Internet Draft
        Document: draft-ietf-ipo-impairments-02.txt     Angela Chiu (Editor)
        Expiration Date: August 2002                       Celion Networks
                                                        John Strand (Editor)
                                                        Robert Tkach
                                                           Celion Networks
                                                        James Luciani
                                                           Crescent Networks
                                                        Ayan Banerjee
                                                        John Drake
                                                        Dan Blumenthal
                                                           Calient Networks
                                                        Andre Fredette
                                                           Hatteras Networks
                                                        Nan Froberg
                                                        Yong Xue
                                                        Taha Landolsi
             Impairments And Other Constraints On Optical Layer Routing
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                       Impairments And Other Constraints     February 2002
                            On Optical Layer Routing
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       Optical networking poses a number challenges for GMPLS. Optical
       technology is fundamentally an analog rather than digital technology;
       and the optical layer is lowest in the transport hierarchy and hence
       has an intimate relationship with the physical geography of the
       network. This contribution surveys some of the aspects of optical
       networks which 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.
     1.        Introduction
        GMPLS [GMPLS] aims to extend MPLS to encompass a number of transport
        architectures. Included are optical networks incorporating 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
        challenges for GMPLS. Optical technology is fundamentally an analog
        rather than digital technology; and the optical layer is lowest in
        the transport hierarchy and hence has an intimate relationship with
        the physical geography of the network.
        GMPLS already has incorporated extensions to deal with some of the
        unique aspects of the optical layer. This contribution surveys some
        of the aspects of optical networks which 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 from the need to
        guarantee the physical diversity of some circuits are discussed.
        Since the purpose of this draft 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 [Strand2001].
        The organization of the contribution is as follows:
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                       Impairments And Other Constraints     February 2002
                            On Optical Layer Routing
             - Section 2 is a section requested by the sub-IP Area
               management for all new drafts. It explains how this document
               fits into the Area and into the IPO WG, and why it is
               appropriate for these groups.
             - Section 3 describes constraints arising from the design of
               new software controllable network elements.
             - Section 4 addresses the constraints in a single all-optical
               domain without wavelength conversion.
             - Section 5 extends the discussion to more complex networks.
               incorporating both all-optical and opaque architectures.
             - Section 6 discusses the impacts of diversity constraints.
             - Section 7 deals with security requirements.
             - Section 8 contains acknowledgments.
     2. Sub-IP Area Summary And Justification Of Work
        This draft merges and extends two previous drafts, draft-chiu-
        strand-unique-olcp-02.txt and draft-banerjee-routing-impairments-
        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
        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. It is
        appropriate work for this WG because the material covered is
        directly aimed at establishing a framework and requirements for
        routing in an optical network.
        Related documents are:
     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
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                       Impairments And Other Constraints     February 2002
                            On Optical Layer Routing
        software reconfigurable elements on the horizon, specifically
        tunable lasers and receivers and reconfigurable optical add-drop
        multiplexers (OADMÆs).  These elements are illustrated in the
        following simple example, which is modeled on announced Optical
        Transport System (OTS) products:
                      +                                       +
          ---+---+    |\                                     /|    +---+---
          ---| A |----|D|          X              Y         |D|----| A |---
          ---+---+    |W|     +--------+     +--------+     |W|    +---+---
               :      |D|-----|  OADM  |-----|  OADM  |-----|D|      :
          ---+---+    |M|     +--------+     +--------+     |M|    +---+---
          ---| A |----| |      |      |       |      |      | |----| A |---
          ---+---+    |/       |      |       |      |       \|    +---+---
                      +      +---+  +---+   +---+  +---+      +
                       D     | A |  | A |   | A |  | A |     E
                             +---+  +---+   +---+  +---+
                              | |    | |     | |    | |
              Figure 3-1: An OTS With OADM's - Functional Architecture
        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 DWDMÆs and X and Y are OADMÆs. 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 which will pass
        transparently to a distant adaptation function. Adaptation
        functionality which 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.
