Impairments and Other Constraints on Optical Layer Routing
draft-ietf-ipo-impairments-05

The information below is for an old version of the document that is already published as an RFC
Document Type RFC Internet-Draft (ipo WG)
Author John Strand 
Last updated 2015-10-14 (latest revision 2003-05-08)
Stream Internet Engineering Task Force (IETF)
Formats plain text pdf htmlized bibtex
Stream WG state (None)
Document shepherd No shepherd assigned
IESG IESG state RFC 4054 (Informational)
Consensus Boilerplate Unknown
Telechat date
Responsible AD Scott Bradner
Send notices to <awduche@isocore.com>
Internet Draft                                  John Strand (Editor) 
        Document: draft-ietf-ipo-impairments-05.txt     Angela Chiu (Editor) 
        Informational Track                                             AT&T  
        Expiration Date: November 2003                  
                                                             
                                                             
                                                        May 2003 
      
     Impairments And Other Constraints On Optical Layer Routing 
         
     Status of this Memo 
         
        This document is an Internet-Draft and is in full conformance with 
        all provisions of Section 10 of RFC2026. Internet-Drafts are 
        working documents of the Internet Engineering Task Force (IETF), its 
        areas, and its working groups.  Note that other groups may also 
        distribute working documents as Internet-Drafts. 
         
        Internet-Drafts are draft documents valid for a maximum of six 
        months and may be updated, replaced, or made obsolete by other 
        documents at any time. It is inappropriate to use Internet-Drafts as 
        reference material or to cite them other than as "work in progress." 
         
        The list of current Internet-Drafts can be accessed at 
        http://www.ietf.org/ietf/1id-abstracts.txt. 
        The list of Internet-Draft Shadow Directories can be accessed at 
        http://www.ietf.org/shadow.html. 
         
     Abstract 
     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 
         
        Chiu, Strand, Eds.       Informational                    [Page 1] 
         

                                         
                       Impairments And Other Constraints          May 2003  
                            On Optical Layer Routing 
                                         
        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: 
             
          - 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. 
          - Section 9 contains references. 
          - Section 10 contains contributing authors' addresses. 
          - Section 11 contains editors' addresses. 
             
            
     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 
         
        Chiu, Strand, Eds.       Informational                    [page 2] 
      

                                         
                       Impairments And Other Constraints          May 2003  
                            On Optical Layer Routing 
                                         
        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: 
        draft-banerjee-routing-impairments-00.txt 
        draft-parent-obgp-01.txt 
        draft-bernstein-optical-bgp-00.txt 
        draft-hayata-ipo-carrier-needs-00.txt 
        draft-many-carrier-framework-uni-01.txt 
        draft-papadimitriou-ipo-non-linear-routing-impairm-01.txt 
      
     3. Reconfigurable Network Elements 
      
     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 (OADM's).  
     These elements are illustrated in the following simple example, which 
     is modeled on announced Optical Transport System (OTS) products: 
                      +                                       + 
          ---+---+    |\                                     /|    +---+--- 
          ---| A |----|D|          X              Y         |D|----| A |--- 
          ---+---+    |W|     +--------+     +--------+     |W|    +---+--- 
               :      |D|-----|  OADM  |-----|  OADM  |-----|D|      : 
          ---+---+    |M|     +--------+     +--------+     |M|    +---+--- 
          ---| A |----| |      |      |       |      |      | |----| A |--- 
          ---+---+    |/       |      |       |      |       \|    +---+--- 
                      +      +---+  +---+   +---+  +---+      + 
                       D     | A |  | A |   | A |  | A |     E 
                             +---+  +---+   +---+  +---+ 
                              | |    | |     | |    | | 
         
              Figure 3-1: An OTS With OADM's - Functional Architecture 
         
        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: 
         
        Chiu, Strand, Eds.       Informational                    [page 3] 
      

                                         
                       Impairments And Other Constraints          May 2003  
                            On Optical Layer Routing 
                                         
          - 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 DWDM's and X and Y are OADM's. 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 
            "adaptation grouping". Examples include so called "wave group" 
            and "waveband" [Passmore01].  Groupings on the same system may 
            differ in basics such as wavelength spacing, which constrain the 
            type of channels that can be accommodated. 
          - Laser Tunability: The lasers producing the LR wavelengths may 
            have a fixed frequency, may be tunable over a limited range, or 
            be tunable over the entire range of wavelengths supported by the 
            DWDM. Tunability speeds may also vary.  
         
