Network Working Group Y. Li
Internet-Draft F. Zhang
Intended status: Standards Track ZTE
Expires: January 5, 2012 R. Casellas
CTTC
July 4, 2011
Flexible Grid Label Format in Wavelength Switched Optical Network
draft-li-ccamp-flexible-grid-label-00
Abstract
Flexible grid is regarded as an efficient way to improve the network
capacity utilization. Mixed bit rate transmission systems can
allocate their channel with different spectral bandwidths so that
they can be optimized for the bandwidth requirements of the
particular bit rate and modulation scheme of the individual channels.
To support the flexible grid technique, this document extends the
wavelength label to accommodate this new specification. It is
demonstrated that the extended label format is compatible to the
rigid one and can be used in the routing and signaling procedure in
the Wavelength Switched Optical Network (WSON).
Status of this Memo
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This Internet-Draft will expire on January 5, 2012.
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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Label format . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Label values . . . . . . . . . . . . . . . . . . . . . . . 4
3.2. Flexible Label . . . . . . . . . . . . . . . . . . . . . . 5
4. Flexible label applications . . . . . . . . . . . . . . . . . 7
4.1. Application for Routing . . . . . . . . . . . . . . . . . 7
4.2. Applications for Signaling . . . . . . . . . . . . . . . . 8
4.3. Applications for PCE . . . . . . . . . . . . . . . . . . . 8
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
7. Security Considerations . . . . . . . . . . . . . . . . . . . 9
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
8.1. Normative references . . . . . . . . . . . . . . . . . . . 9
8.2. Informative References . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10
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1. Introduction
Dense Wavelength Division Multiplexing (DWDM) optical network is
widely deployed by telecom operators to carry their data service.
With the continuing exponential growth of internet traffic, more
efficient utilization of optical network bandwidth for extremely high
data rates is required. Although multi-level modulation formats and
advanced photonics techniques have enabled 100 G/s transmission
within a 50 GHz DWDM fixed gird (or channel spacing), much higher
speed traffic, such as 400 Gbit/s and 1 Tbit/s signals are not
expected to adapt such a narrow channel. So a wider fixed grid like
100 GHz spacing is required to enable these new transmission formats
without inter-channel crosstalk. However, the total available
spectrum resource of the specific band is limited (about 4.4 THz in C
band). If a wider grid is chosen, the fewer wavelengths can be
allocated to carry the data. Not to mention that some low bitrate
signals will occupy too much spectral bandwidth so that the total
utilization efficiency of the spectrum resource is relatively low.
The recent revision of ITU-T Recommendation [G.694.1] has decided to
introduce the flexible grid DWDM technique which provide a new tool
that operators can implement to provide a higher degree of network
optimization than fixed grid systems. Flexible grid network is
composed of arbitrarily assigned spectral slices. That means in such
networks the adjacent channel spacing and assigned spectral bandwidth
per wavelength are variable. Mixed bitrate transmission systems can
allocate their channel with different spectral bandwidths so that
they can be optimized for the bandwidth requirements of the
particular bit rate and modulation scheme of the individual channels.
This technique is regarded to be a promising way to improve the
network utilization efficiency and fundamentally reduce the cost of
the IP core network.
Based on the DWDM technique, Wavelength Switched Optical Network
(WSON) uses the control plane to dynamically provide Label Switched
Paths (LSPs) for the requested end to end connections. The label
switching is performed selectively on wavelength label representing
the center wavelength/frequency of the optical signal. To support
the flexible grid technique, this document extends the wavelength
label defined in [RFC6205] to accommodate the new specification. It
is proved that the extended label format is compatible to the rigid
one and can be used in the routing and signaling procedure in WSON
and generic GMPLS network.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
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"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
3. Label format
3.1. Label values
The wavelength label format is defined in [RFC6205] and the
corresponding wavelength or frequency value is referred to ITU-T
Recommendations [G.694.1] and [G.694.2] for DWDM and CWDM grid
respectively. The ITU-T fixed grid is based on nominal center
frequency/wavelength.
For DWDM system, the nominal center frequency is calculated as:
Frequency (THz)=193.1 THz+n* channel spacing
In the context of rigid grid, the channel spacing of DWDM can support
12.5 GHz, 25 GHz, 50 GHz, or 100 GHz. However, once chosen, the
adjacent channel spacing of the wavelengths is fixed. As mentioned
in the section 1, 50 GHz channel spacing is most commonly used.
The recent revision of [G.694.1] has defined suggested values for the
flexible DWDM grid. The concept of "frequency slot" is introduced to
describe the frequency range allocated to a channel. A frequency
slot is defined by its nominal central frequency and its required
slot width values.
