Today's cellular devices have automatically become an extension of our anthropological arsenal, giving us an additional means of communication that we call wireless or mobile communications.
SIM cards a few millimeters in size that are inserted into mobile devices form a network technology used around the world called Global System for Mobile Communications (GSM). We tend to think of it as a decorative cell phone and a few antennas with scattered cell towers as part of a complex architecture suited to our daily needs. The GSM system can be divided into three main subsystems: radio subsystem, network subsystem and operating subsystem.
1. Wireless subsystem--
As we all know, mobile communications travel in the air in the form of radio waves. The wireless subsystem consists of two main components. These include mobile stations (MS) and base station systems (BSS).
The mobile station (MS) is the user device, essentially our hardware phone and the SIM as part of the software. The SIM card is the soul of our device communication and contains all user-specific data such as serial number, card type, PIN, authentication key, PIN unlock key, International Mobile Subscriber Identity (IMSI) and list of subscription services. The MS also stores dynamic data required for wireless communications, such as location information and keys used for encryption and decryption.
The base station system (BSS) is mainly responsible for maintaining the wireless connection with the MS and encoding and decoding voice communications. Today, BSS does this with the help of radio equipment such as antennas, amplifiers, and signal processors, which we call cell towers and formally known as Base Transceiver Stations (BTS). Each BTS operates within a radius or cell area, and there are multiple BTSs and BSSs, each controlled by a Base Station Controller (BSC). BSC is the primary administrator of BTS. It specifies the radio frequency used for communication and performs handover from one BTS to another within the BSS when the MS signal is weak.
The simplified communication path so far can be expressed as:
MS -> BSS (BTS+BSC)
2. Network subsystem--
This part of the communications system is another layer that handles the handover from one BSS to another. This enables what we know as domestic and international roaming. The handover is performed by the MSC or Mobile Switching Center. The MSC knows the subscriber location through its databases - Home Location Register (HLR) and Visitor Location Register (VLR).
A single HLR database stores MS specific information such as IMSI and subscription services and the current location area. Once the MS leaves the current location area, the MS's movement is tracked and updated in the HLR. This updated HLR is copied into the VLR of the MSC responsible for the MS. Unlike a single HLR per subscriber, multiple VLRs exist within each MSC and it saves all the information of the MSs in its location area in the HLR.
The simplified communication path can now be expressed as:
MS -> BSS (BTS+BSC) -> MSC (HLR+VLR)
3. Operation subsystem--GSM
This part of the network is responsible for the smooth operation of the network, involving traffic monitoring, user management, security, billing and billing. It keeps all information in a unified database of existing mobile devices called the Device Identity Register. In the event of a theft, this database is typically updated to blacklist the stolen device and block communications on the SIM card in question.
The updated communication path can now be expressed as:
MS -> BSS (BTS+BSC) -> MSC (HLR+VLR) +EIR
The wireless network also interacts with the public standard networks - PSTN, ISDN, PSPDN and PLMN - through the gateway MSC to complete the GSM network.
Now the final communication path can be expressed as:
MS -> BSS (BTS+BSC) -> MSC (HLR+VLR)+EIR -> Public Standard Network
The aforementioned GSM is the traditional mobile communications back-end technology that allows 2am conversations to flow back and forth constantly, regardless of physical location.
