Network Working Group G. Bernstein
Internet Draft Grotto Networking
Intended status: Informational Y. Lee
Expires: November 2009 Huawei
Ben Mack-Crane
Huawei
May 21, 2009
WSON Signal Characteristics and Network Element Compatibility
Constraints for GMPLS
draft-bernstein-ccamp-wson-signal-00.txt
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Abstract
While the current GMPLS WSON formalism can deal with many types of
wavelength switching systems there is a desire to extend this control
plane to include other common optical or hybrid electro optical
systems such as OEO switches, regenerators, and wavelength
converters.
This document provides a WSON signal definition and characterization
based on ITU-T interface and signal class standards and describes the
signal compatibility constraints of this extended set of network
elements. The signal characterization and network element
compatibility constraints enable GMPLS routing and signaling to
control these devices and PCE to compute optical light-paths subject
to signal compatibility attributes.
Table of Contents
1. Introduction and Requirements..................................3
1.1. Regenerators..............................................3
1.2. OEO Switches..............................................6
1.3. Wavelength Converters.....................................7
2. Describing Optical Signals in GMPLS............................8
2.1. Optical Interfaces........................................8
2.2. Optical Tributary Signals.................................8
2.3. Proposed GMPLS WSON Signal Definition.....................9
2.4. Implications for GMPLS Signaling and PCEP................10
3. Characterizing WSON Network Elements in GMPLS.................11
3.1. Proposed Link and Network Element (NE) Model Extensions..11
4. Security Considerations.......................................12
5. IANA Considerations...........................................12
6. Acknowledgments...............................................12
7. References....................................................13
7.1. Normative References.....................................13
7.2. Informative References...................................14
Author's Addresses...............................................14
Intellectual Property Statement..................................14
Disclaimer of Validity...........................................15
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1. Introduction and Requirements
While the current GMPLS WSON formalism can deal with many types of
wavelength switching systems, these systems must be located within
optical signal networks to provide useful services. Therefore there
is a desire to extend this control plane to include other common
optical or hybrid electro optical systems required to build a
complete optical signal network. In particular at the March 2009 IETF
meeting the working group expressed a desire to include OEO switches,
regenerators, and wavelength converters within the WSON GMPLS
extensions. In the following we will describe these devices and their
properties. We then show that a combination of additional signal
attributes and network element attributes can be used to accommodate
these devices, relate these attributes to ITU-T recommendations and
describe the implications for GMPLS signaling, PCEP, and the WSON
information model [WSON-Info].
It turns out 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 so we start by describing their properties.
Our approach to efficiently extend WSON GMPLS to networks that
include regenerators, OEO switches and wavelength converters is to
add attributes characterizing the WSON signal in line with ITU-T
standards, and add attributes describing signal compatibility
constraints to WSON network elements. This way the control plane
signaling and path computation functions can ensure "signal"
compatibility between source, sink and any links or network elements
as part of path selection process, and configure devices
appropriately via signaling as part of the connection provisioning
process. This enables integration of a WSON into the operations of a
signal network for which it provides connectivity instead of
requiring the WSON to be separately managed and controlled.
1.1. Regenerators
The various approaches to regeneration are discussed in ITU-T G.872
Annex A [G.872]. They map a number of functions into the so-called
1R, 2R and 3R categories of regenerators as summarized in Table 1
below:
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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 bit rates 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.
--------------------------------------------------------------------
From the previous table we can see that 1R regenerators are generally
independent of signal modulation format (also known as line coding),
but may work over a limited range of wavelength/frequencies. We see
that 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 bit rates (but not a
specific bit rate). Finally, 3R regenerators apply to a single
channel, are dependent upon the modulation format and generally
sensitive to the bit rate of digital signal, i.e., either are
designed to only handle a specific bit rate or need to be programmed
to accept and regenerate a specific bit rate. In all these types of
regenerators the digital bit stream(s) contained within the optical
or electrical is/(are) not modified.
In the most common usage of regenerators the digital bit stream may
be slightly modified for performance monitoring and fault management
purposes. SONET, SDH and G.709 all have a digital signal "envelope"
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, in G.709 this is
known as an OTUk (Optical Channel Transport Unit-k). These signals
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reserve a portion of their frame structure (known as overhead) for
use by regenerators. The nature of this overhead is summarized in
Table 2.
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) | | |
+-----------------------------------------------------------------+
In the previous table we see support for frame alignment, signal
identification, and FEC. What this table also shows by its omission
is 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 not for "layering" to provide the management
functions of 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 and then
a multi-layer or multi-region approach [RFC5212] is called for.
