RFC 6163
Framework for GMPLS and Path Computation Element (PCE) Control of Wavelength Switched Optical Networks (WSONs)
Pages: 51
Informational
Informational
Part 1 of 2 – Pages 1 to 26
Internet Engineering Task Force (IETF) Y. Lee, Ed. Request for Comments: 6163 Huawei Category: Informational G. Bernstein, Ed. ISSN: 2070-1721 Grotto Networking W. Imajuku NTT April 2011 Framework for GMPLS and Path Computation Element (PCE) Control of Wavelength Switched Optical Networks (WSONs)Abstract
This document provides a framework for applying Generalized Multi- Protocol Label Switching (GMPLS) and the Path Computation Element (PCE) architecture to the control of Wavelength Switched Optical Networks (WSONs). In particular, it examines Routing and Wavelength Assignment (RWA) of optical paths. This document focuses on topological elements and path selection constraints that are common across different WSON environments; as such, it does not address optical impairments in any depth. Status of This Memo This document is not an Internet Standards Track specification; it is published for informational purposes. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Not all documents approved by the IESG are a candidate for any level of Internet Standard; see Section 2 of RFC 5741. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc6163.
Copyright Notice Copyright (c) 2011 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.Table of Contents
1. Introduction ....................................................4 2. Terminology .....................................................5 3. Wavelength Switched Optical Networks ............................6 3.1. WDM and CWDM Links .........................................6 3.2. Optical Transmitters and Receivers .........................8 3.3. Optical Signals in WSONs ...................................9 3.3.1. Optical Tributary Signals ..........................10 3.3.2. WSON Signal Characteristics ........................10 3.4. ROADMs, OXCs, Splitters, Combiners, and FOADMs ............11 3.4.1. Reconfigurable Optical Add/Drop Multiplexers and OXCs ..............................11 3.4.2. Splitters ..........................................14 3.4.3. Combiners ..........................................15 3.4.4. Fixed Optical Add/Drop Multiplexers ................15 3.5. Electro-Optical Systems ...................................16 3.5.1. Regenerators .......................................16 3.5.2. OEO Switches .......................................19 3.6. Wavelength Converters .....................................19 3.6.1. Wavelength Converter Pool Modeling .................21 3.7. Characterizing Electro-Optical Network Elements ...........24 3.7.1. Input Constraints ..................................25 3.7.2. Output Constraints .................................25 3.7.3. Processing Capabilities ............................26 4. Routing and Wavelength Assignment and the Control Plane ........26 4.1. Architectural Approaches to RWA ...........................27 4.1.1. Combined RWA (R&WA) ................................27 4.1.2. Separated R and WA (R+WA) ..........................28 4.1.3. Routing and Distributed WA (R+DWA) .................28 4.2. Conveying Information Needed by RWA .......................29
5. Modeling Examples and Control Plane Use Cases ..................30 5.1. Network Modeling for GMPLS/PCE Control ....................30 5.1.1. Describing the WSON Nodes ..........................31 5.1.2. Describing the Links ...............................34 5.2. RWA Path Computation and Establishment ....................34 5.3. Resource Optimization .....................................36 5.4. Support for Rerouting .....................................36 5.5. Electro-Optical Networking Scenarios ......................36 5.5.1. Fixed Regeneration Points ..........................37 5.5.2. Shared Regeneration Pools ..........................37 5.5.3. Reconfigurable Regenerators ........................37 5.5.4. Relation to Translucent Networks ...................38 6. GMPLS and PCE Implications .....................................38 6.1. Implications for GMPLS Signaling ..........................39 6.1.1. Identifying Wavelengths and Signals ................39 6.1.2. WSON Signals and Network Element Processing ........39 6.1.3. Combined RWA/Separate Routing WA support ...........40 6.1.4. Distributed Wavelength Assignment: Unidirectional, No Converters ......................40 6.1.5. Distributed Wavelength Assignment: Unidirectional, Limited Converters .................40 6.1.6. Distributed Wavelength Assignment: Bidirectional, No Converters .......................40 6.2. Implications for GMPLS Routing ............................41 6.2.1. Electro-Optical Element Signal Compatibility .......41 6.2.2. Wavelength-Specific Availability Information .......42 6.2.3. WSON Routing Information Summary ...................43 6.3. Optical Path Computation and Implications for PCE .........44 6.3.1. Optical Path Constraints and Characteristics .......44 6.3.2. Electro-Optical Element Signal Compatibility .......45 6.3.3. Discovery of RWA-Capable PCEs ......................45 7. Security Considerations ........................................46 8. Acknowledgments ................................................46 9. References .....................................................46 9.1. Normative References ......................................46 9.2. Informative References ....................................47
1. Introduction
Wavelength Switched Optical Networks (WSONs) are constructed from subsystems that include Wavelength Division Multiplexing (WDM) links, tunable transmitters and receivers, Reconfigurable Optical Add/Drop Multiplexers (ROADMs), wavelength converters, and electro-optical network elements. A WSON is a WDM-based optical network in which switching is performed selectively based on the center wavelength of an optical signal. WSONs can differ from other types of GMPLS networks in that many types of WSON nodes are highly asymmetric with respect to their switching capabilities, compatibility of signal types and network elements may need to be considered, and label assignment can be non- local. In order to provision an optical connection (an optical path) through a WSON certain wavelength continuity and resource availability constraints must be met to determine viable and optimal paths through the WSON. The determination of paths is known as Routing and Wavelength Assignment (RWA). Generalized Multi-Protocol Label Switching (GMPLS) [RFC3945] includes an architecture and a set of control plane protocols that can be used to operate data networks ranging from packet-switch-capable networks, through those networks that use Time Division Multiplexing, to WDM networks. The Path Computation Element (PCE) architecture [RFC4655] defines functional components that can be used to compute and suggest appropriate paths in connection-oriented traffic-engineered networks. This document provides a framework for applying the GMPLS architecture and protocols [RFC3945] and the PCE architecture [RFC4655] to the control and operation of WSONs. To aid in this process, this document also provides an overview of the subsystems and processes that comprise WSONs and describes RWA so that the information requirements, both static and dynamic, can be identified to explain how the information can be modeled for use by GMPLS and PCE systems. This work will facilitate the development of protocol solution models and protocol extensions within the GMPLS and PCE protocol families. Different WSONs such as access, metro, and long haul may apply different techniques for dealing with optical impairments; hence, this document does not address optical impairments in any depth. Note that this document focuses on the generic properties of links, switches, and path selection constraints that occur in many types of WSONs. See [WSON-Imp] for more information on optical impairments and GMPLS.
