When a cellular phone is turned on, but is not yet engaged in a call, it first scans the group of forward control channels to determine the one with the strongest signal, and then monitors that control channel until the signal drops below a usable level. At this point, it again scans the control channels in search of the strongest base station signal. For each cellular system, the control channels are defined and standardized over the entire geographic area covered and typically make up about 5% of the total number of channels available in the system (the other 95% are dedicated to voice and data traffic for the end-users).Since the control channels are standardized and are identical throughout different markets within the country or continent, every phone scans the same channels while idle. When a telephone call is placed to a mobile user, the MSC dispatches the request to all base stations in the cellular system. The mobile identification number (MIN), which is the subscriber’s telephone number, is then broadcast as a paging message over all of the forward control channels throughout the cellular system. The mobile receives the paging message sent by the base station which it monitors, and responds by identifying itself over the reverse control channel. The base station relays the acknowledgement sent by the mobile and informs the MSC of the handshake.Then,the MSC instructs the base station to move the call to an unused voice channel within the cell(typically, between ten to sixty voice channels and just one control channel are used in each cell’s base station).At this point, the base station signals the mobile to change frequencies to an unused forward and reverse voice channel pair, at which point another data message (called and alert) is transmitted over the forward voice channel to instruct the mobile telephone to ring, thereby instructing the mobile user to answer the phone. All of these events occur within a few seconds and are not noticeable by the user.
To establish a call the different functions of MSC, Base station and Mobile Station can be written like below:
MSC:
1. Receives call initiation request from base station and verifies that the mobile has a valid MIN, ESN pair.
2. Instructs FCC of originating base station to move mobile to a pair of voice channels.
3. Connects the mobile with the called part on the PSTN.
Base station:
1. FCC: Page for called mobile, instructing the mobile to move to voice channel.
2. RCC: Receives call initiation request and MIN, ESN, Station Class Mark.
3. FVC: Begin voice transmission.
4. RVC: Begin voice reception.
Mobile Station or Mobile:
1. FCC: Receives page and matches the MIN with its own MIN.Receives instruction to move to voice channel.
2. RCC: Sends a call initiation request along with subscribe MIN and number of called party.
3. FVC: Begin voice reception.
4. RVC: Begin voice transmission.
Once a call is in progress, the MSC adjust the transmitted power of the mobile and changes the channel of the mobile unit and base stations in order to maintain call quality as the subscriber moves in and out of range of each base station. This is called a handoff. Special control signaling is applied to the voice channels so that the mobile unit may be controlled by the base station and the MSC which a call is in progress.
When a mobile originates a call, a call initiation request is sent on the reverse control channel. With this request the mobile unit transmits its telephone number (MIN), electronic serial number (ESN0,and the telephone number of the called party. The mobile also transmits a station calls mark (SCM) which indicates what the maximum transmitter power level is for the particular user. The cell base station receives this data and sends it to the MSC. The MSC validates the request, makes connection to the called party through the PSTN, and instructs the base station and mobile user to move to an unused forward and reverse voice channel pair to allow the conversation to begin.
When a new call request arrives from the PSTN or a subscriber, and all of the voice channel in a particular base station are occupied, the MSC will hold the PSTN line open while instructing the current base station to issue a directed retry to the subscriber on the FCC.A directed retry forces the subscriber unit to switch to a different control channel.
All cellular systems provide a service called roaming, this allows subscribers to operate in service areas other than the one from which service is subscribed. When a mobile enters a city or geographic area that is different from its home service area, it is registered as a roamer in the new service area. This is accomplished over the FCC, since each roamer is camped on to an FCC at all times. Every several minutes, the MSC issues a global command over each FCC in the system at all times. Every several minutes, the MSC issues a global command over each FCC in the system, asking for all mobiles which are previously unregistered to report their MIN and ESN over the RCC.New unregistered mobiles in the system periodically report back their subscriber information upon receiving the registration request, and the MSC then uses the MIN/ESN data to request billing status from the home location register (HLR) for each roaming mobile. If a particular roamer has roaming authorization for billing purposes, the MSC registers the subscriber as a valid roamer. Once registered, roaming mobiles are allowed to receive and place calls from that area, and billing is routed automatically to the subscriber’s home service provider.
Several factors may contribute to degraded cellular service or dropped or blocked calls. Factors such as the performance of the MSC, the current traffic demand in a geographic area, the specific channel reuse plan, the number of base stations relative to the subscriber population density, the specific propagation conditions between users of the system, and the signal threshold settings for handoffs play major roles in system performance.Maintianing perfect service and call quality in a heavily populated cellular system is practically impossible due to the tremendous system complexity and lack of control in determining radio coverage and customer usage patterns. System operators strive to forecast system growth and do their best to provide suitable coverage and sufficient capacity to avoid co-channel interference within a market, but inevitably some calls will be flopped or blocked. In a large metropolitan market, it is not unusual to have 3-5 percent dropped calls and in excess of 10 percent blocking during extremely heavy traffic conditions.
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