Cellular Solutions LLC
A  CTF certified company

Bill McGirk, CTF Owner/President
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Let Me Explain

As the name implies, cellular networks provide coverage based on dividing up a large geographical service area into smaller areas of coverage called cells. Cells play an important role in reuse of radio frequencies in the limited radio spectrum available to allow more calls to occur than otherwise would be possible. As a mobile phone moves from one cell to another, however, a cellular arrangement requires active connections to be monitored and effectively passed along between cells to maintain the connection. Mobile phones work with certain subsets of the network, typically those associated with the service provider providing the phone and from whom a service agreement was arranged. To administer the cellular network system, provide subscribed services, and accurately bill or debit subscriber accounts, data about the service contract and associated service activities are captured and maintained by the network system.

Despite their differences in technology, cellular networks are organized similarly to one another.

The main components are the radio transceiver equipment that communicates with mobile phones, the controller that manages the transceiver equipment and performs channel assignment, and the switching system for the cellular network. The technical names for these components are respectively the Base Transceiver Station (BTS), the Base Station Controller (BSC), and the Mobile Switching Center (MSC). The BSC and the BTS units it controls are sometimes collectively referred to as a Base Station Subsystem. The transceivers at the BTS can be configured in a variety of ways. A typical configuration involves three distinct sectors of 120 degree coverage: 0 degrees North to 120 degrees Southeast, 120 degrees Southeast to 240 degrees Southwest, and 240 degrees Southwest to 360 degrees North. A cell identifier uniquely identifies the BTS and sector involved in servicing a call. Different cell providers may use different “off-set” configurations on the same tower.

The MSC controls a set of BSCs and manages overall communications throughout the cellular network, including interfacing to the public switch telephone network. To perform its tasks, the MSC uses several databases. A key database is the central repository system for subscriber data and service information, called the Home Location Register (HLR). Another database used in conjunction with the HLR for mobile phones roaming outside of their service area is the Visitor Location Register. Account information, such as data about the subscriber (e.g., a billing address), the subscribed services, and the location update last registered with the network are maintained at the HLR and used by the MSC to route calls and messages and to generate usage records called call detail records (CDRs). The subscriber account data and call detail records are often a valuable source of evidence in an investigation.  Keep in mind billing records are not call detail records, there is a night and day difference in the information afforded from these two forms of records.

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