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1. In LDP the hallo adjacency as well as the session have to be continuously refreshed. Since the hallo adjacencies in a session are continuously refreshed, why is there a need to also refresh the session?
2. Explain the need for loosely explicit routes in CR-LDP. Give an example of an application that requires pinning.
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 mechanism(s) 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.
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 x 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 optical fiber. Each wavelength is a different channel. Conceptually, WDM is the same as frequency division multiplexing (FDM), which is used in microwave radio and satellite 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 Xiy 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
OPTICAL FIBERS AND COMPONENTS
Optical ' [/ Optical
fiber In-line fiber Preamplification 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 x 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 Xt of input port k to the outgoing wavelength Xt of output port m. If it is equipped with converters, it can also switch the optical signal of the incoming wavelength Xt to another outgoing wavelength Xj.