        Connectivity between adaptation functions may also be limited: 
          - As pointed out above, TDM multiplexing and/or adaptation 
            grouping by the adaptation function forces groups of input 
            channels to be delivered together to the same distant adaptation 
            function. 
          - Only adaptation functions whose lasers/receivers are tunable to 
            compatible frequencies can be connected. 
          - The switching capability of the OADM's may also be constrained.  
            For example: 
               o There may be some wavelengths that can not be dropped at 
                 all. 
               o There may be a fixed relationship between the frequency 
                 dropped and the physical port on the OADM to which it is 
                 dropped. 
               o OADM physical design may put an upper bound on the number 
                 of adaptation groupings dropped at any single OADM. 
         
        Chiu, Strand, Eds.       Informational                    [page 4] 
      

                                         
                       Impairments And Other Constraints          May 2003  
                            On Optical Layer Routing 
                                         
         
        For a fixed configuration of the OADM's and adaptation functions 
        connectivity will be fixed: Each input port will essentially be 
        hard-wired to some specific distant port.  However this connectivity 
        can be changed by changing the configurations of the OADM's 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 OS's.  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 
        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 
        OTS. 
      
        The routing of lightpaths through an all-optical network has 
        received extensive attention. (See [Yates99] or [Ramaswami98]).  
         
        Chiu, Strand, Eds.       Informational                    [page 5] 
      

                                         
                       Impairments And Other Constraints          May 2003  
                            On Optical Layer Routing 
                                         
        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 
        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 which does not intend to support 
        bit rates above 2.5 Gb/sec may not be constrained by any of these 
        impairments, while a continental or international network which 
        wished to minimize O/E/O regeneration investment and support 40 
        Gb/sec connections might have to explicitly consider many of them. 
        Also, a network operator may reduce or even eliminate their 
        constraint set by building a relatively small domain of transparency 
        to ensure that all the paths are feasible, or by using some 
        proprietary tools based on rules from the OTS vendor to pre-qualify 
        paths between node pairs and put them in a table that can be 
        accessed each time a routing decision has to be made through that 
        domain.  
      
     4.1  Problem Formulation 
         
         
        Chiu, Strand, Eds.       Informational                    [page 6] 
      

                                         
                       Impairments And Other Constraints          May 2003  
                            On Optical Layer Routing 
                                         
        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 
        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 
        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 
         
        Chiu, Strand, Eds.       Informational                    [page 7] 
      

                                         
                       Impairments And Other Constraints          May 2003  
                            On Optical Layer Routing 
                                         
        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. 
         
        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 
         
        Chiu, Strand, Eds.       Informational                    [page 8] 
      

                                         
                       Impairments And Other Constraints          May 2003  
                            On Optical Layer Routing 
                                         
        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 
        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 
         
        Chiu, Strand, Eds.       Informational                    [page 9] 
      

                                         
                       Impairments And Other Constraints          May 2003  
                            On Optical Layer Routing 
                                         
        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 
        spans. 
         
        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 
        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 
         
        Chiu, Strand, Eds.       Informational                   [page 10] 
      

                                         
                       Impairments And Other Constraints          May 2003  
                            On Optical Layer Routing 
                                         
        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. 
         
