OTN is a global standard that looks, feels, and acts just like SONET/SDH, which prompts several questions: Is OTN a replacement for SONET/SDH? Do I need to throw out my old gear and upgrade to OTN? And how much longer will SONET/SDH exist?
By Vinay Rathore, Ciena
As the world migrates toward packet-based infrastructures, SONET/SDH networks appear to accommodate everything that is thrown at them. Their resilience can be contributed largely to the forward-looking standards published years ago by the American National Standards Institute (ANSI) and International Telecommunications Union (ITU), which helped define SONET/SDH as the way to offer highly reliable infrastructure for voice networks. Over the years, SONET/SDH has evolved to support data services, including ATM and packet services.
However, the original standards struggled to support a growing number of applications, including new services such as storage protocols and high-speed Ethernet. At the same time, the ITU had begun drafting a new standard to make optical networks more transparent, so they could be protocol and service agnostic. Defined as Optical Transport Network (OTN), the new standard supports evolving data services, including higher speed Ethernet, storage, and all SONET/SDH services.
Further, it is a global standard that looks, feels, and acts just like SONET/SDH to ensure ease of operations. But many questions remain: Is OTN a replacement for SONET/SDH? Do I need to throw out my old gear and upgrade to OTN? How much longer will SONET/SDH exist?
Historical perspective on SONET/SDH
For the past 15 years, SONET/SDH has been the technology of choice for service providers' metro and wide area optical transport infrastructures. During this time, TDM-based services have been the dominant service type required by end users. However, recent trends show an ongoing shift in demand toward packet-based services driven by the introduction and growth of Voice-over-Internet Protocol (VoIP), Ethernet services, and IP video, all of which are increasing bandwidth demands for data transport. In fact, growth in demand for packet-based services has surpassed that of TDM-based services, and analysts predict the growth of TDM services will remain flat to declining, indicating that those services are migrating to packet as well.
Supplementing the technology migration from TDM to packet-based services is a shift in the type of interface used to connect end users to the service provider network and to interconnect service provider network elements. Ethernet interfaces are fast becoming the interface of choice due to cost-effectiveness as well as the familiarity and ubiquity of Ethernet technology. To date, Ethernet has been deployed predominantly for enterprise LANs but now has significant presence in the metro and is gaining momentum in the wide area network, which is spawning new, lower cost 10-Gigabit Ethernet (10-GbE) interfaces--including the 10G LAN PHY, which is larger than the current SONET/SDH OC-192/STM-64 can carry.
SONET/SDH networks stepped up support for packet transport through the use of Packet-over-SONET/SDH (PoS) technology, which provides a mapping of Ethernet packets to SONET/SDH frames. Since the operations, administration, and maintenance (OAM) for SONET/SDH is well defined and understood, PoS improves the reliability and availability of packet services. Other examples of how SONET/SDH has evolved to support packet services include Generic Frame Procedure (GFP), Virtual Concatenation (VCAT), and a Link Capacity Adjustment Scheme (LCAS) to address Ethernet-over-SONET/SDH (EoS) requirements. While this evolution has been successful in meeting basic Ethernet requirements, the standards have not yet been able to keep up support for new optical services, including storage protocols (ESCON, FICON, Fibre Channel, etc.) and even video, nor have they been able to address support for the increasingly important Ethernet 10G LAN PHY.
Advancement in standards
The ITU, comprised of service providers and vendors alike, responded to the shifting environment with the forward-looking OTN protocol, which looks and feels like SONET/SDH and is designed to complement SONET/SDH, not replace it. OTN enables 100% transparent delivery of multiple service types across multiple vendor products, while maintaining the same level of reliability, quality, and management as SONET/SDH.
With OTN technology, multiple networks and services can be combined seamlessly onto a common, future-ready infrastructure. Because OTN was designed to be transparent to service type, all services carried over OTN are given individual treatment, preserving any native functionality and performance without compromising the integrity of the underlying services. Through this concept of transparency, even the most complex of services are guaranteed unencumbered delivery from any client to any server regardless of protocol, speed, distance, or number of networks in between. In short, OTN provides the features and durability to ensure carriers can effectively support and manage both traditional and new optical services and networks.
Since its introduction, OTN has steadily gained momentum with deployments in several large service provider networks, which now are carrying SONET/SDH, Ethernet, and storage traffic and have the ability to support just about anything else. In fact, with the defined rates of OTU1 and OTU2, they are assured to carry SONET/SDH OC-48/STM16 and OC-192/STM64 circuits with full transparency, ensuring no adverse impact to the existing service or operational model. Further, OTU2 also transparently carries the 10-GbE LAN PHY--something SONET/SDH cannot support. This high-speed Ethernet interface is becoming a common interface on many new routers and switches and already is a requirement for many service providers.
