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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 1 Network Working Group Tomohiro Otani(Ed.) 2 Internet Draft KDDI 3 Updates: 3471(if approved) Dan Li(Ed.) 4 Category: Standards Track Huawei 6 Expires: July 2011 January 11, 2011 8 Generalized Labels for Lambda-Switching Capable Label Switching 9 Routers 11 draft-ietf-ccamp-gmpls-g-694-lambda-labels-11.txt 13 Status of this Memo 15 This Internet-Draft is submitted to IETF in full conformance with 16 the provisions of BCP 78 and BCP 79. 18 Internet-Drafts are working documents of the Internet Engineering 19 Task Force (IETF), its areas, and its working groups. Note that 20 other groups may also distribute working documents as Internet- 21 Drafts. 23 Internet-Drafts are draft documents valid for a maximum of six 24 months and may be updated, replaced, or obsoleted by other 25 documents at any time. It is inappropriate to use Internet-Drafts 26 as reference material or to cite them other than as "work in 27 progress." 29 The list of current Internet-Drafts can be accessed at 30 http://www.ietf.org/ietf/1id-abstracts.txt 32 The list of Internet-Draft Shadow Directories can be accessed at 33 http://www.ietf.org/shadow.html 35 This Internet-Draft will expire on July 11, 2011. 37 Copyright Notice 39 Copyright (c) 2010 IETF Trust and the persons identified as the 40 document authors. All rights reserved. 42 This document is subject to BCP 78 and the IETF Trust's Legal 43 Provisions Relating to IETF Documents 44 (http://trustee.ietf.org/license-info) in effect on the date of 45 publication of this document. Please review these documents 46 carefully, as they describe your rights and restrictions with 47 respect to this document. Code Components extracted from this 48 document must include Simplified BSD License text as described in 49 Section 4.e of the Trust Legal Provisions and are provided 50 without warranty as described in the Simplified BSD License. 52 This document may contain material from IETF Documents or IETF 53 Contributions published or made publicly available before 54 November 10, 2008. The person(s) controlling the copyright in 55 some of this material may not have granted the IETF Trust the 56 right to allow modifications of such material outside the IETF 57 Standards Process. Without obtaining an adequate license from the 58 person(s) controlling the copyright in such materials, this 59 document may not be modified outside the IETF Standards Process, 60 and derivative works of it may not be created outside the IETF 61 Standards Process, except to format it for publication as an RFC 62 or to translate it into languages other than English. 64 Abstract 66 Technology in the optical domain is constantly evolving and as a 67 consequence new equipment providing lambda switching capability 68 has been developed and is currently being deployed. 70 Generalized MPLS (GMPLS) is a family of protocols that can be 71 used to operate networks built from a range of technologies 72 including wavelength (or lambda) switching. For this purpose, 73 GMPLS defined that a wavelength label only has significance 74 between two neighbors and global wavelength semantics are not 75 considered. 77 In order to facilitate interoperability in a network composed of 78 next generation lambda switch-capable equipment, this document 79 defines a standard lambda label format that is compliant with 80 Dense Wavelength Division Multiplexing and Coarse Wavelength 81 Division Multiplexing grids defined by the International 82 Telecommunication Union Telecommunication Standardization Sector. 83 The label format defined in this document can be used in GMPLS 84 signaling and routing protocols. 