Solutions to Chapter 4 - Communication Networks

More documents

Recommendations

Info

Communication Networks (2 nd Edition) Chapter 4 Solutions a. Compare the star-star topology with a star-ring topology and a ring-ring topology. Explain how information flows in these topologies and consider issues such as efficiency in use of bandwidth and robustness with respect to faults. Star topologies generally provide efficient use of bandwidth because their hierarchical routing structure provides near shortest path routing. However, in situations with low load, ring networks are more efficient. When a user is not transmitting any information in a star topology, the bandwidth of the line to the user is wasted. In a ring topology, since the bandwidth is shared, active users can make use of the extra available bandwidth provided by the inactive users. In general, pure star topologies provide no mechanism for fault line recovery because there is not backup path. Thus, this architecture should be used if the connections are very reliable. The three topologies under examination are shown below. The star-star topology is efficient when the load on each of the lines is high. If one of the lines on a high tier star fails, service for many users is lost. Thus, such architecture is only good if the lines are very reliable. The star-ring topology is good when the traffic produced by each user is relatively sporadic, but the traffic in the “star” links is high. This situation could arise if the bandwidth provided in “star” links is limited in capacity. It still has the same vulnerability as the star-star network in term of large scale loss of service if one of the high-tier links fails. The ring-ring topology is very robust to faults because every path can be backed up by forming the backup path around the ring in the other direction. In every link in the network, bandwidth is shared and therefore used efficiently. b. What role would SONET transmission systems play in the above topologies SONET transmission systems could be used in the architectures described. It would be particularly useful in the rings to provide OAM functions and fault protection. 47. Suppose that the local loop is upgraded so that optical fiber connects the central office to the pedestal and twisted pair of length at most 1000 feet connects the user to the pedestal. a. What bandwidth can be provided to the user Using ADSL a bandwidth of 8 Mbps can be provided. Since the twisted pair length is only 1000 feet, VDSL may be used. In this case, a bandwidth of 55 Mbps can be provided. b. What bit rate does the optical fiber have to carry if each pedestal handles 500 users Leon-Garcia/Widjaja 22
Communication Networks (2 nd Edition) Chapter 4 Solutions If ADSL is used, and each fiber handles 500 users, the total bandwidth on the fiber must be at least 500 × 8 Mbps = 4 Gbps. If VDSL is used, the bandwidth increases to 55 Mbps x 500 = 27.5 Gbps. c. How can SONET equipment be used in this setting SONET can be used at the physical layer in the optical fiber so that the multiplexing of the 500 lines is simplified using tributaries. This functionality could be extended to the pedestal to provide OAM control of the large aggregate bandwidth connecting it to the central office. 48. Let’s consider an approach for providing fiber-to-the-home connectivity from the central office to the user. The telephone conversations of users are time-division multiplexed at the telephone office and broadcast over a “passive optical network” that operates as follows. The TDM signal is broadcast on a fiber up to a “passive optical splitter” which transfers the optical signal to N optical fibers that are connected to N users. Each user receives the entire TDM signal and retrieves its own signal. Solutions follow questions: a. Trace the flow of PCM samples to and from the user. What bit rates are required if N=10, 100 Compare these to the bit rate that is available. Because a passive splitter is used, the fiber must have enough bandwidth to support N connections. Thus, N = 10 640 kbps minimum capacity N = 100 6.4 Mbps Currently bit rates of 2.5 Gbps on a single wavelength fiber is common and 10 Gbps is available. b. Discuss how Internet access service might be provided using this approach. Internet access could be very simply provided by replacing the PCM samples with Internet traffic. c. Discuss how cable television service might be provided using this approach. Since MPEG2 television signals are 2-5 Mbps, on the order of 1000 channels could be provided by this network to each user. Each TDM slot could correspond to one channel. The users could change channels by resynchronizing their receivers to the appropriate time slot. d. What role could WDM transmission play in the connection between the central office and the optical splitter in the connection all the way to the user If WDM is used, the aggregate bandwidth of the optical fiber would be greatly increased. Because the architecture is a passive broadcast system, if WDM were used between the central office and the passive splitter, it would also be on the fiber to the user. The additional bandwidth might still be leveraged in a number of ways. The additional wavelengths provide an additional degree of multiplexing flexibility. For example, the users could tune into one wavelength at a time (with tuning done on a relatively large time scale) and each wavelength could carry a “cable package“ that includes a subset of the provided channels. Although WDM provides increased bandwidth, a WDM receiver typically represents significantly higher hardware costs. If the huge amounts of available bandwidth were used to carry heterogeneous traffic, the hardware costs required would be offset by the benefits. The resulting network could carry television, telephone, and Internet traffic and, in effect, replace all of the currently existing access networks. Leon-Garcia/Widjaja 23
Page 1 and 2: Communication Networks (2 nd Editio
Page 21: Communication Networks (2 nd Editio
Page 35: Communication Networks (2 nd Editio

Solutions to Chapter 4 - Communication Networks

Create successful ePaper yourself

Delete template?

Save as template?