regional optical transport networks

Regional optical transport networks

Paul Bonenfant and Antonio Rodriguez Moral Photuris, Incorporated

20 Corporate Place South, Piscataway, New Jersey 08854 [email protected]

Received 16 November 2001

We provide an overview of current, emerging, and future regional (or metro core) optical transport networking architectures. Particular emphasis is placed on matching optical networking features, protocols, and framing techniques to carrier needs and requirements. © 2001 Optical Society of America

OCIS code: 060.4250.

1. Introduction Why focus on regional optical networks? Optical networking solutions for access and core network segments have been adequately addressed—witness the number of stalwarts and start-ups announcing products for these applications. Regional networks fall in between access and core networks and have been addressed largely with afterthought solutions. We focus on the unique carrier needs and requirements for regional optical networking, based on the wide variety of services—existing (legacy) and emerging— along with the commensurate networking protocols and framing techniques to be supported. The menu of potential networking protocols reads like a recipe for alphabet soup: SONET/SDH (synchronous optical network/synchronous digital hierarchy), Ethernet—in Gigabit Ethernet (GbE) and 10-GbE variations—resilient packet rings, DWDM and CWDM (dense and coarse wavelength division multiplexing), so-called digital wrappers, and the list goes on. With this wide variety of choices in mind, we focus on the key network architectures and networking protocols needed to support both legacy and emerging services in regional optical transport networks.

2. Optical Transport Network Segments: Core, Regional, and Access As shown in Fig. 1, transport network segments are generally categorized as core (or long-haul, wide-area network—WAN, or backbone), regional (or metro interoffice, metro core), and access (or metropolitan area network—MAN). Optical networking applications exist in all three network segments. Only two, however—core and access—are currently served by solutions that are well matched for the requirements of those segments.

Optical networking in the core network will be supported by optical cross connects (OXCs) with large switch fabrics coupled with long-haul or ultra-long-haul point-to-point DWDM systems and high-capacity optical add–drop multiplexer- (OADM-) based rings. These reconfigurable optical switching network elements (OXCs and OADMs) will serve as the underpinning for the optical transport network, providing bandwidth management —including protection and restoration—at wavelength granularity.1

Optical networking in the access networks will be well served by so-called multiservice provisioning platforms (MSPPs) that find ever-more efficient means of packing disparate time-division-multiplex (TDM) and data-networking services onto SONET/SDH (OC-48/STM-16 and OC-192/STM-64) signals. Generally, MSPPs add service-level aggregation to SONET/SDH rings, thereby increasing the effectiveness of delivering services at less than whole wavelength bandwidth granularities. Also entering the access space will be alternatives to traditional SONET/SDH rings, such as Ethernet-centric fail-over techniques and resilient packet rings (RPRs). Both the MSPP and

39291 January 2002 / Vol. 1, No. 1 / JOURNAL OF OPTICAL NETWORKING 9

Ethernet or RPR solutions can coexist with ultra-low-cost CWDM as an inexpensive fiber multiplication technique.

OADM Ring32+ λ

Core

Regional

AccessSONET/SDH,

Ethernet, RPR,CWDM

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EXC,GbE Switch

OXC OLS

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Optical Network Elements:OXC: Optical Cross ConnectOADM: Optical Add/drop Multiplexer OLS: Optical Line System (Long-Haul,

Ultra Long-Haul)OA: Optical Amplifier MSPP: (SONET/SDH) Multi-Service

Provisioning PlatformEXC: Electrical (SONET/SDH) Cross

ConnectGbE: Gigabit EthernetRPR: Resilient Packet RingsCWDM: Coarse WDM

OXC Mesh64+ λ

OA

OLS

OLS

Fig. 1. Emerging optical transport network architecture.

In the regional network segment, a solution that is well matched to the unique requirements has yet to emerge. Carriers have been and are still faced with a variety of mismatched options.

