conservative for others.
+
+ If these assumptions are satisfied, the second issue we need to
+ address is how to treat a transparent network element (e.g., MEMS-
+ based switch) between two adjacent links in terms of a distance
+ constraint since it also introduces an insertion loss. If the
+ network element cannot somehow compensate for this OSNR degradation,
+ one approach is to convert each network element into an equivalent
+ length of fiber based on its loss/ASE contribution. Hence, in
+ general, introducing a set of transparent network elements would
+ effectively result in reducing the overall actual transmission
+ distance between the OEO edges.
+
+ With this approach, the link-specific state information is link-
+ distance, the length of a link. It equals the distance sum of all
+ 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
+
+
+
+
+
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+ all the link-distance over all links of a path should be less than
+ the maximum-path-distance, the upper bound of all paths.
+
+4.7. Other Considerations
+
+ Routing in an all-optical network without wavelength conversion
+ raises several additional issues:
+
+ - Since the route selected must have the chosen wavelength available
+ on all links, this information needs to be considered in the
+ routing process. One approach is to propagate information
+ throughout the network about the state of every wavelength on
+ every link in the network. However, the state required and the
+ overhead involved in processing and maintaining this information
+ is proportional to the total number of links (thus, number of
+ nodes squared), maximum number of wavelengths (which keeps
+ doubling every couple of years), and the frequency of wavelength
+ availability changes, which can be very high. Instead
+ [Hjalmtysson00], proposes an alternative method which probes along
+ a chosen path to determine which wavelengths (if any) are
+ available. This would require a significant addition to the
+ routing logic normally used in OSPF. Others have proposed
+ simultaneously probing along multiple paths.
+
+ - Choosing a path first and then a wavelength along the path is
+ known to give adequate results in simple topologies such as rings
+ and trees ([Yates99]). This does not appear to be true in large
+ mesh networks under realistic provisioning scenarios, however.
+ Instead significantly better results are achieved if wavelength
+ and route are chosen simultaneously ([Strand01b]). This approach
+ would however also have a significant effect on OSPF.
+
+4.8. Implications For Routing and Control Plane Design
+
+ If distributed routing is desired, additional state information will
+ be required by the routing to deal with the impairments described in
+ Sections 4.2 - 4.4:
+
+ - As mentioned earlier, an operator who wants to avoid having to
+ provide impairment-related parameters to the control plane may
+ elect not to deal with them at the routing level, instead treating
+ them at the system design and planning level if that is a viable
+ approach for their network. In this approach the operator can
+ pre-qualify all or a set of feasible end-to-end optical paths
+ through the domain of transparency for each bit rate. This
+ approach may work well with relatively small and sparse networks,
+ but it may not be scalable for large and dense networks where the
+ number of feasible paths can be very large.
+
+
+
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+
+ - If the optical paths are not pre-qualified, additional link-
+ specific state information will be required by the routing
+ algorithm for each type of impairment that has the potential of
+ being limiting for some routes. Note that for one operator, PMD
+ might be the only limiting constraint while for another, ASE might
+ be the only one, or it could be both plus some other constraints
+ considered in this document. Some networks might not be limited
+ by any of these constraints.
+
+ - For an operator needing to deal explicitly with these constraints,
+ the link-dependent information identified above for PMD is link-
+ PMD-square which is the square of the total PMD on a link. For
+ ASE the link-dependent information identified is link-noise which
+ is the total noise on a link. Other link-dependent information
+ includes link-span-length which is the total number of spans on a
+ link, link-crosstalk or OADM-OXC-number which is the total
+ crosstalk or the number of OADM/OXC nodes on a link, respectively,
+ and filter-number which is the number of narrow filters on a link.
+ When the alternative distance-only approach is chosen, the link-
+ specific information is link-distance.
+
+ - In addition to the link-specific information, bounds on each of
+ the impairments need to be quantified. Since these bounds are
+ determined by the system designer's impairment allocations, 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
+
+ - It is likely that the physical layer parameters do not change
+ value rapidly and could be stored in some database; however these
+ are physical layer parameters that today are frequently not known
+ at the granularity required. If the ingress node of a lightpath
+ does path selection these parameters would need to be available at
+ this node.