2. Terminology
Add/Drop Multiplexer (ADM): An optical device used in WDM networks and composed of one or more line side ports and typically many tributary ports. CWDM: Coarse Wavelength Division Multiplexing. DWDM: Dense Wavelength Division Multiplexing. Degree: The degree of an optical device (e.g., ROADM) is given by a count of its line side ports. Drop and continue: A simple multicast feature of some ADMs where a selected wavelength can be switched out of both a tributary (drop) port and a line side port. FOADM: Fixed Optical Add/Drop Multiplexer. GMPLS: Generalized Multi-Protocol Label Switching. Line side: In a WDM system, line side ports and links can typically carry the full multiplex of wavelength signals, as compared to tributary (add or drop) ports that typically carry a few (usually one) wavelength signals. OXC: Optical Cross-Connect. An optical switching element in which a signal on any input port can reach any output port. PCC: Path Computation Client. Any client application requesting a path computation to be performed by the Path Computation Element. PCE: Path Computation Element. An entity (component, application, or network node) that is capable of computing a network path or route based on a network graph and application of computational constraints. PCEP: PCE Communication Protocol. The communication protocol between a Path Computation Client and Path Computation Element. ROADM: Reconfigurable Optical Add/Drop Multiplexer. A wavelength- selective switching element featuring input and output line side ports as well as add/drop tributary ports. RWA: Routing and Wavelength Assignment. Transparent Network: A Wavelength Switched Optical Network that does not contain regenerators or wavelength converters.
Translucent Network: A Wavelength Switched Optical Network that is predominantly transparent but may also contain limited numbers of regenerators and/or wavelength converters. Tributary: A link or port on a WDM system that can carry significantly less than the full multiplex of wavelength signals found on the line side links/ports. Typical tributary ports are the add and drop ports on an ADM, and these support only a single wavelength channel. Wavelength Conversion/Converters: The process of converting an information-bearing optical signal centered at a given wavelength to one with "equivalent" content centered at a different wavelength. Wavelength conversion can be implemented via an optical-electronic- optical (OEO) process or via a strictly optical process. WDM: Wavelength Division Multiplexing. Wavelength Switched Optical Networks (WSONs): WDM-based optical networks in which switching is performed selectively based on the center wavelength of an optical signal.3. Wavelength Switched Optical Networks
WSONs range in size from continent-spanning long-haul networks, to metropolitan networks, to residential access networks. In all these cases, the main concern is those properties that constrain the choice of wavelengths that can be used, i.e., restrict the wavelength Label Set, impact the path selection process, and limit the topological connectivity. In addition, if electro-optical network elements are used in the WSON, additional compatibility constraints may be imposed by the network elements on various optical signal parameters. The subsequent sections review and model some of the major subsystems of a WSON with an emphasis on those aspects that are of relevance to the control plane. In particular, WDM links, optical transmitters, ROADMs, and wavelength converters are examined.3.1. WDM and CWDM Links
WDM and CWDM links run over optical fibers, and optical fibers come in a wide range of types that tend to be optimized for various applications. Examples include access networks, metro, long haul, and submarine links. International Telecommunication Union - Telecommunication Standardization Sector (ITU-T) standards exist for various types of fibers. Although fiber can be categorized into Single-Mode Fibers (SMFs) and Multi-Mode Fibers (MMFs), the latter are typically used for short-reach campus and premise applications. SMFs are used for longer-reach applications and are therefore the
primary concern of this document. The following SMF types are
typically encountered in optical networks:
ITU-T Standard | Common Name
------------------------------------------------------------
G.652 [G.652] | Standard SMF |
G.653 [G.653] | Dispersion shifted SMF |
G.654 [G.654] | Cut-off shifted SMF |
G.655 [G.655] | Non-zero dispersion shifted SMF |
G.656 [G.656] | Wideband non-zero dispersion shifted SMF |
------------------------------------------------------------
Typically, WDM links operate in one or more of the approximately
defined optical bands [G.Sup39]:
Band Range (nm) Common Name Raw Bandwidth (THz)
O-band 1260-1360 Original 17.5
E-band 1360-1460 Extended 15.1
S-band 1460-1530 Short 9.4
C-band 1530-1565 Conventional 4.4
L-band 1565-1625 Long 7.1
U-band 1625-1675 Ultra-long 5.5
Not all of a band may be usable; for example, in many fibers that
support E-band, there is significant attenuation due to a water
absorption peak at 1383 nm. Hence, a discontinuous acceptable
wavelength range for a particular link may be needed and is modeled.
