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Network Working Group                  Ayan Banerjee (Calient Networks)
Internet Draft                            Angela Chiu (Celion Networks)
Expiration Date: November 2001            John Drake (Calient Networks)
                                      Dan Blumenthal (Calient Networks)
                                              Andre Fredette (Photonex)


       Impairment Constraints for Routing in All-Optical Networks

               draft-banerjee-routing-impairments-00.txt



1. Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026 [Bra96].

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that
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   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six
   months and may be updated, replaced, or obsoleted by other documents
   at any time. It is inappropriate to use Internet- Drafts as
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   The list of current Internet-Drafts can be accessed at
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2. Abstract

   In the not too distant future, signals carried between two endpoints
   will be transmitted in an all-optical domain over a multi-hop path.
   Such transparent networks consist of photonic switches, optical
   add/drop multiplexers, optical amplifiers, optical regenerators, and
   fiber. Signaling for such routes needs to account for optical
   impairments in the path. This draft discusses a number of optical
   parameters and proposes optical constraints as enhancements to the
   routing protocols (for a subset of the parameters).
 draft-banerjee-optical-impairments-00.txt                   March 2001

3. Introduction

   Recently, a lot of work has been done to use the Generalized MPLS
   control plane [ABB01] to dynamically provision resources and to
   provide network survivability using protection and restoration
   techniques for all-optical networks. The optical networks presently
   being deployed may be called "opaque"  ([TGN98]) - 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
   networks. Such _transparent_ networks consist of photonic switches,
   optical add/drop multiplexers, optical amplifiers, optical
   regenerators, and fiber.

   Current proposals on routing protocol extensions (see [KRB01a] and
   [KRB01b]) consider opaque networks where all routes have adequate
   signal quality. Here, we consider all-optical networks. In order to
   take full advantages of potential cost and operational efficiencies
   offered by the all-optical networks, we assume that a domain of
   transparency may be too large to ensure that all potential routes
   have adequate signal quality for all connections. In order to obtain
   paths for the connections, physical impairments of various links in
   the all-optical network need to be accounted for. Our goal is to
   understand the impacts of the various types of impairments in this
   environment and to recommend a practical set of parameters that need
   to be accounted for. This necessitates enhancing the routing
   protocols to advertise the selected attributes which are necessary
   to compute constrained shortest paths.

   The organization of the remainder of this document is as follows.
   In Section 4, we discuss the various optical parameters that may
   need to be announced. Furthermore, we outline the TLVs for the
   (specifically for the OSPF and IS-IS routing protocols) parameters
   that are to be flooded into the routing database.

4. Optical Parameters

   In this section, we identify the various attributes that are
   potential candidates for being flooded using the routing protocols.
   We are only concerned with the impairments that may have impacts on
   possible routes chosen through a transparent network. According to
   the requirements specified in [CST00], we account for two key linear
   impairments, namely Polarization Mode Dispersion (PMD) and Optical
   Signal to Noise Ratio (OSNR). There are other performance related
   parameters, e.g., modulator extinction ratio, jitter, Q-factor, etc
   outlined in [CBD00], that need to be taken into account when
   designing the transmission system. These parameters are either not
   route dependent, or implicitly reflected by the PMD and OSNR
   constraints or included in the OSNR margin described in section 4.3.

4.1. Polarization Mode Dispersion (PMD)

 draft-banerjee-optical-impairments-00.txt                   March 2001
   PMD management requires that the time-average differential group
   delay (DGD) between two orthogonal state of polarizations, tau be
   less than a fraction a of the bit duration, T = 1/B, where B is the
   bit rate. The value of a depends on three major factors, 1) margin
   allocated to PMD, e.g., 1dB; 2) targeting outage probability, e.g.,
   4x10-5; 3) sensitivity of receiver to DGD. A typical value for a is
   0.1[ITU].

   Assume that the transparent segment consists of K links, with each
   link k having a PMD value of tau(k). The PMD value of a link tau(k)
   is a function of the length and fiber PMD parameter of each fiber
   span on the link. The constraint on overall path PMD becomes the sum
   of squares of the PMD parameter across all links to be less than
   a^2/B^2. Hence, for routing constraint checking purposes regarding
   PMD, the only link dependent information that needs to be propagated
   or is tau(k)^2 (the square of the polarization mode dispersion).

   In OSPF, the PMD parameter is represented as a sub-TLV of the Link
   TLV in the Traffic Engineering LSA, with type 15. The length of the
   sub-TLV is four-octets and specifies the square of the polarization
   mode dispersion (in IEEE floating point format, the unit being pico
   seconds squared). The format of the PMD sub-TLV is as shown:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Type = 15                    |         Length = 4            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |            Polarization Mode Dispersion Square                |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   In IS-IS, we enhance the sub-TLVs for the extended IS reachability
   TLV. The length of the PMD sub-TLV is four-octets and specifies the
   square of the polarization mode dispersion (in IEEE floating point
   format, the unit being pico seconds squared). Specifically, we add
   the following sub-TLV:
        Sub-TLV type  Length(in bytes)   Name
           21           4              PMD Type