+
+
+
+
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+RFC 4054 Optical Layer Routing May 2005
+
+
+ - The specific constraints required in a given situation will depend
+ on the design and engineering of the domain of transparency; for
+ example it will be essential to know whether chromatic dispersion
+ has been dealt with on a per-link basis, and whether the domain is
+ operating in a linear or nonlinear regime.
+
+ - As optical transport technology evolves, the set of constraints
+ that will need to be considered either explicitly or via a
+ domain-wide margin may change. The routing and control plane
+ design should therefore be as open as possible, allowing
+ parameters to be included as necessary.
+
+ - In the absence of wavelength conversion, the necessity of finding
+ a single wavelength that is available on all links introduces the
+ need to either advertise detailed information on wavelength
+ availability, which probably doesn't scale, or have some mechanism
+ for probing potential routes with or without crankback to
+ determine wavelength availability. Choosing the route first, and
+ then the wavelength, may not yield acceptable utilization levels
+ in mesh-type networks.
+
+5. More Complex Networks
+
+ Mixing optical equipment in a single domain of transparency that has
+ not been explicitly designed to interwork is beyond the scope of this
+ document. This includes most multi-vendor all-optical networks.
+
+ An optical network composed of multiple domains of transparency
+ optically isolated from each other by O/E/O devices (transponders) is
+ more plausible. A network composed of both "opaque" (optically
+ isolated) OLXCs and one or more all-optical "islands" isolated by
+ transponders is of particular interest because this is most likely
+ how all-optical technologies (such as that described in Sec. 2) are
+ going to be introduced. (We use the term "island" in this discussion
+ rather than a term like "domain" or "area" because these terms are
+ associated with specific approaches like BGP or OSPF.)
+
+ We consider the complexities raised by these alternatives now.
+
+ The first requirement for routing in a multi-island network is that
+ the routing process needs to know the extent of each island. There
+ are several reasons for this:
+
+ - When entering or leaving an all-optical island, the regeneration
+ process cleans up the optical impairments discussed in Sec. 3.
+
+ - Each all-optical island may have its own bounds on each
+ impairment.
+
+
+
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+
+
+ - The routing process needs to be sensitive to the costs associated
+ with "island-hopping".
+
+ This last point needs elaboration. It is extremely important to
+ realize that, at least in the short to intermediate term, the
+ resources committed by a single routing decision can be very
+ significant: The equipment tied up by a single coast-to-coast OC-192
+ can easily have a first cost of $10**6, and the holding times on a
+ circuit once established is likely to be measured in months.
+ Carriers will expect the routing algorithms used to be sensitive to
+ these costs. Simplistic measures of cost such as the number of
+ "hops" are not likely to be acceptable.
+
+ Taking the case of an all-optical island consisting of an "ultra
+ long-haul" system like that in Fig. 3-1 embedded in an OEO network of
+ electrical fabric OLXCs as an example: It is likely that the ULH
+ system will be relatively expensive for short hops but relatively
+ economical for longer distances. It is therefore likely to be
+ deployed as a sort of "express backbone". In this scenario a carrier
+ is likely to expect the routing algorithm to balance OEO costs
+ against the additional costs associated with ULH technology and route
+ circuitously to make maximum use of the backbone where appropriate.
+ Note that the metrics used to do this must be consistent throughout
+ the routing domain if this expectation is to be met.
+
+ The first-order implications for GMPLS seem to be:
+
+ - Information about island boundaries needs to be advertised.
+
+ - The routing algorithm needs to be sensitive to island transitions
+ and to the connectivity limitations and impairment constraints
+ 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
+ investigating:
+
+ - Advertise the internal topology and constraints of each island
+ globally; let the ingress node compute an end-to-end strict
+ explicit route sensitive to all constraints and wavelength
+ availabilities. In this approach the routing algorithm used by
+ the ingress node must be able to deal with the details of routing
+ within each island.
+
+
+
+
+
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+RFC 4054 Optical Layer Routing May 2005
+
+
+ - Have the EMS or control plane of each island determine and
+ advertise the connectivity between its boundary nodes together
+ with additional information such as costs and the bit rates and
+ formats supported. As the spare capacity situation changes,
+ updates would be advertised. In this approach impairment
+ constraints are handled within each island and impairment-related
+ parameters need not be advertised outside of the island. The
+ ingress node would then do a loose explicit route and leave the
+ routing and wavelength selection within each island to the island.
