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Unformatted text preview: 26 Introduction to Optical Networks The mission of optical packet switching is to enable packet-switching capabilities at rates that cannot be contemplated using electronic packet switching. However, designers are handicapped by several limitations with respect to processing signals in the optical domain. One important factor is the lack of optical random access memory for buffering. Optical buffers are realized by using a length of fiber and are just simple delay lines, not fully functional memories. Packet switches include a high amount of intelligent real-time software and dedicated hardware to control the network and provide quality-of-service guarantees. These functions are difficult to perform in the optical domain. Another factor is the relatively primitive state of fast optical-switching technology, compared to electronics. For these reasons, optical packet switching is not yet viable commercially, though many of their aspects have been demonstrated in research laboratories. Chapter 12 covers all these aspects in detail. 1.7 Transmission Basics In this section, we introduce and define the units for common parameters associated with optical communication systems. 1.7.1 Wavelengths, Frequencies, and Channel Spacing When we talk about WDM signals, we will be talking about the wavelength, or frequency, of these signals. The wavelength and frequency f are related by the equation c = f , where c denotes the speed of light in free space, which is 3 10 8 m/s. We will reference all parameters to free space. The speed of light in fiber is actually somewhat lower (closer to 2 10 8 m/s), and the wavelengths are also correspondingly different. To characterize a WDM signal, we can use either its frequency or wavelength interchangeably. Wavelength is measured in units of nanometers (nm) or microme- ters ( m or microns). 1 nm = 10 9 m, 1 m = 10 6 m. The wavelengths of interest to optical fiber communication are centered around 0.8, 1.3, and 1.55 m. These wavelengths lie in the infrared band, which is not visible to the human eye. Frequen- cies are measured in units of hertz (or cycles per second), more typically in megahertz ( 1 MHz = 10 6 Hz), gigahertz ( 1 GHz = 10 9 Hz), or terahertz (...
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This note was uploaded on 01/15/2011 for the course ECE 6543 taught by Professor Boussert during the Spring '09 term at Georgia Institute of Technology.
- Spring '09