Internet DRAFT - draft-banerjee-routing-impairments
draft-banerjee-routing-impairments
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].
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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
