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has been acceptedfrom for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: Downloaded https://iranpaper.ir https://www.tarjomano.com/order Lightwave Technology

JLT-20891-2017

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OTN Interface Standards for Rates beyond 100 Gbit/s Steve Gorshe, Fellow, IEEE

Abstract—The optical transport network (OTN) protocol defined by the ITU-T (Recommendation G.709) has become the backbone of service provider long haul and metro networks. Its flexibility has allowed it to support the converged transparent transport for the full variety of both constant bit rate (CBR) and packet-oriented digital WAN client signals, and its overhead is optimized to reduce the equipment and operational costs for service provider networks. The emerging technologies that enable interface rates beyond 100Gbit/s have created multiple challenges and opportunities for the next generation of OTN. This paper first discusses the challenges and considerations behind the new extension of OTN for rates beyond 100 Gbit/s (OTN B100G). The paper then provides a tutorial for the new OTN B100G standard in light of the flexible and modular manner in which it addressed these considerations. The paper also gives a tutorial overview of the new Flexible OTN “FlexO” protocol. FlexO was developed as a modular PHY technology for OTN B100G, allowing it to reuse IP and modules from the Ethernet ecosystem. In addition, the paper provides a tutorial overview of the new Flexible Ethernet “FlexE” technology from the OIF. FlexE provides both an important new client for transport over OTN, and technology that has been incorporated into the OTN B100G standard. The complementary synergies between OTN and 200G/400G Ethernet and FlexE will become apparent. Index Terms—DWDM, transport network, WDM

optical

network,

OTN,

WDM,

I. INTRODUCTION

T

HE ITU-T G.709 Optical Transport Network (OTN) standard was originally developed for simple transport of signals over a DWDM (dense wavelength division multiplexed) network. The OTN signal frame format provided a, flexible digital “wrapper” that used the same frame format and percentage of signal overhead regardless of its rate. Since its initial release, shortly after 2000, G.709 has evolved to become an increasingly flexible transport protocol. OTN supports converged transparent transport for the full variety of both constant bit rate (CBR) and packet-oriented digital WAN client signals with rates ≥1 Gbit/s. This flexibility allows maximum utilization of optical resources with low protocol This paper is an invited paper based on the “Beyond 100G OTN Interface Standardization” tutorial Th1I.1 at OFC 2017. Steve Gorshe is a Distinguished Engineer with Microsemi Corp. in its Carrier Products Business Unit, in Portland, Oregon USA (e-mail: [email protected]) Copyright © 2017 IEEE. Personal use of this material permitted. However, permission to use this material for any other purposes must be obtained from the IEEE by sending a request to [email protected].

complexity. When signal rates beyond 100 Gbit/s were on the horizon, it was recognized that a new approach should be taken to evolving OTN for higher rates. After providing some background on OTN, this paper discusses some of the challenges and motivations for a different approach, and then describes the resulting recently released version of G.709 to support rates up through multiple Tbit/s. As explained in this paper, the new extension to G.709 for rates beyond 100 Gbit/s (B100G) differs from the legacy OTN in four significant ways: 1. OTN B100G adopts a frame structure to support a variety of rates in increments of 100 Gbit/s rather than specifying certain discrete rates. 2. The method for mapping packet clients into OTN B100G is based on Ethernet techniques rather than the ITU-T G.7041 Generic Framing Procedure (GFP). 3. Forward error correction (FEC) is now required, however it is specified separately for each interface type rather than made a part of the base frame format as it was for legacy OTN. 4. A modular physical layer interface, called “FlexO” has been added in a companion ITU-T Recommendation. Notes on terminology: Except where specified, all rates mentioned in this paper are nominal rates rounded to the nearest convenient value. In reality, most of the actual rates are slightly higher in order to accommodate the required overhead, etc. Ethernet signals are named based on their MAC rate. For example, 10GbE is 10 Gbit/s Ethernet and 100GbE is 100 Gbit/s Ethernet. II. BACKGROUND The OTN signal frame is illustrated in Fig. 1. An ODU (Optical Data Unit) is the basic wrapper signal that includes the payload area for carrying client signals and the overhead for network management. The OTU (Optical Transport Unit) adds information for frame and multiframe recovery that are required when the signal is transmitted over the optical media, and additional performance monitoring overhead for the optical span. The OPU (Optical Payload Unit) is the portion of the frame in which client signals are carried. The OPU can contain either a single client signal or multiple clients multiplexed into the OPU. The OPU payload area is structured as a set of tributary slots (TS), with each client signal occupying an integer number of TS. The OPU overhead identifies the client signal occupying each TS and provides the means for adapting between the client signal rate and the aggregate rate of the TS set used by that client. This

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adaptation is called justification, and provides the method for adding padding or “stuff” words to the client signal to match the TS set rate.