          - 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
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            "adaptation grouping". Examples include so called "wave group"
            and ôwavebandö [Passmore01].  Groupings on the same system may
            differ in basics such as wavelength spacing, which constrain the
            type of channels that can be accommodated.
          - Laser Tunability: The lasers producing the LR wavelengths may
            have a fixed frequency, may be tunable over a limited range, or
            be tunable over the entire range of wavelengths supported by the
            DWDM. Tunability speeds may also vary.
        Connectivity between adaptation functions may also be limited:
          - 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
          - Only adaptation functions whose lasers/receivers are tunable to
            compatible frequencies can be connected.
          - The switching capability of the OADMÆs may also be constrained.
            For example:
               o There may be some wavelengths that can not be dropped at
               o There may be a fixed relationship between the frequency
                 dropped and the physical port on the OADM to which it is
               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 OADMÆs 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 OADMÆs 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
        operatorÆs OSÆs.  However in principle the EMS could participate in
        the GMPLS routing process.
     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
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        wavebands.  If these were reconfigured by the external EMS the
        resulting connectivity changes would need to be detected and
        advertised within GMPLS.  If the topology shown in Fig. 3-1 became a
        tree or a mesh instead of the linear topology shown, the
        connectivity changes could result in SRLG changes.
        Alternatively, GMPLS could attempt to directly control this port
        connectivity. The state information needed to do this is likely to
        be voluminous and vendor specific.
     4.   Wavelength Routed All-Optical Networks
        The optical networks presently being deployed 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
        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 which are optically isolated from other parts of the
        optical layer by transponders.  This approach is very practical and
        has been applied to date, e.g. when determining the maximum length
        of an Optical Transport System (OTS).  Furthermore operational
        considerations like fault isolation also make limiting the size of
        domains of transparency attractive.
        There are however reasons to consider contained domains of
        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
        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
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        Note that as we describe later in the section there are many types
        of physical impairments. Which of these need 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 which does not intend to support
        bit rates above 2.5 Gb/sec may not be constrained by any of these
        impairments, while a continental or international network which
        wished to minimize O/E/O regeneration investment and support 40
        Gb/sec connections might have to explicitly consider many of them.
        Also, a network operator may reduce or even eliminate their
        constraint set by building a relatively small domain of transparency
        to ensure that all the paths are feasible, or by using some
        proprietary tools based on rules from the OTS vendor to pre-qualify
        paths between node pairs and put them in a table that can be
        accessed each time a routing decision has to be made through that
     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 which optically isolates the portion within our
        domain.  We assume that we know the bit rate of the circuit. Also,
        we assume that the adaptation function at X may apply some Forward
        Error Correction (FEC) method to the circuit. We also assume we know
        the launch power of the laser at X.
        Impairments can be classified into two categories, linear and
        nonlinear. (See [Tkach98] 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
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        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 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
        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 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.
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        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
     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.) LetÆs take a typical example. Assuming P=4dBm,
        SNRmin=20dB with FEC, B=12.5GHz, n=2.5, G=25dB, based on the
        constraint, the maximum number of spans is at most 10. However, if
        FEC is not used and the requirement on SNRmin becomes 25dB, the
        maximum number of spans drops down to 3.
        For ASE the only link-dependent information needed by the routing
        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
     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
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        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 designerÆs 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 to maintain the total PDL on the path to be 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
        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 DCF (Dispersion Compensation Fiber). 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
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        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. û25dB 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.
        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
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        [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 maybe valid for some but not for others. However, it
        is likely that there is an advantage in designing systems which are
        less aggressive with respect to nonlinearities, and therefore
        somewhat sub-optimal, in exchange for improved scalability,
        simplicity and flexibility in routing and control plane design.
     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 by the system
        designer or by the transmission engineers at the time the system is
        installed. If dealt with successfully in this manner they should not
        need to be considered in the dynamic routing process.