For the flexible DWDM grid, the allowed frequency slots have a
nominal central frequency (in THz) defined by:
Frequency (THz)=193.1 THz + n * 0.00625
and a slot width (the same meaning as the spectral bandwidth) defined
by:
12.5 GHz * m
where m is a positive integer.
The nominal center frequency representations of the fixed grid and
flexible grid types are similar except that the latter has a more
precise channel spacing granularity (6.25 GHz). Meanwhile the
adjacent channel spacing (the spacing of the adjacent nominal center
frequency) is implied to be (n1-n2) * 6.25 GHz, where n1 and n2
represent the n number defined above for the nominal center frequency
of the adjacent frequency slots respectively (n is an integer
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including positive, negative integer and 0). The slot width assigned
to a frequency slot is arbitrary times of the slot width granularity.
It was agreed on flexible grids with a granularity of 6.25 GHz for
the central frequency and slot width of a multiple of 12.5 GHz. The
slot width granularity is twice the channel spacing granularity, so
that by carefully choosing n and m, the spectral resources can be
allocated without leaving any gaps between slots. Therefore, in
contrast to the rigid label, the new flexible label should have a
capability to indicating the slot width allocation.
Note that in this document, the concepts "slot width" and "frequency
slot" are similar to "spectral bandwidth" and "wavelength channel"
respectively.
3.2. Flexible Label
To accommodate the new feature mentioned above, the wavelength label
supporting flexible grid is illustrated as follows :
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Grid | C.S. | Identifier | n |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Additional slot width parameters |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Additional slot width parameters:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| m | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Grid:
One new Grid type called "Flexible DWDM" is defined.
+---------------+-------+
| Grid | Value |
+---------------+-------+
| Reserved | 0 |
+---------------+-------+
| ITU-T DWDM | 1 |
+---------------+-------+
| ITU-T CWDM | 2 |
+---------------+-------+
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+---------------+-------+
| Flexible DWDM | 3 |
+---------------+-------+
| Future use | 4-7 |
+---------------+-------+
C.S.:
For Grid=1 and 2, C.S. is referred to DWDM and CWDM channel spacing
[RFC6205], which indicates that the adjacent channel spacing is
constant. In this situation, the spectral bandwidth value allocated
to every single channel is equal to value of the channel spacing.
For Grid=3, C.S. is referred to channel spacing granularity,
accordingly the slot width granularity is twice of the C.S.. Minimum
channel spacing granularity of 6.25 GHz with a slot width granularity
of 12.5 GHz is supported.
+------------+-------+
| C.S. (GHz) | Value |
+------------+-------+
| Reserved | 0 |
+------------+-------+
| 100 | 1 |
+------------+-------+
| 50 | 2 |
+------------+-------+
| 25 | 3 |
+------------+-------+
| 12.5 | 4 |
+------------+-------+
| 6.25 | 5 |
+------------+-------+
| Future use | 6-15 |
+------------+-------+
Identifier:
The identifier field in the flexible label format is left unmodified
compared with [RFC6205]. It is defined to distinguish which
transmitter is used to carry the lambda. This identifier only has a
local significance that should be indicated in the signaling message
for LSP establishment. For routing information flooding, this filed
is meaningless and should be ignored on receipt.
n:
This field is used to compute the nominal center frequency/wavelength
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of the channel mentioned above. Together with the channel spacing
granularity (C.S.), the spacing of the adjacent channel is (n1-n2) *
6.25 GHz in flexible grid network (see definition of n1 and n2 in
section 3.1).
Additional slot width parameters.
The slot width parameters field is mandatory only when Grid is set to
3 for flexible grid condition. These 5 bits field are used to
represent how many slot width granularity the label has occupied. As
the granularity is defined to be twice of the channel spacing
granularity, so the slot width is calculated to be m * 2 * C.S..
4. Flexible label applications
This section illustrated the routing, signaling, PCE application of
the extended flexible grid label.
4.1. Application for Routing
Flexible grid is regarded as an enabler for another kind of networks,
requiring network elements, or nodes, that go past beyond the
functional requirements of OXCs or ROADMs, in the sense that they do
switching based on a frequency range. This means that a new swithing
type called e.g. "Spectrum Selective Switching" in Interface
Switching Capability Descriptor (ISCD) SHOULD be defined. However
this is beyond the scope of this document and will be studied in the
routing draft.
