Solutions to Chapter 4 - Communication Networks

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Communication Networks (2 nd Edition) Chapter 4 Solutions If the two telephones are in different LATAs then the digital voice signal must traverse tandem switches that connect the two LATAs. c. In parts (a) and (b) identify the points at which a call request during setup can be blocked because resources are unavailable. Call requests can be blocked when there are insufficient resources to complete the call. These resources can include time slots and crosspoints in the switches and time slots in the digital lines. Blocking can also result from unavailability of signaling processing equipment. 41. Suppose that an Internet service provider has a pool of modems located in a telephone office and that a T-1 digital leased line is used to connect to the ISP’s office. Explain how the 56K modem (that was discussed in Chapter 3) can be used to provide 56 kbps transfer rate from the ISP to the user. Sketch a diagram showing the various equipment and facilities involved. Solution: The ISP office is connected to the user’s local central office, which is in turn connected to the user. The ISP can use the T-1 line to send digital information at a rate of 56 kbps into and across the telephone network to the line card that connects to the user. The line card can then deliver information at 56 kbps to the user. Recall that the user cannot send at 56 kbps because of limitations on the available SNR. Modem User’s comp Twisted Pair CO T-1 line ISP office Modem bank 42. Why does a conventional telephone still work when the electrical power is out Solution: The telephone company supplies each of its telephone lines with power at the central office. This power is stored in the form of wet batteries that can alternately be charged by a backup battery in the event of a power failure at the central office. These huge batteries occupy entire floors in telephone offices. 43. In Figure 4.44b, how does the network know which inter-exchange carrier is to be used to route a long distance call Solution: The inter-exchange carrier that handles the long-distance calls for a specific user is specified a priori by the user’s service contract. The user “signs-up” with one long distance plan over another, and it is the responsibility of the local service provider to route the call over the appropriate inter-exchange carriers. 44. ADSL was designed to provide high-speed digital access using existing telephone facilities. a. Explain how ADSL is deployed in the local loop. Leon-Garcia/Widjaja 20
Communication Networks (2 nd Edition) Chapter 4 Solutions An ADSL modem is required at each end of the user’s twisted pair connection. Since ADSL coexists with regular phone service, it uses higher frequencies for data transmission. Bandwidthlimiting elements, such as loading coils, must be removed. ADSL can only be offered within certain distances of the central office. b. What happens after the twisted pairs enter the telephone office At the telephone office, the voice and data are separated/filtered and the demultiplexed signals are routed accordingly (for example, voice may be routed over the phone network and data over the Internet). c. Can ADSL and ISDN services be provided together Explain why or why not. No, ADSL and ISDN use overlapping parts of the signal spectrum so they cannot operate simultaneously. 45. In this problem we compare the local loop topology of the telephone network with the coaxial cable topology of cable television networks (discussed in Chapter 3). a. Explain how telephone service may be provided using the cable television network. Since the television network is inherently a broadcast, shared medium, a method such as the cable modem method described in chapter 3 would be needed to accommodate the traffic of predominantly point-to-point bidirectional telephone service. The mechanism used would include a MAC protocol of some form to provide a fair method of partitioning the total available bandwidth among callers. b. Explain how cable television service may be provided using the local loop. It would be very difficult to provide cable television on the local loop network due to the huge bandwidth currently provided by the cable broadcast network. The only effective means of providing full cable service over the local loop would likely involve sending a single channel to each user. Changing channels would require an upstream request to the local switch, which would then replace the originally transmitted channel with the requested one. By moving the pedestal that connects to the home, it may be possible to provide sufficiently high speeds to offer several simultaneous channels to a home. c. Compare both topologies in terms of providing Internet access service. Both these networks currently provide Internet access. Cable modems have the advantage of providing huge peak bandwidth via that large bandwidth (up to 30 Mbps) of the cable network. However, as has been discovered by many subscribers to this service, since the cable network is a shared broadcast medium, the bandwidth provided does not scale well to many users. During peak hours, the total bandwidth of the network is divided among all of the users connected to the final branching point of the network. This leads to noticeable and significant decrease in service. Access to the Internet via the phone network using DSL modems has the advantage of providing a dedicated access line to its users. As a result, each user’s service access rate is independent of the number of users. The tradeoff is the lower bandwidths available. Currently ADSL typically provides bandwidth of up to 1 Mbps. 46. The local loop was described as having a star topology in the feeder plant and a star topology in the distribution plant. Leon-Garcia/Widjaja 21
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Solutions to Chapter 4 - Communication Networks

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