WDM Technology and Issues in WDM Optical Networks
The influence of "networking" on the organization of computer systems has been tremendous, especially in the last 20 years. The old model of a single computer catering to the computational needs of an organization (company or university) has been replaced by one in which a number of separate but interconnected computers carry out the job. Broadly speaking, a computer network is an interconnected (via copper cable, fiber optics, microwaves, or satellites) collection of independent computers that aids communication in numerous ways. Apart from providing a good communication medium, sharing of available resources (programs and data on a computer are available to anyone on the network, without regard to the physical location of the computer and user), improved reliability of service (because of the presence of multiple computers), and cost-effectiveness (as small computers have better price/performance ratio than large ones) are some of the advantages of networking. From the time the ARPANET1 was conceived to the current-day high-speed networks, the design and technology associated with this (computer networks) field have come a long way. The need for error-free, high-bandwidth communication channels has been on the rise. The services provided by computer networks include remote information access (communication between a person and a remote databasefor example, World Wide Web browsing) and electronic mail (person-to-person communication) used by millions of people around the globe. The explosive growth of the Internet and bandwidth-intensive applications such as video-on-demand (for example, selecting a movie located at some remote site and watching it online) and multimedia conferencing (which requires setting up high-bandwidth connections among different people, for a virtual meeting, and guaranteeing the desired quality-of- service [QoS] levelshigh bandwidth, low latency, and reasonable packet loss ratefor the virtual meeting) require high-bandwidth transport networks whose capacity (bandwidth) is much beyond what current high-speed networks such as asynchronous transfer mode (ATM)2 networks can provide. Thus, a continuous demand for networks of high capacities at low costs is seen now. This can be achieved with the help of optical networks, as the optical fiber provides an excellent medium for transfer of huge amounts of data (nearly 50 terabits per second [Tb/s]). Apart from providing such huge bandwidth, optical fiber has low cost (approximately $0.30 per yard), extremely low bit error rates (fractions of bits that are received in error, typically 10-12, compared to 10-6 in copper cable), low signal attenuation (0.2 decibels per kilometer [dB/km]), low signal distortion, low power requirement, low material use, and small space requirement. In addition, optical fibers are more secure, compared to copper cables, from tapping (as light does not radiate from the fiber, it is nearly impossible to tap into it secretly without detection) and are also immune to interference and crosstalk. Optical networks, employing wavelength division multiplexing (WDM), is seen as the technology of the future for a variety of other reasons which we will mention in Section 1.3.