OADM Ring32+ λ

Core

Regional

AccessSONET/SDH,

Ethernet, RPR,CWDM

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OADM

OADM

MSPP

OADM

MSPP

MSPP

MSPP

MSPP IP ATM TDM WDMRPRSome combination of…

1st Generation Regional Solutions

2nd Generation Regional Solutions

Fig. 2. Historic approaches to regional optical networks.

As illustrated in Fig. 2, many first-generation solutions were initially based on simplified long-haul DWDM systems burdened with the high costs of core network technologies. Of late, second-generation MSPP systems with bolt-on point-to-point DWDM or CWDM, or even fixed add–drop DWDM or CWDM, have been touted as regional network solutions. And, for some time, even fixed add–drop DWDM solutions have been advertised as dynamic optical networking systems. Unfortunately, the needs of the regional network are unique and cannot be satisfied by retrofitted core or access solutions. The bandwidth explosion in the regional network—especially in light of rapidly emerging industry groundswell activities such as 10 GbE and RPR—coupled with the need for fast service


provisioning and emerging new applications, dictates the need for true optical layer bandwidth management in this segment.

3. Regional Carrier Needs and Network Requirements Carriers and service providers that seek to differentiate themselves in the regional market space require network solutions that enable them to design, deploy, operate, and manage regional optical transport networks that support both traditional and new revenue-generating services at reduced cost points. These include managed wavelength services at 2.5 and 10 Gbit/s, optical Ethernet services (with GbE and 10-GbE interfaces), carrier interconnection services (with SONET/SDH interfaces from OC-3/STM-1 to OC-192/STM-64), and optical storage area network (SAN) extension services.

3.A. Revenue-Generating Services The explosion of data traffic, not only with respect to the number of users but also with respect to the bandwidth required by emergent applications and services, is a key factor in the evolution of regional networks. Service interfaces on optical networking equipment in regional networks are quickly approaching the OC-48 and OC-192 line rates. This is driven by a number of factors, including rapidly emerging industry groundswell activities, such as 10 GbE, and the emergence of Ethernet-centric carriers that plan to offer 100- Mbit/s (100 BaseT) Internet connections to businesses—at roughly the price of a T1—over an Ethernet-over-DWDM infrastructure.

However, beyond solving capacity-exhaust problems, regional network carriers are clearly looking for a means of improving their networks to become more competitive, and of creating new revenue-generating services from their embedded base that leverage the capabilities of optical networking technology in the regional space. Some of these revenue-generating services, which can be offered to other carrier and service providers (carrier services) or to end users, typically medium-to-large corporations (end-user services) include the following: Carrier interconnection services. Provide Internet infrastructure services—high-speed interconnection of Internet data centers and points of presence (PoPs), bandwidth brokering, and colocation services. Traffic aggregation and multiservice transport services. Consolidate traffic from customers that use diverse technologies and protocols [SONET/SDH, ATM (asynchronous transfer mode), IP (Internet protocol), Ethernet] into a common transport network infrastructure. Managed wavelength services. Support 2.5- and 10-Gbit/s wavelength connectivity with powerful performance monitoring and protection options, allowing carriers to offer gigabit service-level agreements to other carriers and service providers. Wavelength on demand. Allow the provisioning wavelength services by (or for) the end user, on demand. Optical LAN extension services and Optical Virtual Private Networks (VPNs). Interconnect enterprise or MAN sites via native GbE, ATM, or SONET interfaces. High-speed Internet access services. Provide business with high-speed, high-quality, reliable, and cost-effective access to their service providers for Internet, intranet, and extranet applications and for mission-critical business Internet applications; in Optical Ethernet solutions this can be done at gigabit speeds in native Ethernet format.


High-speed WAN connectivity services. Connect customer routers, ATM switches and other high-speed equipment to WAN and long-haul networks at multi megabit and gigabit rates. Storage connectivity and SAN optical extension services. Expand storage networking across multiple sites over access and regional segments, for global corporations that require storage on-demand, disk mirroring, and disaster recovery.

