Solutions to Chapter 4 - Communication Networks

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Communication Networks (2 nd Edition) Chapter 4 Solutions Mobile Station Base Station Mobile Switching Center Base Station Mobile Station Mobile Station Base Station Mobile Switching Center Telephone Switch b. Repeat part (a) in the case where the two telephone calls are mobile. To telephone network In this case once the mobile telephone switching office received the call it determines the cell where the requested mobile phone is located and connects the call to the corresponding base station. If the two mobile phones are in the area covered by the mobile telephone switching office, then the signals do not traverse the public telephone network. If the two mobiles are covered by different telephone switches then the signals must be transmitted over digital lines, and possibly across the public telephone network. c. In parts (a) and (b) identify the points where a call request can be blocked during call setup because resources are unavailable. The call can be blocked between the mobile user and the base station if no frequency channels are available. The call cannot likely be blocked between the Base station and the mobile switching center since it is assumed that this link has a capacity at least equal to that of the aggregate traffic from the base station. For this same reason, it is unlikely that the call can be blocked between the mobile switching center and the telephone switch (in part a). The cellular service provider leases a link to the ground-based telephone network, so it may be possible that a call can be blocked at this point if the entire network experiences a lot of traffic. 67. Explain the signaling events that take place when call is set up and released in a cellular telephone system. Identify the equipment and facilities involved in each phase of the call. Consider the following cases. Solutions follow questions: a. The source and destination mobile phones are in the same cell. Whether or not the destination phone is in the same cell as the source phone, the procedure for call setup may still be the same, because the base station does not know a priori whether the destination phone is in its cell. • Mobile station sends a request in reverse setup channel along with user information such as phone number, serial number, and authentication • Base station verifies information with MSC • MSC sends call request to all base stations Leon-Garcia/Widjaja 32
Communication Networks (2 nd Edition) Chapter 4 Solutions • Base Stations broadcast request to all forward setup channels • Receiving phone replies on reverse setup channel • All base stations that receive this reply forward it to the MSC along with other information such as the power level of the received signal. This information helps the MSC to assign the connection channels • MSC identifies which base station is best suited to service the call for the receiving phone • The base station assigns a forward and reverse channel and the receiving phone is rung b. The source and destination mobile phones are in different cells, but in the same MSC. See answer for part (a). c. The source and destination mobile phones are in different cellular networks. If the number is not stored in the MSC database, it must go through the wire-line phone network which in turn will interface with the receiving phone’s cellular network as described above via the MSC and the closest base station. At the cellular-wire-line network interface, the data must be reformatted to accommodate the signaling systems of the downstream network. 68. Explain the signaling events that take place when call is handed off from one cell to another cell. Suppose first that the two cells are under the same MSC, and then that they are under different MSCs. Solution: The signal level must be periodically monitored when a call is in progress. When it drops below a certain threshold, the base station instructs the mobile station to transmit on a specific setup channel. The base station also signals the MSC, which signals all neighboring base stations to monitor the setup channel. The signal strength and, possibly, other information about the signal is recorded at each base station and sent to the MSC, which uses it to decide which cell is best suited to receive the handoff. The selected base station is informed and the old connection from the MSC to mobile station via the original base station is broken and a new connection via the new base station is established. The service to the mobile phone is briefly interrupted during this process. The process is centralized if the mobile station moves to a cell that is connected to the same MSC as the original cell. Otherwise it is possible that a more distributed control mechanism is used. If, after receiving the information about the signal quality at each base station, the MSC decides that none of the cells are suitable to receive the handoff, the MSC communicates to neighboring MSCs to locate the mobile station. If the mobile station is found in a cell of one of these neighboring MSCs, the base station is informed and the new connection is made through the new MSC. 69. Bob Smiley, star salesman, has just arrived in Los Angeles from his home base in Chicago. He turns on his cell phone to contact his Los Angeles client. a. Sketch the sequence of events that takes place to set up his call. Assume he subscribes to roaming service. Bob Smiley turns on his phone, which sends user information to the MSC via the nearest base station. The MSC contacts Bob’s home location register (in Chicago), and copies Bob’s database information into its own visitor location register (in LA). Further, Bob’s home MSC is not aware that his calls should be forwarded to LA. Now Bob can use his phone in the new network area. b. Next he calls home in Chicago to inform his wife that he forgot to take out his son’s hockey gear from the trunk of his car and to give her the parking spot where he left the car in the Leon-Garcia/Widjaja 33
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Solutions to Chapter 4 - Communication Networks

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