Achieving transparency and switching in metro DWDM equipment

Metro networks of today are facing significant challenges in their development.  Facing increasing demand for voice, data and broadband services from enterprise customers, today's metro networks are struggling to keep ahead of that demand. As enterprises' needs evolve and grow in sophistication, voice circuits and private lines are being supplemented with Gigabit Ethernet, fiber channel and Enterprise System Connections (ESCON). In addition to new traffic types, enterprises are also connecting to a greater number of suppliers, customers and information sources. A single enterprise, for example, could conceivably have external voice circuits, private lines, an Ethernet Internet connection, and fiber channel or ESCON.

Options that enterprise customers have are to procure their own "layer 1" optical infrastructure through dark fiber IRUs, or in some cases privately build fiber connections, managing their own service delivery infrastructure.  But as connectivity needs grow in sophistication, there is a shift away from layer 1 transport infrastructure, such as leased lines, towards layer 2 services, such as Frame Relay and ATM, and more complete layer 3 service offerings based on Internet protocols. Sensing a market opportunity, a new breed of specialty service providers has emerged, setting market expectations for reliable, low-cost, ubiquitous bandwidth and services on demand. Often theses services are vertically focused in terms of market segment, or technology focused in terms of converging all the enterprises service over a single technology such as IP, ATM or SONET/SDH.

The need for transparency

Key in meeting the needs of metro optical networking services is the ability to support the broader range of optical transports in use today. Services based on SONET/SDH still account for a significant amount -- and arguably the majority of -- profitable revenue from service providers today. Data services, which tend to be more elastic in terms of bandwidth requirements and more dynamic in terms of connection models, present a completely different traffic pattern than SONET/SDH. In addition to IP and Ethernet data services, metro networks deliver storage area networks, high-speed video delivery and other specialized services over fiber. Typically these special services provide higher revenues, generated from large enterprise customers.

Given the differences in traffic speeds and types and the distribution of connection end-points, the metro network requires an optical network capable of delivering a dynamic range of services depending on customer requirements. The design challenge presented by all these diverse services is how to evolve an infrastructure to accommodate the profitable revenue base of voice and circuit-oriented traffic, as well as new services such as variable and dynamic IP/Gigabit Ethernet. Keep in mind that despite double- and triple-digit traffic growth predictions, IP and Gigabit Ethernet have yet to provide solid, profitable revenue streams for carriers. The common approach to supporting the growth in data services often entails adapting IP or storage traffic at the customer premise for transport over SONET/SDH infrastructure, alongside voice and circuit traffic. While this approach meets customer demand, it does so sub-optimally by further complicating protocol conversions and adaptations.  These complexities translate directly into increased costs, often higher than the customer is willing to readily bear. Building a new infrastructure optimized for the delivery of IP and Ethernet services over fiber, and avoiding the complexity of SONET/SDH adaptation for non-SONET services, is one possibility for many new and established service providers. However, this approach results in increased service management complexity and inefficient utilization of fiber network resources. A transparent metro optical infrastructure is the service provider's best option for accommodating the diversity of traffic in the metro.

Transparent but not blind

While transparency allows for support of a broad range of traffic types, monitoring the performance of the services delivered over the optical network is just as critical for service providers, especially in fulfilling service level agreements.  As a result, the network cannot be blind to the traffic or services it carries.  Being able to determining the performance of a Gigabit Ethernet, SONET/SDH or fiber channel connection at any point in the network is critical to maintaining high levels of service availability. The network must be able to respond not only to catastrophic events such as fiber cuts, but also to degraded bit-level performance of the signals riding over the fibers.

Flexibility

Different application services require different connectivity models. IP and Gigabit Ethernet traffic is best served by a mesh network environment, whereas SANs are typically best served in point-to-point configurations, and voice traffic by using native SONET/SDH rings. Deploying just one specific topology either limits the service provider to supporting only a limited set of services or, more often, forces them to incur the extra cost and complexity of adapting traffic from one topology to another.  Flexibility is not limited to topology alone. Intelligent utilization of wavelength resources is also important.  Wavelength translation is the ability to assign an incoming lambda from a given ingress frequency to an available frequency on the desired egress, and it avoids the "grid blocking," to which simple DWDM systems are prone.  Universal interfaces also allow for the translation of customer-side optics such as multimode, short reach, long reach, and in some cases WDM to various trunk-side coarse and dense WDM interface types.

Intelligence

The different services offered in the metro network require different path characteristics in terms of wavelength topology and protection. By supporting intelligence through protocols for topology, path selection, protection and signaling, the optical wavelength network can provide the appropriate transport infrastructure for each application. Providing the right protection for a given service increases the efficiency and utilization of available optical bandwidth.

Integration of optical switching/transport

By integrating transparent wavelength transport with a wavelength switch, the optical infrastructure evolves into a flexible optical service delivery network that can transparently deliver all the optical services required in the metro. At last, carriers can respond to the needs of their customer base, while simultaneously having a base from which to innovate new services.  Another key advantage of the integration of the wavelength functions is a reduction in the capital and operational expenses of deploying the network.  The consolidation of two previously independent network elements in the Central Office or Point of Presence onto a single shelf simplifies the network architecture, removes redundant banks of lasers and muxes, frees up rack space, and simplifies sparing logistics.  At the operational layer, the management infrastructure becomes simplified. Now service providers can manage optical connections as end-to-end light paths from a single system, rather than a series of independent transport structures managed by separate element management systems.

Benefits of integrated optical switching/transport

The impact of the integrated switching and transmission is dramatic, even in today's predominantly SONET/SDH environment. With the increase of access ring traffic, the use of IOF (Inter Office Facility) networks to interconnect these rings, presents a challenge. Interconnecting multiple access rings via a combination of OADMs and OXCs (Optical Cross-Connects) requires multiple boxes, adding cost and complexity. Using WDM to increase the capacity of the IOF fiber plant relieves fiber exhaustion, but it does not decrease the number of IOF rings, or the OADM and OXC port count.

The integrated solution replaces the OXCs and OADMs with a single element called an OADX (Optical Add/Drop Switch) and reduces the fiber connectivity required for interconnecting the IOF. The resulting OADX network offers significantly better fiber utilization with the same levels of protection in a much simpler implementation, while also providing increased capacity for future growth

Capitalizing on the transparency afforded by the OADX, transparent optical connections can be made between access rings across the IOF without forcing a conversion to SONET/SDH transport. Now service providers can provision native services such as SANs, Gigabit Ethernet/IP quickly and seamlessly across their entire metro infrastructure in response to customer connectivity requirements. This ability to support the current SONET/SDH voice infrastructure while accommodating the tremendous growth of data traffic with a scaleable transparent switched optical transport, allows the service provider to employ a best-of-breed technology and vendor selection strategy for all the services they currently support, as well as any future services that may emerge.

The integration of transparent transmission and switching into a next generation intelligent metro WDM platform combines a fiber optic transport infrastructure with a transparent optical network capable of delivering services whenever, wherever and however customers require them.  It is now possible to deliver multiple services using the best architecture for the job rather than merely adapting architectures to the infrastructure at hand. And more importantly, service providers can prepare for new service offerings such as virtual dark fiber and protected wavelengths, without the replacing the entirety of their existing networks.

David Frost is Director of Product Marketing for Meriton Networks.

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