3.B. Network Requirements The market for regional networks differs significantly from current DWDM long-haul deployments in which multiwavelength technology is used almost exclusively to increase the signal-carrying capacity of any given fiber-optic transmission link. In regional applications, whereas link capacity, measured by the number of wavelengths per fiber, remains important, the following features are critical: Optical bandwidth flexibility. Historically, carriers have found it difficult to accurately predict emerging or growing traffic patterns in the Internet. This difficulty will persist as traffic demands grow and use of network capacity increases. Network planners need some flexibility in the network to absorb this uncertainty without the need of costly redesign and without locking into architectures that result in stranded capacity. As a result, carriers in the regional space demand solutions that are capable of accommodating, with an efficient use of optical bandwidth, diverse traffic patterns (e.g., a combination of hub-and-spoke and mesh) as well as unpredictable variations and growth of those traffic patterns. Support for higher bit rates. On the basis of emerging applications, carriers in the regional network space demand solutions that are capable of supporting higher-bit-rate services. At the same time, edge–core connectivity of IP routers and ATM switches at OC-48/STM-16 and OC-192/STM-64 rates have become prevalent. This bandwidth explosion in the regional space dictates the need for 10-Gbit/s optimized solutions in the regional network. Efficient transport and switching of subwavelength services. The trend toward higher bit rates and managed wavelength services does not reduce or eliminate the need for carriers to support existing bread-and-butter SONET/SDH services at OC-3/STM-1 (155 Mbit/s) and OC-12/STM-4 (622 Mbit/s) rates. The transition to gigabit services will of course not happen overnight; until gigabit services and wavelength services predominate, ways of transporting and switching TDM signals at lower rates are needed, since these constitute the bulk of revenue-generating services provided by transport carriers today. Therefore seamless integration of legacy SONET/SDH services by means of efficient transport and switching of OC-3/STM-1 and OC-12/STM-4 signals, as well as TDM traffic switching and grooming with STS-1/VC-3 granularity, is an essential piece of the regional networking puzzle. Data over transport. Carriers aiming to leverage their existing network infrastructure are seeking ways of obtaining the efficient multiplexing gains found in traditional data networks coupled with the service reliability provided by traditional transport networks (the ubiquitous five-nines reliability). Thus, along with protocol transparency, carriers are looking for solutions that are capable of providing efficient transport and grooming of diverse data services [ATM, packet over SONET/SDH (POS), Frame Relay, Ethernet, Fibre Channel, Enterprise System Connect Connect (ESCON), and Fiber Connection (FICON)] transported over wavelengths on their optical network infrastructure, without costly upgrades to currently unsupported protocols or framing techniques. At first glance


this may seem like a daunting requirement—witness the variability in protocol mappings and framing techniques, for example, for IP over fiber, as illustrated in Fig. 3.

Why the variability? Although network economics may drive toward a tighter coupling of optical/transport and data-networking functions and management, differing degrees of integration are appropriate, depending upon carrier business models, customer service offerings, and corresponding service requirements.

• POS = Packet over SONET/SDH• EOS = Ethernet over SONET/SDH (proprietary mappings)• GFP = Generic Framing Procedure

IP

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IEEE 802.2 LLCIEEE 802.2 LLC

STS-192c/VC-64c adaptation

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• RPR = Resilient Packet Rings, IEEE 802.17• POS = Point to Point Protocol• PHY = Physical Layer

Fig. 3. Networking over the optical layer: IP over fiber protocol stacks (after Ref. 2). PPP, point-to-point protocol; PHY, physical layer.

