Optical crossconnects for the new generation network

Optical transport switches are key building blocks for simpler, more manageable, and cost effective metro backbone networks.

By CHERYL GRAY
Polaris Networks Inc.

As the pressure mounts for service providers to focus on cost reduction and service acceleration, the metro core network has taken center stage. Functioning as the critical bridge between access and long-haul network segments, metro networks are challenged to adapt to a highly dynamic set of service provider requirements--build to demand, not to forecast. This paradigm shift has exposed inefficiencies and complexities associated with current metro backbones.

In a service provider's network, metro core facilities such as tandem offices, large points-of-presence (PoPs) and regional hub sites, present the greatest economic and technical challenge. These facilities typically house large wideband and broadband digital crossconnect systems (DCS) and a multitude of stacked SONET add/drop multiplexers (ADMs). These systems form the underlying infrastructure, supporting the transport, grooming, and switching of traffic.

Network challenges

Although new technology has been introduced to address deficiencies with broadband DCS, wideband DCS have not sufficiently evolved since their introduction almost 10 years ago. Often referred to as "the wideband problem" by service providers, these systems have become a major economic and operational pain point. They are expensive and have become increasingly difficult to maintain and scale--they occupy large footprints, are power intensive, and lack the scalability and density needed to meet new demands.

While these systems are critical for managing and grooming revenue-bearing traffic, such as VT1.5/DS1-based voice, private-line and lease-line services, they have become recognized as the most expensive and operationally tedious elements in the network. To make matters worse, in an effort to meet increasing bandwidth demand, service providers are being forced to deploy more overlays of this equipment, making the network even more complex, expensive, and problematic (see Figure 1).

Given that up to 70% of a service providers' overall costs are in the metro network, and most of that cost is operational, the impact of these inefficiencies becomes quite clear.

New generation solutions

Migrating today's metro networks to a more simplified, converged architecture will help service providers respond to market demands more rapidly and cost-effectively. In addition to slashing infrastructure expenses, a metro network that is substantially more software-driven will enable service providers to migrate their networks in a smoother, non-disruptive manner. It will also allow the introduction and rapid provisioning of new broadband services, such as Gigabit Ethernet, storage area network (SAN) services and high-speed virtual private networks (VPNs).

To address these requirements, new-generation optical transport switches (OTS) are now emerging in the industry. OTS combine the functions of SONET ADM transport systems with DCS grooming and switching, as well as advanced provisioning capabilities driven by Generalized Multi-protocol Label Switching (GMPLS). These new network elements are also capable of supporting a diverse range of traffic types (TDM, ATM, IP, Ethernet). Since metro backbone networks require a high level of scalability, these systems should be capable of expanding to multiple-terabits of capacity without service disruption.

While OTS are typically all-optical platforms (supporting all OC-N interfaces), the requirement in metro networks is for highly granular bandwidth grooming capability - extending from VT1.5 /DS-1 (wideband) levels to STS-Nc (super-broadband) levels to support the massive T-1-base.

Despite the market fixation with broadband service opportunities, T-1 and T-3 private lines continue to grow steadily and represent over 80% of a service providers' revenue stream. For this reason, the OTS platform forms an integral part of the new generation metro network, bridging the gap between the legacy TDM-centric network and the increasingly more data-centric architecture. This flexibility allows service providers to leverage and grow their existing revenue-generating base while eliminating network overlays and operational complexities.

Network architecture migration is clearly not a simple undertaking. Service providers must carefully balance capital and operational investments against revenue and margin considerations, while also addressing competitive forces and new market opportunities. Network strategies based on marginal improvements are insufficient--service providers looking to introduce new products into their networks must consider the incremental costs associated with network integration, OSS integration, testing, and interoperability.

For a successful business case, service providers require substantial economic and operational improvements that meet immediate economic-driven objectives, while migrating to a new-generation architecture that opens new avenues for profitability. The realities of risk management and cash flow/budget constraints also dictate a network migration strategy that is set out in phases. Often the initial phase of migration entails simplification of the operator's ADM and DCS transport network (see Figure 2)--typically the most expensive and operationally intensive part of the metro backbone network.

Once the optical transport network has been collapsed into a single-layered OTS-based architecture, the service provider can plan and implement a second phase of simplification - the introduction of multi-service intelligence into the new-generation architecture. Today, service providers typically use ATM switches exclusively for cell grooming, and routers exclusively for packet grooming at metro core hub sites.

While necessary at network access service points, these service-switching platforms are prohibitively expensive if used solely as multi-service bandwidth managers. If the OTS has been designed with cell and packet bandwidth management intelligence, service providers have the opportunity to cut over this functionality to the OTS and extend the efficiency of the network to full multi-service bandwidth management.

Service acceleration

The speed with which services can be created and provisioned has become vital as carriers compress their time-to-revenue for new offerings. As noted previously, most services travel substantially over TDM network segments, so increasing service velocity for TDM networks becomes even more important.

TDM-based networks are inherently more rigid than data networks, and typically require hop-by-hop manual provisioning. This old paradigm does not support high-service velocity, Consequently, the new generation metro should ideally feature point-and-click end-to-end provisioning--irrespective of service type.

GMPLS has garnered serious attention among service providers for its ability to automate traffic engineering and end-to-end provisioning through a common control plane. GMPLS takes MPLS features, such as label-switched paths, and extends them to optical switching media, a scope not originally encompassed in MPLS.

GMPLS not only includes packet switching, but also TDM, wavelength (lambdas), and spatial-switching. Its common control plane seeks to manage complexity, simplify traffic-engineering problems, and dramatically reduce network bottlenecks. This protocol promises to automate network-resource management and provisioning for all service types, and provide the basis for an open and interoperable next-generation optical network.

What service providers ultimately require is a network architecture comprising multi-vendor end-to-end "point and click" provisioning for all traffic. With GMPLS, metro access products--such as multi-service provisioning platforms (MSPPs) and ADMs--can signal each other and the OTS in the metro core to provision connections.

In turn, the OTS can signal long-haul transport systems to provision the long-distance connections. Using products that comply with the Optical Interworking Forum's (OIF) UNI 1.0 specification, service providers have a solution to migrate their networks toward an open, end-to-end architecture that can deliver service-agnostic provisioning.

While the metro network is in the midst of great flux, it has also become an area of tremendous excitement. Even as this part of the network presents some of the biggest challenges for service providers, it also presents the some of the biggest opportunities for growth and deployment of new-generation technologies. The migration of the metro network to a flatter, simpler and more flexible network will give carriers the ability to respond and adapt to changing demands without expensive network rebuilds.

Using optical transport switches as key building blocks for the new-generation metro backbone network, service providers can implement a network architecture that is simple, cost-effective, and easy to manage. This architecture will also be future-proofed for multi-service and support rapid provisioning of all services. Service providers will be able to respond faster and more efficiently to changing and variable demands--scaling the network cost-effectively, building to demand, and software-enabling new services for increased revenue opportunities.