Also, some systems will utilize more than one band. This is
particularly true for CWDM systems.
Current technology subdivides the bandwidth capacity of fibers into
distinct channels based on either wavelength or frequency. There are
two standards covering wavelengths and channel spacing. ITU-T
Recommendation G.694.1, "Spectral grids for WDM applications: DWDM
frequency grid" [G.694.1], describes a DWDM grid defined in terms of
frequency grids of 12.5 GHz, 25 GHz, 50 GHz, 100 GHz, and other
multiples of 100 GHz around a 193.1 THz center frequency. At the
narrowest channel spacing, this provides less than 4800 channels
across the O through U bands. ITU-T Recommendation G.694.2,
"Spectral grids for WDM applications: CWDM wavelength grid"
[G.694.2], describes a CWDM grid defined in terms of wavelength
increments of 20 nm running from 1271 nm to 1611 nm for 18 or so
channels. The number of channels is significantly smaller than the
32-bit GMPLS Label space defined for GMPLS (see [RFC3471]). A label
representation for these ITU-T grids is given in [RFC6205] and
provides a common label format to be used in signaling optical paths.
Further, these ITU-T grid-based labels can also be used to describe WDM links, ROADM ports, and wavelength converters for the purposes of path selection. Many WDM links are designed to take advantage of particular fiber characteristics or to try to avoid undesirable properties. For example, dispersion-shifted SMF [G.653] was originally designed for good long-distance performance in single-channel systems; however, putting WDM over this type of fiber requires significant system engineering and a fairly limited range of wavelengths. Hence, the following information is needed as parameters to perform basic, impairment-unaware modeling of a WDM link: o Wavelength range(s): Given a mapping between labels and the ITU-T grids, each range could be expressed in terms of a tuple, (lambda1, lambda2) or (freq1, freq2), where the lambdas or frequencies can be represented by 32-bit integers. o Channel spacing: Currently, there are five channel spacings used in DWDM systems and a single channel spacing defined for CWDM systems. For a particular link, this information is relatively static, as changes to these properties generally require hardware upgrades. Such information may be used locally during wavelength assignment via signaling, similar to label restrictions in MPLS, or used by a PCE in providing combined RWA.3.2. Optical Transmitters and Receivers
WDM optical systems make use of optical transmitters and receivers utilizing different wavelengths (frequencies). Some transmitters are manufactured for a specific wavelength of operation; that is, the manufactured frequency cannot be changed. First introduced to reduce inventory costs, tunable optical transmitters and receivers are deployed in some systems and allow flexibility in the wavelength used for optical transmission/reception. Such tunable optics aid in path selection. Fundamental modeling parameters for optical transmitters and receivers from the control plane perspective are: o Tunable: Do the transmitters and receivers operate at variable or fixed wavelength? o Tuning range: This is the frequency or wavelength range over which the optics can be tuned. With the fixed mapping of labels to lambdas as proposed in [RFC6205], this can be expressed as a
tuple, (lambda1, lambda2) or (freq1, freq2), where lambda1 and
lambda2 or freq1 and freq2 are the labels representing the lower
and upper bounds in wavelength.
o Tuning time: Tuning times highly depend on the technology used.
Thermal-drift-based tuning may take seconds to stabilize, whilst
electronic tuning might provide sub-ms tuning times. Depending on
the application, this might be critical. For example, thermal
drift might not be usable for fast protection applications.
o Spectral characteristics and stability: The spectral shape of a
laser's emissions and its frequency stability put limits on
various properties of the overall WDM system. One constraint that
is relatively easy to characterize is the closest channel spacing
with which the transmitter can be used.
Note that ITU-T recommendations specify many aspects of an optical
transmitter. Many of these parameters, such as spectral
characteristics and stability, are used in the design of WDM
subsystems consisting of transmitters, WDM links, and receivers.
However, they do not furnish additional information that will
influence the Label Switched Path (LSP) provisioning in a properly
designed system.
Also, note that optical components can degrade and fail over time.
This presents the possibility of the failure of an LSP (optical path)
without either a node or link failure. Hence, additional mechanisms
may be necessary to detect and differentiate this failure from the
others; for example, one does not want to initiate mesh restoration
if the source transmitter has failed since the optical transmitter
will still be failed on the alternate optical path.
3.3. Optical Signals in WSONs
The fundamental unit of switching in WSONs is intuitively that of a
"wavelength". The transmitters and receivers in these networks will
deal with one wavelength at a time, while the switching systems
themselves can deal with multiple wavelengths at a time. Hence,
multi-channel DWDM networks with single-channel interfaces are the
prime focus of this document as opposed to multi-channel interfaces.