In a sense dependence on client signal type represents a fourth
regenerator type, i.e., 4R, that includes all the capabilities and
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restrictions of a 3R, 2R, and 1R, and in addition is depending upon
the format of the digital stream, i.e., these regenerators can accept
only one type of stream or must be programmed to accommodate
different stream types.
Hence we see that depending upon the regenerator technology we may
have the following constraints imposed by a regenerator device:
List 1. Network Element Compatibility Constraints
1. Limited wavelength range (1R) -- Already modeled in GMPLS for
WSON
2. Modulation type restriction (2R)
3. Bit rate range restriction (2R, 3R)
4. Exact bit rate restriction (3R)
5. Client signal dependence (4R)
1.2. OEO Switches
A common place where optical-to-electrical-to-optical (OEO)
processing may take place is in WSON switches that utilize (or
contain) regenerators. A vendor may add regenerators to a switching
system for a number of reasons. One obvious reason is to restore
signal quality either before or after optical processing (switching).
Another reason may be to convert the signal to an electronic form for
switching then reconverting to an optical signal prior to egress from
the switch. In this later 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 the following signal
processing restrictions that are essentially the same as those we
described for regenerators in List 1.
Note that a common system integration function in transport networks
is to add multi-channel WDM interfaces to electro-optical switching
systems such as G.709, SONET, SDH, IP, or Ethernet switching systems.
Although such systems may have high layer switching functionality
they, by their nature contain WSON functionality, though this maybe
in the form of fixed WDM multiplexing and de-multiplexing
functionality. See [WSON-FRAME] for how GMPLS WSON can model fixed
devices. If they only contain higher layer (IP, Ethernet, SONET path,
etc...) functionality then these systems act as a termination point
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for the WSON switching layer, otherwise they look like a combination
of WSON end system and WSON switching system and could contain OEO
conversions.
Integrating WSON capabilities into electro-optical switching systems
brings the WSON network into the operational domain of these systems
and higher layer networks. By adding optical tributary attributes to
the GMPLS control protocols this draft enables the integration of
WSON subnetworks into the higher layer networks within which they
reside and to which they provide flexible connectivity. This
streamlines network operations by enabling a single request to
establish a connection across both electro-optical and all optical
elements within a higher layer network. The optical tributary
attributes for a connection may be set based on the related
attributes of the network element at the boundary of each new WSON
subnetwork traversed by the connection.
1.3. Wavelength Converters
In [WSON-FRAME] the motivation for utilizing wavelength converters
was discussed. In essence a wavelength converter would take one or
more optical channels on specific wavelengths and convert them to
corresponding new specific wavelengths. Currently all optical
wavelength converters exist but have not been widely deployed, hence
the majority of wavelength converters are based on demodulation to an
electrical signal and then re-modulation onto a new optical carrier,
i.e., an OEO process. This process is very similar to that used for a
regenerator except that the output optical wavelength will be
different from the input optical wavelength. Hence in general
wavelength converters have signal processing restrictions that are
essentially the same as those we described for regenerators in List
1:
(a) Limited input wavelength range (1R), Limited output wavelength
range
(b) Modulation type restriction (2R)
(c) Bit rate range restriction (2R, 3R)
(d) Exact bit rate restriction (3R)
(e) Client signal dependence (4R)
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2. Describing Optical Signals in GMPLS
In the previous section we saw that each of the additional network
elements (OEO switches, regenerators, and wavelength converters) can
impose constraints on the types of signals they can "process". Hence
to enable the use of a larger set of network elements the first step
is to define and characterize our "optical signal".
2.1. Optical Interfaces
In wavelength switched optical networks (WSONs) our fundamental unit
of switching 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 we are generally concerned with
multichannel dense wavelength division multiplexing (DWDM) networks
with single channel interfaces. Interfaces of this type are defined
in ITU-T recommendations [G.698.1] and [G.698.1]. Key non-impairment
related parameters defined in [G.698.1] and [G.698.2] are:
(a) Minimum Channel Spacing (GHz)
(b) Bit-rate/Line coding of optical tributary signals
(c) Minimum and Maximum central frequency
We see that (a) and (c) above are related to properties of the link
and have been modeled in [Otani],[WSON-FRAME], [WSON-Info] and (b) is
related to the "signal".
2.2. 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 multichannel optical signal. This is our candidate
terminology for the entity that we will be controlling in our GMPLS
extensions for WSONs.
There are a currently a number of different "flavors" of optical
tributary signals, known as "optical tributary signal classes". These
are currently characterized by a modulation format and bit rate range
[G.959.1]:
(a) optical tributary signal class NRZ 1.25G
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(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 RZ 40G
Note that with advances in technology more optical tributary signal
classes will be added and that this is currently an active area for
standardization.
