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Optical Networks: A Practical Perspective - Part 24. 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. | 200 Components Table 3.3 Applications for optical switches and their switching time and port count requirements. Application Switching Time Required Number of Ports Provisioning 1-10 ms 1000 Protection switching 1-10 ms 2-1000 Packet switching 1 ns 100 External modulation 10 ps 1 possible and based on the scheme used the number of switch ports needed may vary from two ports to several hundreds to thousands of ports when used in a wavelength crossconnect. Switches are also important components in high-speed optical packet-switched networks. In these networks switches are used to switch signals on a packet-by-packet basis. For this application the switching time must be much smaller than a packet duration and large switches will be needed. For example a 53-byte packet one cell in an ATM network at 10 Gb s is 42 ns long so the switching time required for efficient operation is on the order of a few nanoseconds. Optical packet switching is still in its infancy and is the subject of Chapter 12. Yet another use for switches is as external modulators to turn on and off the data in front of a laser source. In this case the switching time must be a small fraction of the bit duration. So an external modulator for a 10 Gb s signal with a bit duration of 100 ps must have a switching time or equivalently a rise and fall time of about 10 ps. In addition to the switching time and the number of ports the other important parameters used to characterize the suitability of a switch for optical networking applications are the following 1. The extinction ratio of an on-off switch is the ratio of the output power in the on state to the output power in the off state. This ratio should be as large as possible and is particularly important in external modulators. Whereas simple mechanical switches have extinction ratios of 40-50 dB high-speed external modulators tend to have extinction ratios of 10-25 dB. 2. The insertion loss of a switch is the fraction of power usually expressed in .