86 Conventions used in this document 88 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL 89 NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and 90 "OPTIONAL" in this document are to be interpreted as described in 91 [RFC2119]. 93 1. Introduction 95 As described in [RFC3945], Generalized MPLS (GMPLS) extends MPLS 96 from supporting only packet (Packet Switching Capable - PSC) 97 interfaces and switching to also include support for four new 98 classes of interfaces and switching: 100 o Layer-2 Switch Capable (L2SC) 102 o Time-Division Multiplex (TDM) 104 o Lambda Switch Capable (LSC) 106 o Fiber-Switch Capable (FSC). 108 A functional description of the extensions to MPLS signaling 109 needed to support new classes of interfaces and switching is 110 provided in [RFC3471]. 112 This document presents details that are specific to the use of 113 GMPLS with Lambda Switch Capable (LSC) equipment. Technologies 114 such as Reconfigurable Optical Add/Drop Multiplex (ROADM) and 115 Wavelength Cross-Connect (WXC) operate at the wavelength 116 switching level. [RFC3471] has defined that a wavelength label 117 (section 3.2.1.1) "only has significance between two neighbors" 118 and global wavelength semantics is not considered. In order to 119 facilitate interoperability in a network composed of lambda 120 switch-capable equipment, this document defines a standard lambda 121 label format, which is compliant with both [G.694.1](Dense 122 Wavelength Division Multiplexing (DWDM)-grid) or [G.694.2](Coarse 123 Wavelength Division Multiplexing (CWDM)-grid). 125 2. Assumed Network Model and Related Problem Statement 127 Figure 1 depicts an all-optically switched network consisting of 128 different vendors' optical network domains. Vendor A's network 129 consists of ROADM or WXC, and vendor B's network consists of a 130 number of photonic cross-connect (PXC) and DWDM multiplexer & 131 demultiplexer, otherwise both vendors' networks might be based on 132 the same technology. 134 In this case, the use of standardized wavelength label 135 information is quite significant to establish a wavelength-based 136 LSP. It is also an important constraint when conducting CSPF 137 calculation for use by Generalized Multi-Protocol Label Switching 138 (GMPLS) RSVP-TE signaling, [RFC3473]. The way the Constrained 139 Shortest Path First (CSPF) is performed is outside the scope of 140 this document. 142 It is needless to say, an LSP must be appropriately provisioned 143 between a selected pair of ports not only within Domain A but 144 also over multiple domains satisfying wavelength constraints. 146 Figure 2 illustrates in detail the interconnection between Domain 147 A and Domain B. 149 | 150 Domain A (or Vendor A) | Domain B (or Vendor B) 151 | 152 Node-1 Node-2 | Node-6 Node-7 153 +--------+ +--------+ | +-------+ +-+ +-+ +-------+ 154 | ROADM | | ROADM +---|------+ PXC +-+D| |D+-+ PXC | 155 | or WXC +========+ or WXC +---|------+ +-+W+=====+W+-+ | 156 | (LSC) | | (LSC) +---|------+ (LSC) +-+D| |D+-+ (LSC) | 157 +--------+ +--------+ | | +-|M| |M+-+ | 158 || || | +++++++++ +-+ +-+ +++++++++ 159 || Node-3 || | ||||||| ||||||| 160 || +--------+ || | +++++++++ +++++++++ 161 ||===| WXC +===|| | | DWDM | | DWDM | 162 | (LSC) | | +--++---+ +--++---+ 163 ||===+ +===|| | || || 164 || +--------+ || | +--++---+ +--++---+ 165 || || | | DWDM | | DWDM | 166 +--------+ +--------+ | +++++++++ +++++++++ 167 | ROADM | | ROADM | | ||||||| ||||||| 168 | or WXC +========+ or WXC +=+ | +-+ +++++++++ +-+ +-+ +++++++++ 169 | (LSC) | | (LSC) | | | |D|-| PXC +-+D| |D+-+ PXC | 170 +--------+ +--------+ +=|==+W|-| +-+W+=====+W+-+ | 171 Node-4 Node-5 | |D|-| (LSC) +-+D| |D+-+ (LSC) | 172 | |M|-| +-+M| |M+-+ | 173 | +-+ +-------+ +-+ +-+ +-------+ 174 | Node-8 Node-9 176 Figure 1 Wavelength-based network model 178 +-------------------------------------------------------------+ 179 | Domain A | Domain B | 180 | | | 181 | +---+ lambda 1 | +---+ | 182 | | |---------------|---------| | | 183 | WDM | N | lambda 2 | | N | WDM | 184 | =====| O |---------------|---------| O |===== | 185 | O | D | . | | D | O | 186 | T WDM | E | . | | E | WDM T | 187 | H =====| 2 | lambda n | | 6 |===== H | 188 | E | |---------------|---------| | E | 189 | R +---+ | +---+ R | 190 | | | 191 | N +---+ | +---+ N | 192 | O | | | | | O | 193 | D WDM | N | | | N | WDM D | 194 | E =====| O | WDM | | O |===== E | 195 | S | D |=========================| D | S | 196 | WDM | E | | | E | WDM | 197 | =====| 5 | | | 8 |===== | 198 | | | | | | | 199 | +---+ | +---+ | 200 +-------------------------------------------------------------+ 202 Figure 2 Interconnecting details between two domains 204 In the scenario of Figure 1, consider the setting up of a 205 bidirectional LSP from ingress switch 1 to egress switch 9 using 206 GMPLS RSVP-TE. In order to satisfy wavelength continuity 207 constraint, a fixed wavelength (lambda 1) needs to be used in 208 domain A and domain B. A Path message will be used for signaling. 209 The Path message will contain the Upstream_Label object and a 210 Label_Set object; both containing the same value. The Label_set 211 object shall contain a single sub-channel that must be the same 212 as the Upstream_Label object. The Path setup will continue 213 downstream to switch 9 by configuring each lambda switch based on 214 the wavelength label. If a node has a tunable wavelength 215 transponder, the tuning wavelength is considered as a part of 216 wavelength switching operation. 218 Not using a standardized label would add undue burden on the 219 operator to enforce policy as each manufacturer may decide on a 220 different representation and therefore each domain may have its 221 own label formats. Moreover, manual provisioning may lead to 222 misconfiguration if domain-specific labels are used. 224 Therefore, a wavelength label should be standardized in order to 225 allow interoperability between multiple domains; otherwise 226 appropriate existing labels are identified in support of 227 wavelength availability. As identical wavelength information, the 228 ITU-T frequency grid specified in [G.694.1] for DWDM and 229 wavelength information in [G.694.2] for CWDM are used by Label 230 Switching Routers (LSRs) and should be followed as a wavelength 231 label. 233 3. Label Related Formats 235 To deal with the widening scope of MPLS into the optical and time 236 domains, several new forms of "label" have been defined in 237 [RFC3471]. This section contains a definition of a Wavelength 238 label based on [G.694.1] or [G.694.2] for use by LSC LSRs. 240 3.1. Wavelength Labels 242 In section 3.2.1.1 of [RFC3471], a Wavelength label is defined to 243 have significance between two neighbors, and the receiver may 244 need to convert the received value into a value that has local 245 significance. 247 We do not need to define a new type as the information stored is 248 either a port label or a wavelength label. Only the wavelength 249 label as below needs to be defined. 251 LSC equipment uses multiple wavelengths controlled by a single 252 control channel. In a case, the label indicates the wavelength to 253 be used for the LSP. This document defines a standardized 254 wavelength label format. As an example of wavelength values, the 255 reader is referred to [G.694.1] which lists the frequencies from 256 the ITU-T DWDM frequency grid. The same can be done for CWDM 257 technology by using the wavelength defined in [G.694.2]. 259 Since the ITU-T DWDM grid is based on nominal central frequencies, 260 we need to indicate the appropriate table, the channel spacing in 261 the grid and a value n that allows the calculation of the 262 frequency. That value can be positive or negative. 