The evolution of OTN
As an ITU standard originally dating back to 1998, OTN--sometimes referred to as "Digital Wrapper" technology because of its ability to wrap any service into a digital optical container--was unified from competing standards developed by both the ITU and ANSI. According to ITU standards, an OTN comprises a set of optical network elements that are connected by optical fiber links, providing the functionality of transport, multiplexing, routing, management, supervision, and survivability of optical channels and carrying client signals.
Unlike SDH transport standards, OTN is a globally accepted standard that defines management of next-generation optical networks and is described by a host of ITU Recommendations, starting with G.872, which illustrates the network architecture on OTN, and including G.709, which focuses on structure, interfaces, and mapping. These recommendations provide equipment manufacturers with the necessary tools to produce interoperable products that allow carriers to build and manage ultra-high capacity optical networks, facilitating end-to-end connectivity between optical transport elements in a global network.
Another distinguishing characteristic of OTN is its ability to provision transport for any digital signal, independent of client-specific aspects. As such, according to the general functional modeling described in the ITU's Recommendation G.805, the OTN boundary is placed across the Optical Channel/Client adaptation in a way that includes the server-specific processes and leaves out the client-specific processes.
In simpler terms, OTN is designed to manage multiple wavelength transmissions per fiber and consists of a header, in which overhead bytes are carried, and a trailer that performs Forward Error Correction (FEC). The payload section of OTN allows for all existing network protocols to be mapped with no disruptions to the protocol. FEC has the capability to correct errors and allows carriers to offer and support the varying service level agreements (SLAs) they offer to users. By minimizing errors in the network, FEC can extend the reach of the fiber, allowing further high transmission rates of traffic (See Figure 1).Prior to OTN, a common approach to networking involved connecting IP routers to multiservice provisioning platforms (MSPP). The drawback to this approach is that the MSPP can break the end-to-end communications and topology discovery when it removes the client SONET/SDH overhead bytes and terminates the Data Communications Channel (DCC). These two elements are critical to maintaining client management connectivity and inter-nodal communications across the network used to determine network paths and link states. Loss of this information also prevents proper end-to-end management and makes it difficult to troubleshoot both customer and network problems.
Ready for the future
How OTN overcomes this issue can be summed up in one word: transparency. For this reason, OTN is the ideal technology for both operators and enterprises to build converged networks. As operators and standards bodies look for ways to support new services on SONET/SDH networks, both the cost and the network complexity grow with little added value. OTN offers transparency as well as a separate overhead for performance monitoring and fault signaling, as well as a General Communications Channel (GCC) for remote management, software downloads, and other control functions.
OTN specifications provide a robust management overhead analogous to SONET/SDH that ensures management of both the payload and service. In fact, an OTN payload can fully encapsulate a SONET/SDH frame without terminating the SONET DCC so remote add/drop multiplexers (ADMs) can continue to be managed in the same manner, and topology discovery still works between customer equipment, even if the signal traverses multiple rings or networks.
However, OTN does much more than transparently transport SONET/SDH. It also is the only transport layer standard in the industry that can carry a fully managed 10-GbE circuit from IP Ethernet switches and routers at full bandwidth, including the proprietary overhead associated with the many specific vendor implementations. Highly effective in supporting asynchronous data services such as Gigabit Ethernet, OTN also supports the various speeds of Fibre Channel, ESCON, and FICON, which do not have the same physical layer performance monitoring capabilities and fault isolation necessary for high quality of service. OTN brings those capabilities to asynchronous services without sacrificing the qualities that make those services attractive in the first place, such as lower costs and ease of installation.
OTN's inherent flexibility is enabled by its ability to extend transparency to the timing plane. This ability allows the mixing of both synchronous and asynchronous signal types on a common wavelength. Moreover, synchronous services with different clock sources can be transported side-by-side, which is not possible in a SONET/SDH network.
Outlined below is a diagram that demonstrates OTN for carrier-grade transport of asynchronous services (See Figure 2).
The forward-and-backward looking nature of OTN standards is the driving force behind its success. Considering the important heritage of the SONET/SDH operational model, the need to support a variety of new services options, and the ability to interoperate in a multi-vendor environment, OTN rapidly is becoming the de facto choice for optical networks.
Conclusion
With the rapid migration toward IP/Ethernet-based infrastructure and changing network requirements, the networks of tomorrow will demand the capabilities of OTN. Given the global scope of fiber-optic transport networks, the growth of the OTN market is inevitable. As operators commit to building out next-generation networks, the challenge of convergence becomes greater. By implementing an OTN standard, operators will be able to continue offering value-added services of any type and stay ahead of the game in an increasingly competitive industry.
Vinay Rathore is director of service provider marketing at Ciena. He may be reached via the company's Web site at www.ciena.com.