Hence next-generation optical networking systems aimed at the regional market should be capable of efficiently supporting both TDM (SONET/SDH) and data-signal formats such as ATM, POS, GbE, 10 GbE, ESCON, FICON, Fibre Channel, and the emerging IEEE 802.17 RPR standard. Fast, dynamic provisioning. In the emerging regional market, the need for more bandwidth and higher bit rates emerges, accompanied by the need for provisioning and turning up new services in a timeframe much shorter than in the past. The traditional, static model of network operations that required weeks for activating a new service is an obstacle to carriers that want to differentiate themselves by offering services and bandwidth anywhere, anytime. This translates into a requirement for solutions that are capable of adding and removing services from the network by means of simple, intuitive procedures (point-and-click wavelength and circuit provisioning). Optical bandwidth scalability and modularity. To minimize capital expenses, carriers are looking for a way of adding optical bandwidth to their networks as service needs dictate. This pay-as-you-grow is a critical requirement for adjusting network cost to revenues at any moment, as opposed to having to buy and deploy network equipment that provides unused optical bandwidth. The granularity offered by network equipment to deploy additional optical bandwidth should, ideally, consist of individual wavelengths.


4. Next-Generation Regional Optical Networks To meet these emerging requirements, a new generation of optical network elements is required for regional networks that resemble SONET/SDH in their flexibility for adding and dropping wavelengths and provide a broad set of features needed for an increasingly wider variety of service offerings.

4.A. Flexible (SONET/SDH-like) Wavelength Add–Drop Providing flexible, programmable wavelength add–drop for regional networks has thus far proven too costly from two perspectives: first, the cost of the optical components to build a truly flexible add–drop capability, and second, from the increased cost of amplifiers required for overcoming the higher resultant loss. This has been the historical reason for the deployment of first- and second-generation regional networking solutions—although they failed to meet carrier needs, they represented what was minimally acceptable and were the only building blocks available to build regional optical networks. Emerging innovations in optical component technology should enable a next generation of optical network elements that provide flexible and programmable wavelength add–drop, at an attractive cost, and with reduced optical loss.

4.B. Use of Standard and Widely Applicable Framing Techniques: SONET/SDH The use of SONET/SDH for carrying signals for which SONET/SDH was not initially designed (i.e., data signals) is now almost ubiquitous. Witness the recent definition of SONET/SDH OC-192/STM-64 framing as the WAN PHY for 10 GbE, which will serve only to perpetuate the ubiquity of SONET/SDH-framed signals. In large part this is caused by the desire to leverage over a decade of SONET/SDH-specific hardware development, software development, craft personnel/network operator/carrier familiarity and embedded network infrastructure—across the access, regional, and core network environments. In the absence of a standard for mapping unconventional data signals into SONET frames, a variety of proprietary solutions have emerged [e.g., current methods for providing Ethernet over SONET/SDH (EoS) mappings]. To promote vendor equipment and carrier interworking, an effort is underway to define standard mappings for data over SONET/SDH; this effort falls under the guise of work to develop a generic framing procedure (GFP).3

GFP provides a generic mechanism to adapt traffic from higher-layer client signals over SONET/SDH or OTN transport networks. Client signals include GbE, ESCON, FICON, and Fibre Channel.

The GFP work for SONET/SDH leverages a parallel activity to standardize so-called virtual concatenation of SONET/SDH paths. Virtual concatenation allows for relaxation of the rigidity of SONET/SDH payload bit rates, which were originally designed on the basis of the digital hierarchy defined for the telephone (voice) network. In combination with virtual concatenation, GFP will allow the standard mapping of a wide variety of data signals over SONET/SDH, as shown in Fig. 4.

Finally, the data over SONET/SDH initiative is also leveraging the emerging capability of dynamically sizing virtually concatenated SONET/SDH paths using a link capacity adjustment scheme (LCAS)4 recently defined for SONET/SDH. LCAS provides a control mechanism for “hitlessly” increasing or decreasing the capacity of a link to meet the bandwidth needs of the application. It also provides a means of removing member links that have experienced failure. The LCAS assumes that in cases of capacity initiation, or capacity increases or decreases, the construction or destruction of the end-to-end path is the responsibility of the network and element management systems, or of a distributed control plane.

Combined, virtual concatenation, GFP, and LCAS offer an attractive option for carrying data networking protocols over transport networks.


