As the telecommunications industry approaches the optical networking era, the need for optical interconnects with network element consolidation capabilities becomes increasingly important.

However, service providers want a solution that is cost-effective and space-efficient. This has created the need for hybrid Sonet add/drop multiplexing systems that combine the functionality of a fiber optic terminal, digital multiplexer and digital cross-connect into a single optical networking node.

Advanced OC-192 transport systems provide the first opportunity to mix and match OC-3, OC-12 and OC-48 tributary support within the same network element. The addition of integrated cross-connect matrices and subtending ring functionality allows service providers to effectively reduce equipment size, power and costs while meeting subscribers' growing service demands.

The evolution of all-optical networking requires high-capacity and highly reliable optical transport, and 10 Gb/s line rates quickly are becoming the standard for long-haul and interoffice backbone networks. With the addition of International Telecommunication Union-compliant narrowband optics, OC-192 systems can be integrated seamlessly with dense wavelength division multiplexing (DWDM) systems to further increase network efficiency and reduce costs.

Tending the rings

A subtending ring is an advanced dual-ring configuration in which a node's tributary optics support a secondary ring, offering substantial cost savings by allowing a single network element to be used in place of multiple co-located shelves. OC-12 and OC-48 tributary interfaces allow for lower-rate subtending rings via an OC-192 backbone. A hybrid Sonet ADM's ability to interface with lower-speed systems allows the network operator to significantly reduce cost and complexity at the subtending node.

A hybrid time division multiplexing (TDM)-based system that supports subtending - or integrated - rings effectively allows the replacement of entire shelves of equipment with a single card or cards. This means that multiple unidirectional path-switched rings (UPSRs) or bidirectional line-switched rings (BLSRs) can be terminated at an optical hubbing node with full Sonet protection switching support (Figure 1).

As a result, equipment space and costs can be reduced significantly, depending on the network configuration. The Sonet data communications channel provides full visibility of subtending nodes and tributaries to increase network capacity while simplifying network planning.

Welcome to the matrix

An integrated cross-connect matrix provides additional benefits when used in conjunction with the subtending ring functionality. For instance, a 768-by-768 STS-1 matrix - the largest OC-192 matrix available to date - can route traffic across a four-fiber BLSR configuration to provide 40 Gb/s of actual cross-connect capacity.

Support for non-blocking connectivity is provided for the following types of configurations:

- High speed to low speed

- Low speed to high speed

- Low speed to low speed

Also called "hairpinning," the low-speed to low-speed routing capability allows tributary traffic to be routed between network elements without traversing the high-speed OC-192 line. Hairpinning further reduces equipment requirements.

The large matrix provides maximum cross-connect flexibility, allowing carriers to route and groom individual STS channels within the 10 Gb/s system. Its ability to groom traffic within the OC-192 signal provides efficiency benefits when interfacing with a standard digital cross-connect system (DCS). For instance, instead of using two OC-12 ports on a DCS that may only be half full (six STS-1s each), service providers can mix and match traffic to provide a full OC-12. This increases port usage while decreasing overall port costs.

Show me the savings

So what are the savings? By using a hybrid ADM system, service providers can effectively reduce their equipment requirements, which translates into the following quantifiable benefits: space savings, power savings and, most important, cost savings.

Less space. With floor space in central offices (COs) becoming scarce and real estate costs rising, it is essential for service providers to pack more functionality into smaller areas. Subtending rings eliminate entire racks of equipment while maintaining equivalent network element functionality and visibility. This drives network element consolidation for a corresponding reduction in equipment space (Figure 2).

Optical hubbing allows the interconnection of lower-rate Sonet equipment at the optical level, eliminating the need for costly optical-to-electrical and electrical-to-optical conversions. The ability to support multiple tributary connections (for example, OC-3, OC-12 and OC-48) is a key to maximizing the associated space and cost savings. High-capacity transport systems such as those operating at OC-192 line rates will become the hubbing backbone for the evolving optical network.