Optional OTUk FEC or all 0 extension

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Fig. 1. OTN base frame format

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The initial version of G.709 was optimized for carrying the widely deployed SONET/SDH (Synchronous Optical Network / Synchronous Digital Hierarchy) optical transport signals. Consequently, it was defined with a set of discrete rates at roughly 2.5, 10 and 40 Gbit/s, corresponding to the standard rates of those client signals. As data client signals (e.g., Ethernet and Fiber Channel) became increasingly important, G.709 was amended and revised to include new ODU and OTU rates to support their transport. Specifically, by 2010, G.709 had defined a discrete set of OTUk transport signals, with k = 1, 2, 3 and 4 corresponding to 2.5, 10, 40 and 100 Gbit/s OTN signals. The OTU4 was optimized for carrying 100GbE. The ODUj signals carried by these OTUk included the same four nominal rates, but also included the 1.25 Gbit/s ODU0 and an m × 1.25 Gbit/s ODUflex that could be multiplexed into the OPUk of the OTUk. The ODU0 was added primarily for carrying Gbit/s Ethernet (GbE). The ODUflex was introduced for encapsulating both arbitrary-rate CBR signals that are carried transparently, and packet data client signals. ODUflex signals carrying CBR clients are referred to as ODUflex(CBR), and those carrying packet data streams are referred to as ODUflex(GFP) due to the client data packets being first encapsulated by G.7041 GFP frames. The need and challenges for OTN rates beyond 100 Gbit/s provided an opportunity to take a fresh look at OTN. The challenges included: • Higher bit rate signals, beginning with 40GbE and 100GbE Ethernet, had already focused on interfaces with multiple physical lanes rather than a single serial interface. For example, 40GbE and 100GbE initially specified 4 x 10 Gbit/s and 10 x 10 Gbit/s interfaces, respectively, where the different physical lanes could be transmitted over separate electrical wires, separate fibers, or separate wavelengths within the same fiber. OTN had specified single lane serial interfaces, but adopted this multi-lane option, primarily for the electrical intra-system

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interfaces, so that the PHY modules developed for 40GbE and 100GbE could be reused for transport interfaces. The Shannon channel capacity limits are catching up to optical transport network capabilities. Specifically, the with currently available technology, the standard 50 GHz channel spacing used for dense wavelength division multiplexing (DWDM) imposes limits on transporting signals over reasonable distances when they have rates much over 100 Gbit/s. The old paradigm of adding new discrete rates for OTN had largely reached its practical limits, making a modular rate and frame structure approach more attractive. Different B100G interface types have different FEC performance capability requirements. Consequently, while the legacy OTUk frame format had fixed dedicated overhead for an RS(255,239,8) Reed-Solomon, it was more appropriate to specify the FEC on a per-interface basis and not make the FEC overhead in integral part of the frame structure. The 1.25 Gbit/s Tributary Slot (TS) rate of legacy OTN is too fine-grained to be practical with B100G signals. The higher OTN data rates and the emerging new data client signals, such as 200GbE, 400GbE and the OIF’s Flexible Ethernet (FlexE), have made the legacy byteoriented mapping approach for data clients impractical. This motivated the desire for a wide-word type of mapping. In order to optimize the use of each wavelength, including for transmission reach, there was a desire to be able to transmit the OTN signals at the rate required for the client payload being carried rather than at the full discrete rate of the OTUCn signal B100G interfaces should re-use as much IP from the 100 Gbit/s OTN interfaces as practical. At that time, the IEEE 802.3bs Task Force working on 400 Gbit/s Ethernet was examining several new approaches that were different from its previous interfaces. The new OTN format needed to not only carry 400GbE, but also re-use its technology and PHY components whenever possible in order to benefit from the Ethernet component cost curves.