Client Payload (bandwidth) SONET/SDH VC Path (bandwidth)ESCON (160/200 Mbit/s) STS-1-4v/VC-3-4v (196 Mbit/s)Fibre Channel - FC100 (850/1062.5 Mbit/s) STS-3c-6v/VC-4-6v (900 Mbit/s)FICON (850/1062.5 Mbit/s) STS-3c-6v/VC-4-6v (900 Mbit/s)Gigabit Ethernet (1000/1250 Mbit/s) STS-3c-7v/VC-4-7v (1050 Mbit/s)

SONET/SDHPayload

(STS-1s / VC-3,4’s)

Section/RSOH

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Path

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Virtually Concatenated Path Size for Standard 8B/10B Client Signals Fig. 4. Efficient Data over SONET/SDH: GFP + Virtual Concatenation + LCAS.

4.C. Judicious Layer and Interface Reduction Insofar as we predict a convergence of optical/transport and data networking (e.g., see Ref. 5), many large, multiservice carriers have yet to see the marriage of their optical/transport and data organizations, let alone their networks. This implies that it is important to have an optical transport platform capable of scaling toward advanced packet/data features as carrier needs dictate. Ultimately, judicious integration (not simply integration for integration’s sake) of key technologies and layer network functions, resulting in appropriate collapsing of layers, will translate into cost savings for carriers reflected in the reduction of boxes, interfaces, and disparate management systems.

For regional carriers this implies the need for next-generation optical transport systems with WDM channels supporting legacy services, as well as emerging data over transport applications. The latter might be powered by GFP and LCAS over traditional SONET/SDH framing, or Ethernet-centric RPRs; this implies the need for TDM-on-a-wavelength and RPR-on-a-wavelength functions on WDM channels for data over transport, alongside the WDM channels supporting DSN and OCN wavelength services.

4.D. Mix and Match Optical/Transport Layer Protection Options The emerging services mix dictates a closer look at traditional all-or-nothing protection switching approaches, for example, the traditional view that transport rings support either dedicated protection or shared protection, but not both. In emerging optical networks, services that historically lack protection schemes may benefit from optical layer protection or restoration.6 For these services, traffic patterns will dictate whether optical layer dedicated restoration schemes (for hubbed traffic) or shared restoration schemes (for mesh-like or distributed traffic) are more appropriate. Furthermore, supporting enhanced TDM-on-a-wavelength, Ethernet, or RPR-on-a-wavelength applications requires that these wavelengths be unprotected from the optical layer perspective. This reflects a requirement that optical, TDM, RPR, or other client layer protection be selectable on a per-wavelength basis—effectively translating into a palette of service levels that a carrier can offer to customers, selectable on a per-wavelength basis, over the same physical optical network topology. This concept is illustrated in Fig. 5, as applied to an optical ring.


Fiber 1

Fiber 2

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Optical Layer Unprotected Services, or TDM SONET Ring Protection, or

RPR Protection Fig. 5. Mix-and-match protection on a per-channel basis for diverse services.

4.E. 10-Gbit/s-Optimized DWDM Transport, with an Eye toward 40-Gbit/s Scalability A 10-Gbit/s-optimized regional network is obviously required for supporting applications at OC-192/STM-64 rates and for the deployment of 10-GbE networks. Some of the optimizations required for designing a 10-Gbit/s-optimized optical platform include active and passive dispersion-compensating modules, dynamic gain equalizers for managing channel powers during add–drop, optical amplifiers designed to transmit 10-Gbit/s level powers, and optical filter shapes capable of handling 10-Gbit/s channels without concatenation and interference effects. 40-Gbit/s systems will likely first appear in long-haul applications, but carriers will quickly demand that at least some wavelengths in a regional network can transport OC-768/STM-256 signals. To avoid the same lack of forethought that rendered existing 2.5Gbit/s-optimized systems effectively unusable for 10-Gbit/s applications, next-generation optical transport networks should be designed with 40-Gbit/s scalability in mind.