+
+ - Have the ingress node send out probes or queries to nearby gateway
+ nodes or to an NMS to get routing guidance.
+
+6. Diversity
+
+6.1. Background on Diversity
+
+ "Diversity" is a relationship between lightpaths. Two lightpaths are
+ said to be diverse if they have no single point of failure. In
+ traditional telephony the dominant transport failure mode is a
+ failure in the interoffice plant, such as a fiber cut inflicted by a
+ backhoe.
+
+ Why is diversity a unique problem that needs to be considered for
+ optical networks? Traditionally, data network operators have relied
+ on their private line providers to ensure diversity and so have not
+ had to deal directly with the problem. GMPLS makes the complexities
+ handled by the private line provisioning process, including
+ diversity, part of the common control plane and so visible to all.
+
+ To determine whether two lightpath routings are diverse it is
+ necessary to identify single points of failure in the interoffice
+ plant. To do so we will use the following terms: A fiber cable is a
+ uniform group of fibers contained in a sheath. An Optical Transport
+ System will occupy fibers in a sequence of fiber cables. Each fiber
+ cable will be placed in a sequence of conduits - buried honeycomb
+ structures through which fiber cables may be pulled - or buried in a
+ right of way (ROW). A ROW is land in which the network operator has
+ the right to install his conduit or fiber cable. It is worth noting
+ that for economic reasons, ROWs are frequently obtained from
+ railroads, pipeline companies, or thruways. It is frequently the
+ case that several carriers may lease ROW from the same source; this
+ makes it common to have a number of carriers' fiber cables in close
+ proximity to each other. Similarly, in a metropolitan network,
+ several carriers might be leasing duct space in the same RBOC
+ conduit. There are also "carrier's carriers" - optical networks
+ which provide fibers to multiple carriers, all of whom could be
+ affected by a single failure in the "carrier's carrier" network. In
+
+
+
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+
+RFC 4054 Optical Layer Routing May 2005
+
+
+ a typical intercity facility network there might be on the order of
+ 100 offices that are candidates for OLXCs. To represent the inter-
+ office fiber network accurately a network with an order of magnitude
+ more nodes is required. In addition to Optical Amplifier (OA) sites,
+ these additional nodes include:
+
+ - Places where fiber cables enter/leave a conduit or right of way;
+
+ - Locations where fiber cables cross; Locations where fiber splices
+ are used to interchange fibers between fiber cables.
+
+ An example of the first might be:
+
+ A B
+ A-------------B \ /
+ \ /
+ X-----Y
+ / \
+ C-------------D / \
+ C D
+
+ (a) Fiber Cable Topology (b) Right-Of-Way/Conduit Topology
+
+ Figure 6-1: Fiber Cable vs. ROW Topologies
+
+ Here the A-B fiber cable would be physically routed A-X-Y-B and the
+ C-D cable would be physically routed C-X-Y-D. This topology might
+ arise because of some physical bottleneck: X-Y might be the Lincoln
+ Tunnel, for example, or the Bay Bridge.
+
+ Fiber route crossing (the second case) is really a special case of
+ this, where X and Y coincide. In this case the crossing point may
+ not even be a manhole; the fiber routes might just be buried at
+ different depths.
+
+ Fiber splicing (the third case) often occurs when a major fiber route
+ passes near to a small office. To avoid the expense and additional
+ transmission loss only a small number of fibers are spliced out of
+ the major route into a smaller route going to the small office. This
+ might well occur in a manhole or hut. An example is shown in Fig.
+ 6-2(a), where A-X-B is the major route, X the manhole, and C the
+ smaller office. The actual fiber topology 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 might have different numbers of
+ fibers. (One of the latter might even be missing.)
+
+
+
+
+
+
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+RFC 4054 Optical Layer Routing May 2005
+
+
+ C C
+ | / \
+ | / \
+ | / \
+ A------X------B A---------------B
+
+ (a) Fiber Cable Topology (b) Fiber Topology
+
+ Figure 6-2. Fiber Cable vs Fiber Topologies
+
+ The imminent deployment of ultra-long (>1000 km) Optical Transport
+ Systems introduces a further complexity: Two OTSes could interact a
+ number of times. To make up a hypothetical example: A New York -
+ 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
+ Georgia. They might also cross at Raleigh or some other intermediate
+ node without sharing right of way.