Interfaces of this type are defined in ITU-T Recommendations
[G.698.1] and [G.698.2]. Key non-impairment-related parameters
defined in [G.698.1] and [G.698.2] are:
(a) Minimum channel spacing (GHz)
(b) Minimum and maximum central frequency
(c) Bitrate/Line coding (modulation) of optical tributary signals For the purposes of modeling the WSON in the control plane, (a) and (b) are considered properties of the link and restrictions on the GMPLS Labels while (c) is a property of the "signal".3.3.1. Optical Tributary Signals
The optical interface specifications [G.698.1], [G.698.2], and [G.959.1] all use the concept of an optical tributary signal, which is defined as "a single channel signal that is placed within an optical channel for transport across the optical network". Note the use of the qualifier "tributary" to indicate that this is a single- channel entity and not a multi-channel optical signal. There are currently a number of different types of optical tributary signals, which are known as "optical tributary signal classes". These are currently characterized by a modulation format and bitrate range [G.959.1]: (a) Optical tributary signal class Non-Return-to-Zero (NRZ) 1.25G (b) Optical tributary signal class NRZ 2.5G (c) Optical tributary signal class NRZ 10G (d) Optical tributary signal class NRZ 40G (e) Optical tributary signal class Return-to-Zero (RZ) 40G Note that, with advances in technology, more optical tributary signal classes may be added and that this is currently an active area for development and standardization. In particular, at the 40G rate, there are a number of non-standardized advanced modulation formats that have seen significant deployment, including Differential Phase Shift Keying (DPSK) and Phase Shaped Binary Transmission (PSBT). According to [G.698.2], it is important to fully specify the bitrate of the optical tributary signal. Hence, modulation format (optical tributary signal class) and bitrate are key parameters in characterizing the optical tributary signal.3.3.2. WSON Signal Characteristics
The optical tributary signal referenced in ITU-T Recommendations [G.698.1] and [G.698.2] is referred to as the "signal" in this document. This corresponds to the "lambda" LSP in GMPLS. For signal
compatibility purposes with electro-optical network elements, the
following signal characteristics are considered:
1. Optical tributary signal class (modulation format)
2. Forward Error Correction (FEC): whether forward error correction
is used in the digital stream and what type of error correcting
code is used
3. Center frequency (wavelength)
4. Bitrate
5. General Protocol Identifier (G-PID) for the information format
The first three items on this list can change as a WSON signal
traverses the optical network with elements that include
regenerators, OEO switches, or wavelength converters.
Bitrate and G-PID would not change since they describe the encoded
bitstream. A set of G-PID values is already defined for lambda
switching in [RFC3471] and [RFC4328].
Note that a number of non-standard or proprietary modulation formats
and FEC codes are commonly used in WSONs. For some digital
bitstreams, the presence of FEC can be detected; for example, in
[G.707], this is indicated in the signal itself via the FEC Status
Indication (FSI) byte while in [G.709], this can be inferred from
whether or not the FEC field of the Optical Channel Transport Unit-k
(OTUk) is all zeros.
3.4. ROADMs, OXCs, Splitters, Combiners, and FOADMs
Definitions of various optical devices such as ROADMs, Optical Cross-
Connects (OXCs), splitters, combiners, and Fixed Optical Add/Drop
Multiplexers (FOADMs) and their parameters can be found in [G.671].
Only a subset of these relevant to the control plane and their non-
impairment-related properties are considered in the following
sections.
3.4.1. Reconfigurable Optical Add/Drop Multiplexers and OXCs
ROADMs are available in different forms and technologies. This is a
key technology that allows wavelength-based optical switching. A
classic degree-2 ROADM is shown in Figure 1.
Line side input +---------------------+ Line side output --->| |---> | | | ROADM | | | | | +---------------------+ | | | | o o o o | | | | | | | | O O O O | | | | Tributary Side: Drop (output) Add (input) Figure 1. Degree-2 Unidirectional ROADM The key feature across all ROADM types is their highly asymmetric switching capability. In the ROADM of Figure 1, signals introduced via the add ports can only be sent on the line side output port and not on any of the drop ports. The term "degree" is used to refer to the number of line side ports (input and output) of a ROADM and does not include the number of "add" or "drop" ports. The add and drop ports are sometimes also called tributary ports. As the degree of the ROADM increases beyond two, it can have properties of both a switch (OXC) and a multiplexer; hence, it is necessary to know the switched connectivity offered by such a network element to effectively utilize it. A straightforward way to represent this is via a "switched connectivity" matrix A where Amn = 0 or 1, depending upon whether a wavelength on input port m can be connected to output port n [Imajuku]. For the ROADM shown in Figure 1, the switched connectivity matrix can be expressed as: Input Output Port Port #1 #2 #3 #4 #5 -------------- #1: 1 1 1 1 1 #2 1 0 0 0 0 A = #3 1 0 0 0 0 #4 1 0 0 0 0 #5 1 0 0 0 0 where input ports 2-5 are add ports, output ports 2-5 are drop ports, and input port #1 and output port #1 are the line side (WDM) ports. For ROADMs, this matrix will be very sparse, and for OXCs, the matrix will be very dense. Compact encodings and examples, including high- degree ROADMs/OXCs, are given in [Gen-Encode]. A degree-4 ROADM is shown in Figure 2.
