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Versions: 00 01 02                                                      
                                                             Angela Chiu
                                                             John Strand
                                                                    AT&T
   Internet Draft
   Document: draft-chiu-strand-unique-olcp-00.txt            July, 2000


   Unique 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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   http://www.ietf.org/ietf/1id-abstracts.txt.
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Abstract

   Advances in the Optical Layer control plane is critical to ensure
   tremendous amount of bandwidth generated by the 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

   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; cross-connect technologies, both electrical and optical,
   promise the ability to establish very high bandwidth connections
   within milliseconds; and the insatiable appetite of the Internet for

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                   Unique Features and Requirements          July 2000
                 For The Optical Layer Control Plane

   high capacity "pipes" 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 are widely acknowledged to be incapable of
   scaling to meet exploding demand or to establish connections as
   rapidly as needed.  Consequently in the last year much attention has
   been paid to new control plane architectures based on data
   networking protocols from IETF such as RSVP-TE and CR-LDP from MPLS,
   IGPs including OSPF and IS-IS.  These architectures feature
   distributed routing and control logic, auto discovery and self
   inventorying, and many other advantages.

   The potential of these new architectures for optical networking are
   enormous; however, to be successful they need to be adapted to the
   specific technological, service, and business context characteristic
   of optical networking.  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.

   We most definitely are not claiming that these differences are fatal
   to these new architectures, only that the new 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


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   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.

   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

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       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".
     - 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 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 between the DWDMÆs and OADMÆs 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.

   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, etc. have been deployed. Thus the state information that
   would need to be disseminated is likely to be voluminous, possibly
   vendor specific, and likely to be hard to pin down. However it is
   very desirable to solve these issues, possibly by advertising only

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   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 from other links.  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.

   The routing of lightpaths through an all-optical network has
   received extensive attention. (For recent reviews, see [Yates99],
   [Ramaswami98], [Mukherjee97]).  One aspect of this problem that is
   still troublesome is the impact of transmission impairments on
   signal quality. 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.  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:
   Maximum private line bit rates have rapidly increased from DS3 (45
   Mb/sec) through OC-3 (155 Mb/sec), OC-12 (622 Mb/s) and OC-48 (2.5
   Gb/sec) to OC-192 (10 Gb/sec).  OC-768 (40 Gb/sec) is now under
   discussion. As bit rates increase it is necessary to increase power.
   This makes impairments and nonlinearities more troublesome. Thus a
   contained domain of transparency sized so all routes can support an
   OC-768 would necessarily be quite small (perhaps <100 km in
   diameter). A domain sized for OC-192 very likely be significantly
   smaller than one sized for OC-48. This suggests that more aggressive
   domain sizing might have some benefits.

   Optical technology is advancing very rapidly and is making ever-
   larger domains possible. Of particular importance in this respect
   are advances in optical cross-connects (OXCÆs) and ultra-long OTSÆs
   employing Raman amplification and other techniques. The use of all-

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                 For The Optical Layer Control Plane

   optical networking in metro and access applications is under very
   rapid evolution also.

   Optical layer control plane architectures are under intense
   discussion in the ITU, IETF, OIF, and other standards bodies. The
   general approach being considered for routing is to adapt the Open
   Shortest Path First (OSPF) protocol from IP ([Moy98]). A number of
   the specifics of this adaptation depend strongly on whether
   transmission impairments need to be explicitly considered in the
   routing process. To give one example, the link state information
   advertised might need to contain information about the specific
   impairments on the link.

   In this document we assume that these considerations have led to the
   deployment of a domain of transparency that is too large to ensure
   that all potential routes have adequate signal quality. Our goal is
   to understand the impacts of the various types of impairments in
   this environment and to recommend a practical method for doing
   routing in this situation.

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.

