A Cellular device today has become unthinkingly an extension of our anthropological arms that has endowed us with an added means of communication, which we term as wireless communication or mobile telecommunication.
A few millimeters sized SIM card plugged into a mobile device forms a worldwide used network technology called the Global System for Mobile Communication (GSM). What we tend to view as an ornamental handset and a few sprung out antennas with scattered cell towers is part of a complex architecture that fits into our daily requirement. This GSM system can be dissected into three main subsystems: Radio subsystem, Network subsystem, and Operation subsystem.
Lets discuss this hierarchical system breakdown to get a brief overview of how telecom providers are engaged in providing the freedom of speech to mobile users all over.
1. Radio Subsystem –
As we all know mobile communication occurs in the form of radio waves that travel through the medium-air.The radio subsystem includes two main components. These include a mobile station (MS) and the Base Station System (BSS).
- The Mobile station (MS) is the user equipment, essentially our cellphones in hardware and the SIM as part of the software. The SIM card is the soul of communication for our device and contains all the user-specific data such as serial number, card type, PIN, authentication key, PIN unblocking key, International Mobile Subscriber Identity (IMSI), and list of subscribed services. The MS also stores dynamic data required for wireless communication such as location information and a cipher key for encryption and decryption.
- The Base station system (BSS) is mainly responsible for maintaining the radio connection to the MS and performing coding and decoding of the voice communication. Now, the BSS is able to do this with the help of radio equipment such as antennas, amplifiers and signal processors, which we see as the cell tower, officially termed as the Base Transceiver Station (BTS). Each BTS acts within a radius or cell region and there are several BTSs and BSSs, each controlled by a Base Station Controller (BSC). The BSC is the prime manager of the BTS. It designates radio frequencies for communication and performs handover from one BTS to another within the BSS when the MS signal is weak.
The simplified communication pathway thus far can be represented as:
MS -> BSS (BTS+BSC)
2. Network Subsystem –
This part of the communication system is another level up and handles the handover from one BSS to another. This enables what we know as national and international roaming. This handover is performed by the MSC or the Mobile Switching Center. The MSC is aware of the subscriber location via its databases- the Home Location Register (HLR) and Visitor Location Register (VLR).
A single HLR database stores the MS specific information such as IMSI and subscribed services as well as current location area. The movement of the MS is traced and updated in the HLR as soon as it leaves the current location area. This updated HLR is copied into the VLR of the MSC responsible for the MS. Unlike a single HLR for each subscriber, there exists multiple VLRs within each MSC and it holds all information in the HLR for a MS in its location area.
The simplified communication pathway now can be represented as:
MS -> BSS (BTS+BSC) -> MSC (HLR+VLR)
3. Operation Subsystem –
This part of the GSM is responsible for the smooth operations of the network, which involves traffic monitoring, subscriber management, security, accounting and billing. It holds all information in a consolidated database of existing mobile devices called Equipment Identity Register. In case of theft, it is usually this database that is updated to blacklist the stolen device and block communication on the associated SIM.
The updated communication pathway now can be represented as:
MS -> BSS (BTS+BSC) -> MSC (HLR+VLR)+EIR
The wireless network also interacts with the public standard networks – PSTN, ISDN, PSPDN and PLMN via the gateway MSC to complete the GSM network.
The final communication pathway now can be represented as:
MS -> BSS (BTS+BSC) -> MSC (HLR+VLR)+EIR -> Public Standard Network
The GSM described above is the traditional mobile communication backend technique in place allowing 2A.M.conversations to flow back and forth incessantly, barring physical location constraints.
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