Optical-network technologies have reached an enormous capacity in recent years, yet carriers still cannot keep up with bandwidth demand. For example, while the demand for high-speed data services has driven interconnect vendors to introduce high-capacity interfaces (OC-12, OC-48) for backbone routers, now there is growing demand for DS-3 and OC-3 service payloads. In turn, this has created the need for more capacity at network access points that will accommodate OC-12/OC-48 trunks capable of supporting these large payloads.
Handling such emerging requirements—and enable carriers to position themselves for more demanding ones in the future—requires a new paradigm for optical-network service deployment in the next century. This new paradigm will give carriers the means to provide wavelength-allocated bandwidth, wavelength routing, and wavelength-translation capabilities.
Wavelength-allocated bandwidth will provide the capacity necessary for large users, who will need ubiquitous access throughout the country via optical virtual private networks. Wavelength routing will enable diverse point-to-point and point-to-multipoint optical payload transport for users in regional networks. Wavelength translation will enable optical services to traverse multiple carriers, regardless of vendor technologies. It enhances optical-network topology designs by allowing a wavelength frequency to be selected at one port and switched over to another frequency for acceptance by another port in the network.
Architecture
The key elements of the all-optical network architecture are dense wavelength division multiplexers (DWDM), optical add-drop multiplexers (ADM), and optical cross-connects.
DWDM DWDM systems are already in operation today, particularly for long-distance and so-called next-generation networks. These systems multiplex channels on a single fiber to vastly increase their capacity for point-to-point applications. Current DWDM systems multiplex up to 16 SONET 2.5-Gbps signals into one 40-Gbps composite multiwavelength signal. Carriers are optimizing the best attributes of SONET ring protection and survivability with DWDM virtual capacity. This allows them to maintain existing SONET rings by deploying DWDM as fiber expansion instead of using DWDM for network replacement. With DWDM systems now in deployment, the next step is to internetwork them with a variety of tributary interfaces such as asynchronous transfer mode (ATM), Internet Protocol (IP), and Gigabit Ethernet. After that, more diverse wide-area network topologies must be developed, such as mesh and ring.
ADM Optical ADMs are being integrated into DWDM-based networks. This type of multiplexer enables carriers to access wavelength-based services and create route diversity for network topologies, and also provide a migration path to optical ADM rings. In conjunction with ADMs, rings provide a fail-safe means of surviving the most severe disasters. ADMs work together to take traffic off the affected ring segment and place it onto another ring with spare capacity.
CROSS-CONNECTS Optical cross-connect systems give carriers more options in building network topologies consistent with today’s telecommunications infrastructure, including mesh, ring, and star. Mesh designs, for example, make it possible to build network topologies with enhanced survivability. Essentially, optical cross-connects let carriers establish mesh designs at the optical layer, regardless of payload or service application. Optical cross-connects will complement optical ADM by centralizing all network topologies.
Transparency
The new paradigm for optical-network services also includes the concept of transparency—the use of the light path itself as the transmission medium, which eliminates the need for optical-to-electrical conversions in the network.
Existing transmission networks—which consist of fiber, DWDM multiplication products, and SONET transmission equipment—are subject to potential congestion because they require optical signals to be converted to electrical signals and then converted back into optical. Congestion can be relieved by eliminating the need for these conversions and by using the light path as the transport medium rather than the fiber.
Sycamore Networks Inc., for example, envisions a network that will at first coexist with SONET and DWDM equipment, before phasing into an all-optical network. The first phase will be optical-networking products aimed at increasing fiber capacity to relieve congestion.
To fulfill its vision, Sycamore’s first products in 1999 will be for distances of less than 500 km, for such applications as corporate Internet access and virtual private networks. Sycamore’s plan is for the carrier networks to evolve into first-generation optical networks and what the company calls Lambda, or light-path networking, where services are mapped to light paths, optical/electrical conversions are eliminated, and wide-area network services are delivered at local-area network speeds. In future rollouts, Sycamore will introduce products for distances of 1,000 km and then 10,000 km.
Last Word
All-optical networks are essential to provide the bandwidths required for the future. Improved technology in optical components—such as lasers, amplifiers, and filters—will enable information to be sent over longer distances and at lower cost. Being able to use light waves that can flow freely without having to be converted into electrical energy and then back again will enable carriers to offer OC-3 service (155 Mbps) at the price of T3 (45 Mbps) service. This transparency also provides additional advantages, such as reduced delay through the elimination of optical-electrical conversion, reduced cost due to a need for fewer system components, and added flexibility carriers will have in service provisioning and management. See Also Next Generation Networks, Synchronous Optical Network