        Gain Nonuniformity and Gain Transients For simple noise estimates to
        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
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                       Impairments And Other Constraints     February 2002
                            On Optical Layer Routing
        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
     4.6 An Alternative Approach û Using Maximum Distance As The only
        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:
          - 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) If a span is shorter than a
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                       Impairments And Other Constraints     February 2002
                            On Optical Layer Routing
            typical span length D, unless certain mechanism is built in the
            OTS to take advantages of shorter spans, it needs to be treated
            as a span of length D instead of with its real length. 2) When
            there are spans that are longer than D, it means that paths with
            these longer spans would have higher average span loss. In
            general, the maximum system reach decreases when the average
            span loss increases. Thus, in order to accommodate longer spans
            in the network, the maximum distance upper bound has to be set
            with respect to the average span loss of the worst path in the
            network. This sub-optimality may be acceptable for some networks
            if the variance is not too large, but may be too conservative
            for others.
        If these assumptions are satisfied, the second issue we need to
        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 to 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
        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. This is discussed in [Chaudhuri00], where
           it is concluded that advertising detailed wavelength
           availabilities on each link is not likely to scale. Instead they
           propose 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.
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                       Impairments And Other Constraints     February 2002
                            On Optical Layer Routing
        -  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-qualifies all or a set of feasible end-to-end optical paths
          through the domain of transparency for each bit rate. This
          approach may work well with relatively small and sparse networks,
          but it may not be scalable for large and dense networks where the
          number of feasible paths can be very large.
        - If the optical paths are not pre-qualified, additional link-
          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
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                       Impairments And Other Constraints     February 2002
                            On Optical Layer Routing
          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 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
          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.
        . 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
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                       Impairments And Other Constraints     February 2002
                            On Optical Layer Routing
        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) OLXC's and one or more all-optical "islands" isolated by
        transponders is of particular interest because this is most likely
        how all-optical technologies (such as that described in Sec. 2) are
        going to be introduced. (We use the term "island" in this discussion
        rather than a term like "domain" or "area" because these terms are
        associated with specific approaches like BGP or OSPF.)
        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
        . 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 OLXC's 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:
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                       Impairments And Other Constraints     February 2002
                            On Optical Layer Routing
        . 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
        . 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.
        . 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
        Why is diversity a unique problem that needs to be considered for
        optical networks? So far, 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
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                       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
        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, ROWÆs 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 carriersÆ fiber cables in close
        proximity to each other. Similarly, in a metropolitan network,
        several carriers might be leasing duct space in the same RBOC
        conduit.  There are also "carrier's carriers" - optical networks
        which provide fibers to multiple carriers, all of whom could be
        affected by a single failure in the "carrier's carrier" network.
        In a typical intercity facility network there might be on the order
        of 100 offices that are candidates for OLXCÆs. 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                 \             /
                                                     \         /
                                                     /         \
                   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
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                       Impairments And Other Constraints     February 2002
                            On Optical Layer Routing
        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.)
                         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 OTS's 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:
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                       Impairments And Other Constraints     February 2002
                            On Optical Layer Routing
          - 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
               o Office Outages: Although less frequent than route failures,
                 fires, power outages, and floods do occur.  Many network
                 managers require that diverse routes have no (intermediate)
                 nodes in common. In other cases an intermediate node might
                 be acceptable as long as there is power diversity within
                 the office.
               o Shared Rings: Many applications are willing to allow
                 "diverse" circuits to share a SONET ring-protected link;
                 presumably they would allow the same for optical layer
               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 ROWÆs will
            need to live with some level of compromise in the immediate
            vicinity of the office.
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                       Impairments And Other Constraints     February 2002
                            On Optical Layer Routing
        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.
        [Chaudhuri00] introduced the term "Shared Risk Link Group" (SRLG) to
        describe the relationship between two non-diverse links.  The above
        discussion suggests that an SRLG should be characterized by 2
          - 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.