+-----------------------+ Line side-1 --->| |---> Line side-2 Input (I1) | | Output (E2) Line side-1 <---| |<--- Line side-2 Output (E1) | | Input (I2) | ROADM | Line side-3 --->| |---> Line side-4 Input (I3) | | Output (E4) Line side-3 <---| |<--- Line side-4 Output (E3) | | Input (I4) | | +-----------------------+ | O | O | O | O | | | | | | | | O | O | O | O | Tributary Side: E5 I5 E6 I6 E7 I7 E8 I8 Figure 2. Degree-4 Bidirectional ROADM Note that this is a 4-degree example with one (potentially multi- channel) add/drop per line side port. Note also that the connectivity constraints for typical ROADM designs are "bidirectional"; that is, if input port X can be connected to output port Y, typically input port Y can be connected to output port X, assuming the numbering is done in such a way that input X and output X correspond to the same line side direction or the same add/drop port. This makes the connectivity matrix symmetrical as shown below. Input Output Port Port E1 E2 E3 E4 E5 E6 E7 E8 ----------------------- I1 0 1 1 1 0 1 0 0 I2 1 0 1 1 0 0 1 0 A = I3 1 1 0 1 1 0 0 0 I4 1 1 1 0 0 0 0 1 I5 0 0 1 0 0 0 0 0 I6 1 0 0 0 0 0 0 0 I7 0 1 0 0 0 0 0 0 I8 0 0 0 1 0 0 0 0 where I5/E5 are add/drop ports to/from line side-3, I6/E6 are add/drop ports to/from line side-1, I7/E7 are add/drop ports to/from line side-2, and I8/E8 are add/drop ports to/from line side-4. Note that diagonal elements are zero since loopback is not supported in the example. If ports support loopback, diagonal elements would be set to one.
Additional constraints may also apply to the various ports in a
ROADM/OXC. The following restrictions and terms may be used:
o Colored port: an input or, more typically, an output (drop) port
restricted to a single channel of fixed wavelength
o Colorless port: an input or, more typically, an output (drop) port
restricted to a single channel of arbitrary wavelength
In general, a port on a ROADM could have any of the following
wavelength restrictions:
o Multiple wavelengths, full range port
o Single wavelength, full range port
o Single wavelength, fixed lambda port
o Multiple wavelengths, reduced range port (for example wave band
switching)
To model these restrictions, it is necessary to have two pieces of
information for each port: (a) the number of wavelengths and (b) the
wavelength range and spacing. Note that this information is
relatively static. More complicated wavelength constraints are
modeled in [WSON-Info].
3.4.2. Splitters
An optical splitter consists of a single input port and two or more
output ports. The input optical signaled is essentially copied (with
power loss) to all output ports.
Using the modeling notions of Section 3.4.1, the input and output
ports of a splitter would have the same wavelength restrictions. In
addition, a splitter is modeled by a connectivity matrix Amn as
follows:
Input Output Port
Port #1 #2 #3 ... #N
-----------------
A = #1 1 1 1 ... 1
The difference from a simple ROADM is that this is not a switched
connectivity matrix but the fixed connectivity matrix of the device.
3.4.3. Combiners
An optical combiner is a device that combines the optical wavelengths carried by multiple input ports into a single multi-wavelength output port. The various ports may have different wavelength restrictions. It is generally the responsibility of those using the combiner to ensure that wavelength collision does not occur on the output port. The fixed connectivity matrix Amn for a combiner would look like: Input Output Port Port #1 --- #1: 1 #2 1 A = #3 1 ... 1 #N 13.4.4. Fixed Optical Add/Drop Multiplexers
A Fixed Optical Add/Drop Multiplexer can alter the course of an input wavelength in a preset way. In particular, a given wavelength (or waveband) from a line side input port would be dropped to a fixed "tributary" output port. Depending on the device's construction, that same wavelength may or may not also be sent out the line side output port. This is commonly referred to as a "drop and continue" operation. Tributary input ports ("add" ports) whose signals are combined with each other and other line side signals may also exist. In general, to represent the routing properties of an FOADM, it is necessary to have both a fixed connectivity matrix Amn, as previously discussed, and the precise wavelength restrictions for all input and output ports. From the wavelength restrictions on the tributary output ports, the wavelengths that have been selected can be derived. From the wavelength restrictions on the tributary input ports, it can be seen which wavelengths have been added to the line side output port. Finally, from the added wavelength information and the line side output wavelength restrictions, it can be inferred which wavelengths have been continued. To summarize, the modeling methodology introduced in Section 3.4.1, which consists of a connectivity matrix and port wavelength restrictions, can be used to describe a large set of fixed optical devices such as combiners, splitters, and FOADMs. Hybrid devices consisting of both switched and fixed parts are modeled in [WSON-Info].
