Solutions to Chapter 4 - Communication Networks

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Communication Networks (2 nd Edition) Chapter 4 Solutions 17. Consider the synchronous multiplexing in Figure 4.15. Explain how the pointers in the outgoing STS-1 signals are determined. Solution: The section and line overhead of each incoming STS-1 signal is terminated and its payload is mapped into a new STS-1 frame that is synchronous to a local clock. The outgoing pointers are all adjusted according to a common clock such that all the outgoing STS-1 frames are synchronized with respect to each other. 18. Draw a sketch to explain the relationship between a virtual tributary and the synchronous payload envelope. Show how 28 T-1 signals can be carried in an STS-1. Solution: Seven tributaries are multiplexed in stream. Each tributary carries four T-1 channels and occupies twelve columns of the SPE, as shown below: 87 Columns T 1 T 2 T 3 T 4 T 5 T 6 T 7 9 rows t The first column is path overhead. The next 84 columns are the seven tributaries and the last two columns are unused. 19. Do a web search for DWDM equipment available from telecommunications equipment vendors. What specifications do you see in terms of number of channels, spacing between channels, bit rates and formats of the signals that can be carried Solution: Some specs for WDM and DWDM multiplexers: Wavelength: 632- 820 nm, 980-1064 nm, 980-1480 nm, 980-1550 nm, 1064-1310 nm, 1310-1550 nm, 1310-1625 nm, 1480-1550 nm, 1550-1625 nm, etc. Channel Spacing: 50 GHz (0.1 nm spacing between channels), 100 GHz (0.2 nm spacing between channels), 200 GHz (0.4 nm spacing between channels), etc. Per channel bit rates: 100 Mbps, 1.2 Gbps, 2.4 Gbps, 10 Gbps, etc. Number of Channels: 4, 8, 16, 32, 80, 160 20. Consider WDM systems with 100, 200, and 400 wavelengths operating at the 1550 nm region and each carrying an STS-48 signal. Solutions follow questions: Leon-Garcia/Widjaja 10
Communication Networks (2 nd Edition) Chapter 4 Solutions a. How close do these systems come to using the available bandwidth in the 1550 nm range One STS-48 signal carries 2488.32 Mbps of traffic. Thus, the aggregate bit rate of each of these systems is: System Bit rate 100 wavelengths 248.832 Gbps 200 wavelengths 497.664 Gbps 400 wavelengths 995.328 Gbps The available bandwidth of this window is 12.49 THz. Thus, none of these systems even comes close to using all of the available bandwidth. b. How many telephone calls can be carried by each of these systems How many MPEG2 television signals The number of telephone calls (64 Kbps) and MPEG2 video signals (5 Mbps) that can be carried each system is shown below: System Telephone Calls MPEG2 Video Signals 100 wavelengths 3.888 × 10 6 49 766 200 wavelengths 7.776 × 10 6 99 532 400 wavelengths 15.552 × 10 6 199 065 Although, none of the systems comes close to using all of the available bandwidth, they can still carry huge amounts of traffic. This shows how WDM provides the potential for huge amounts of bandwidth and radically expands the bandwidth of existing optical links. 21. Consider the SONET fault protection schemes described earlier in the chapter. Explain whether these schemes can be used with WDM rings. Solution: The fault protection schemes described in problem 15 are applicable to any ring network structure. To use these schemes in WDM, the exact same procedures can be carried out except that in the case of WDM, instead of switching streams, wavelengths are switched. Furthermore, the switching can be done in optical domain by all optical protection switches that detect the signals and switch to protection signal instantaneously. Optical protection switching can be faster and more efficient. 22. Calculate the spacing between the WDM component signals in Figure 4.8. What is the spacing in hertz and how does it compare to the bandwidth of each component signal Solution: In Figure 4.8 there are 16 wavelengths distributed equally over the spectrum from 1545 nm to 1557 nm. Thus the spacing between each wavelength is: 1557 −1545 = 0.75nm 16 Leon-Garcia/Widjaja 11
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Solutions to Chapter 4 - Communication Networks

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