This analysis will focus only on OC-12 UPSR subtending rings, though such an analysis could be performed for any standard Sonet tributary rate. Depending on the actual number of OC-12 drops configured, an OC-192 hybrid ADM implementation could provide floor space savings of up to 67% when compared to implementations using traditional Sonet ADM equipment.

Less power. The reduction in space requirements yields a corresponding decrease in power consumption, or lower power grid requirements. For a full OC-12 drop configuration, the OC-192 hybrid ADM implementation provides power consumption savings of more than 34% over traditional implementations (Figure 3). This translates into more power available for other essential CO operations or network elements. Any environmental savings would be in addition to the aforementioned power savings.

Lower costs. In addition to the space and power savings that result from consolidating network element equipment, overall equipment costs decline. This includes costs associated with equipment operation and maintenance and air conditioning. The combination of subtending rings and integrated cross-connect capability results in cost savings for any tributary drop configuration.

For example, for a full OC-12 drop configuration, the OC-192 hybrid ADM implementation provides cost savings of more than 52% over traditional implementations (Figure 4). Any real estate or environmental savings would be in addition to these savings.

Bandwidth booster

The addition of ITU-compliant narrowband optics on the OC-192 line and DWDM equipment can increase the bandwidth efficiency of single-mode fiber facilities substantially. The elements of optical hubbing discussed previously are applicable to DWDM systems, and their benefits are multiplied accordingly. The fault isolation and protection schemes inherent in Sonet equipment can form a basis for high-capacity, highly reliable transmission systems.

Some DWDM systems today can support up to 32 wavelengths of OC-192 transport for a total capacity of 320 Gb/s on a single fiber. Future systems will expand this capacity well into the terabit region with more than 160 wavelengths. At the same time, the distance between the repeaters required for electrical regeneration of the optical signal is increasing to more than 3000 kilometers from 600 kilometers.

These optical hubbing elements are attractive to the long-haul, interexchange market segment, where OC-192 transmission first saw widespread deployment. As networks evolve, the industry will see TDM and DWDM technologies working together - rather than competing - to achieve increased fiber bandwidth and decreased system cost.

A unique application of the OC-192 hybrid Sonet ADM is its ability to combine the functions of an OC-192 multiplexer and transponder without incurring the cost of an OC-192 transponder card. Operating together, the advanced ADM expands the traditional functionality of an optical transponder to provide a cost-effective bridge between the large installed base of OC-48 equipment and newer OC-192 and DWDM technologies.

Transponders have become popular for their ability to provide a transparent interface between one manufacturer's transmission system (including legacy asynchronous transmission equipment) and another manufacturer's DWDM system. For example, a transponder can accept a wideband OC-192 signal and output a narrowband OC-192 signal of the appropriate wavelength and optical characteristics for the target DWDM system.

OC-192 systems are capable of multiplexing and demultiplexing multiple OC-48 tributaries in a single rack. This capability enhances the transponder concept when narrowband optics are used on the OC-192 interface with the target DWDM system. Not only does it eliminate the need for a dedicated transponder card on the DWDM system, but the effective interface capacity of that system is multiplied by four because there are four OC-48 tributaries for every OC-192 line.

Tomorrow's solution

A new generation of hybrid Sonet ADM systems will enable the essential ingredients of optical networking - optical interconnect flexibility and network element consolidation - while providing the immediate benefits of reduced equipment size, power and costs. Advanced OC-192 transport systems will provide the ability to gracefully scale from two- to four-fiber configurations and from partial bandwidth drops to full bandwidth drops (of OC-3, OC-12 and OC-48) using a service provider's existing equipment.

These capabilities will provide substantial cost savings while simplifying network planning. In addition, the emergence of 10 Gb/s line rates as the preferred transport data rate in populating DWDM systems will further drive the capacity evolution. Such scalability will be a key to supporting network growth as we approach the optical networking era.