3.5. Electro-Optical Systems
This section describes how Electro-Optical Systems (e.g., OEO switches, wavelength converters, and regenerators) interact with the WSON signal characteristics listed in Section 3.3.2. OEO switches, wavelength converters, and regenerators all share a similar property: they can be more or less "transparent" to an "optical signal" depending on their functionality and/or implementation. Regenerators have been fairly well characterized in this regard and hence their properties can be described first.3.5.1. Regenerators
The various approaches to regeneration are discussed in ITU-T [G.872], Annex A. They map a number of functions into the so-called 1R, 2R, and 3R categories of regenerators as summarized in Table 1 below: Table 1. Regenerator Functionality Mapped to General Regenerator Classes from [G.872] -------------------------------------------------------------------- 1R | Equal amplification of all frequencies within the amplification | bandwidth. There is no restriction upon information formats. +---------------------------------------------------------------- | Amplification with different gain for frequencies within the | amplification bandwidth. This could be applied to both single- | channel and multi-channel systems. +---------------------------------------------------------------- | Dispersion compensation (phase distortion). This analogue | process can be applied in either single-channel or multi- | channel systems. -------------------------------------------------------------------- 2R | Any or all 1R functions. Noise suppression. +---------------------------------------------------------------- | Digital reshaping (Schmitt Trigger function) with no clock | recovery. This is applicable to individual channels and can be | used for different bitrates but is not transparent to line | coding (modulation). -------------------------------------------------------------------- 3R | Any or all 1R and 2R functions. Complete regeneration of the | pulse shape including clock recovery and retiming within | required jitter limits. -------------------------------------------------------------------- This table shows that 1R regenerators are generally independent of signal modulation format (also known as line coding) but may work over a limited range of wavelengths/frequencies. 2R regenerators are
generally applicable to a single digital stream and are dependent upon modulation format (line coding) and, to a lesser extent, are limited to a range of bitrates (but not a specific bitrate). Finally, 3R regenerators apply to a single channel, are dependent upon the modulation format, and are generally sensitive to the bitrate of digital signal, i.e., either are designed to only handle a specific bitrate or need to be programmed to accept and regenerate a specific bitrate. In all these types of regenerators, the digital bitstream contained within the optical or electrical signal is not modified. It is common for regenerators to modify the digital bitstream for performance monitoring and fault management purposes. Synchronous Optical Networking (SONET), Synchronous Digital Hierarchy (SDH), and Interfaces for the Optical Transport Network [G.709] all have digital signal "envelopes" designed to be used between "regenerators" (in this case, 3R regenerators). In SONET, this is known as the "section" signal; in SDH, this is known as the "regenerator section" signal; and, in G.709, this is known as an OTUk. These signals reserve a portion of their frame structure (known as overhead) for use by regenerators. The nature of this overhead is summarized in Table 2 below.
Table 2. SONET, SDH, and G.709 Regenerator-Related Overhead
+-----------------------------------------------------------------+
|Function | SONET/SDH | G.709 OTUk |
| | Regenerator | |
| | Section | |
|------------------+----------------------+-----------------------|
|Signal | J0 (section | Trail Trace |
|Identifier | trace) | Identifier (TTI) |
|------------------+----------------------+-----------------------|
|Performance | BIP-8 (B1) | BIP-8 (within SM) |
|Monitoring | | |
|------------------+----------------------+-----------------------|
|Management | D1-D3 bytes | GCC0 (general |
|Communications | | communications |
| | | channel) |
|------------------+----------------------+-----------------------|
|Fault Management | A1, A2 framing | FAS (frame alignment |
| | bytes | signal), BDI (backward|
| | | defect indication), |
| | | BEI (backward error |
| | | indication) |
+------------------+----------------------+-----------------------|
|Forward Error | P1,Q1 bytes | OTUk FEC |
|Correction (FEC) | | |
+-----------------------------------------------------------------+
Table 2 shows that frame alignment, signal identification, and FEC
are supported. By omission, Table 2 also shows that no switching or
multiplexing occurs at this layer. This is a significant
simplification for the control plane since control plane standards
require a multi-layer approach when there are multiple switching
layers but do not require the "layering" to provide the management
functions shown in Table 2. That is, many existing technologies
covered by GMPLS contain extra management-related layers that are
essentially ignored by the control plane (though not by the
management plane). Hence, the approach here is to include
regenerators and other devices at the WSON layer unless they provide
higher layer switching; then, a multi-layer or multi-region approach
[RFC5212] is called for. However, this can result in regenerators
having a dependence on the client signal type.
Hence, depending upon the regenerator technology, the constraints
listed in Table 3 may be imposed by a regenerator device:
Table 3. Regenerator Compatibility Constraints
+--------------------------------------------------------+
| Constraints | 1R | 2R | 3R |
+--------------------------------------------------------+
| Limited Wavelength Range | x | x | x |
+--------------------------------------------------------+
| Modulation Type Restriction | | x | x |
+--------------------------------------------------------+
| Bitrate Range Restriction | | x | x |
+--------------------------------------------------------+
| Exact Bitrate Restriction | | | x |
+--------------------------------------------------------+
| Client Signal Dependence | | | x |
+--------------------------------------------------------+
Note that the limited wavelength range constraint can be modeled for
GMPLS signaling with the Label Set defined in [RFC3471] and that the
modulation type restriction constraint includes FEC.
3.5.2. OEO Switches
A common place where OEO processing may take place is within WSON
switches that utilize (or contain) regenerators. This may be to
convert the signal to an electronic form for switching then reconvert
to an optical signal prior to output from the switch. Another common
technique is to add regenerators to restore signal quality either
before or after optical processing (switching). In the former case,
the regeneration is applied to adapt the signal to the switch fabric
regardless of whether or not it is needed from a signal-quality
perspective.
In either case, these optical switches have essentially the same
compatibility constraints as those described for regenerators in
Table 3.
