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                                                             Angela Chiu
                                                         Celion Networks

                                                             John Strand
                                                                    AT&T
   Internet Draft
   Document: draft-chiu-strand-unique-olcp-02.txt          Robert Tkach
   Expiration Date: August 2001                         Celion Networks

                                                          James Luciani
                                                             Tollbridge


       Features and Requirements for The Optical Layer Control Plane


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
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Abstract

   Advances in the Optical Layer control plane are critical to ensure
   that the tremendous amount of bandwidth generated by DWDM technology
   be provided to upper layer services in a timely, reliable, and cost
   effective fashion. This document describes some unique features and
   requirements for the Optical Layer control plane that protocol
   designers need to take into consideration.


1.        Introduction


   Chiu/Strand                                                [Page 1]


                      Features and Requirements         February 2001
                 For The Optical Layer Control Plane

   The confluence of technical advances and service needs has focused
   intense interest on optical networking.  Dense Wave Division
   Multiplexing (DWDM) is allowing unprecedented growth in raw optical
   bandwidth; new cross-connect technologies promise the ability to
   establish very high bandwidth connections within milliseconds; and
   the insatiable appetite of the Internet for high capacity ``pipesÆÆ
   has caused transport network operators to tear up their forecasts
   and add optical capacity as fast as they can.

   Critical to these advances are improvements to the "Optical Layer
   Control Plane" -the software used to determine routings and
   establish and maintain connections. Traditional centralized
   transport operations systems (OSÆs) are widely acknowledged to be
   incapable of scaling to meet exploding demand or establishing
   connections as rapidly as needed.  Consequently much attention has
   been paid recently to new control plane architectures based on data
   networking protocols such as MPLS and OSPF/IS-IS), under the
   Generalized MPLS (GMPLS) umbrella.  These architectures feature
   distributed routing and control logic, auto discovery and self
   inventorying, and many other advantages. OSPF/IS-IS provides a
   constraint-based routing capability that takes bandwidth
   availability into account.

   The potential of these new architectures for optical networking is
   enormous; however, to be successful they need to be adapted to the
   specific technological, service, and business context characteristic
   of optical networking. In order to identify some of the enhancements
   necessary in GMPLS to make it applicable to optical networks, this
   document attempts to describe several aspects of optical networking
   which differ from those in the data networking environment inspiring
   these new architectures:

     - Section 2 describes some distinctive technological and
       networking aspects of optical networking that will constrain
       routing in an optical network, and

     - Section 3 gives a transport network operatorÆs perspective on
       business and operational realities that optical networks are
       likely to face which are unlike those in data networking.

   Particular emphasis is placed on the multihop optical network
   problem. We most definitely are not claiming that these differences
   are fatal to these new architectures, only that the new

   Chiu/Strand et al                                          [page 2]


                      Features and Requirements         February 2001
                 For The Optical Layer Control Plane

   architectures must be built upon a detailed appreciation of the
   unique characteristics of the optical world.

2.   Constraints On Routing

   Optical Layer routing is less insulated from details of physical
   implementation than routing in higher layers.  In this section we
   give examples of constraints arising from the design of network
   elements, from the accumulation of signal impairments, and from the
   need to guarantee the physical diversity of some circuits.

2.1       Reconfigurable Network Elements

   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 (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 2-1: An OTS With OADM's - Functional Architecture

   In Fig.2-1, 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.


   Chiu/Strand et al                                          [page 3]


                      Features and Requirements         February 2001
                 For The Optical Layer Control Plane

   In Fig. 2-1, 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
       "adaptation grouping". Another term frequently used is "wave
       group".
     - 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. Note that tunable
       receivers are becoming a reality and for certain applications.

   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 OADMÆs may also be constrained.
       For example:
          o There may be some wavelengths that can not be dropped at
            all.
          o There may be a fixed relationship between the frequency
            dropped and the physical port on the OADM to which it is
            dropped.
          o OADM physical design may put an upper bound on the number
            of adaptation groupings dropped at any single OADM.

   For a fixed configuration of the 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.