Note that according to [G.698.2] it is important to fully specify the
bit rate of the optical tributary signal:
"When an optical system uses one of these codes, therefore, it is
necessary to specify both the application code and also the exact bit
rate of the system. In other words, there is no requirement for
equipment compliant with one of these codes to operate over the
complete range of bit rates specified for its optical tributary
signal class."
Hence we see that modulation format (optical tributary signal class)
and bit rate are key in characterizing the optical tributary signal.
2.3. Proposed GMPLS WSON Signal Definition
We proposed to call the signal that we will be working with an
optical tributary signal like that defined in ITU-T G.698.1 and .2.
This is an "entity" that can be put on an optical communications
channel formed from links and network elements in a WSON.
An optical tributary signal has the following attributes:
List 2. Optical Tributary Signal Attributes
1. Optical tributary signal class: This relates to the specifics of
modulation format, and bit rate range. Could possibly change along
the path. For example when running through a 3R regenerator a
different output modulation format could be used. This could be
more prevalent if we are controlling combined metro and long haul
networks.
2. FEC: Indicates whether forward error correction is used in the
digital stream. Note that in [G.707] this is indicated in the
signal itself via the FEC status indication (FSI) byte, while in
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[G.709] this can be inferred from whether the FEC field of the OTUk
is all zeros or not.
3. Bit rate. This typically would not change since we are not changing
the digital bit stream in any end-to-end meaningful way.
4. Center frequency (wavelength). Can change along path if there are
wavelength converters. This is already modeled via labels in GMPLS.
5. G-PID: General Protocol Identifier for the information format. This
would not change since this describes the encoded bit stream. This
is already present in GMPLS signaling. A set of G-PID values are
already defined for lambda switching in [RFC3471], [RFC4328].
6. (Optional) A signal identifier or name distinguishing a particular
tributary signal from others in the network that may be used to
detect misconnection of signals. For example this can be used in
setting up the section trace in SDH or the trail trace identifier
in G.709 between format aware regenerators. This is not used in
determining signal compatibility with network elements and hence is
optional.
These attributes are used during RWA to select a compatible path for
the optical tributary signal. These attributes are used during
signaling to configure devices such as wavelength converters or
parameter sensitive devices such as 3R regenerators. Some of these
attributes such as wavelength may change as the optical tributary
signal traverses the path from source to sink.
2.4. Implications for GMPLS Signaling and PCEP
When establishing a connection or requesting a path computation the
attributes of the optical tributary signal given in List 2 in section
2.3. needs to be furnished. However of these five attributes two are
already supplied in GMPLS signaling: wavelength and G-PID. This
leaves only four new types of attributes:
1. Signal Class with possible qualifying parameters
2. Bit Rate
3. FEC information
4. Optional signal identifier
For RSVP-TE signaling these could be put in a new WSON T_SPEC object.
For PCEP these signal attributes would need to be included in various
request and response messages.
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3. Characterizing WSON Network Elements in GMPLS
A number of processes may operate on an "optical tributary signal" as
it traverses a path through a network these include: Generation
(including modulation), Regeneration, Wavelength Conversion,
Switching and Reception (including demodulation). In any of these
processes a number of attributes of the "optical tributary signal"
may be either constrained or incompatible with those of the
processing elements. These attributes include:
(a) Optical tributary signal class (modulation format and
approximate bit rate, FEC)
(b) Exact bit rate
(c) Center frequency (wavelength)
(d) Digital stream format information
Qualification of a route involves determining that the route provides
a signal path capable of propagating the physical layer network
signal and meeting the input signal requirements of the termination
sink function (receiver).
Some of the previously mentioned attributes of our optical tributary
signal may change as the signal traverses its path across a network.
The most common of these would be center frequency (wavelength).
GMPLS signaling currently supports the specification of wavelength to
be used at a given point on a path. Less common, although, possible
would be a change in modulation format of the signal, particularly
after some type of OEO regeneration or switching. Currently GMPLS
signaling doesn't support indicating a change of modulation at a
particular point in the network.
The bulk of compatibility checking of network element capabilities
against optical tributary signal attributes would fall on the path
computation entity whose traffic engineering database is typically
constructed with the help of a link state IGP. Currently, only layer
type information is given in the form of the interface switching
capability descriptor (ISCD) from [RFC4202].