3.6. Wavelength Converters
Wavelength converters take an input optical signal at one wavelength
and emit an equivalent content optical signal at another wavelength
on output. There are multiple approaches to building wavelength
converters. One approach is based on OEO conversion with fixed or
tunable optics on output. This approach can be dependent upon the
signal rate and format; that is, this is basically an electrical
regenerator combined with a laser/receiver. Hence, this type of
wavelength converter has signal-processing restrictions that are
essentially the same as those described for regenerators in Table 3
of Section 3.5.1.
Another approach performs the wavelength conversion optically via
non-linear optical effects, similar in spirit to the familiar
frequency mixing used in radio frequency systems but significantly
harder to implement. Such processes/effects may place limits on the
range of achievable conversion. These may depend on the wavelength
of the input signal and the properties of the converter as opposed to
only the properties of the converter in the OEO case. Different WSON
system designs may choose to utilize this component to varying
degrees or not at all.
Current or envisioned contexts for wavelength converters are:
1. Wavelength conversion associated with OEO switches and fixed or
tunable optics. In this case, there are typically multiple
converters available since each use of an OEO switch can be
thought of as a potential wavelength converter.
2. Wavelength conversion associated with ROADMs/OXCs. In this case,
there may be a limited pool of wavelength converters available.
Conversion could be either all optical or via an OEO method.
3. Wavelength conversion associated with fixed devices such as
FOADMs. In this case, there may be a limited amount of
conversion. Also, the conversion may be used as part of optical
path routing.
Based on the above considerations, wavelength converters are modeled
as follows:
1. Wavelength converters can always be modeled as associated with
network elements. This includes fixed wavelength routing
elements.
2. A network element may have full wavelength conversion capability
(i.e., any input port and wavelength) or a limited number of
wavelengths and ports. On a box with a limited number of
converters, there also may exist restrictions on which ports can
reach the converters. Hence, regardless of where the converters
actually are, they can be associated with input ports.
3. Wavelength converters have range restrictions that are either
independent or dependent upon the input wavelength.
In WSONs where wavelength converters are sparse, an optical path may
appear to loop or "backtrack" upon itself in order to reach a
wavelength converter prior to continuing on to its destination. The
lambda used on input to the wavelength converter would be different
from the lambda coming back from the wavelength converter.
A model for an individual OEO wavelength converter would consist of: o Input lambda or frequency range o Output lambda or frequency range3.6.1. Wavelength Converter Pool Modeling
A WSON node may include multiple wavelength converters. These are usually arranged into some type of pool to promote resource sharing. There are a number of different approaches used in the design of switches with converter pools. However, from the point of view of path computation, it is necessary to know the following: 1. The nodes that support wavelength conversion 2. The accessibility and availability of a wavelength converter to convert from a given input wavelength on a particular input port to a desired output wavelength on a particular output port 3. Limitations on the types of signals that can be converted and the conversions that can be performed To model point 2 above, a technique similar to that used to model ROADMs and optical switches can be used, i.e., matrices to indicate possible connectivity along with wavelength constraints for links/ports. Since wavelength converters are considered a scarce resource, it is desirable to include, at a minimum, the usage state of individual wavelength converters in the pool. A three stage model is used as shown schematically in Figure 3. This model represents N input ports (fibers), P wavelength converters, and M output ports (fibers). Since not all input ports can necessarily reach the converter pool, the model starts with a wavelength pool input matrix WI(i,p) = {0,1}, where input port i can potentially reach wavelength converter p. Since not all wavelengths can necessarily reach all the converters or the converters may have a limited input wavelength range, there is a set of input port constraints for each wavelength converter. Currently, it is assumed that a wavelength converter can only take a single wavelength on input. Each wavelength converter input port constraint can be modeled via a wavelength set mechanism. Next, there is a state vector WC(j) = {0,1} dependent upon whether wavelength converter j in the pool is in use. This is the only state kept in the converter pool model. This state is not necessary for modeling "fixed" transponder system, i.e., systems where there is no
sharing. In addition, this state information may be encoded in a much more compact form depending on the overall connectivity structure [Gen-Encode]. After that, a set of wavelength converter output wavelength constraints is used. These constraints indicate what wavelengths a particular wavelength converter can generate or are restricted to generating due to internal switch structure. Finally, a wavelength pool output matrix WE(p,k) = {0,1} indicates whether the output from wavelength converter p can reach output port k. Examples of this method being used to model wavelength converter pools for several switch architectures are given in [Gen-Encode]. I1 +-------------+ +-------------+ E1 ----->| | +--------+ | |-----> I2 | +------+ WC #1 +-------+ | E2 ----->| | +--------+ | |-----> | Wavelength | | Wavelength | | Converter | +--------+ | Converter | | Pool +------+ WC #2 +-------+ Pool | | | +--------+ | | | Input | | Output | | Connection | . | Connection | | Matrix | . | Matrix | | | . | | | | | | IN | | +--------+ | | EM ----->| +------+ WC #P +-------+ |-----> | | +--------+ | | +-------------+ ^ ^ +-------------+ | | | | | | | | Input wavelength Output wavelength constraints for constraints for each converter each converter Figure 3. Schematic Diagram of Wavelength Converter Pool Model Figure 4 shows a simple optical switch in a four-wavelength DWDM system sharing wavelength converters in a general shared "per-node" fashion.