   Chiu/Strand et al                                          [page 4]


                      Features and Requirements         February 2001
                 For The Optical Layer Control Plane


   This capability 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 routing
   process. The constraints on reconfiguration are likely to be quite
   complex, dependent on the vendor design and also on exactly what
   line cards have been deployed. Thus the state information needed for
   routing  is likely to be voluminous and possibly vendor specific.
   However it is very desirable to solve these issues, possibly by
   advertising only an abstraction of the complex configuration options
   to the external world via the control plane.

2.2       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 eliminated transparency to bit
   rates and frame format. These transponders are quite expensive and
   they also constrain the rapid evolution to new services - for
   example, they tend to be bit rate and format specific.  Thus there
   are strong motivators to introduce "domains of transparency" - all-
   optical subnetworks - larger than an OTS.

   The routing of lightpaths through an all-optical network has
   received extensive attention. (See [Yates99] or [Ramaswami98]).
   When discussing routing in an all-optical network it is usually
   assumed that all routes have adequate signal quality. This may be
   ensured by limiting all-optical networks to subnetworks of limited
   geographic size 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

   Chiu/Strand et al                                          [page 5]


                      Features and Requirements         February 2001
                 For The Optical Layer Control Plane

   understand the impacts of the various types of impairments in this
   environment.

2.2.1     Problem Formulation

   We consider a single domain of transparency. 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 applies 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 dispersion 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.

2.2.2     Polarization Mode Dispersion

   For a transparent fiber segment, the general rule for the PMD
   requirement is that the time-average differential time delay between
   two orthogonal state of polarizations should be less than a% of the
   bit duration. (A typical value for a is 10 [ITU]. More aggressive
   designs to compensate for PMD may allow higher than 10%. This would
   be a system parameter known to the routing process.) This results in
   a upper bound on the maximum length of an M-fiber-span transparent
   segment, which is inverse proportion to the square of bit rate and
   fiber PMD parameter where a fiber span in a transparent network
   refers to a segment between two optical amplifiers (The detailed
   equation is omitted due to the format constraint). For typical
   fibers with PMD parameter of 0.5 picosecond 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. With newer fibers assuming PMD parameter
   equals to 0.1 picosecond per square root of km, the maximum length

   Chiu/Strand et al                                          [page 6]


                      Features and Requirements         February 2001
                 For The Optical Layer Control Plane

   of the transparent segment should not exceed 10000km and 625km for
   bit rates of 10Gb/s and 40Gb/, respectively. 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.

2.2.3     Amplifier Spontaneous Emission

   ASE degrades the signal to noise ratio. An acceptable optical SNR
   level (SNRmin) which depends on the bit rate and transmitter-
   receiver technology (e.g., FEC) needs to be maintained at the
   receiver. It also includes all the margins for those impairments
   that are not treated explicitly as described in the following
   subsection. 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 with 80km
   long in each. Startups have announced ultra long haul systems that
   are claimed to be able to support up to thousands of km. Although
   these general rules are helpful in network planning, more detailed
   information on the SNR reduction in each component should be used to
   determine whether the SNR level through a given transparent segment
   is within the required value. This would provide flexibility in
   provisioning or restoring a lightpath through a transparent
   subnetwork. Here, we assume that the average optical power launched
   at the transmitter is known as 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 nonlineararity. A constraint
   on the maximum number of spans can be obtained [Kaminow97] which is
   proportional to P and inverse proportional to SNRmin, optical
   bandwidth B, amplifier gain G-1 and spontaneous emission factor n of
   the optical amplifier. (Again, the detailed equation is omitted due
   to the format constraint.) 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.

2.2.4     Other Impairments

   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 it is generally required to maintain the total PDL on the
   path to be within some acceptable limit, typically 1dB margin in
   OSNR.

   Chiu/Strand et al                                          [page 7]


                      Features and Requirements         February 2001
                 For The Optical Layer Control Plane


   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.

   Crosstalk Since crosstalk in the system affects Q which is a measure
   of the electrical signal-to-noise ratio assuming Gaussian noise
   statistics, it can be factored in with some margin in Q. As a
   result, one can increase the OSNR requirement by some modified
   margin.

