The RSVP-TE New Objects

180 LABEL DISTRIBUTION PROTOCOLS 3. Could CR-LDP work using unsolicited downstream label allocation and independent order? Why? 4. Consider the traffic parameters for the delay sensitive service class given in Table 7.1. Do these parameters suffice to provide this service? What additional mechanisms is are required? 5. Explain why in RSVP the Path message contains the RSVP HOP object. 6. Explain the difference between the fixed-filter style and the shared explicit style. 7. Is it possible for RSVP-TE to set up a CR-LSP based on the next hop routing information? How? 8. Compare CR-LDP with RSVP-TE. What are the common features in these two protocols? Identify some of the main differences in these two protocols. 8 Optical Fibers and Components This chapter deals with the physical layer of wavelength division multiplexing WDM optical networks. We first give a general overview of WDM optical networks. We then proceed to describe how light is transmitted through an optical fiber. Specifically, we discuss the index of refraction, step-index and graded-index optical fibers, multi-mode and single mode optical fibers, and various optical effects that occur when light is transmitted through an optical fiber, known as impairments. Finally, we conclude this chapter by describing some of the components used in WDM optical networks, such as lasers, optical amplifiers, 2 × 2 couplers and star couplers, and optical cross-connects OXCs. This chapter, somewhat stretches the intended scope of this book, which focuses on layers higher than the physical layer. However, due to the novelty of optical networks, it is important to have some knowledge of the underlying WDM technology. It is not necessary to read this chapter in detail in order to understand the subsequent chapters on optical networks. The key sections to study are the introductory section, Section 8.1 and the section on components Section 8.3.

8.1 WDM OPTICAL NETWORKS

WDM refers to the technology of combining multiple wavelengths onto the same opti- cal fiber. Each wavelength is a different channel. Conceptually, WDM is the same as frequency division multiplexing FDM , which is used in microwave radio and satel- lite systems. A typical point-to-point connection is shown in Figure 8.1. At the transmitting end, there are W independent transmitters. Each transmitter Tx is a light source, such as a laser, and is independently modulated with a data stream. The output of each transmitter is an optical signal on a unique wavelength λ i , i = 1, 2, . . . , W . The optical signals from the W transmitters are combined into a single optical signal at the wavelength multiplexer and transmitted out onto a single optical fiber. At the other end, the combined optical signal is demultiplexed into the W individual signals, and each one is directed to the appropriate receiver Rx, where it is terminated and converted to the electric domain. Amplification is used immediately after the wavelength multiplexer and before the wavelength demultiplexer. Also, if the fiber is very long, the signal is further amplified using in-line amplifiers. As can be seen, this point-to-point system provides W independent channels, all on the same fiber. As the WDM technology improves, the number of wavelengths that can Connection-oriented Networks Harry Perros  2005 John Wiley Sons, Ltd ISBN: 0-470-02163-2 182 OPTICAL FIBERS AND COMPONENTS In-line amplification Optical fiber Optical fiber l 1 l W l 1 l W Tx Wavelength multiplexer Power amplifier Tx Rx Rx Wavelength demultiplexer Pre- amplifier • • • • • • Figure 8.1 A WDM point-to-point link. be transmitted onto the same fiber increases as well. Thus, the capacity of a link can be increased by utilizing the WDM technology rather than adding new fibers. The latter solution is significantly more expensive than the upgrading of components necessary for the introduction of WDM. More complex WDM optical networks can be built using optical cross-connects OXC. An OXC is an N × N optical switch, with N input fibers and N output fibers. The OXC can switch optically all of the incoming wavelengths of the input fibers to the outgoing wavelengths of the output fibers, assuming no external conflicts at the output fibers. For instance, it can switch the optical signal on incoming wavelength λ i of input port k to the outgoing wavelength λ i of output port m. If it is equipped with converters, it can also switch the optical signal of the incoming wavelength λ i to another outgoing wavelength λ j . An OXC can also be used as an optical adddrop multiplexer OADM. That is, it can terminate the optical signal of a number of incoming wavelengths and insert new optical signals on the same wavelengths in an output port. The remaining wavelengths are switched as described above. An OXC can switch wavelengths in a static or dynamic manner. In the static case, the OXC is configured to switch permanently the incoming wavelengths to the outgoing wavelength. In the dynamic case, the OXC will switch a particular incoming wavelength to an outgoing wavelength on demand. An OADM can also adddrop wavelengths either in a static manner or dynamically i.e., on demand. Mesh network Ring 1 Ring 2 Ring 4 Ring 3 Figure 8.2 An example of an optical network. HOW LIGHT IS TRANSMITTED THROUGH AN OPTICAL FIBER 183 A typical WDM optical network, as operated by a telecommunication company, con- sists of WDM metro i.e., metropolitan rings, interconnected by a mesh WDM optical network, i.e. a network of OXCs arbitrarily interconnected. An example of such a network is shown in Figure 8.2. There are many different types of optical components used in a WDM optical network, and some of these components are described in Section 8.3. We now proceed to examine some of the basic principles of light transmission through an optical fiber.

8.2 HOW LIGHT IS TRANSMITTED THROUGH AN OPTICAL FIBER

Light radiated by a source can be seen as consisting of a series of propagating electromag- netic spherical waves see Figure 8.3. Along each wave, one can measure the electric field, indicated in Figure 8.3 by a dotted line, which is vertical to the direction of the light. The magnetic field not shown in Figure 8.3 is perpendicular to the electric field. The intensity of the electrical field oscillates following a sinusoidal function. Let us mark a particular point, say the peak, on this sinusoidal function. The number of times that this particular point occurs per unit of time is called the frequency. The frequency is measured in Hertz. For example, if this point occurs 100 times, then the frequency is 100 Hertz. An electromagnetic wave has a frequency f , a speed v, and a wavelength λ . In vacuum or in air, the speed v is approximately the speed of light which is 3 × 10 8 meterssec. The frequency is related to the wavelength through the expression: v = f λ. An optical fiber consists of a transparent cylindrical inner core which is surrounded by a transparent cladding see Figure 8.4. The fiber is covered with a plastic protective cover. Both the core and the cladding are typically made of silica SiO 2 , but they are made so that to have different index of refraction. Silica occurs naturally in impure forms, such as quartz and sand. The index of refraction, known as the refractive index, of a transparent medium is the ratio of the velocity of light in a vacuum c to the velocity of light in that medium v, that Source Electric field Wave Figure 8.3 Waves and electrical fields. Core Cladding Cladding Figure 8.4 An optical fiber.