+
+ Diversity is often equated to routing two lightpaths between a single
+ pair of points, or different pairs of points so that no single route
+ failure will disrupt them both. This is too simplistic, for a number
+ of reasons:
+
+ - A sophisticated client of an optical network will want to derive
+ diversity needs from his/her end customers' availability
+ requirements. These often lead to more complex diversity
+ requirements than simply providing diversity between two
+ lightpaths. For example, a common requirement is that no single
+ failure should isolate a node or nodes. If a node A has single
+ lightpaths to nodes B and C, this requires A-B and A-C to be
+ diverse. In real applications, a large data network with N
+ lightpaths between its routers might describe their needs in an
+ NxN matrix, where (i,j) defines whether lightpaths i and j must be
+ diverse.
+
+ - Two circuits that might be considered diverse for one application
+ might not be considered diverse for in another situation.
+ Diversity is usually thought of as a reaction to interoffice route
+ failures. High reliability applications may require other types
+ of failures to be taken into account. Some examples:
+
+ o Office Outages: Although less frequent than route failures,
+ fires, power outages, and floods do occur. Many network
+ managers require that diverse routes have no (intermediate)
+ nodes in common. In other cases an intermediate node might be
+ acceptable as long as there is power diversity within the
+ office.
+
+
+
+
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+
+ o Shared Rings: Many applications are willing to allow "diverse"
+ circuits to share a SONET ring-protected link; presumably they
+ would allow the same for optical layer rings.
+
+ o Disasters: Earthquakes and floods can cause failures over an
+ extended area. Defense Department circuits might need to be
+ routed with nuclear damage radii taken into account.
+
+ - Conversely, some networks may be willing to take somewhat larger
+ risks. Taking route failures as an example: Such a network might
+ be willing to consider two fiber cables in heavy duty concrete
+ conduit as having a low enough chance of simultaneous failure to
+ be considered "diverse". They might also be willing to view two
+ fiber cables buried on opposite sides of a railroad track as being
+ diverse because there is minimal danger of a single backhoe
+ disrupting them both even though a bad train wreck might
+ jeopardize them both. A network seeking N mutually diverse paths
+ from an office with less than N diverse ROWs will need to live
+ with some level of compromise in the immediate vicinity of the
+ office.
+
+ These considerations strongly suggest that the routing algorithm
+ should be sensitive to the types of threat considered unacceptable by
+ the requester. Note that the impairment constraints described in the
+ previous section may eliminate some of the long circuitous routes
+ sometimes needed to provide diversity. This would make it harder to
+ find many diverse paths through an all-optical network than an opaque
+ one.
+
+ [Hjalmtysson00] introduced the term "Shared Risk Link Group" (SRLG)
+ to describe the relationship between two non-diverse links. The
+ above examples and discussion given at the start of this section
+ suggests that an SRLG should be characterized by 2 parameters:
+
+ - Type of Compromise: Examples would be shared fiber cable, shared
+ conduit, shared ROW, shared optical ring, shared office without
+ power sharing, etc.)
+
+ - Extent of Compromise: For compromised outside plant, this would
+ be the length of the sharing.
+
+ A CSPF algorithm could then penalize a diversity compromise by an
+ amount dependent on these two parameters.
+
+
+
+
+
+
+
+
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+
+
+ Two links could be related by many SRLGs. (AT&T's experience
+ indicates that a link may belong to over 100 SRLGs, each
+ corresponding to a separate fiber group.) Each SRLG might relate a
+ single link to many other links. For the optical layer, similar
+ situations can be expected where a link is an ultra-long OTS.
+
+ The mapping between links and different types of SRLGs is in general
+ defined by network operators based on the definition of each SRLG
+ type. Since SRLG information is not yet ready to be discoverable by
+ a network element and does not change dynamically, it need not be
+ advertised with other resource availability information by network
+ elements. It could be configured in some central database and be
+ distributed to or retrieved by the nodes, or advertised by network
+ elements at the topology discovery stage.
