tailieunhanh - Optical Networks: A Practical Perspective - Part 7

Optical Networks: A Practical Perspective - Part 7. This book describes a revolution within a revolution, the opening up of the capacity of the now-familiar optical fiber to carry more messages, handle a wider variety of transmission types, and provide improved reliabilities and ease of use. In many places where fiber has been installed simply as a better form of copper, even the gigabit capacities that result have not proved adequate to keep up with the demand. The inborn human voracity for more and more bandwidth, plus the growing realization that there are other flexibilities to be had by imaginative use of the fiber, have led people. | 30 Introduction to Optical Networks Wavelength nm Figure The 100 GHz ITU frequency grid based on a reference frequency of THz. A 50 GHz grid has also been defined around the same reference frequency. a bit rate of 56 kb s over this phone line. This ratio of bit rate to available bandwidth is called spectral efficiency. Optical communication systems use rather simple modulation techniques that achieve a spectral efficiency of about bits s Hz and it is reasonable to assume therefore that a signal at a bit rate of 10 Gb s uses up bandwidth of approximately 25 GHz. Note that the signal bandwidth needs to be sufficiently smaller than the channel spacing otherwise we would have undesirable interference between adjacent channels and distortion of the signal itself. Wavelength Standards WDM systems today primarily use the m wavelength region for two reasons the inherent loss in optical fiber is the lowest in that region and excellent optical amplifiers are available in that region. We will discuss this in more detail in later chapters. The wavelengths and frequencies used in WDM systems have been standardized on a frequency grid by the International Telecommunications Union ITU . It is an infinite grid centered at THz a segment of which is shown in Figure . The ITU decided to standardize the grid in the frequency domain based on equal channel spacings of 50 GHz or 100 GHz. Observe that if multiple channels are spaced apart equally in wavelength they are not spaced apart exactly equally in frequency and vice versa. The figure also shows the power spectrum of two channels 400 GHz apart in the grid populated by traffic-bearing signals as indicated by the increased signal bandwidth on those channels. The ITU grid only tells part of the story. Today we are already starting to see systems using 25 GHz channel spacings. We are also seeing the use of several transmission bands. The early WDM systems .