        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 
         
        Chiu, Strand, Eds.       Informational                   [page 11] 
      

                                         
                       Impairments And Other Constraints          May 2003  
                            On Optical Layer Routing 
                                         
        margin approach may be valid for some but not for others. However, 
        it is likely that there is an advantage in designing systems that 
        are less aggressive with respect to nonlinearities, and therefore 
        somewhat sub-optimal, in exchange for improved scalability, 
        simplicity and flexibility in routing and control plane design. 
         
     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 
        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.  
         
         
        Chiu, Strand, Eds.       Informational                   [page 12] 
      

                                         
                       Impairments And Other Constraints          May 2003  
                            On Optical Layer Routing 
                                         
        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 
            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 
         
        Chiu, Strand, Eds.       Informational                   [page 13] 
      

                                         
                       Impairments And Other Constraints          May 2003  
                            On Optical Layer Routing 
                                         
        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. 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 [Hjßlmt²sson00] 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: 
         
         
        Chiu, Strand, Eds.       Informational                   [page 14] 
      

                                         
                       Impairments And Other Constraints          May 2003  
                            On Optical Layer Routing 
                                         
          - 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. 
           
          - 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 
         
        Chiu, Strand, Eds.       Informational                   [page 15] 
      

                                         
                       Impairments And Other Constraints          May 2003  
                            On Optical Layer Routing 
                                         
            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 
      
        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.)  
         
         
        Chiu, Strand, Eds.       Informational                   [page 16] 
      

                                         
                       Impairments And Other Constraints          May 2003  
                            On Optical Layer Routing 
                                         
        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. 
          - 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: 
          - 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 
         
        Chiu, Strand, Eds.       Informational                   [page 17] 
      

                                         
                       Impairments And Other Constraints          May 2003  
                            On Optical Layer Routing 
                                         
            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 
        backhoe.  
         
        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 
        plant. To do so we will use the following terms: A fiber cable is a 
        uniform group of fibers contained in a sheath.  An Optical Transport 
        System will occupy fibers in a sequence of fiber cables. Each fiber 
        cable will be placed in a sequence of conduits - buried honeycomb 
        structures through which fiber cables may be pulled - or buried in a 
        right of way (ROW). A ROW is land in which the network operator has 
        the right to install his conduit or fiber cable.  It is worth noting 
        that for economic reasons, ROW's are frequently obtained from 
        railroads, pipeline companies, or thruways. It is frequently the 
        case that several carriers may lease ROW from the same source; this 
        makes it common to have a number of carriers' fiber cables in close 
        proximity to each other. Similarly, in a metropolitan network, 
         
        Chiu, Strand, Eds.       Informational                   [page 18] 
      

                                         
                       Impairments And Other Constraints          May 2003  
                            On Optical Layer Routing 
                                         
        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 OLXC's. 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 
         
        Chiu, Strand, Eds.       Informational                   [page 19] 
      

                                         
                       Impairments And Other Constraints          May 2003  
                            On Optical Layer Routing 
                                         
        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: 
         
          - 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 
         
        Chiu, Strand, Eds.       Informational                   [page 20] 
      

                                         
                       Impairments And Other Constraints          May 2003  
                            On Optical Layer Routing 
                                         
            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. 
               o Shared Rings: Many applications are willing to allow 
                 "diverse" circuits to share a SONET ring-protected link; 
                 presumably they would allow the same for optical layer 
                 rings. 
               o Disasters: Earthquakes and floods can cause failures over 
                 an extended area.  Defense Department circuits might need 
                 to be routed with nuclear damage radii taken into account. 
          - Conversely, some networks may be willing to take somewhat larger 
            risks.  Taking route failures as an example: Such a network 
            might be willing to consider two fiber cables in heavy duty 
            concrete conduit as having a low enough chance of simultaneous 
            failure to be considered "diverse". They might also be willing 
            to view two fiber cables buried on opposite sides of a railroad 
            track as being diverse because there is minimal danger of a 
            single backhoe disrupting them both even though a bad train 
            wreck might jeopardize them both. A network seeking N mutually 
            diverse paths from an office with less than N diverse ROW's will 
            need to live with some level of compromise in the immediate 
            vicinity of the office. 
                