264 The frequency is calculated as such in [G.694.1]: 266 Frequency (THz) = 193.1 THz + n * channel spacing (THz) 268 Where "n" is a two's-complement integer (positive, negative or 0) 269 and "channel spacing" is defined to be 0.0125, 0.025, 0.05 or 0.1 270 THz. When wider channel spacing such as 0.2 THz is utilized, the 271 combination of narrower channel spacing and the value "n" can 272 provide proper frequency with that channel spacing. Channel 273 spacing is not utilized to indicate the LSR capability but only 274 to specify a frequency in signaling. 276 For the other example of the case of the ITU-T CWDM grid, the 277 spacing between different channels was defined to be 20nm, so we 278 need to pass the wavelength value in nanometers(nm) in this case. 279 Examples of CWDM wavelengths are 1471, 1491, etc. nm. 281 The wavelength is calculated as follows 283 Wavelength (nm) = 1471 nm + n * 20 nm 285 Where "n" is a two's-complement integer (positive, negative or 0). 286 The grids listed in [G.694.1] and [G.694.2] are not numbered and 287 change with the changing frequency spacing as technology advances, 288 so an index is not appropriate in this case. 290 3.2. DWDM Wavelength Label 292 For the case of lambda switching (LSC) of DWDM, the information 293 carried in a Wavelength label is: 295 0 1 2 3 296 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 1 297 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 298 |Grid | C.S | Identifier | n | 299 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 301 (1) Grid: 3 bits 303 The value for grid is set to 1 for ITU-T DWDM Grid as defined in 304 [G.694.1]. 306 +----------+---------+ 307 | Grid | Value | 308 +----------+---------+ 309 | Reserved | 0 | 310 +----------+---------+ 311 |ITU-T DWDM| 1 | 312 +----------+---------+ 313 |ITU-T CWDM| 2 | 314 +----------+---------+ 315 |Future use| 3 - 7 | 316 +----------+---------+ 318 (2) C.S.(channel spacing): 4 bits 320 DWDM channel spacing is defined as follows. 322 +----------+---------+ 323 | C.S(GHz) | Value | 324 +----------+---------+ 325 | Reserved | 0 | 326 +----------+---------+ 327 | 100 | 1 | 328 +----------+---------+ 329 | 50 | 2 | 330 +----------+---------+ 331 | 25 | 3 | 332 +----------+---------+ 333 | 12.5 | 4 | 334 +----------+---------+ 335 |Future use| 5 - 15 | 336 +----------+---------+ 338 (3) Identifier: 9 bits 340 The identifier field in lambda label format is used to 341 distinguish different lasers (in one node) when they can transmit 342 the same frequency lambda. The identifier field is a per-node 343 assigned and scoped value. This field MAY change on a per-hop 344 basis. In all cases but one, a node MAY select any value, 345 including zero (0), for this field. Once selected, the value MUST 346 NOT change until the LSP is torn down and the value MUST be used 347 in all LSP related messages, e.g., in Resv messages and label RRO 348 subobjects. The sole special case occurs when this label format 349 is used in a label ERO subobject. In this case, the special value 350 of zero (0) means that the referenced node MAY assign any 351 Identifier field value, including zero (0), when establishing the 352 corresponding LSP. When non-zero value is assigned to the 353 identifier field in a label ERO subobject, the referenced node 354 MUST use the assigned value for the identifier field in the 355 corresponding LSP related messages. 357 (4) n: 16 bits 359 n is a two's-complement integer to take either a negative, zero 360 or a positive value. The value used to compute the frequency as 361 shown above. 363 3.3. CWDM Wavelength Label 365 For the case of lambda switching (LSC) of CWDM, the information 366 carried in a Wavelength label is: 368 0 1 2 3 369 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 1 370 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 371 |Grid | C.S | Identifier | n | 372 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 374 The structure of the label in the case of CWDM is the same as 375 that of DWDM case. 377 (1) Grid: 3 bits 379 The value for grid is set to 2 for ITU-T CWDM Grid as defined in 380 [G.694.2]. 