   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. 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.
   Further work is required to determine the validity of this approach.
   However, it is possible that there could be 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.

2.2.5     Implications For Routing and Control Plane Design

     - Additional state information will be required by the routing
       algorithm for each type of impairment that has the potential of
       being limiting for some routes.


   Chiu/Strand et al                                          [page 8]


                      Features and Requirements         February 2001
                 For The Optical Layer Control Plane

     -  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 important to know whether
       chromatic dispersion has been dealt with on per-link basis, and
       whether the domain is operating in a linear or nonlinear regime.

     - In situations where only PMD and/or ASE impairments are
       potentially binding the optimal routing problem as two
       constraints OSPF algorithm enhancements will be needed. However,
       it is likely that relatively simple heuristics could be used in
       practice.

   Additionally, 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.

     - 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.  This approach
       would however also have a significant affect on OSPF.

2.3       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.


   Chiu/Strand et al                                          [page 9]


                      Features and Requirements         February 2001
                 For The Optical Layer Control Plane

   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, 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:



   Chiu/Strand et al                                         [page 10]


                      Features and Requirements         February 2001
                 For The Optical Layer Control Plane

                                       A                 B
              A-------------B                 \             /
                                                \         /
                                                  X-----Y
                                                /         \
              C-------------D                 /             \
                                            C                 D

      (a) Fiber Cable Topology       (b) Right-Of-Way/Conduit Topology

                Figure 2-2:  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. 2-3(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. 2-3(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 2-3.  Fiber Cable vs Fiber Topologies


   Chiu/Strand et al                                         [page 11]


                      Features and Requirements         February 2001
                 For The Optical Layer Control Plane

   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
       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.

   Chiu/Strand et al                                         [page 12]


                      Features and Requirements         February 2001
                 For The Optical Layer Control Plane

     - 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.

   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
   parameters:
     - Type of Compromise: Examples would be shared fiber cable, shared
       conduit, shared ROW, shared optical ring, shared office without
       power sharing, etc.)
     - Extent of Compromise:  For compromised outside plant, this would
       be the length of the sharing.

   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 (3000 km)
   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. On
   the other hand, in order to be able to perform distribute path
   selection at each node that satisfies certain diverse routing

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                      Features and Requirements         February 2001
                 For The Optical Layer Control Plane

   criterion, each network element may need to propagate the
   information of number of channels available for each channel type
   (e.g., OC48, OC192) on each channel group, where channel group is
   defined as a set of channels that are routed identically and should
   be given unique identification. Each channel group can be mapped
   into a sequence of fiber cables while each fiber cable can belong to
   multiple SRLGÆs based on their definitions.

2.4       Other Unique Features of Optical Networks

   There are other major differences between optical networks and IP
   networks that have significant impacts on the design of the Optical
   Layer control plane. They include the following two areas.

     - Bi-directionality: In an IP network, Label Switched Paths (LSPs)
       are inherently unidirectional. However, current transport
       networks are bi-directional oriented, mostly due to the
       evolution of two-way transmission in Public Switched Telephone
       Network and by SONET/SDH line protection schemes [Doverspike00].
       This often requires the bi-directional connections provided by
       the optical layer to use the same numbered channel in each
       direction. As a result, a channel contention problem may occur
       between two bi-directional request traveling in opposite
       directions. Signaling mechanisms have been proposed to resolve
       this type of contention [Ashwood00].

     - Protection and restoration: In an IP network, when a backup LSP
       is pre-established to protect against failure(s) on a working
       LSP, the backup LSP does not occupy any physical resources
       before a failure occurs. However, in an optical network, a pre-
       established optical connection for backup does occupy the ports
       and channels on the path of the connection. This can be used for
       the 1+1 protection, but not for shared mesh protection. Instead
       with shared mesh protection, the backup path can be pre-selected
       with or without the associated channels being chosen prior to
       any failure, then cross-connect ports/channels physically after
       a failure on the working path has been detected. See
       [Doverspike00] for more detailed discussions on various
       protection/restoration schemes.

