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12.3 INTRODUCTION TO AMPS SIGNALING 339functions between the mobile switching center (MSC) and its base stations isimplementation-dependent. We assume here that all “logic” of the CMN residesin the MSC, and that the BS merely transmits messages as directed by the MSCand reports all received messages to the MSC. At this point, only a small numberof message parameters are discussed.Channel Number (CHAN). This identifies a voice channel.Voice-Channel Mobile Attenuation Code (VMAC). This indicates the powerlevel at which the mobile should transmit on the voice channel.System Identification (SID). This identifies a particular mobile system. By convention,the “A” and “B” cellular systems have odd and even SIDs,respectively.Mobile Identification Number (MIN). A 10-digit national number that identifies amobile. In the United States, the numbering plan for mobile network is integratedinto the PSTN numbering plan. A MIN consists of a three-digit areaand exchange codes AC-EC, followed by a four-digit “line number” LN(Section 1.2.1). The AC-EC of a MIN identify the “home” MSC of the MS.Calls to a MS are routed by the PSTN to its home MSC.Mobile Serial Number (MSN) or Electronic Serial Number (ESN). Uniquelyidentifies a mobile station.A mobile station has variable, semipermanent, and nonalterable memory devices.The SID of the “home” cellular system and the MIN assigned to the mobile areentered into semipermanent memory by an agent of the “home” cellular system.The MSN of a mobile station is assigned by the manufacturer and is stored in nonalterablememory.12.3.1 InitializationWhen a mobile is turned on, it has to establish contact with a cellular network. Amobile whose home is an “A” system first tries to establish contact with the “A”system that serves the area where the MS is located. If this fails, it tries the “B”system. Mobiles with “B” home systems first try to contact the “B” system.In the example of Fig. 12.3-1, we assume that the mobile has an “A” home system.It starts by scanning the 21 dedicated forward control channels of “A” systems andtunes to the strongest one (i.e., a control channel transmitted by the nearest BS).All forward control channels of a CMN broadcast overhead parameter messages,at intervals of 0.8 s. These messages contain system-specific parameters. A mobilehas to acquire this information before it can access (send messages to) a cellularsystem. Examples of system-specific information are the SID of the CMN, dataabout its access and paging channels, and indicators that represent the characteristicsand capabilities of the system.When the mobile receives its first overhead message (a), it stores the receivedSID, and information about paging channels, in its variable memory. By comparing