382 +----------+---------+ 383 | Grid | Value | 384 +----------+---------+ 385 | Reserved | 0 | 386 +----------+---------+ 387 |ITU-T DWDM| 1 | 388 +----------+---------+ 389 |ITU-T CWDM| 2 | 390 +----------+---------+ 391 |Future use| 3 - 7 | 392 +----------+---------+ 394 (2) C.S.(channel spacing): 4 bits 396 CWDM channel spacing is defined as follows. 398 +----------+---------+ 399 | C.S(nm) | Value | 400 +----------+---------+ 401 | Reserved | 0 | 402 +----------+---------+ 403 | 20 | 1 | 404 +----------+---------+ 405 |Future use| 2 - 15 | 406 +----------+---------+ 408 (3) Identifier: 9 bits 410 The identifier field in lambda label format is used to 411 distinguish different lasers (in one node) when they can transmit 412 the same frequency lambda. The identifier field is a per-node 413 assigned and scoped value. This field MAY change on a per-hop 414 basis. In all cases but one, a node MAY select any value, 415 including zero (0), for this field. Once selected, the value MUST 416 NOT change until the LSP is torn down and the value MUST be used 417 in all LSP related messages, e.g., in Resv messages and label RRO 418 subobjects. The sole special case occurs when this label format 419 is used in a label ERO subobject. In this case, the special value 420 of zero (0) means that the referenced node MAY assign any 421 Identifier field value, including zero (0), when establishing the 422 corresponding LSP. When non-zero value is assigned to the 423 identifier field in a label ERO subobject, the referenced node 424 MUST use the assigned value for the identifier field in the 425 corresponding LSP related messages. 427 (4) n: 16 bits 429 n is a two's-complement integer. The value used to compute the 430 wavelength as shown above. 432 4. Security Considerations 434 This document introduces no new security considerations to 435 [RFC3471] and [RFC3473]. For a general discussion on MPLS and 436 GMPLS related security issues, see the MPLS/GMPLS security 437 framework [RFC5920]. 439 5. IANA Considerations 441 IANA maintains the "Generalized Multi-Protocol Label Switching 442 (GMPLS) Signaling Parameters" registry. IANA is requested to add 443 three new subregistries to track the codepoints (Grid and C.S.) 444 used in the DWDM and CWDM Wavelength Labels, which are described 445 in the following sections. 447 5.1. Grid Subregistry 449 Initial entries in this subregistry are as follows: 451 Value Grid Reference 452 ----- ------------------------- ---------- 453 0 Reserved [This.I-D] 454 1 ITU-T DWDM [This.I-D] 455 2 ITU-T CWDM [This.I-D] 456 3-7 Not assigned at this time [This.I-D] 458 New values are assigned according to Standards Action. 460 5.2. DWDM Channel Spacing Subregistry 462 Initial entries in this subregistry are as follows: 464 Value Channel Spacing (GHz) Reference 465 ----- ------------------------- ---------- 466 0 Reserved [This.I-D] 467 1 100 [This.I-D] 468 2 50 [This.I-D] 469 3 25 [This.I-D] 470 4 12.5 [This.I-D] 471 5-15 Not assigned at this time [This.I-D] 473 New values are assigned according to Standards Action. 475 5.3. CWDM Channel Spacing Subregistry 477 Initial entries in this subregistry are as follows: 479 Value Channel Spacing (nm) Reference 480 ----- ------------------------- ---------- 481 0 Reserved [This.I-D] 482 1 20 [This.I-D] 483 2-15 Not assigned at this time [This.I-D] 485 New values are assigned according to Standards Action. 487 6. Acknowledgments 489 The authors would like to thank Adrian Farrel, Lou Berger, 490 Lawrence Mao, Zafar Ali and Daniele Ceccarelli for the discussion 491 and their comments. 