In the early 1980s, a revolution in telecommunications networks began that was spawned by the use of a relatively unassuming technology, fiber-optic cable. Since then, the tremendous cost savings and increased network quality has led to many advances in the technologies required for optical networks, the benefits of which are only beginning to be realized.

History

Telecommunication networks have evolved during a century-long history of technological advances and social changes. The networks that once provided basic telephone service through a friendly local operator are now transmitting the equivalent of thousands of encyclopedias per second. Throughout this history, the digital network has evolved in three fundamental stages: asynchronous, synchronous, and optical.

Asynchronous

The first digital networks were asynchronous networks. In asynchronous networks, each network element's internal clock source timed its transmitted signal. Because each clock had a certain amount of variation, signals arriving and transmitting could have a large variation in timing, which often resulted in bit errors.

More importantly, as optical-fiber deployment increased, no standards existed to mandate how network elements should format the optical signal. A myriad of proprietary methods appeared, making it difficult for network providers to interconnect equipment from different vendors.

Synchronous

The need for optical standards led to the creation of the synchronous optical network (SONET). SONET standardized line rates, coding schemes, bit-rate hierarchies, and operations and maintenance functionality. SONET also defined the types of network elements required, network architectures that vendors could implement, and the functionality that each node must perform. Network providers could now use different vendor's optical equipment with the confidence of at least basic interoperability.

Optical

The one aspect of SONET that has allowed it to survive during a time of tremendous changes in network capacity needs is its scalability. Based on its open-ended growth plan for higher bit rates, theoretically no upper limit exists for SONET bit rates. However, as higher bit rates are used, physical limitations in the laser sources and optical fiber begin to make the practice of endlessly increasing the bit rate on each signal an impractical solution. Additionally, connection to the networks through access rings has also had increased requirements. Customers are demanding more services and options and are carrying more and different types of data traffic. To provide full end-to-end connectivity, a new paradigm was needed to meet all the high-capacity and varied needs. Optical networks provide the required bandwidth and flexibility to enable end-to-end wavelength services (see Figure 1).

Figure 1. End-to-End Wavelength Services

Optical networks began with wavelength division multiplexing (WDM), which arose to provide additional capacity on existing fibers. Like SONET, defined network elements and architectures provide the basis of the optical network. However, unlike SONET, rather than using a defined bit-rate and frame structure as its basic building block, the optical network will be based on wavelengths. The components of the optical network will be defined according to how the wavelengths are transmitted, groomed, or implemented in the network. Viewing the network from a layered approach, the optical network requires the addition of an optical layer. To help define network functionality, networks are divided into several different physical or virtual layers. The first layer, the services layer, is where the services—such as data traffic—enter the telecommunications network. The next layer, SONET, provides restoration, performance monitoring, and provisioning that is transparent to the first layer.

Emerging with the optical network is a third layer, the optical layer. Standards bodies are still defining the optical layer, but it will eventually provide the same functionality as the SONET layer, while operating entirely in the optical domain. The optical network also has the additional requirement of carrying varied types of high bit-rate nonSONET optical signals that bypass the SONET layer altogether. Just as the SONET layer is transparent to the services layer, the optical layer will ideally be transparent to the SONET layer, providing restoration, performance monitoring, and provisioning of individual wavelengths instead of electrical SONET signals.