2.4  Discussion and Summary

   Dealing with diversity seems to be an unavoidable requirement on
   optical layer routing. It requires dealing with additional

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                      Features and Requirements         February 2001
                 For The Optical Layer Control Plane

   constraints in the routing process but most importantly requires
   additional state information to be available to the routing process.

   The physical constraints of optical technology apply inside an all-
   optical ``domains of transparencyÆÆ.  TodayÆs OTS is a simple
   ``domain of transparencyÆÆ consisting of WDM Mux/Demuxers and
   Optical Amplifiers.  Because an OTS is not easily reconfigurable
   these constraints are dealt with at the time of installation and
   donÆt complicate routing and the control plane.

   As domains of transparency become both larger and software
   reconfigurable as discussed earlier, these physical constraints on
   connectivity and transmission quality become increasingly of concern
   to the control plane.   It is important to note that at present this
   evolution is largely technology driven:  vendors pushing the
   technology envelope are competing fiercely to provide solutions
   which have higher capacity, can go further all-optically, are more
   reconfigurable, and are more cost-effective.  Routing constraints,
   which are essentially a by-product of this competitive dynamic, may
   well become more complex. As vendors pursue their diverse visions it
   is quite plausible that the optical layer of the future will be made
   up of heterogeneous technologies which differ significantly in their
   routing implications.

   What are the control plane architecture choices in such an
   eventuality? Alternative approaches that deserve consideration are:

     - Per-Domain Routing: In this approach each domain could have its
       own tuned approach to routing. Inter-domain routing would be
       handled by a multi-domain or hierarchical protocol that allowed
       the hiding of local complexity.  Single vendor domains might
       have proprietary intra-domain routing strategies. This approach
       has the advantage of providing carriers a flexible way to digest
       technologies and subnetworks that for whatever reason are not
       using a standard intra-domain routing protocol.

     - Enforced Homogeneity: The capabilities of the control plane
       would impose constraints on system design and network
       engineering.  As examples: If control plane protocols did not
       deal with non-linear impairments carriers would require their
       vendors to provide transport systems where these constraints
       were never binding. Transmission engineers could be required to
       only deploy domains where every possible route met all
       constraints not handled explicitly by the control plane even if
       the cost penalties were severe.

     - Additional Regeneration: At (selected) OLXCÆs within a domain of
       transparency, the control plane could insert O/E/O regeneration
       into routes with transmission problems. This might make all

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                      Features and Requirements         February 2001
                 For The Optical Layer Control Plane

       routes feasible again, but at the cost of additional cost and
       complexity and with some loss of rate and format transparency.

     - Standardized Intra-Domain Routing Protocol: The examples
       discussed in Section 2 suggest that a single standardized
       protocol which tries to deal with the full range of possible
       topological and transmission constraints will be extremely
       complex and will require a lot of state information. However
       when combined with limited application of the two previous
       approaches it might be more plausible.

   Given the complexity of physical and connectivity impairments and
   diversity requirements, a valid question to ask is whether a
   centralized routing model, where routing is done centrally using a
   centralized database with a global network view would be better than
   the distributed model favored in the Internet. Here, we provide some
   pros and cons on each model.

   To the extent that the per-domain routing approach just discussed is
   used, the choice of model might be different depending on the
   characteristics of the domain.  For example, in a domain like Fig.
   2-1 it seems likely that a centralized model is more appropriate
   because network elements like tunable lasers and reconfigurable
   OADM's seem on the surface to be unlikely peers to much more complex
   devices like OXC's or routers. On the other hand, a purely "opaque"
   domain where impairment constraints play no role in routing would
   appear to be an excellent candidate for the distributed model.

   In the context of the complexities discussed in this paper, a
   centralized model has some advantages:

     - Information such as SRLGÆs and performance parameters which
       change infrequently and are unlikely to be amenable to self-
       discovery could reside in a central database and would not need
       to be advertised.

     - Routing dependencies among circuits (to ensure diversity, for
       example) is more easily handled centrally when the circuits do
       not share terminals since the necessary state information should
       be more easily accessible in a centralized model.