493 7. References 495 7.1. Normative References 497 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 498 Requirement Levels", BCP 14, RFC 2119, March 1997. 500 [RFC3471] Berger, L., "Generalized Multi-Protocol Label Switching 501 (MPLS) Signaling Functional Description", RFC 3471, 502 January 2003. 504 [RFC3473] Berger, L., "Generalized Multi-Protocol Label Switching 505 (MPLS) Signaling - Resource ReserVation Protocol Traffic 506 Engineering (RSVP-TE) Extensions", RFC 3473, January 507 2003. 509 [RFC3945] Mannie, E., Ed., "Generalized Multiprotocol Label 510 Switching (GMPLS) Architecture", RFC 3945, October 2004. 512 7.2. Informative References 514 [G.694.1] ITU-T Recommendation G.694.1, "Spectral grids for WDM 515 applications: DWDM frequency grid", June 2002. 517 [G.694.2] ITU-T Recommendation G.694.2, "Spectral grids for WDM 518 applications: CWDM wavelength grid", December 2003. 520 [RFC5920] Fang, L., "Security Framework for MPLS and GMPLS 521 Networks", RFC 5920, July 2010. 523 8. Authors' Address 525 Tomohiro Otani 526 KDDI Corporation 527 2-3-2 Nishishinjuku Shinjuku-ku 528 Tokyo, 163-8003, Japan 529 Phone: +81-3-3347-6006 530 Email: tm-otani@kddi.com 532 Richard Rabbat 533 Google, Inc. 534 1600 Amphitheatre Pkwy 535 Mountain View, CA 94043 536 Email: rabbat@alum.mit.edu 538 Sidney Shiba 539 Email: sidney.shiba@att.net 541 Hongxiang Guo 542 Email: hongxiang.guo@gmail.com 544 Keiji Miyazaki 545 Fujitsu Laboratories Ltd 546 4-1-1 Kotanaka Nakahara-ku, 547 Kawasaki Kanagawa, 211-8588, Japan 548 Phone: +81-44-754-2765 549 Email: miyazaki.keiji@jp.fujitsu.com 551 Diego Caviglia 552 Ericsson 553 16153 Genova Cornigliano, ITALY 554 Phone: +390106003736 555 Email: diego.caviglia@ericsson.com 557 Dan Li 558 Huawei Technologies 559 F3-5-B R&D Center, Huawei Base, 560 Shenzhen 518129 China 561 Phone: +86 755-289-70230 562 Email: danli@huawei.com 564 Takehiro Tsuritani 565 KDDI R&D Laboratories Inc. 566 2-1-15 Ohara Fujimino-shi 567 Saitama, 356-8502, Japan 568 Phone: +81-49-278-7806 569 Email: tsuri@kddilabs.jp 571 9. Appendix A. DWDM Example 573 Considering the network displayed in figure 1 it is possible to 574 show an example of LSP set up using the lambda labels. 576 Node 1 receives the request for establishing an LSP from itself 577 to Node 9. The ITU-T grid to be used is the DWDM one, the channel 578 spacing is 50Ghz and the wavelength to be used is 193,35 THz. 580 Node 1 signals the LSP via a Path message including a Wavelength 581 Label structured as defined in section 3.2: 583 0 1 2 3 584 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 1 585 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 586 |Grid | C.S | Identifier | n | 587 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 589 Where: 591 Grid = 1 : ITU-T DWDM grid 593 C.S. = 2 : 50 GHz channel spacing 595 n = 5 : 597 Frequency (THz) = 193.1 THz + n * channel spacing (THz) 599 193.35 (THz) = 193.1 (THz) + n* 0.05 (THz) 601 n = (193.35-193.1)/0.05 = 5 603 10. Appendix B. CWDM Example 605 The network displayed in figure 1 can be used also to display an 606 example of signaling using the Wavelength Label in a CWDM 607 environment. 609 This time the signaling of an LSP from Node 4 to Node 7 is 610 considered. Such LSP exploits the CWDM ITU-T grid with a 20nm 611 channel spacing and is to established using wavelength equal to 612 1331 nm. 614 Node 4 signals the LSP via a Path message including a Wavelength 615 Label structured as defined in section 3.3: 617 0 1 2 3 618 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 1 619 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 620 |Grid | C.S | Identifier | n | 621 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 623 Where: 625 Grid = 2 : ITU-T CWDM grid 627 C.S. = 1 : 20 nm channel spacing 629 n = -7 : 631 Wavelength (nm) = 1471 nm + n * 20 nm 633 1331 (nm) = 1471 (nm) + n * 20 nm 635 n = (1331-1471)/20 = -7