In addition to the topology information, wavelength constraints
information like Port Label Restrictions, Shared Backup Labels,
Resource Pool Wavelength Constrains, Resource Block Available
Wavelengths detailed in [I-D.ietf-ccamp-rwa-info] should be flooded
in the network through routing protocol like OSPF-TE. All the
information is described by the label set object. The general label
set is described in [RFC3471] and specific wavelength label set in
[I-D.ietf-ccamp-general-constraint-encode] . There are 5 ways to
represent the wavelength label set
1. Inclusive list
2. Exclusive list
3. Inclusive range
4. Exclusive range
5. Bitmap set
For flexible grid optical network, the label set should be more
actually to represent the spectral resources constraints. For type 1
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and 2, flexible label with different slot width is acceptable to put
into the list. For type 3 and 4, start label and end label with
minimal slot width (while it is not mandatory) is RECOMMENDED. For
type 5, the base label/frequency slot is REQUIRED to have a minimum
slot width (m=1). As there MAY exist some situations that the unused
bandwidth between two occupied bandwidth is odd times of the channel
spacing granularity (not integral times of the slot with
granularity), two bits are needed to represent a single slot. It can
be seen that these 5 types of representations can be easily inherited
by incorporating the new flexible label into the object. Note that
in the procedure of wavelength constraints flooding, any combination
of the 5 types of label sets is feasible.
4.2. Applications for Signaling
In flexibel grid network, flexible label representing frequency
"slots" or "ranges" rather than individual wavelengths is requested
to establish the LSP. The extensions to the Genralized Label Request
object and TSPEC object are needed, this will be studied in the
future.
To establish a label switched path, an available wavelength label
satisfying the wavelength continuity constraints is reserved with
signaling protocol like RSVP-TE. For the flexible grid DWDM network,
this procedure should be modified to assign available spectral
resources. In other words, the label is not only assigning the
nominal center frequency of wavelength but also the slot width for
the LSP. The slot width is definitely clarified through the field m
in the label. Nevertheless in the procedure, wavelength continuity
constraint is unchanged.
4.3. Applications for PCE
[RFC6163] describes a Path Computation Element (PCE) can be used to
performing routing and wavelength assignment in WSON. [RFC5440]
details the path computation element communication protocol messages
for this purpose. According to the modulation format, FEC type,
client
bitrates[I-D.ietf-ccamp-rwa-info][I-D.ietf-ccamp-rwa-wson-encode],
and physical impairment, the required frequency slot indicated by
flexible label should be calculated out by the PCE to carry the
client signal.
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5. Acknowledgements
6. IANA Considerations
A future revision of this document will present requests to IANA for
codepoint allocation.
7. Security Considerations
8. References
8.1. Normative references
[G.694.1] International Telecommunications Union, "Spectral grids
for WDM applications: DWDM frequency grid", Recommendation
G.694.1, June 2002 .
[G.694.2] International Telecommunications Union, "Spectral grids
for WDM applications: CWDM wavelength grid",
Recommendation G.694.2, December 2003 .
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3471] Berger, L., "Generalized Multi-Protocol Label Switching
(GMPLS) Signaling Functional Description", RFC 3471,
January 2003.
[RFC5440] Vasseur, JP. and JL. Le Roux, "Path Computation Element
(PCE) Communication Protocol (PCEP)", RFC 5440,
March 2009.
[RFC6163] Lee, Y., Bernstein, G., and W. Imajuku, "Framework for
GMPLS and Path Computation Element (PCE) Control of
Wavelength Switched Optical Networks (WSONs)", RFC 6163,
April 2011.
[RFC6205] Otani, T. and D. Li, "Generalized Labels for Lambda-
Switch-Capable (LSC) Label Switching Routers", RFC 6205,
March 2011.
8.2. Informative References
[I-D.ietf-ccamp-general-constraint-encode]
Bernstein, G., Lee, Y., Li, D., and W. Imajuku, "General
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Network Element Constraint Encoding for GMPLS Controlled
Networks", draft-ietf-ccamp-general-constraint-encode-05
(work in progress), May 2011.
[I-D.ietf-ccamp-rwa-info]
Bernstein, G., Lee, Y., Li, D., and W. Imajuku, "Routing
and Wavelength Assignment Information Model for Wavelength
Switched Optical Networks", draft-ietf-ccamp-rwa-info-11
(work in progress), March 2011.
[I-D.ietf-ccamp-rwa-wson-encode]
Bernstein, G., Lee, Y., Li, D., Imajuku, W., and J. Han,
"Routing and Wavelength Assignment Information Encoding
for Wavelength Switched Optical Networks",
draft-ietf-ccamp-rwa-wson-encode-11 (work in progress),
March 2011.
Authors' Addresses
Yao Li
ZTE
P.R.China
Phone: +86 025 52871109
Email: li.yao3@zte.com.cn
Zhang Fei
ZTE
P.R.China
Phone: +86 025 52871109
Email: zhang.fei3@zte.com.cn
Ramon Casellas
CTTC
Spain
Phone: +34 936452916
Email: ramon.casellas@cttc.es
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