        These considerations strongly suggest that the routing algorithm 
        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. 
         
        [Hjßlmt²sson00] 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.) 
         
        Chiu, Strand, Eds.       Informational                   [page 21] 
      

                                         
                       Impairments And Other Constraints          May 2003  
                            On Optical Layer Routing 
                                         
          - 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 SRLG's 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. 
         
        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 SRLG's appropriate to a particular routing request to be 
        easily identified. 
         
        Chiu, Strand, Eds.       Informational                   [page 22] 
      

                                         
                       Impairments And Other Constraints          May 2003  
                            On Optical Layer Routing 
                                         
         
     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 
        domain, network security issues would arise.  Specifically, 
        unauthorized usage in the form of signals at improper wavelengths or 
        with power levels or impairments inconsistent with those assumed by 
        the domain would be possible. With OEO interfaces, these types of 
        layer one threats should be controllable. 
         
        A key layer one security issue is resilience in the face of physical 
        attack.  Diversity, as describe in Sec. 6, is a part of the 
        solution. However, it is ineffective if there is not sufficient 
        spare capacity available to make the network whole after an attack. 
        Several major related issues are: 
          - Defining the threat: If, for example, an electro-magnetic 
            interference (EMI) burst is an in-scope threat, then (in the 
            terminology of Sec. 6) all of the links sufficiently close 
            together to be disrupted by such a burst must be included in a 
            single SRLG.  Similarly for other threats: For each in-scope 
            threat, SRLG's must be defined so that all links vulnerable to a 
            single incident of the threat must be grouped together in a 
            single SRLG. 
          - Allocating responsibility for responding to a layer one failure 
            between the various layers (especially the optical and IP 
            layers): This must be clearly specified to avoid churning and 
            unnecessary service interruptions. 
         
        The whole proposed process depends on the integrity of the 
        impairment characterization information (PMD parameters, etc.) and 
        also the SRLG definitions. Security of this information, both when 
        stored and when distributed, is essential.  
         
        This document does not address control plane issues, and so control-
        plane security is out of scope. IPO control plane security 
        considerations are discussed in [Rajagopalam02].  Security 
        considerations for GMPLS, a likely control plane candidate, are 
        discussed in [Mannie02].  
         
     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. 
         
        Chiu, Strand, Eds.       Informational                   [page 23] 
      

                                         
                       Impairments And Other Constraints          May 2003  
                            On Optical Layer Routing 
                                         
         
      
     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. 
         
        [Hjßlmt²sson00] Gsli Hjßlmt²sson, 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. 
         
        [Mannie02] Mannie, E. (ed.), "Generalized Multi-Protocol Label 
        Switching (GMPLS) Architecture", Interned Draft, draft-ietf-ccamp- 
        gmpls-architecture-03.txt, August, 2002. 
         
        [Rajagopalam02] Rajagopalam, B., et. al., "IP over Optical Networks: 
        A Framework", Internet Draft, draft-ietf-ipo-framework-02.txt June, 
        2002. 
         
        [Strand01] J. Strand, A. Chiu, and R. Tkach, "Issues for Routing in 
        the Optical Layer", IEEE Communications Magazine, Feb. 2001, vol. 39 
        No. 2, pp. 81-88. 
         
        [Strand01b] J. Strand, R. Doverspike, and G. Li, "Importance of 
        Wavelength Conversion In An Optical Network", Optical Networks 
        Magazine, May/June 2001, pp. 33-44. 
         
        [Yates99] Yates, J. M., Rumsewicz, M. P. and Lacey, J. P. R., 
        "Wavelength Converters in Dynamically-Reconfigurable WDM Networks", 
        IEEE Communications Surveys, 2Q1999 (online at 
        www.comsoc.org/pubs/surveys/2q99issue/yates.html). 
         
