The Methods by Which Mobile Phones Communicate


When a mobile phone is powered on but not yet connected to a call, it scans a set of forward control channels to find the one with the strongest signal and then keeps an eye on that channel until the strength of the signal drops below an unusable threshold. This triggers a new round of scanning the control channels for the strongest base station signal. The control channels for each cellular system are predetermined and uniform over the entire service area, taking up just a tiny fraction of the total number of channels (the other 95% are reserved for user voice and data traffic). When a phone is idle, it scans the same control channels since they are uniform throughout all markets within a country or continent. To learn about imac pro i7 4k, click here.

When a call is placed to a mobile user, the MSC notifies all of the base stations in the cellular network to prepare to receive the call. The subscriber’s mobile phone number, or mobile identification number (MIN), is paged by sending a signal across the cellular network’s forward control channels. The base station pages the mobile device it is keeping tabs on, and in response, it provides its identity through the reverse control channel. The mobile’s acknowledgment is sent to the base station, which reports the handshake to the MSC. Each cell’s base station commonly employs ten to sixty speech channels and a single control channel; thus, the MSC asks the base station to transfer the call to an available voice channel.

An additional data message (called an alert) is then transmitted over the forward voice channel to instruct the mobile telephone to ring, requiring the mobile user to answer. This is done by the base station signaling the mobile to switch frequencies to an unused forward and reverse voice channel pair. All of this happens in seconds and is invisible to the user.

The various roles of the Mobile Switching Center, the Base Station, and the Mobile Station in establishing a call can be expressed in the following ways:

First, the MSC takes the base station’s request to initiate a call and ensures the mobile device has a valid Mobile Identification Number and Electronic Serial Number pair.
Second, the sending base station tells the receiving mobile to switch to the voice channels.
Thirdly, it links the mobile phone to the PSTN’s dialed number.

Home base:
First, the FCC will page the phoned mobile and tell it to switch to the voice channel.
Second, the RCC receives the call setup data (MIN, ESN, and Station Class Mark).
The third FVC: Start the voice transmission.
4. RVC: Voice transmission has begun.

Terminal-Free or On-the-Go:
First, FCC checks the MIN against its own MIN after receiving the page. He is told to switch to the voice channel.
Second, the RCC initiates a call by sending the subscriber’s MIN and the destination number.
Third, initiate voice reception with FVC.
4) RVC, we’re ready to start talking.

As a subscriber moves in and out of range of each base station while on a call, the MSC adjusts the transmitted power of the mobile and switches the channel of the mobile unit and base stations to keep the call stable. The term for this is “handoff.” For the base station and the MSC to exert control over the mobile unit while a call is in process, exceptional control signaling is applied to the voice channels.

When a mobile phone makes the first move in a call, it uses the opposite control channel to request to do so. The mobile device includes its phone number (MIN), the calling party’s phone number (PIN), and its electronic serial number (ESN) in this request. The mobile also sends a station call mark (SCM) specifying the user’s permitted transmission power range. This information is relayed from the cell base station to the MSC. After verifying the request, the MSC establishes a connection with the called party via the PSTN and advises the base station and mobile user to switch to an idle forward and reverse voice channel pair.

A directed retry is issued by the current base station to the subscriber on the FCC when a new call request comes in from the PSTN or a subscriber, and all of the voice channels in that base station are occupied. Directed retries need the subscriber device to change control channels.

Roaming is a feature of all cellular networks that enable users to make phone calls and send text messages from beyond their home network’s coverage region. A mobile phone is considered a roamer when it leaves its home service area and travels to a new city or region. This is done via the FCC, as each roamer is permanently camped on one. The MSC continuously issues a global command to all FCCs in the system at intervals of several minutes. The MSC sends a system-wide instruction via each FCC every few minutes, telling all unregistered mobiles to submit their MIN and ESN over the RCC. After receiving the registration request, newly added mobiles periodically report back their subscriber information to the MSC, and the MSC uses the MIN/ESN data to query the HLR for billing status for each roaming mobile. The MSC marks a subscriber as a valid roamer if the user has been granted walking authority for billing purposes. Roaming mobiles can make and receive calls once registered, with charges being returned to the subscriber’s home service provider.

Several potential causes of poor cell phone reception include call dropping and blocking. System performance is heavily influenced by MSC efficiency, regional traffic demand, channel reuse strategy, base station density to user population, user-to-user propagation conditions, and handoff signal threshold settings. The immense complexity of the system and the lack of control in defining radio coverage and customer usage patterns make it almost difficult to maintain perfect service and call quality in a densely populated cellular system. Some calls will be bounced or rejected even if system operators do their best to predict system expansion, maintain enough coverage, and prevent co-channel interference within a market. It is common for between 3 and 5 percent of dropped calls and more than 10 percent of blocking in a highly congested urban area.

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Master of Science in Wireless Communication Candidate at National Taipei University, Himadri Subrah Saha.

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