+
+6.2. Implications For Routing
+
+ Dealing with diversity is an unavoidable requirement for routing in
+ the optical layer. It requires dealing with constraints in the
+ routing process, but most importantly requires additional state
+ information (e.g., the SRLG relationships). The routings of any
+ existing circuits from which the new circuit must be diverse must
+ also be available to the routing process.
+
+ At present SRLG information cannot be self-discovered. Indeed, in a
+ large network it is very difficult to maintain accurate SRLG
+ information. The problem becomes particularly daunting whenever
+ multiple administrative domains are involved, for instance after the
+ acquisition of one network by another, because there normally is a
+ likelihood that there are diversity violations between the domains.
+ It is very unlikely that diversity relationships between carriers
+ will be known any time in the near future.
+
+ Considerable variation in what different customers will mean by
+ acceptable diversity should be anticipated. Consequently we suggest
+ that an SRLG should be defined as follows: (i) It is a relationship
+ between two or more links, and (ii) it is characterized by two
+ parameters, the type of compromise (shared conduit, shared ROW,
+ shared optical ring, etc.) and the extent of the compromise (e.g.,
+ the number of miles over which the compromise persisted). This will
+ allow the SRLGs appropriate to a particular routing request to be
+ easily identified.
+
+7. Security Considerations
+
+ We are assuming OEO interfaces to the domain(s) covered by our
+ discussion (see, e.g., Sec. 4.1 above). If this assumption were to
+ be relaxed and externally generated optical signals allowed into the
+
+
+
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+
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+
+ 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, SRLGs 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. IPO control plane security
+ considerations are discussed in [Rajagopalam04]. Security
+ considerations for GMPLS, a likely control plane candidate, are
+ discussed in [Mannie04].
+
+8. Acknowledgments
+
+ This document has benefited from discussions with Michael Eiselt,
+ Jonathan Lang, Mark Shtaif, Jennifer Yates, Dongmei Wang, Guangzhi
+ Li, Robert Doverspike, Albert Greenberg, Jim Maloney, John Jacob,
+ Katie Hall, Diego Caviglia, D. Papadimitriou, O. Audouin, J. P.
+ Faure, L. Noirie, and with our OIF colleagues.
+
+
+
+
+
+
+
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+
+9. References
+
+9.1. Normative References
+
+ [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.
+
+ [Hjalmtysson00] Gsli Hjalmtysson, Jennifer Yates, Sid Chaudhuri and
+ Albert Greenberg, "Smart Routers - Simple Optics: An
+ Architecture for the Optical Internet, IEEE/OSA
+ Journal of Lightwave Technology, December 2000, Vo
+ 18, Issue 12, Dec. 2000, pp. 1880-1891.
+
+ [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.
+
+ [Mannie04] Mannie, E., Ed., "Generalized Multi-Protocol Label
+ Switching (GMPLS) Architecture", RFC 3945, October
+ 2004.
+
+ [Rajagopalam04] Rajagopalan, B., Luciani, J., and D. Awduche, "IP
+ over Optical Networks: A Framework", RFC 3717, March
+ 2004.
+
+ [Strand01] Strand, J., Chiu, A., and R. Tkach, "Issues for
+ Routing in the Optical Layer", IEEE Communications
+ Magazine, Feb. 2001, vol. 39 No. 2, pp. 81-88.
+
+ [Strand01b] Strand, J., Doverspike, R., 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 J. P. R. Lacey,
+ "Wavelength Converters in Dynamically-Reconfigurable
+ WDM Networks", IEEE Communications Surveys, 2Q1999
+ (online at
+ www.comsoc.org/pubs/surveys/2q99issue/yates.html).
+
+
+
+
+
+
+
+
+
+Strand & Chiu Informational [Page 25]
+
+RFC 4054 Optical Layer Routing May 2005
+
+
+9.2. Informative References
+
+ [Awduche99] Awduche, D. O., Rekhter, Y., Drake, J., R. and
+ Coltun, "Multi-Protocol Lambda Switching: Combining
+ MPLS Traffic Engineering Control With Optical
+ Crossconnects", Work in Progress.
