As the optical network evolves, network planners must understand a dilemma in best utilizing the optical network. On the one hand, access networks require a transparent optical network that is bit-rate and format independent. This would provide flexibility and allow connection to the network directly with asynchronous transfer mode (ATM), transmission control protocol/Internet protocol (TCP/IP), SONET, or any other signal format without additional equipment costs. It also would allow wavelengths be added and dropped completely optically without affecting the original signal format.

Unfortunately, this transparent model for the access network falls completely apart when applied to metropolitan or long-haul networks. As the distances increase, carriers need to maximize the capacity to reduce costs, and allow any signal data rate onto the network would greatly increase costs. Therein lies the dilemma: networks need the flexibility to provide a variety of end-user services without inefficiencies in the long-haul network. The solution is the optical gateway, which will integrate with existing optical-network elements.

Discussed below are some of the optical-network elements that make end-to-end wavelength services a reality, and how they will be integrated into the network.

Dense Wavelength Division Multiplexing

As discussed earlier, many network providers are already deploying DWDM on a large scale. Fiber-congested, point-to-point segments of long-distance networks were one of the first applications for WDM terminals. Today, 16-channel dense WDM terminals are widely deployed to enhance the bandwidth capacity of the long-haul network backbone. Throughout 1998, the industry will shift to 32 and 40 channel systems, and in following years even more.

Optical Add/Drop Multiplexers (OADM)

The OADM enhances the WDM terminals by adding several significant features (see Figure 16). The OADM systems have the capacity of up to 40 optical wavelengths. They efficiently drop and add various wavelengths at intermediate sites along the network—resolving a significant challenge for existing WDM.

Figure 16. Optical ADM Functionality

Most important, OADM technology introduces asynchronous transponders to allow the optical-network element to interface directly to high revenue–generating services. It is now possible for ATM, frame relay (FR), native local-area networks (LANs), high-bandwidth Internet protocol (IP), and others to connect directly to the network via a wavelength in the optical layer. Transponder technology also extends the life of older lightwave systems by accepting its bandwidth directly into the optical layer, converting its frequency to an acceptable standard, and providing protection and restoration. The OADM also is the foundation of optical bidirectional line switched rings (OBLSRs), which are described in the next module.

Optical Gateways

In order to access the optical network efficiently and maximize bandwidth capacity and transport-protocol transparency, the optical gateway becomes a critical network element (see Figure 17). As a variety of bit rates and signal formats, ranging from asynchronous legacy networks to 10–Gbps SONET systems, a common transport structure must groom and provision traffic entering the optical layer. The emerging basic format for high-speed transparent transport is ATM, and optical gateways will allow a mix of standard SONET and ATM services. By providing a link between the variety of electrical protocols and allowing flexible deployment of any mix of them, optical gateways provide networks the maximum benefits of optical networks.

The optical gateway will be the key element to allow smooth transition to optical networks. As more intelligence is added to the optical layer, costs can be reduced in the SONET layer. For example, as optical rings are implemented, the optical gateway can interface lower cost 1:N protected SONET system with the optical ring. By partitioning wavelengths, existing SONET rings can be kept intact, while new systems are lower-cost integrated 1:N tributaries to the optical layer.

Figure 17. Optical Gateways