     - Pre-computation of restoration paths and other computations that
       can benefit from the use of global state information may also
       benefit from centralization.

   There are, of course, significant disadvantages to the centralized
   model when compared to a distributed model:

     - If rapid restoration is required, it is not possible to rely on
       a centralized routing system to compute a recovery path for each

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                      Features and Requirements         February 2001
                 For The Optical Layer Control Plane

       failed lightpath on demand after a failure has been detected.
       The distributed model arguably will not have this problem.

     - The centralized approach is not consistent with the distributed
       routing philosophy prevalent in the Internet. The reasons which
       drove the InternetÆs architecture-scalability, the inherent
       problems with hard state information, etc.-are largely relevant
       to optical networking. In addition there is the major
       disadvantage that a centralized approach would seem to preclude
       integrated routing across the IP and optical boundary.

   A related issue is whether routes should be pre-computed. It has
   been suggested, for example, that all routes (or at least a large
   number) be pre-computed and stored in a central database. This
   potentially might allow more sophisticated algorithms to be used to
   filter out the routes violating transmission constraints. There are
   however serious disadvantages (in addition to the disadvantages of
   the centralized model given above):

     - In a large national network there are just too many routes that
       might be needed, by orders of magnitude. This is particularly
       true when diversity constraints and restoration routing may
       force weird routings.
     - Every time any parameter changes anywhere in the network all
       routes using the impacted resource will need to be reexamined.


3.        Business and Operational Realities

   The Internet technologies being applied to define the new Optical
   Layer control plane evolved in a very different business and
   operational environment than that of today's transport network
   provider. The differences need to be clearly understood and dealt
   with if the new control plane is going to be a success. The Optical
   Interworking Forum, one of the principal standards groups in this
   area, has recently formed a Carrier Subgroup to provide guidance
   from this perspective for their standards activities.

   In this section we touch on two aspects of this problem: Business
   Models and the management of the introduction of new technology.

3.1       Business Models

   The cost of providing gigabit connections is expected to drop
   rapidly, but will still require dedicated use of expensive and
   periodically scarce capacity and equipments.  Therefore the ability
   to control network access, and to measure and bill for usage, will

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                      Features and Requirements         February 2001
                 For The Optical Layer Control Plane

   be critical. Also, lightpath connections are expected to have quite
   long holding times (weeks-months) compared to LSPs in an IP network.
   Therefore the collection of usage data and the nature of the
   connection establishment process have very different characteristics
   in the Optical Network than in an IP network.

   In addition, industry revenues from legacy services (voice and
   private line) are expected to dwarf those from IP transport for the
   next few years. ?? John: could you find a reference from RHK?
   Meeting the needs of these services and migrating them to the
   operatorÆs newer service platforms will also be a critical need for
   operators with extensive embedded revenues.  Thus the needs of
   services based on SONET/SDH, Ethernet, ATM, etc. will need to be
   given attention.  In addition most operators hope that they will
   have many different ISP's and Intranets as customers. Thus the
   customer base for most operators will be quite diverse.

   Another area of prime concern is Operations Systems (OSÆs). The
   opportunity to create a thinner and more nimble network management
   plane by off-loading many provisioning and data-basing functions
   onto a vendor-provided control plane and/or Element Management
   System (EMS) holds the promise of large and immediate benefits to
   operators in the form of reduced software development and more rapid
   deployment of new functionality.  This is a critical area to achieve
   scalability.

   In the short term the principal benefits of the proposed control
   plane are two: rapid provisioning and a reduction in the cost and
   complexity of OSÆs and operations. Both of these benefits require
   that circuits be controlled end-to-end by the new control plane, for
   otherwise the provisioning times will be determined by those of the
   older, much slower segments and OS costs and OS and operations
   complexity may actually go up because of the need to interwork the
   old and the new worlds. To avoid this the capabilities of the new
   control plane need to be available end-to-end as soon as possible.
   This will put a premium on the rapid development of standards for
   interworking across trust boundaries, for example between Local
   Exchange Carrier's and national networks.