+
+ [Gerstel2000] Gorstel, O., "Optical Layer Signaling: How Much Is
+ Really Needed?" IEEE Communications Magazine, vol. 38
+ no. 10, Oct. 2000, pp. 154-160
+
+ [Kaminow02] Ivan P. Kaminow and Tingye Li (editors), "Optical
+ Fiber Communications IV: Systems and Impairments",
+ Elsevier Press, 2002.
+
+ [Passmore01] Passmore, D., "Managing Fatter Pipes," Business
+ Communications Review, August 2001, pp. 20-21.
+
+ [Ramaswami98] Ramaswami, R. and K. N. Sivarajan, Optical Networks:
+ A Practical Perspective, Morgan Kaufmann Publishers,
+ 1998.
+
+ [Strand02] John Strand, "Optical Network Architecture
+ Evolution", in [Kaminow02].
+
+ [Tkach98] Tkach, R., Goldstein, E., Nagel, J., and J. Strand,
+ "Fundamental Limits of Optical Transparency", Optical
+ Fiber Communication Conf., Feb. 1998, pp. 161-162.
+
+10. Contributing Authors
+
+ This document was a collective work of a number of people. The text
+ and content of this document was contributed by the editors and the
+ co-authors listed below.
+
+ Ayan Banerjee
+ Calient Networks
+ 6620 Via Del Oro
+ San Jose, CA 95119
+ EMail: abanerjee@calient.net
+
+
+ Prof. Dan Blumenthal
+ Eng. Science Bldg., Room 2221F
+ Department of Electrical and Computer Engineering
+ University of California
+ Santa Barbara, CA 93106-9560
+ EMail: danb@ece.ucsb.edu
+
+
+
+Strand & Chiu Informational [Page 26]
+
+RFC 4054 Optical Layer Routing May 2005
+
+
+ Dr. John Drake
+ Boeing
+ 2260 E Imperial Highway
+ El Segundo, Ca 90245
+ EMail: John.E.Drake2@boeing.com
+
+
+ Andre Fredette
+ Hatteras Networks
+ PO Box 110025
+ Research Triangle Park, NC 27709
+ EMail: afredette@hatterasnetworks.com
+
+
+ Change Nan Froberg's reach info to:
+ Dr. Nan Froberg
+ Photonic Systems, Inc.
+ 900 Middlesex Turnpike, Bldg #5
+ Billerica, MA 01821
+ EMail: nfroberg@photonicsinc.com
+
+
+ Dr. Taha Landolsi
+ King Fahd University
+ KFUPM Mail Box 1026
+ Dhahran 31261, Saudi Arabia
+ EMail: landolsi@kfupm.edu.sa
+
+
+ James V. Luciani
+ 900 Chelmsford St.
+ Lowell, MA 01851
+ EMail: james_luciani@mindspring.com
+
+
+ Dr. Robert Tkach
+ 32 Carriage House Lane
+ Little Silver, NJ 07739
+ 908 246 5048
+ EMail: tkach@ieee.org
+
+
+
+
+
+
+
+
+
+
+
+Strand & Chiu Informational [Page 27]
+
+RFC 4054 Optical Layer Routing May 2005
+
+
+ Yong Xue
+ Dr. Yong Xue
+ DoD/DISA
+ 5600 Columbia Pike
+ Falls Church VA 22041
+ EMail: yong.xue@disa.mil
+
+Editors' Addresses
+
+ Angela Chiu
+ AT&T Labs
+ 200 Laurel Ave., Rm A5-1F13
+ Middletown, NJ 07748
+
+ Phone: (732) 420-9061
+ EMail: chiu@research.att.com
+
+
+ John Strand
+ AT&T Labs
+ 200 Laurel Ave., Rm A5-1D33
+ Middletown, NJ 07748
+
+ Phone: (732) 420-9036
+ EMail: jls@research.att.com
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Strand & Chiu Informational [Page 28]
+
+RFC 4054 Optical Layer Routing May 2005
+
+
+Full Copyright Statement
+
+ Copyright (C) The Internet Society (2005).
+
+ This document is subject to the rights, licenses and restrictions
+ contained in BCP 78, and except as set forth therein, the authors
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+
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+
+Acknowledgement
+
+ Funding for the RFC Editor function is currently provided by the
+ Internet Society.
+
+
+
+
+
+
+
+Strand & Chiu Informational [Page 29]
+