        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
        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 SRLGÆs 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 û the SRLG relationships and also the routings of any
        existing circuits from the new circuit is to be diverse û to be
        available to the routing process.
        draft-ietf-ipo-impairments-02.txt                         [page 22]

                       Impairments And Other Constraints     February 2002
                            On Optical Layer Routing
        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 SRLGÆs appropriate to a particular routing request to be
        easily identified.
     7.        Security Considerations
        The solution developed to address the requirements defined in this
        document must address security aspects.
     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.
        [Awduche99] Awduche, D. O., Rekhter, Y., Drake, J., and Coltun, R.,
        "Multi-Protocol Lambda Switching: Combining MPLS Traffic Engineering
        Control With Optical Crossconnects", Work in Progress, draft-
        [Bra96] Bradner, S., "The Internet Standards Process -- Revision 3,"
        BCP 9, RFC 2026, October 1996.
        [CBD00] Ceuppens, L., Blumenthal, D., Drake, J., Chrostowski, J.,
        Edwards, W., "Performance Monitoring in Photonic Networks in Support
        of MPL(ambda)S", Internet draft, work in progress, March 2000.
        draft-ietf-ipo-impairments-02.txt                         [page 23]

                       Impairments And Other Constraints     February 2002
                            On Optical Layer Routing
        [Chaudhuri00] Chaudhuri, S., Hjalmtysson, G., and Yates, J.,
        "Control of Lightpaths in an Optical Network", Work in Progress,
        [Doverspike00] Doverspike, R. and Yates, J., "Challenges For MPLS in
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        [Gerstel 2000] O. Gorstel, "Optical Layer Signaling: How Much Is
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        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.
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        [KRB01a] Kompella, K.,, "IS-IS extensions in support of
        Generalized MPLS," Internet Draft, draft-ietf-gmpls- extensions-
        01.txt, work in progress, 2001.
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        [Passmore01] Passmore, D. ôManaging Fatter Pipes,ö Business
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        A Practical Perspective, Morgan Kaufmann Publishers, 1998.
        draft-ietf-ipo-impairments-02.txt                         [page 24]

                       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
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        unique-olcp-01.txt, work in progress, November 2000.
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        "Fundamental Limits of Optical Transparency", Optical Fiber
        Communication Conf., Feb. 1998, pp. 161-162.
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        "Wavelength Converters in Dynamically-Reconfigurable WDM Networks",
        IEEE Communications Surveys, 2Q1999 (online at
     Authors' Addresses:
        Ayan Banerjee
        Calient Networks
        5853 Rue Ferrari
        San Jose, CA 95138
        Dan Blumenthal
        Calient Networks
        5853 Rue Ferrari
        San Jose, CA 95138
        Angela Chiu
        Celion Networks
        1 Sheila Dr., Suite 2
        Tinton Falls, NJ 07724
        Phone:(732) 747-9987
        John Drake
        Calient Networks
        5853 Rue Ferrari
        San Jose, CA 95138
        Andre Fredette
        draft-ietf-ipo-impairments-02.txt                         [page 25]

                       Impairments And Other Constraints     February 2002
                            On Optical Layer Routing
        Hatteras Networks
        PO Box 110025
        Research Triangle Park, NC 27709
        Nan Froberg
        PhotonEx Corporation
        200 Metrowest Technology Dr.
        Maynard, MA 01754
        Taha Landolsi
        2400 North Glenville Drive
        Richardson, TX 75082
        Telephone: 972-729-5201
        James V. Luciani
        900 Chelmsford St.
        Lowell, MA 01851
        +1 978 275 3182
        John Strand
        AT&T Labs
        200 Laurel Ave., Rm A5-1D06
        Middletown, NJ 07748
        Phone:(732) 420-9036
        Robert Tkach
        Celion Networks
        1 Sheila Dr., Suite 2
        Tinton Falls, NJ 07724
        Phone:(732) 747-9909
        Yong Xue
        WorldCom, Inc.
        22001 Loudoun County Parkway
        Ashburn, VA 20147
        Telephone: (703) 886-5358
        draft-ietf-ipo-impairments-02.txt                         [page 26]