Solutions to Chapter 4 - Communication Networks

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Communication Networks (2 nd Edition) Chapter 4 Solutions airport (“somewhere on the third level of parking lot A”). Sketch the sequence of events that take place to set up this call. (Don’t concern yourself with the specifics of the conversation.) Bob’s Mobile Station Base Station L. A. MSC Phone Switch Telephone Network Visitor’s location register Wife’s phone c. In the meantime, Bob’s college roommate, Kelly, who now works in Hollywood calls Bob’s cell phone. Note that Bob’s cell phone begins with the Chicago area code. Sketch the sequence of events that take place to set up this call. Should this call be billed as local or long distance Kelly’s long-distance service provider sees Bob’s Chicago area code and proceeds with connecting her to Bob’s Chicago cellular network. When the call arrives at Bob’s cellular network (in the form of signaling traffic), Bob’s home MSC checks its home location register and sees that Bob is in LA (recall that it found out about his whereabouts in part a)). The call is then forwarded back to LA where the connection can be made via Bob’s temporary MSC and its visitors’ location register. Whether Bob or Kelley should be billed long distance depends on the setup of Bob’s “roaming service”. If the three networks involved (two cellular and one wireline) are coordinated enough, Kelly’s final connection could be routed directly from Hollywood to Bob’s mobile station directly. In this case it could be possible to bill them both locally. However, it is more likely that the connection will actually be made from Kelly to Bob’s home network and then back to L. A. By dialing long distance, Kelly implicitly agrees that she will pay the long distance charges for a call to Chicago. By using his phone in L. A. Bob expects to pay long distance fees for all calls he receives (even local ones) because if someone from Chicago calls him, the call must be carried by a long distance carrier. In this case, both Bob and Kelly will pay long distance charges. She will pay the same as if she were dialing a regular Chicago number (which she is), and Bob will pay as if he is receiving a call from a Chicago caller, say, his wife. 70. Compare cellular wireless networks to the local loop and to the coaxial cable television system in terms of their suitability as an integrated access network. In particular comment on the ability to support telephone service, high-speed Internet access, and digital television service. Consider the following two cases: In question 45, the local loop and coaxial cable network were compared in terms of their ability to act as integrated access networks. Here, we examine the ability of cellular wireless networks to provide these services. Each type of traffic on an integrated access network has very different characteristics. Digital television requires high bandwidth and low loss. Although the signal can be compressed using Leon-Garcia/Widjaja 34
Communication Networks (2 nd Edition) Chapter 4 Solutions MPEG encoding, the compressed signal still requires a minimum of 1-2 Mbps. Service can most efficiently be provided in a broadcast network because it lacks the interactivity of phone and Internet, and each user can be sent the same information. Telephone service is less constrained by bandwidth. Because the vast majority of calls are between two users, point-to-point networks are well suited for carrying phone traffic. Current high speed Internet access has medium bandwidth requirements. IP traffic can also be adaptive, in the sense that if need be, the connection can usually be slowed down. This affects the quality of service, but unlike the other two (phone and TV), it does not interrupt service completely. a. The band of frequencies available spans 300 MHz. Current cellular networks have only moderate bandwidth. Cellular networks share the same scalability problem as the coaxial television network. If the band of frequencies in the network span 300 MHz (about 10x the current band), the bandwidth available to each user is inversely proportional to the number of users. Of course cells can be made small, and CDMA can be used so that the reuse factor is 1, but a bottleneck still exists. Clearly, the cellular network can support telephone traffic, since it currently already does. Successful efforts have been made to provide Internet service on mobile cellular devices, but due to the bandwidth restriction mentioned, the service quality is moderate. New protocols are currently being defined that reduce bandwidth for Internet services that run on devices of these networks. It would be difficult to provide digital television using the above-described cellular network. Because of the high bandwidth required, by partitioning the network into a number of parallel channels, a broadcast network could provide digital television service. b. The band of frequencies available spans 2 GHz. If the band of frequencies available spanned 2 Ghz, all of the above obstacles would be significantly reduced. Since the aggregate bandwidth of this system is 100 times that of the current cellular networks, most of the problems mentioned above would melt away. With a rough calculation, assuming each cell has on the order of 100 users, and the frequency reuse factor is 1, we see that a bandwidth on the order of 10 Mbps can be provided to each user (or to each channel in a 100-station digital television network). This is comparable to current ADSL in the local loop network, and would likely be sufficient to serve as an integrated access network. A difficulty with this scenario is that the region of spectrum where 2 GHz is available would be in the 28 GHz band or beyond. These frequencies tend to be suitable to point-to-point or point-tomultipoint but not to omnidirectional communications. Leon-Garcia/Widjaja 35
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Solutions to Chapter 4 - Communication Networks

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