         
        Chiu, Strand, Eds.       Informational                   [page 24] 
      

                                         
                       Impairments And Other Constraints          May 2003  
                            On Optical Layer Routing 
                                         
         
     9.2  Informative References 
         
        [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-
        awduche-mpls-te-optical-01.txt. 
         
        [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. 
         
        [Doverspike00] Doverspike, R. and Yates, J., "Challenges For MPLS in 
        Optical Network Restoration", IEEE Communication Magazine, February, 
        2001. 
         
        [Gerstel 2000] O. Gorstel, "Optical Layer Signaling: How Much Is 
        Really Needed?" IEEE Communications Magazine, vol. 38 no. 10, Oct. 
        2000, pp. 154-160 
         
        [KRB01a] Kompella, K., et.al., "IS-IS extensions in support of 
        Generalized MPLS," Internet Draft, draft-ietf-gmpls- extensions-
        01.txt, work in progress, 2001.  
         
        [KRB01b] Kompella, K., et. al., "OSPF extensions in support of 
        Generalized MPLS," Internet draft, draft-ospf-generalized- mpls-
        00.txt, work in progress, March 2001. 
         
        [Moy98] Moy, John T., OSPF: Anatomy of an Internet Routing Protocol, 
        Addison-Wesley, 1998. 
      
        [Passmore01] Passmore, D., "Managing Fatter Pipes," Business 
        Communications Review, August 2001, pp. 20-21. 
      
        [Ramaswami98] Ramaswami, R. and Sivarajan, K. N., Optical Networks: 
        A Practical Perspective, Morgan Kaufmann Publishers, 1998. 
      
        [Tkach98] Tkach, R., Goldstein, E., Nagel, J., and Strand, J., 
        "Fundamental Limits of Optical Transparency", Optical Fiber 
        Communication Conf., Feb. 1998, pp. 161-162. 
         
         
        Chiu, Strand, Eds.       Informational                   [page 25] 
      

                                         
                       Impairments And Other Constraints          May 2003  
                            On Optical Layer Routing 
                                         
         
     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                      
        5853 Rue Ferrari 
        San Jose, CA 95138               
        Email: abanerjee@calient.net 
         
        Dan Blumenthal 
        Calient Networks                      
        5853 Rue Ferrari 
        San Jose, CA 95138 
        Email: dblumenthal@calient.net 
         
        John Drake                        
        Calient Networks                 
        5853 Rue Ferrari                 
        San Jose, CA 95138                
        Email: jdrake@calient.net        
         
        Andre Fredette 
        Hatteras Networks 
        PO Box 110025 
        Research Triangle Park, NC 27709 
        Email: afredette@hatterasnetworks.com 
         
        Nan Froberg 
        PhotonEx Corporation 
        200 Metrowest Technology Dr. 
        Maynard, MA 01754 
        Email: nfroberg@photonex.com 
         
        Taha Landolsi 
        WorldCom, Inc. 
        2400 North Glenville Drive 
        Richardson, TX 75082 
        Email: taha.landolsi@wcom.com 
         
        James V. Luciani 
        900 Chelmsford St. 
        Lowell, MA 01851 
        Email: james_luciani@mindspring.com 
      
        Robert Tkach 
         
        Chiu, Strand, Eds.       Informational                   [page 26] 
      

                                         
                       Impairments And Other Constraints          May 2003  
                            On Optical Layer Routing 
                                         
        Celion Networks 
        1 Sheila Dr., Suite 2  
        Tinton Falls, NJ 07724 
        Email: bob.tkach@celion.com 
         
        Yong Xue 
        WorldCom, Inc. 
        22001 Loudoun County Parkway 
        Ashburn, VA 20147 
        Email: yxue@cox.com 
         
         
     11. 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 
      
         
        Chiu, Strand, Eds.       Informational                   [page 27]