The first three challenges all motivated defining OTN B100G as a modular structure that included support for multilane interface structures. This was the first of the foundational agreements. Other foundational agreements will be discussed below as they relate to the topic. III. OTN B100G FRAME STRUCTURE As mentioned above, legacy OTN signals used an identical frame structure regardless of the ODU rate. In contrast, OTN B100G borrowed the modular approach of SONET/SDH and interleaves base frames to create higher rate signals. Specifically, the B100G frame format is an interleaving of n of the basic frames. The basic frame is an ODUC (ODU, where the “C” reflects its 100 Gbit/s nominal rate), and the transmitted signal is an OTUCn. Unlike with SONET/SDH, the entire OPUCn payload area of the resulting signal is a

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JLT-20891-2017

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specific overhead fields. The rate of the base OTUC signal was chosen in order to meet the criteria that the OPUC1 must be capable of carrying an ODU4 client, the OPUC4 must be capable of carrying a 400GbE client, and the resulting signal rate should be reasonable efficient within the constraints of the first two criteria. The signal rate information is summarized in Table 1. As noted above, there is a desire to be able to transmit the B100G signal at only the rate required for the payload that it carries, hence optimizing the wavelength utilization and signal reach. As described below in section V, the way this is accomplished with OTN B100G is by keeping all the overhead of the OTUCn signal, but removing every TS that will not be used on this interface.

JC4-6 for TS A.i JC1-3 for TS A.i

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single entity with n times the size of the OPUC. A key early decision was that the new B100G signals would not be switched within the network, since the ability to switch would make them a new layer network for the carrier to manage. Instead, B100G was defined for only point-to-point connectivity, with the client signals carried within the B100G signal being switched. Another closely related foundational agreement is related to the need to be able to use multiple wavelengths when transmitting an OTUCn signal. It was agreed that the entire group of wavelengths associated with an OTUCn interface signal would go through the same fiber and optical switches (i.e., the same Optical Multiplex Section trails) such that the OTUCn signal can be managed as a single entity. Consequently, very limited deskew is required if multiple wavelengths are used. Note that with the recently revised ITU-T nomenclature, a WDM wavelength used to carry an OTN signal is referred to as an Optical Tributary Signal (OTSi). An Optical Tributary Signal Group (OTSiG) is the set of OTSi signals that support carrying a single client such as an OTUCn. The resulting signal is shown in Fig. 2. As is apparent, the basic ODUC frame format is identical to the ODUk except for some differences in the overhead field definitions that will be noted below. This allows IP reuse from legacy OTN. Note that the format for interleaving the ODUC overhead into the ODUCn is not specified, since it depends on the interface.

a) OTUC/ODUC/OPUC slice frame format

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Fig. 2. OTN B100G frame format

The OTUCn, ODUCn, and OPUCn overhead fields are essentially the same as for the previous generation of OTN. Overhead that pertains to the entire interface (e.g., Trail Trace Identifier for proper connectivity checking, remote defect indications, and delay measurement overhead) only appear on the first OTUC/ODUC of the OTUCn/ODUCn rather than on all n slices. See [1], [3], or [4] for more information on the

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b) Across all n OPUC slices Fig. 3. OPUCn TS and overhead structure

The TS size was chosen to be nominally 5 Gbit/s, which gives (100n/5) = 20n TS in an OPUCn. This TS rate provided the desired coarser granularity, and was the lowest rate that

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JLT-20891-2017 would efficiently accommodate the new data client signals, including 25GbE and 16 Gbit/s Fiber Channel. Newer data clients typically have rates that are divisible by 5 Gbit/s. Legacy data client signals with rates below 5 Gbit/s will be first mapped into a legacy OTN signal. The exact TS and OPUCn payload rates are shown in Table I. In order to reduce device and management complexity, it was agreed that there would be a maximum of 10n tributaries in the OPUCn. TABLE I OTN B100G RATE INFORMATION OTUCn/ODUCn OPUCn payload Tributary Slot rate signal rate area rate n×(239/226)× (OPUCn rate)×(MF/bit) n×(238/226)× 99.5328 ×(bit/TS word) 99.5328 = n×105.258138 ×(TS word/MF) = n×104.817727 = 5.24089 Note: All rates are in Gbit/s ±20 ppm

Frame period 1.163μs

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