3.1. Proposed Link and Network Element (NE) Model Extensions
Other drafts [WSON-FRAME],[WSON-Info] provide NE models that include
switching asymmetry and port wavelength constraints here we add
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parameters to our existing node and link models to take into account
restrictions on the optical tributary signal attributes that a
network element can accept. These are:
1. Permitted optical tributary signal classes: A list of optical
tributary signal classes that can be processed by this network
element or carried over this link.
2. Acceptable Bit Rate Set: A list of specific bit rates or bit rate
ranges that the device can accommodate. Coarse bit rate info is
included with the optical tributary signal class restrictions.
3. Acceptable G-PID list: A list of G-PIDs corresponding to the
"client" digital streams that are compatible with this device.
Note that such parameters could be specified on an (a) Network
element wide basis, (b) a per port basis, (c) on a per regenerator
basis. Typically such information has been on a per port basis,
e.g., the GMPLS interface switching capability descriptor [RFC4202].
However, in [WSON-FRAME] we give examples of shared wavelength
converters within a switching system, and hence this would be on a
subsystem basis. The exact form would be defined in the [WSON-Info]
and [WSON-Encoding] drafts.
4. Security Considerations
This document has no requirement for a change to the security models
within GMPLS and associated protocols. That is the OSPF-TE, RSVP-TE,
and PCEP [RFC5540] security models could be operated unchanged.
Furthermore the additional information distributed in order to extend
GMPLS capabilities to the additional network elements discussed in
this document represents a disclosure of network capabilities that an
operator may wish to keep private. Consideration should be given to
securing this information.
5. IANA Considerations
This document makes no request for IANA actions.
6. Acknowledgments
This document was prepared using 2-Word-v2.0.template.dot.
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7. References
7.1. Normative References
[RFC3471] Berger, L., "Generalized Multi-Protocol Label Switching
(GMPLS) Signaling Functional Description", RFC 3471,
January 2003.
[RFC4202] Kompella, K. and Y. Rekhter, "Routing Extensions in Support
of Generalized Multi-Protocol Label Switching (GMPLS)", RFC
4202, October 2005.
[RFC4328] Papadimitriou, D., "Generalized Multi-Protocol Label
Switching (GMPLS) Signaling Extensions for G.709 Optical
Transport Networks Control", RFC 4328, January 2006.
[G.694.1] ITU-T Recommendation G.694.1, "Spectral grids for WDM
applications: DWDM frequency grid", June, 2002.
[RFC5212] Shiomoto, K., Papadimitriou, D., Le Roux, JL., Vigoureux,
M., and D. Brungard, "Requirements for GMPLS-Based Multi-
Region and Multi-Layer Networks (MRN/MLN)", RFC 5212, July
2008.
[RFC5540] J.P. Vasseur and J.L. Le Roux (Editors), "Path Computation
Element (PCE) Communication Protocol (PCEP)", RFC 5540,
March 2009.
[WSON-FRAME] Y. Lee, G. Bernstein, W. Imajuku, "Framework for GMPLS
and PCE Control of Wavelength Switched Optical Networks
(WSON)", draft-ietf-ccamp-rwa-wson-framework-02.txt, March
2009.
[WSON-Info] Y. Lee, G. Bernstein, D. Li, W. Imajuku, "Routing and
Wavelength Assignment Information for Wavelength Switched
Optical Networks", draft-bernstein-ccamp-wson-info-03.txt,
March, 2009.
[WSON-Encoding] G. Bernstein, Y. Lee, D. Li, W. Imajuku, "Routing and
Wavelength Assignment Information Encoding for Wavelength
Switched Optical networks", work in progress, draft-ietf-
ccamp-rwa-wson-encode-01.txt, March 2009.
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7.2. Informative References
[Otani] T. Otani, H. Guo, K. Miyazaki, D. Caviglia, "Generalized
Labels for G.694 Lambda-Switching Capable Label Switching
Routers (LSR)", work in progress, draft-ietf-ccamp-gmpls-g-
694-lambda-labels-04.txt
[G.872] ITU-T Recommendation G.872, Architecture of optical
transport networks, November 2001.
[G.959.1] ITU-T Recommendation G.959.1, Optical Transport Network
Physical Layer Interfaces, March 2006.
Author's Addresses
Greg M. Bernstein
Grotto Networking
Fremont California, USA
Phone: (510) 573-2237
Email: gregb@grotto-networking.com
Young Lee
Huawei Technologies
1700 Alma Drive, Suite 100
Plano, TX 75075
USA
Phone: (972) 509-5599 (x2240)
Email: ylee@huawei.com
T. Benjamin Mack-Crane
Huawei Technologies
Downers Grove, Illinois
Email: tmackcrane@huawei.com
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