4.F. Distributed (GMPLS) Control Planes That operators want a more intelligent network with less reliance on centralized operations systems is reflected in the flurry of standards and marketing activity around the concept of generalized multi-protocol label switching (GMPLS). While a distributed control plane as applied to transport networks is a novel and powerful concept, carrier approaches to network management are not necessarily GMPLS ready.7 If designed to complement rather than outright replace traditional network management systems, GMPLS control planes will allow for faster service provisioning times and create opportunities for new network services.8

5. Summary and Conclusions The emergence of terabit routers, OXCs, and next-generation digital cross connects coupled with long-haul and ultra-long-haul DWDM systems are well matched to the surge of bandwidth in the core network, whereas so-called MSPPs providing multiservice over SONET/SDH optimized for access network applications are entering their second generation of development. Although enhanced functionality, layer integration and box reduction have been adequately addressed for the core and access portions of the optical transport network, we contend that solutions appropriately matched to the needs of regional networks have yet to emerge.


Between the bulk transport and switching requirements of the core transport network, and the multiservice aggregation requirements of the access transport network, lies the regional transport network space, with a unique set of requirements for efficient multiservice traffic distribution. Emerging applications dictate the need for the following features in next-generation regional optical transport networks: flexible and programmable (SONET/SDH-like) wavelength add–drop, freeing the network from the tyranny of fixed add–drop solutions; the use of standard—and widely applicable—framing techniques, such as SONET/SDH, to support both bread-and-butter legacy services and emerging data over transport services; judicious layer and interface reduction, combining wavelength services with TDM and RPR on a wavelength applications; mix and match optical/transport layer protection options for support of an increasingly wide variety of service-level agreements and traffic patterns; 10-Gbit/s optimized solutions, for 10 GbE and data over transport 10-Gbit/s applications; and distributed (GMPLS-based) control planes, appropriately integrated with existing network management systems.

And, while it almost goes without saying, these next-generation solutions must—while supporting a large menu of enhanced features—lead to a reduction first and foremost in capital expense, as well as operational expense and service turn-up time. Finally, given the state of the marketplace, these next-generation solutions must lessen the degree to which a paradigm shift is required for supporting new services. In other words, they must allow for the creation of new revenue-generating services that, to the extent possible, leverage the existing network infrastructure and have minimal impact on operating procedures, rather than require a wholesale replacement of a carrier’s network infrastructure.

References and Links 1. A. Rodriguez-Moral, P. Bonenfant, S. Baroni, and R. Wu, “Optical data networking:

protocols, technologies and architectures for next generation optical transport networks and optical internetworks,” IEEE J. Lightwave Technol. (December 2000), pp. 1855–1870.

2. P. Bonenfant and A. Rodriguez-Moral, “Framing techniques for IP over fiber,” IEEE Netw. Mag. (July/August 2001), pp. 12–18.

3. P. Bonenfant, “Optical data networking in the 21st century: a sea change,” Industry Forum column, Opt. Netw. Mag. (October 2000), pp. 8–10.

4. “Generic framing procedure (GFP),” ITU-T draft Rec. G.7041 (International Telecommunication Union, October 2000), http://www.itu.int/publications/online/index.html.

5. “Link capacity adjustment scheme (LCAS) for virtual concatenated signals,” ITU-T draft Rec. G.7042 (International Telecommunication Union, October 2000), http://www.itu.int/publications/online/index.html.

6. P. Bonenfant, “Optical layer protection and restoration,” in Optical Networking, A. Bononi, ed. (Springer-Verlag, Berlin, 1999), pp. 77–88.

7. O. Gerstel, “Optical layer signaling: how much is really needed?” IEEE Commun. Mag. (October 2000), pp. 154–160.

8. A. McGuire, S. Mirza, and D. Freeland, “Application of control plane technology to dynamic configuration management,” IEEE Commun. Mag. (September 2001), pp. 94–99.


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