2.2.2     Impairment Constraints ([Tkach98])

   Impairment constraints can be classified into two categories, linear
   and nonlinear. Linear effects are independent of signal power and
   affect wavelengths individually. Amplifier spontaneous emission
   (ASE) and Polarization Mode Dispersion (PMD) are examples. On the
   other hand, fiber nonlinearities are significantly more complex:
   they generate not only dispersion on individual channel, but also
   crosstalk between channels which causes dependency across channels.
   Examples include four-photon mixing, cross-phase modulation, self-
   phase modulation, stimulated Brillouin scattering, and stimulated
   Raman scattering. Here, we assume that proper system design will
   compensate for those effects (for example, Raman amplification and
   FEC mechanisms both serve to allow lower power to be used and thus
   move systems towards the linear regime) and/or simulation studies

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   can be used to rule out certain fiber type(s). We consider only the
   linear regime in this document. We assume that chromatic dispersion
   is compensated in fiber lines and can be neglected. Hence PMD and
   signal to noise ratio (SNR) are the only impairment constraints that
   need to be considered in determining the path of a lightpath through
   a transparent optical subnetwork. We examine the role of each in
   this regard.

     - PMD: 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 10% of the bit duration [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 constraint on the maximum length of an M-fiber-span
       transparent segment where a fiber span in a transparent network
       refers to a segment between two optical amplifiers. The
       constraint depends on a set of parameters including the length
       and the fiber PMD parameter of each of the M fiber spans. (The
       detailed equation is omitted due to the format constraint.) For
       typical fibers with PMD parameter of 1 picosecond per square
       root of km, based on the constraint, the maximum length of the
       transparent segment should not exceed 100km and 6.75km 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 of the transparent segment should not
       exceed 10000km and 675km 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.

     - SNR: Based on the bit rate and type of transmitter-receiver
       technology (e.g., FEC), an acceptable optical SNR level (SNRmin)
       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 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. A constraint
       on the maximum number of spans can be obtained [Kaminow97] which

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       depends on a set of parameters including SNRmin, P, optical
       bandwidth B, amplifier gain G and spontaneous emission factor n
       for each 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 without FEC where the
       requirement on SNRmin becomes 25dB, the maximum number of spans
       drops down to 3.

2.2.3     Implications For Routing and Control Plane Design

   Here, we describe the main implications that these two main
   impairment constraints have on routing algorithm and control plane
   design.

   The optimal routing problem with the two constraints is in general
   more computational intensive. However, relatively simple heuristics
   can be used in practice. If the ingress node of a lightpath does
   path selection, in order to check whether the two constraints are
   satisfied or not for a given path, it needs to obtain all the
   relevant parameters for each span on the path if they vary from one
   span to another. These parameters typically do not change
   dynamically, and are often stored in some database. So the ingress
   node (or some other node that makes the path selection decision) can
   retrieve the information from the database when needed, or the
   information can be advertised by the node attached to the span at
   the topology discovery stage.

   Note that in some circumstances, it may be useful to consider
   nonlinear effects also. Nevertheless, the two constraints described
   here are enough to illustrate the impact of the impairment
   constraints on the routing algorithm and control plane design in
   transparent subnetworks.

   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

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       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 the routing
       logic normally used in 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.

   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:

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     - 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 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.)




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                    C                             C
                    |                           /   \
                    |                         /       \
                    |                       /           \
             A------X------B              A---------------B

        (a) Fiber Cable Topology         (b) Fiber Topology

               Figure 2-3.  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
       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)

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                 For The Optical Layer Control Plane

            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 Natural 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.
          o 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.

   [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

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                 For The Optical Layer Control Plane

   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
   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

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       [Doverspike00] for more detailed discussions on various
       protection/restoration schemes.

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
   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. 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

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   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
   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

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   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.

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,
   Mark Shtaif, Bob Tkach, and our other AT&T colleagues.

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.


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                   Unique Features and Requirements          July 2000
                 For The Optical Layer Control Plane

   [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
   Protocols in the Optical Network Control Plane", submitted for
   journal publication, June, 2000 (online at
   http://www.research.att.com/~jyates/IPoverWDMpublications.html).

   [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.

   [Mukherjee97] Mukherjee, B., Optical Communication Networks, McGraw
   Hill, 1997.

   [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.

   [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
   100 Schulz Dr., Rm 4-204
   Red Bank, NJ 07701, USA
   Phone: +1 (732) 345-3441
   Email: alchiu@att.com

   John Strand
   AT&T Labs
   100 Schulz Dr., Rm 4-212

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                   Unique Features and Requirements          July 2000
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   Red Bank, NJ 07701, USA
   Phone: +1 (732) 345-3255
   Email: jls@att.com

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