3.2       Managing The Introduction Of New Technology

   We expect optical layer hardware technology to continue to evolve
   very rapidly, with a very real possibility of additional
   "disruptive" advances. The analog nature of optical technology

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                      Features and Requirements         February 2001
                 For The Optical Layer Control Plane

   compounds this problem for the control planes because these advances
   are likely to be accompanied by complex technology-specific
   constraints on routing and functionality. (Sections 2.1 and 2.2
   above provide examples of this.)  An architecture which allows the
   gradual and seamless introduction of new technologies into the
   network without time-consuming and costly changes to embedded
   technologies and especially control planes is highly desirable.

   When compared to the IP experience several distinctions stand out:
     - The optical layer control plane seems more likely to be buffeted
       by hardware changes than is the IP control plane.
     - Optical layer innovations are currently being driven by start-up
       companies, with product innovation well ahead of the standards
       process.  Efforts at control plane standardization are much less
       mature than comparable IP efforts.  This is a matter of
       considerable concern because neither rapid provisioning nor the
       operational improvements desired are likely if each vendor has a
       proprietary control plane, with interworking between vendors
       (and hence between networks, in most cases) left as a problem
       for operators' OS's to solve.

3.3       Service Framework Suggestions

   For the reasons given above and others, we expect that the best
   model for an optical layer control plane within a trust domain is
   one that pays heavy attention to the management of heterogeneous
   technologies and associated service capabilities. This might be done
   by hiding complexities in subnetworks. These subnetworks would then
   advertise only a standardized abstraction of their connectivity,
   capacity, and functionality capabilities. Hopefully this would allow
   even disruptive technologies such as all-optical subnetworks to be
   introduced with a minimum of impact on preexisting parts of the
   trust domain.

   Each network operator will have a need to define "branded" services
   - bundles of service functionality and SLA's with a specific price
   structure. In a heterogeneous network it will be necessary to map a
   customer request for such a "branded" service onto the specific
   capabilities of each subnetwork. This suggests a hierarchical model,
   decisions about these mappings, and also about policies for peering
   with other networks and overall management of the service offerings
   available to specific customers managed centrally but application of
   these policies handled at the local or subnetwork level.


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                      Features and Requirements         February 2001
                 For The Optical Layer Control Plane

4.        Security Considerations

   The solution developed to address the requirements defined in this
   document must address security aspects.

5.        Acknowledgments

   This document has benefited from discussions with Michael Eiselt and
   Mark Shtaif.

References:

   [Ashwood00] Ashwood-Smith, P. et al., "MPLS Optical/Switching
   Signaling Functional Description", Work in Progress, draft-ashwood-
   generalized-mpls-signaling-00.txt.

   [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.

   [Chaudhuri00] Chaudhuri, S., Hjalmtysson, G., and Yates, J.,
   "Control of Lightpaths in an Optical Network", Work in Progress,
   draft-chaudhuri-ip-olxc-control-00.txt.

   [Doverspike00] Doverspike, R. and Yates, J., "Challenges For MPLS in
   Optical Network Restoration", IEEE Communication Magazine, February,
   2001.

   [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.

   [Moy98] Moy, John T., OSPF: Anatomy of an Internet Routing Protocol,
   Addison-Wesley, 1998.

   [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 et al                                         [page 20]


                      Features and Requirements         February 2001
                 For The Optical Layer Control Plane


   [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).

Authors' Addresses:

   Angela Chiu
   AT&T Labs
   200 Laurel Ave., Rm A5-1F06
   Middletown, NJ 07748
   Phone:(732) 420-9057
   Email: alchiu@att.com

   John Strand
   AT&T Labs
   200 Laurel Ave., Rm A5-1D06
   Middletown, NJ 07748
   Phone:(732) 420-9036
   Email: jls@att.com

   Robert Tkach
   Celion Networks
   1 Shiela Dr., Suite 2
   Tinton Falls, NJ 07724
   Phone:(732) 747-9909
   Email: bob.tkach@celion.com

   James Luciani
   PO Box 1010
   Concord, MA 01742
   Phone:(978) 266-2854
   james_luciani@mindspring.com

   Chiu/Strand et al                                         [page 21]


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