4.2 Optical Signal to Noise Ratio (OSNR)

   Amplifier Spontaneous Emission (ASE) degrades the signal to noise
   ratio. An acceptable optical SNR level (SNRmin) which depends on the
   bit rate, transmitter-receiver technology (e.g., FEC), and margins
   allocated for other impairments, needs to be maintained at the
   receiver. Vendors currently provide OTS engineering rules defining
   maximum span length and number of spans that ensure that all routes
   meet this requirement. For larger transparent domains, more detailed
   OSNR computations will be needed to determine whether the OSNR level
   on a given all-optical service or restoration route has acceptable
   OSNR.

 draft-banerjee-optical-impairments-00.txt                   March 2001

   Assume P is the average optical power launched at the transmitter,
   and each link k generates noise power N(k). The OSNR constraint for
   path computation becomes the sum of the noise power across all links
   in the path must be less than P/ SNRmin.

   In OSPF, the Noise parameter is represented as a sub-TLV of the Link
   TLV in the Traffic Engineering LSA, with type 16. The length of the
   sub-TLV is four-octets and specifies the noise power (in IEEE
   floating point format, the unit being dBm). The format of the Noise
   sub-TLV is as shown:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Type = 16                    |         Length = 4            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                      Noise Parameter                          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   In IS-IS, we enhance the sub-TLVs for the extended IS reachability
   TLV. The length of the noise sub-TLV is four-octets and specifies
   the noise power of the link (in IEEE floating point format, the unit
   being dBm ). Specifically, we add the following sub-TLV:
        Sub-TLV type  Length(in bytes)   Name
           21           4              Noise parameter Type



4.3 OSNR Margin and Receiver OSNR requirements

   As an additional constraint, a network-wide margin on the OSNR
   accounts for a number of other additional parameters that are not
   spelled out explicitly in the above TLVs. For example, other major
   impairments are:
   1. Polarization-Dependent Loss (PDL): It is required that the total
      PDL on the path to be within some acceptable limit, typically 1dB
      margin in OSNR.
   2. 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.
   3. Crosstalk: Since crosstalk in the system affects Q, it can be
      factored in with some margin in Q. As a result, one can increase
      the OSNR requirement by some modified margin.
   4. Nonlinear Impairments: One could assume that nonlinear
      impairments are bounded and increase the required OSNR level by X
      dB, 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 limit to be longer than that imposed by the constraints
      which can be treated explicitly.

   Furthermore, it is assumed that all nodes in the network have a
   table of the minimum value of the OSNR required to transmit
 draft-banerjee-optical-impairments-00.txt                   March 2001

   information at a specified bit rate for a given transceiver
   technology (e.g. FEC).


5. Security Considerations

   The enhancements do not introduce any additional security
   considerations.

6. Acknowledgments

  This document has benefited from discussions with Michael Eiselt and
  Jonathan Lang.

7. References

   [ABB01] Ashwood-Smith, P., et. al., "Generalized MPLS Signaling
           Functional Description,_ Internet draft, draft-ietf-
           generalized-mpls-signaling-00.txt, work in progress, March
           2001.
   [Bra96] Bradner, S., "The Internet Standards Process -- Revision 3,"
           BCP 9, RFC 2026, October 1996.
   [CBD00] Ceuppens, L., Blumenthal, D., Drake, J., Chrostowski, J.,
           Edwards, W., "Performance Monitoring in Photonic Networks in
           Support of MPL(ambda)S", Internet draft, work in progress,
           March 2000.
   [CST00]  A. Chiu, J. Strand, and R. Tkach, "Unique Features and
           Requirements for The Optical Layer Control Plane", Internet
           Draft, draft-chiu-strand-unique-olcp-01.txt, work in
           progress, November 2000.
   [KRB01a] Kompella, K., et.al., "IS-IS extensions in support of
           Generalized MPLS," Internet Draft, draft-ietf-gmpls-
           extensions-01.txt, work in progress, 2001.
   [KRB01b] Kompella, K., et. al., "OSPF extensions in support of
           Generalized MPLS," Internet draft, draft-ospf-generalized-
           mpls-00.txt, work in progress, March 2001.
   [TGN98] Tkach, K., Goldstein, E., Nagel, J., and Strand, J.,
           "Fundamental Limits of Optical Transparency," Optical Fiber
           Communication Conference, February 1998.

7. Author's Addresses

   Ayan Banerjee                   Angela Chiu
   Calient Networks                Celion Networks
   5853 Rue Ferrari                1 Sheila Drive, Suite 2
   San Jose, CA 95138              Tinton Falls, NJ 07724
   Email: abanerjee@calient.net    email: angela.chiu@celion.com

   John Drake                      Dan Blumenthal
   Calient Networks                Calient Networks
   5853 Rue Ferrari                5853 Rue Ferrari
   San Jose, CA 95138              San Jose, CA 95138
   Email: jdrake@calient.net       Email: dblumenthal@calient.net

 draft-banerjee-optical-impairments-00.txt                   March 2001

   Andre Fredette
   8C Preston Court
   Bedford, MA 01730
   Photonex Corporation
   Email: fredette@photonex.com


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