+-----------+ ___________ +------+ | |--------------------------->| | | |--------------------------->| C | /| | |--------------------------->| o | E1 I1 /D+--->| |--------------------------->| m | + e+--->| | | b |====> ====>| M| | Optical | +-----------+ +----+ | i | + u+--->| Switch | | WC Pool | |O S|-->| n | \x+--->| | | +-----+ | |p w|-->| e | \| | +----+->|WC #1|--+->|t i| | r | | | | +-----+ | |i t| +------+ | | | | |c c| +------+ /| | | | +-----+ | |a h|-->| | I2 /D+--->| +----+->|WC #2|--+->|l |-->| C | E2 + e+--->| | | +-----+ | | | | o | ====>| M| | | +-----------+ +----+ | m |====> + u+--->| | | b | \x+--->| |--------------------------->| i | \| | |--------------------------->| n | | |--------------------------->| e | |___________|--------------------------->| r | +-----------+ +------+ Figure 4. An Optical Switch Featuring a Shared Per-Node Wavelength Converter Pool Architecture In this case, the input and output pool matrices are simply: +-----+ +-----+ | 1 1 | | 1 1 | WI =| |, WE =| | | 1 1 | | 1 1 | +-----+ +-----+ Figure 5 shows a different wavelength pool architecture known as "shared per fiber". In this case, the input and output pool matrices are simply: +-----+ +-----+ | 1 1 | | 1 0 | WI =| |, WE =| | | 1 1 | | 0 1 | +-----+ +-----+
+-----------+ +------+ | |--------------------------->| | | |--------------------------->| C | /| | |--------------------------->| o | E1 I1 /D+--->| |--------------------------->| m | + e+--->| | | b |====> ====>| M| | Optical | +-----------+ | i | + u+--->| Switch | | WC Pool | | n | \x+--->| | | +-----+ | | e | \| | +----+->|WC #1|--+---------->| r | | | | +-----+ | +------+ | | | | +------+ /| | | | +-----+ | | | I2 /D+--->| +----+->|WC #2|--+---------->| C | E2 + e+--->| | | +-----+ | | o | ====>| M| | | +-----------+ | m |====> + u+--->| | | b | \x+--->| |--------------------------->| i | \| | |--------------------------->| n | | |--------------------------->| e | |___________|--------------------------->| r | +-----------+ +------+ Figure 5. An Optical Switch Featuring a Shared Per-Fiber Wavelength Converter Pool Architecture3.7. Characterizing Electro-Optical Network Elements
In this section, electro-optical WSON network elements are characterized by the three key functional components: input constraints, output constraints, and processing capabilities. WSON Network Element +-----------------------+ WSON Signal | | | | WSON Signal | | | | ---------------> | | | | -----------------> | | | | +-----------------------+ <-----> <-------> <-----> Input Processing Output Figure 6. WSON Network Element
3.7.1. Input Constraints
Sections 3.5 and 3.6 discuss the basic properties of regenerators, OEO switches, and wavelength converters. From these, the following possible types of input constraints and properties are derived: 1. Acceptable modulation formats 2. Client signal (G-PID) restrictions 3. Bitrate restrictions 4. FEC coding restrictions 5. Configurability: (a) none, (b) self-configuring, (c) required These constraints are represented via simple lists. Note that the device may need to be "provisioned" via signaling or some other means to accept signals with some attributes versus others. In other cases, the devices may be relatively transparent to some attributes, e.g., a 2R regenerator to bitrate. Finally, some devices may be able to auto-detect some attributes and configure themselves, e.g., a 3R regenerator with bitrate detection mechanisms and flexible phase locking circuitry. To account for these different cases, item 5 has been added, which describes the device's configurability. Note that such input constraints also apply to the termination of the WSON signal.3.7.2. Output Constraints
None of the network elements considered here modifies either the bitrate or the basic type of the client signal. However, they may modify the modulation format or the FEC code. Typically, the following types of output constraints are seen: 1. Output modulation is the same as input modulation (default) 2. A limited set of output modulations is available 3. Output FEC is the same as input FEC code (default) 4. A limited set of output FEC codes is available Note that in cases 2 and 4 above, where there is more than one choice in the output modulation or FEC code, the network element will need to be configured on a per-LSP basis as to which choice to use.
3.7.3. Processing Capabilities
A general WSON network element (NE) can perform a number of signal processing functions including: (A) Regeneration (possibly different types) (B) Fault and performance monitoring (C) Wavelength conversion (D) Switching An NE may or may not have the ability to perform regeneration (of one of the types previously discussed). In addition, some nodes may have limited regeneration capability, i.e., a shared pool, which may be applied to selected signals traversing the NE. Hence, to describe the regeneration capability of a link or node, it is necessary to have, at a minimum: 1. Regeneration capability: (a) fixed, (b) selective, (c) none 2. Regeneration type: 1R, 2R, 3R 3. Regeneration pool properties for the case of selective regeneration (input and output restrictions, availability) Note that the properties of shared regenerator pools would be essentially the same as that of wavelength converter pools modeled in Section 3.6.1. Item B (fault and performance monitoring) is typically outside the scope of the control plane. However, when the operations are to be performed on an LSP basis or on part of an LSP, the control plane can be of assistance in their configuration. Per-LSP, per-node, and fault and performance monitoring examples include setting up a "section trace" (a regenerator overhead identifier) between two nodes or intermediate optical performance monitoring at selected nodes along a path.
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